Towhee Trouble

Alright guys, you got me. I made a mistake on my last blogpost. When I described finding some brushfinches in the mountains outside Mexico City, I said "their phylogeny is quite interesting, but maybe I'll save that topic for another blog post," and you smug bastards were on to me immediately. Against overwhelming pressure from my readers and haters, I have no choice. I admit it. I was referring to Charles Sibley's classic 1950s research on the hybrid zone between Collared and Spotted Towhees in central Mexico, and I absentmindedly thought it was a paper about brushfinches. I don't think I ever actually read the paper, I think I must have just seen it discussed somewhere. I had the image of the brushfinch-like Collared Towhee in my head and got mixed up. No one is perfect and I'm admitting my mistake, so please, take it easy with the death threats and hate mail. My inbox is overflowing!

In any event, I'd like to wipe the slate clean and dig into this research a little bit. Charles Sibley's work is blogpost-worthy for several reasons. Firstly, I get a kick out of reading these classic research papers from decades ago, and I hope you will too. Gone are the days when a biologist could go swashbuckling into Mexico with nothing but a small team of student assistants, and collect samples across the entire latitudinal gradient of the country. Sibley's paper contains a tinge of the old adventure writing style that one might find in naturalist narratives for instance from Darwin, or Bartram's Travels. 

Another amusing quirk of science writing from this time is that you can find prominent researchers with polar opposite views on now well-established subjects. Moreover, they were not afraid to express their extreme opinions with hostile confidence. In his introduction, Sibley illustrates how controversial the subject of hybridization was at the time by first quoting Edgar Anderson's view that "introgressive hybridization is certainly more important than all other factors combined in providing raw materials for natural selection to work upon," and immediately follows this with G. Ledyard Stebbins' view that "introgressive hybridization, whatever may be its importance in modifying and amplifying the variation pattern of certain individual species, is nevertheless by its very nature not a way of producing new morphological or physiological characteristics, and therefore of progressive evolution." Science is simply more fun when you have one researcher who's certain that hybridization is the most important factor by far in evolution, while another believes it isn't a factor at all. Nowadays, we know much more about this subject, and these types of arguments tend to quickly fizzle out into a middle-of-the-road "it depends." 

Getting to the meat of the paper, Sibley took on the unique and troubling case of a hybrid zone between two bird species in Mexico: the Collared and Spotted Towhee. Hybrid zones can take on many forms and result from many different situations, but one textbook process that is a useful baseline example is as follows: one freely breeding population gets bisected, possibly by some geographic change such as a river or mountain formation, and reproductive isolation ensues. Over time, the two populations diverge genetically, and eventually, through range shifts/expansion, come into "secondary contact" with each other. At this point, genetic incompatibilities have built up during their isolation, but perhaps not enough for them to consistently recognize each other as different. They interbreed, but because of these genetic differences their hybrids are inviable or less fit for any number of reasons. Total reproductive isolation is rarely achieved in nature, but if hybrids have lower fitness, various mechanisms may discourage interbreeding, and the hybrid zone will gradually grow smaller and less relevant. The exceptions to this, where stable hybrid zones are maintained instead of diminished, often end up being more nuanced and interesting. In some cases, hybrids form fairly stable and recognizable types, think for instance of Brewster's and Lawrence's Warblers, or hybrids between red-shafted and yellow-shafted Norther Flickers that end up with predictable intergrade orange shafts. What makes Collared and Spotted Towhee hybrids so unique and challenging is that they exhibit less predictable variation across several traits (Sibley identifies six key traits that vary between species), and hybrids are far-flung across a range of hundreds of miles. 

Broadly speaking, the pure-blooded Collared Towhees are in western Mexico, and moving east one begins to find specimens with more black on their cheeks and throats. Eventually, their distinctive white supercilium disappears altogether as it is filled in with black. Moving even further into the center of this range, the classic Spotted Towhee rufous flanks begin to show up. The olive green back remnant of Collared Towhees doesn't disappear in specimens until one reaches all the way over very nearly to the eastern coast (DeRaad et al. 2023, a paper I will be discussing shortly, has some terrific graphics that illustrate these various types). This is a bit of an oversimplification however, as across this range researchers found a huge amount of variability, and Sibley identifies several different clines operating at once. Despite these complications, Sibley writes that there's no doubt that this pattern of variation resulted from hybridization between two distinct species. But where did the hybrid border initially form? Why are introgressed traits so wide ranging in this case? Are the different plumage characteristics adaptive in some way? What are the implications of these results for the role of hybridization in evolution writ large? These questions Sibley offers some initial thoughts on, but for the most part he leaves them standing as a challenge to simplistic and idealized conceptualizations of the species question.  

This is a perfectly reasonable conclusion to leave us with, especially given the relatively limited tools Sibley had at his disposal. The truly astounding follow-up however, is that researchers reevaluated Sibley's hybrid zone in 2023, marshalling all the modern genetic resources available to them to answer these questions, and they still wound up short. DeRaad et al. were able to dismiss one of Sibley's hypotheses that recent human-made habitat alterations had caused the unusual hybrid distribution by showing that in the 70 years since Sibley collected his specimens, the genetic picture remained largely the same. This finding displays more stability in towhee hybrid forms than would be expected of a recently formed hybrid zone, and lends more credence to the theory that a longer time frame of hybridization caused the resulting variation. DeRaad et al. postulated that secondary contact may have occurred more than one thousand years ago. DeRaad cleans up a few of Sibley's other misapprehensions and provides a wealth of new genetic context to the picture, but ultimately finds that these towhees are pretty much just as tricky today as they were 75 years ago. 

DeRaad et al.'s results confirm that Collared and Spotted Towhees do not form a typical "tension zone," where two distinct species' ranges meet in the middle and suffer from low hybrid fitness. The hybrids are too wide ranging and have "erased nearly all fixed autosomal differences between the two ends of the cline" (DeRaad et al. 2023). They also consider the possibility that the range represents a complex of multiple semi-isolated lineages of towhees that are in the process of developing reproductive isolation, and potentially even full species status, but yet again come up short of anything conclusive. It is possible that hybrids are hybridizing with other hybrids, creating novel variation and complicating the picture even further! DeRaad painstakingly characterizes the individual trait variation and provides compelling evidence of how various traits could be adaptive in different parts of the complex patchwork of habitat, but since many traits seem to vary individually and sometimes along different gradients from other traits, it is quite difficult to explain in total. Researchers find several candidate regions where more genetic differentiation exists between populations, but never enough to confidently establish species or even subspecies delineations. 

No doubt somewhat exasperated, DeRaad et al. conclude "it is humbling to recognize that many of the novel genomic patterns we document here bear out Sibley's (1950) taxonomic recommendations." The unique biogeography of their range, and the complex and thorny patchwork of their hybrid interactions bely simple categorization, even after 75 years of study. Echoing the sentiments of a paper I covered in this blogpost, modern genetic methods, as comprehensive as they've become, do not always offer easy solutions to nature's pesky idiosyncrasies. From purely phenotypic work done 75 years ago up to the modern genomic techniques of today, the specific nature of this species relationship remain hidden away, locked in mystery, like Excalibur locked in stone. Will there ever be a hero strong enough to pull out the truth? 

Bibliography:

Sibley, Charles G. “Hybridization in the red-eyed towhees of Mexico.” Evolution (1954), vol. 8, no. 3.

DeRaad, Devon A. et al. “Hybrid zone or hybrid lineage: a genomic reevaluation of Sibley's classic species conundrum in Pipilo towhees.” Evolution; international journal of organic evolution (2022): n. pag.

Birding CDMX

The first thing to understand about Mexico City is that we Americans do not belong there. This may come as a shock to some, especially as the city is currently overwhelmed by remote-working product manager types from our great nation, but the problem relates to a spiritual rift that has been growing between our neighboring countries for centuries. To put it simply, we took their land 175 years ago and turned it into Tempe Arizona. After the Mexican-American war, we chose the path of superhighways, 3-ton pickup trucks, and salad bowl chain restaurants, which locked our national spirit into an existence that is incompatible with life in Mexico’s capital city. It is for this reason that while spending a long weekend in Mexico City two weeks ago, when I felt vaguely too tired and a little uncomfortable throughout the trip, I knew it wasn’t the cervezas, or the sudden change in altitude, or even the spicy plates of chilaquiles with which I’d start each morning. I knew deep down I was being spiritually punished for trespassing into such a wonderful place. Fleeing the concrete-mega-gas-station-manicured-lawn-sprawling-subdivision-fast-food matrix of central Florida, Mexico City’s pleasantly walkable streets lined with lush vegetation and colorfully painted buildings felt almost too good, bordering on unreal. I spent the weekend enjoying the delicious food, warm atmosphere, and of course the very good birding in the area.  

I was there with my girlfriend Nada and her family. Her parents had managed to get tickets to the World Cup match that took place the preceding Wednesday between South Korea and South Africa in Monterrey, and they very kindly invited me to meet them afterwards in Mexico City for the second half of their trip. Nada and I flew out of Orlando on Thursday night, and since I had left for a Snail Kite banding session at one of our most remote sites at 6 o’clock that morning, by the time we arrived in Mexico City at around midnight EST, I was wiped out. We had planned to rent a car for the weekend, which I had been trepidatious about, not exactly knowing the driving culture of the area. On arrival, dead-tired and wanting nothing more than to get to our Airbnb, we found that we had rented a car from the one rental agency that only had an office at the terminal across the airport from the one we landed in. Fatigue-driven irritation welling up in me, I held my nerve and found the shuttle to the relevant terminal. The shuttle bus driver informed us it would cost us to get there, and we could pay “solo en efectivo.” Of course I had not yet converted any currency, and this minor inconvenience was the final straw. Boiling over, I petulantly told Nada, “I never thought a rental car was a good idea anyway.” We decided to take a taxi to our Airbnb 20 minutes away, and the fight spiraled in the car. “You agreed it was a good idea when I planned this,” she retorted, “I did nothing of the sort,” was my reply. It got out of control from there. After getting to the Airbnb, maneuvering around Nada’s impulse to leave me and walk through the midnight streets of CDMX to her parent’s Airbnb for the night, we finally were in bed by 1 am, or 3 am EST. Needless to say we would not be getting up early for our first morning of birding in the country.  

Day One:

The beautiful weather the following morning contributed nicely to our amelioration. How could we be mad at each other as we ambled our way amongst the fragrant taco stands and mom-and-pop shops of the Roma neighborhood, as Inca Doves foraged seeds on the sidewalk, and Rufous-backed Robins sang from the trees shading our passage? Stopping at Hule café for an espresso and our first plate of chilaquiles of the trip, we finally had a chance to laugh about our Airbnb host’s obviously google-translated welcome message. Jokes about my girlfriend’s name are virtually always annoying and never welcome, but we agreed that the way our host began with “Dear Nothing,” was somewhat funny. Caffeinated and full-bellied, we embarked on the remaining 20 minutes of our walk to our first birding destination.

Bosque de Chapultepec is a large and relatively well conserved park in downtown Mexico City with an eBird list of 266 species. It is a convenient and exciting spot to bird, especially for a gringo like myself enjoying his first visit to Mexico. Entering the park, we immediately found a few more Rufous-backed Robins and a flock of Bushtits. The Bushtits there represent the unique “melanotis” or “black-eared” subspecies and caused me some initial confusion. These birds are everywhere in the city, and the first time I laid bins on them, despite immediately recognizing their “giss” (birding lingo short for “gestalt”) as “Bushtit-like,” their handsome black mask gave me pause, since the northern types that were more familiar to me are uniformly gray. I was not expecting to see such a unique subspecies, but it was a pleasant surprise.

                                                              A Rufous-backed Robin hunting for worms.

A little further into the park, Nada pointed out a seemingly orphaned group of ducklings in a small pond. Looking everywhere, I couldn’t find any adults, but by process of elimination we decided they were Mexican Ducks, and we encountered a few adults later, paddling around a bigger pond. Following a somewhat crowded footpath, we stopped to dig into some activity around a gently sloping hillside. In this area we found a few calling Bewick’s Wrens, a Beryline Hummingbird, Broad-billed Hummingbird, some lovely breeding plumage Lesser Goldfinches, and a singing Black-headed Grosbeak. So far, along with the common and garrulous Great-tailed Grackles, this list of birds could easily have come from Southeast Arizona, which is a region I have birded extensively. Beryline Hummingbird was really the only semi-rare hummingbird I missed while I was in Arizona however, so I was quite happy to pick them up here. I also was never lucky enough to find a vagrant Rufous-backed Robin in the state, so that was another much-appreciated lifer. 

Suddenly, we ended up in a busy market area. Vendors surrounded us, and speaker-blasted music drowned out the birdsong. Anxiously seeking shelter at a nearby creek, we were rewarded with a pair of calling Harris’s Hawks perched high up in some trees and being mobbed by grackles. Chapultapec is large enough to be separated by a highway into two distinct sections, and at this point I realized we were still barely into the eastern “Primera Seccion” of the park and would be potentially an hour’s walk from the better birding of the more interior “Tercero Seccion.” Greedily, and conscious of the clock ticking towards midday, we hired an Uber to take us into the western depths of Tercero.

                                                          This Harris's Hawk was calling from its high perch.

Upon arrival, within a small group of House Finches, Nada pointed out a diminutive Northern Beardless Tyrannulet (despite only getting into birding a few months ago, she’s really a natural). We found a little trail through a dense and hilly area, but we were approaching peak midday heat, and Nada’s prioritization of fashion over comfort brought her to the park in a stylish pair of cowboy boots, which while attractive, were actively digging a hole in her heel skin, and we both felt we ought to return to our Airbnb soon for recuperation. Bird activity had dropped off significantly, but before we left, we were able to get some great views of a decent sized group of Vermilion Flycatchers, including a stunning red male, along with some Vaux’s Swifts soaring high up in the sky amongst some of the more abundant Barn Swallows. While our lazy late morning visit did not produce the most impressive bird list, and despite a few sore misses such as Blue Mockingbird, under the circumstances we left the park happy with our efforts and comforted by a few lifers.

Our first afternoon in the city was less about birds than relaxation. We took some rest at our Airbnb before enjoying a local pizza dinner, and spent the remainder of the evening with Nada’s parents, who have no interest whatsoever in birding. We went to sleep giddy with anticipation for our following morning in the mountains.

Day Two:

We rose early on Saturday and took a thirty-minute uber from the valley within which Mexico City is nestled, up into the mountains to the entrance of Parque Nacional Desierto de los Leones, which was the site I was most excited to explore on our trip. Leaving the Uber, we were greeted by cool air and large montane conifers that triggered memories of the Pacific Northwest. On the outskirts of the park, we scanned the edge of the forest and found several lifer White-throated Thrushes, as well as a more familiar Hutton’s Vireo and Mexican Chickadee. Finding some American Robins felt like bumping into an old friend at a party full of people you don’t know.

A muddy dirt trail took us further into the vast forest. Western Flycatchers were calling all around, and the first bird we laid eyes on in the area was the much anticipated and stunning Crescent-chested Warbler. After enjoying this unique and colorful species, its distant cousin, the Golden-browed Warbler, flew in and gave phenomenal views as well. Later, a skulky Central Mexican endemic Russet Nightingale-Thrush gave brief views as it hopped through a dense bush. Nada unfortunately missed this individual, but we heard several others singing throughout our hike and eventually got good views of a couple.

Taking pride in being a snobbish birding purist, I don’t tend to use the Merlin app much to identify bird calls, but Nada was using it throughout our walk and I must say it helped us quite a bit, as I hadn’t done my homework and was largely unfamiliar with these unique Mexican bird sounds. At one point Merlin recognized the high-pitched whistle of a Brown-backed Solitaire, but it was far too high up in the canopy for us to pick out. As we strained for a window through the tree branches to see where it was perched, another individual called from deeper in the forest. After a brief duet, we could hear our closer bird’s calls growing more distant, as it presumably flew off towards the other solitaire to sort out their territorial dispute. We heard one other solitaire later in our walk but sadly left our trip without a view of any.

Birding the forest was quite difficult, as the healthy towering conifers did a good job of obscuring many of the little flitting birds in the canopy. Our main target of the day (and really of the entire trip) was the stunning endemic Red Warbler, and for the first hour or so of our walk, we had merely been teased by its delicate song and had yet to get a view of one. I knew from preparing for this trip that these warblers could be observed quite reliably at this site, but I had the same impression about the solitaires that escaped me, and as time passed, I couldn’t help wondering, would we really leave without laying eyes on one? Suddenly, we noticed a darting flash of bright red through the branches. This was it; I would have to act quickly. Scrambling to get an angle, the bird obliged, and we were blessed with incredible views for a solid few minutes before the little gem took off. Red Warblers are a unique highland specialist with a small range, disjunct between a Sinaloa population to our north and a somewhat contiguous central/southern population that extends from Guadalajara in the west to Oaxaca in the east. The northern subspecies has a dark gray to black cheek, while our southern subspecies’ cheek is a handsome silvery-white. Needless to say, I was buzzing, and if I did not lay eyes on a single other bird for the rest of the trip, I would have returned home satisfied.

                    I debated whether I should include this horrible record shot, but this warbler is simply too cool not to share!

Luckily that would not be an issue, as the last few hours of our walk produced several more species, notably two endemics that were significant targets for us: the Green-striped and Rufous-capped Brushfinch. These charismatic sparrow-relatives are always a treat to see, and their phylogeny is quite interesting, but maybe I’ll save that topic for another blog post.

I felt as though I could’ve birded there all day, but around noon we decided we ought to call it, and we returned to the city to meet Nada’s parents for lunch. While birding is conventionally viewed as a hobby, it can also be viewed as an affliction, or a derangement. In hard cases such as myself, the birding sickness tends to manifest in especially devious ways during trips abroad. As I sat with Nada and her parents at lunch discussing plans for the rest of the day, I was at war internally. Her parents suggested making a trip to the Frida Kahlo museum, or perhaps some ancient ruins outside the city, and the human part of me thought that sounded nice, but the birder in me rebelled. I tried to quiet him, to force him back into the depths of my being, but he was too powerful. “You know, the university botanical gardens are quite close to the Frida Kahlo museum, perhaps we could go for a short walk there first?” I blurted out. “Oh, ok. What can we do there?” was Nada’s mom’s polite challenge. “Well, they have all these lovely plants, and the university campus is famous for its unique architecture,” was the angle I came up with. “Hmm. Ok, why not.” Lukewarm at best, but the drug addict isn’t concerned with debasing himself as he hunts for a fix, and neither was I at that point.

The four of us crammed into an Uber and off we went. On the way there I realized with frustration that the gardens close at 3 pm on Saturdays, and we would be arriving just after 2. I would not have nearly as much time as I would have liked. When we arrived, before we even passed through the gate, I noticed some kind of flycatcher perched on a tree branch. Upon closer inspection I was glad to see it was a lifer Buff-breasted Flycatcher. Staying true to the theme of picking up birds I had missed while living in Arizona, Buff-breasted Flycatchers had been a sore subject for me ever since my brother and I missed them on our trip to the Chiricahua mountains a few years ago. I was glad to finally see one here.

The botanical gardens ended up being quite pleasant. The first area we explored was a nicely constructed desert landscape, featuring many beautiful desert plants and some Curve-billed Thrashers, which were lifers for Nada and a new bird for our trip list. It was here that we found our first herp of the trip, the oddly named crevice swift (Sceloporus torquatus). These are large spiny lizards endemic to central Mexico, and even Nada’s parents seemed excited to see them basking gingerly on the black volcanic rocks. With less than an hour to explore the huge gardens, Nada and I agreed that we ought to break from her parent’s leisurely pace. As we sped off deeper down the main path, we quickly came upon a group of one of my top remaining targets for the trip, the Gray Silky-flycatcher. These unique and elegant long-tailed birds flew into the crown of a large tree near us, and despite flying actively in a chaotic jumble, Nada and I both eventually got great views of them before they left to find more berries elsewhere.

Stopping in a little cul-de-sac with some flowers that looked good for hummingbirds, we found a Beryline and a Broad-billed Hummingbird. In the distance I noticed a Ladder-backed Woodpecker climbing up a bare tree branch. A pair of Canyon Towhees hopped around on the ground nearby, foraging seeds seemingly without a care in the world. Before I knew it, it was nearly 3 pm, and reluctantly, we had to turn around. On our way back to the entrance, we passed another flowering bush, and noticed a few people gathered around pointing to something in it. As we approached, we found a lovely female White-eared Hummingbird buzzing from flower to flower, as well as a Cinnamon-bellied Flowerpecker delicately hopping amongst the branches. It was nice to see two distinct strategies for consuming nectar side-by-side. I left the gardens wishing for more time but supremely happy with the birds (and one lizard) we managed to encounter.

                                      Cinnamon-bellied Flowerpecker. Low bar but probably my best picture from the trip.

These early afternoon periods in the day seemed to be when the altitude affected us flatlanders most, so when we got to the Frida Kahlo museum a little later and were told they were fully booked, the prospect of a little afternoon rest more than made up for our disappointment. Sunday would be my 27th birthday, but since we would be traveling for most of it, we decided to enjoy a cheeky early birthday dinner on Saturday night. Nada and I went to El Califa de Leon, a famous and bustling taqueria known among other things for their wide selection of vegan options. Naturally, we went a little overboard and sampled quite a few of them. They were all delicious, but what blew us away most was the spread of six or seven different vibrant salsas that accompanied the meal. We fell asleep that night doing our best to avoid the harsh truth that the following morning would be our last for the foreseeable future in CDMX, but with a resolve to make the best of our final bit of birding.

Day 3:

Our last birding session of the trip would be along the Canal Nacional, in the southeast part of town. This stop was mainly driven by an obsession I had developed over the last few weeks with finding a Sparkle-tailed Hummingbird. This stunner is not so easy to see anywhere in Mexico City, but it seemed like our best bet would be to scan the flowering plants along the lush footpath accompanying the canal. It was a beautiful morning, but unfortunately, we were not the only ones with the good sense to spend it there. Maybe the park is less crowded on weekdays, but on that Sunday morning it was full of people strolling and jogging. Not quite ideal for birding, but it was still enjoyable, and we quickly picked up a few common but new species for our trip list, including Green Heron, Common Gallinule, Song Sparrow, and Eurasian Collared Dove. It was also funny to see a large group of exotic Monk Parakeets noisily foraging in some trees.

Hearing a song that I did not recognize, I looked up and was ecstatic to find a last-minute lifer Black-backed Oriole, with its striking orange belly and black upperparts. Sadly, it quickly flew off, and we did not see another. The canal was lined with orange flowering plants, and I could not help being repeatedly annoyed by the lack of hummingbirds in them. Any bit of motion around these flowers, whether from insect or wind or Song Sparrow, drew my anxious attention. Finally, I noticed the unmistakable emerald back of a hovering hummingbird and wished with all my might for a sparkle-tailed but discovered it was merely a common Beryline. As birders we must be vigilant against jadedness, and I had to remind myself that Beryline Hummingbirds are extremely cool too, despite my undeniable disappointment. As we left the park for the airport, even without my hummingbird target, it was hard to feel anything other than satisfaction overall.

                                           Beryline Hummingbirds were the most common hummingbird in the area.

My first visit to Mexico was tantalizingly brief but very lovely. The 58 species we recorded over three days were a nice introduction but only added fuel to my fire for birding more extensively in the country. Maybe Oaxaca ought to be next?

Here’s my eBird trip report from the weekend:

https://ebird.org/tripreport/544132

Invasive Anole Evolution

One of the major challenges of studying evolution is that animals tend to evolve over time periods that extend far beyond the lifetimes of the scientists who study them. Evolutionary biologists generally make do with reconstructing evolutionary histories via clues left over in the present, and with decades of technological innovation and creative science, this approach has led to a robust patchwork of theory. That said, it is always nice when a capable scientist is blessed with a rare opportunity to study evolution in nature in real time. One of the most memorable papers I've come across in recent years happens to be written about exactly this kind of blessing. Stroud et al.'s "Observing character displacement from process to pattern in a novel vertebrate community," from November 2024, is the fascinating result of the kind of crafty and opportunistic science that I find exhilarating. 

The paper rests on a significant body of theoretical work on adaptive radiation, which is a subject that is a little broad to be covered thoroughly in a casual blogpost (and probably beyond my capability in any format). For those unfamiliar with the concept and looking for some resources to check out, I recommend starting with the work of Dolph Schluter, i.e. the adaptive radiation GOAT, and/or Peter and Rosemary Grant's prolific research on the Darwin's finch study system. Darwin's finches are the textbook example of an adaptive radiation, but other famous examples exist, for instance the cichlids of the central African crater lakes, the silverswords of Hawaii, and--most relevant to today's paper--the Anolis lizards of the Caribbean.

Stroud et al.'s 2024 paper builds upon theoretical work done particularly by Jonathan B Losos on the Anolis lizard system, and I would therefore be remiss if I didn't at least mention Losos' classic 1998 paper "Contingency and Determinism in Replicated Adaptive Radiations of Island Lizards." In this paper Losos shows that on each major Caribbean island, several species of anoles exist and fall into consistent "ecomorphs," or categories of behavior, habitat use, and corresponding morphological characteristics. On each island, anoles consistently occupy six distinct microhabitats that Losos defined as "crown-giant, grass-bush, trunk, trunk-ground, and twig," based on which parts of trees these arboreal lizards primarily spend their time. Observing a system with such remarkably consistent ecomorphs across multiple islands and dozens of species, Losos judged only two explanations to be feasible: either anoles evolved once into these distinct niches and then spread from island to island (which would be the parsimonious option), or anoles colonizing each island were repeatedly and separately driven into the same ecomorph categories. If the former hypothesis were true, anoles in each ecomorph would be more related to anoles in the same ecomorph class across islands, whereas if the latter hypothesis were true, this pattern would not be detected. After some genetic tests, Losos showed that anole relatedness was no stronger among ecomorph classes across islands than across species within islands, and the likelihood of these patterns being driven by repeated and regular adaptive radiation skyrocketed. 

Using genetics and carefully collected behavioral and morphological data, Losos provided strong evidence of some degree of determinism in evolution, as well as the power of adaptive radiations in generating diverse natural forms. While a massive contribution to science, Losos' conclusions still relied on a theoretical reconstruction of past events, leaving adaptive radiation-heads such as myself yearning for real-time, observational confirmation. This is where Stroud et al. 2024 (finally) comes into the picture. Stroud and his collaborators were busy studying invasive anoles in South Florida when one of his study sites that initially only had brown anoles (Anolis sagrei) was invaded and eventually dominated by crested anoles (Anolis cristatellus). Stroud jumped on the opportunity to observe the early phases of these novel competitive interactions, as these two species don't overlap in their native ranges. He and his collaborators observed that while in allopatry, both crested and brown anoles were happy to spend their time occupying similar regions of the lower sections of tree trunks, only spending 2% of their time on the ground. In Lososian terms, both species instantiated the same ecomorph class. In sympatry however, Stroud et al. were able to show that cristatellus individuals moved further up the trunk and sagrei further down, to the point where sagrei individuals were observed spending an average of 33% of their time on the ground. Novel competition had driven these anoles into separate ecomorphs! Aside from these behavioral observations, Stroud et al. showed that the newly ground-dwelling sagrei had developed significantly longer fore and hindlimbs, a result that conforms perfectly to the morphological characterizations made by Losos of Caribbean ground-dwelling ecomorphs. 

Fully-fledged adaptive radiations such as those found among anoles in the Caribbean may take thousands of years to develop, and what Stroud and his collaborators observed was character displacement, which is only a first step towards radiation. However, it is a crucial step, and one that is rarely observed so clearly in nature. The way that the results conform so exactly to the theoretical work laid out by Losos and others is also a lovely vindication of theory. Aside from these great qualities, as a biologist currently working in Florida, I also particularly love this paper because it draws a beautiful result from the invasive disaster zone that is South Florida and interrupts the steady stream of doomsday-level invasive news coming from that region with a terrifically exciting bit of science. 

Works Cited:

Stroud, J.T., et al. Observing character displacement from process to pattern in a novel vertebrate community. Nat Commun 15, 9862 (2024). https://doi.org/10.1038/s41467-024-54302-1
   
Losos, J. B., et al. Contingency and determinism in replicated adaptive radiations of Island Lizards. Science, vol. 279, no. 5359 (1998). https://doi.org/10.1126/science.279.5359.2115.

Snail Kites Prefer Escargot?

Two weeks ago I observed something quite unusual during a Snail Kite survey. I was tasked with meeting the project's Kissimmee area crew to survey a section of the Kissimmee river with them. As field lead of the state wide Snail Kite monitoring crew, I'm used to making some long drives to get to our daily surveys, but this site was only fifteen minutes from our field house. Basking in the luxury of a short drive, I rolled out of bed at the civilized hour of seven a.m., and met the crew for what I expected would be a short and kite-less survey. Kissimmee River can be an incredible spot to see kites when the river floods and creates a healthy floodplain. A rushing river is not much use to a Snail Kite, but when the grassy plains surrounding a river channel flood, shallow, vegetation-rich conditions can arise and provide perfect foraging habitat. A good rule of thumb for kite activity is too little water=no snails, while too much water=too deep for kites to successfully hunt snails. Anyway, in the fall the river floods, and in October we counted nearly 150 kites in one survey, which was among the highest counts we achieved in the state all month. Over the rest of the year however, water levels tend to drop, and our monthly surveys reflect the degraded conditions. Over winter, with water levels clearly already dwindling, we could muster a fifty kite survey, but across our last three spring and summer surveys, the river banks looked bone dry and we counted one measly, probably confused and disoriented kite. I had no reason to expect my most recent survey to be any different. 

As we launched our boat I planned to be on the water for no more than three hours, but rounding a bend in the river six miles in, I was rudely shocked. We came upon a section of the river where land managers had clearly done a prescribed burn recently, and my colleague Melisa pointed to a kite perched on some vegetation. "Oh nice," I thought, "at least we'll have one kite this survey." Shutting the boat off and pulling up to a steep river bank to inspect the bird more closely, I called out to Sandy, who on this day was recording data for us, and I asked her to note one un-banded male kite on the datasheet. Melisa quickly and politely contradicted me by pointing out that the bird was actually a banded female kite. Slightly embarrassed, I double check, and am filled with righteous indignation when I realize I had not made any mistake! My kite had orange legs unmarked by the color bands we apply, a fully blue head, striking red eyes, a bright orange cere, and could not have been anything but adult male. Before I had a chance to argue, it became clear that we were simply looking at different birds. After a cursory scan of the burnt area of the river bank, the three of us realized there were not one, not two, but dozens of kites present. 

What the hell was going on? Prescribed burns provide a whole bunch of ecological benefits, but I had never heard of them helping Snail Kites. Everything I knew about the species told me they need nice, shallow, freshwater wetlands where they can hunt live snails, but here I was watching them in a half mile section of bone dry, scorched, floodplain, scavenging dead apple snails. Melisa and I disembarked on the river bank while Sandy was left with the unenviable task of hanging back to make sure our airboat didn't float away down the river. Sensing I was witnessing an unusual ecological event, I took pictures of the chalky, scorched snail shells and their boiled meat, along with the coal-black mud and stripped bases of plant stems that the dead snails were resting on top of, and then I got to work counting kites. When so many kites are swirling around, actively hunting (or scavenging in this case), and territorially chasing each other, my job becomes quite tricky. In moments like these it's crucial to have a cool, calm, and collected voice assisting you, and luckily, as I was stressing myself out trying to keep track of which kites I had counted and which I hadn't got to yet, Melisa was my guiding light. With her help, after about an hour and a half, we were pretty confident in our count of fifty three total birds. 

After finally getting an accurate count, we continued with the remaining thirty miles of our survey track down the river channel, which turned out about as I expected it to that morning. The unburned river banks were lush and thick, and while beautiful to look at, would have completely concealed the scant puddles of water where apple snails may or may not have been eking out an existence, preventing kite foraging. We saw only two other kites for the rest of our survey. On my drive home that afternoon, I texted my boss to see if she had ever heard of kites scavenging dead snails on this scale, and she hadn't. Performing a cursory literature search online, I did not find any published accounts of such behavior either. 

Hopefully we'll be able to publish some account of this experience in a scientific journal, and maybe more experienced scientists than I can shed some light on this. For what it's worth, with my two years of involvement in this project, my best guess would be that the terrible drought plaguing the state is probably the root cause of this novel foraging strategy. Already, over the last few months, we have been making note of increased instances of unusual foraging. While apple snails make up ~99% of Snail Kite's typical diet, lately, we've observed unusually high instances of them deviating and eating musk turtles, dead fish, a few dead birds, some kind of large crab (pictured below), and now this scorched snail feeding frenzy. We have also observed some fierce intraspecific fighting, which I'm told is also generally uncommon for these kites, and may be another sign of resource scarcity and increased competition. Concealed within the excitement of observing a new and unexpected behavior, it's hard to avoid feeling that the root cause is related to degrading kite conditions throughout the state. 

Curious Side Effects of Invasive Species

I am currently working as the field crew lead of the University of Florida’s Snail Kite monitoring project, which comes with several perks. The most obvious is that I have the daily privilege of getting to zip around on airboats through beautiful, remote, wetland habitat across the southern half of Florida. I spend hours a day observing and occasionally handling and banding Snail Kites, which are unique and beautiful birds that I can’t imagine ever tiring of watching. On our crew we sometimes joke about how they lack the intimidating presence of a typical bird of prey; they’re simply too goofy. While they spend their days deftly plucking apple snails from fresh water marshes, unthreatening to anything other than their mollusk victims (outside of the rare times they find an unlucky musk turtle to gruesomely excavate and consume), in this cruel and unforgiving world of ours they still get relentlessly mobbed by grackles and red-winged blackbirds, as if they were any other egg-stealing, fledgling-hunting, run-of-the-mill bird of prey. Their strained, nasal, almost mechanical calls are another incongruity separating them from their raptor relatives, but as with their other quirks, they are simply delightful. It’s definitely worth a Google if you’ve never heard the various calls they make, by the way.    

Young male Snail Kite with a musk turtle. ~99% of their diet is made up of apple snails but they will rarely catch other prey.

Getting back to the point, one of the other major benefits of my job is that I get to be part of one of the longest running wildlife monitoring projects in the country, and I can claim some association with the incredible wealth of research that biologists in the lab have been putting out about Snail Kites for decades. A few years ago, the UF Snail Kite lab published a paper that was exciting enough to generate a certain amount of mainstream attention. Cattau et al.’s 2018 paper, “Rapid morphological change of a top predator with the invasion of novel prey,” has been covered by all sorts of outlets including the New York Times. The paper is an elegant and reassuring example of what the daily toil of my field crew and the many field crews before us can contribute to. Cattau et al. analyzed the previous 11 years of meticulously collected nestling morphological data and coupled it with quantitative genetic analyses to show conclusively that since the invasion of the larger Chinese apple snail species into Florida in the late 2000s, Snail Kites had grown significantly larger beaks, talons, and tarsi. A fascinating result, especially given that in 2007, just 11 years before Cattau et al.’s paper, Snail Kite researchers Darby et al. published “Food-handling Difficulties for Snail Kites Capturing Non-native Snails,” in which they recount numerous observations of kites dropping the significantly larger Asian snails and otherwise failing to extract their meat. In a matter of a mere 11 years, researchers went from worrying about invasive snails acting as ecological traps that could hurt the kite population, to showing clearly that kites have adapted in a very short time span and are now actually benefiting from their new invasive prey.

A crucial and underrepresented detail I want to highlight from the paper is that the researchers make it very clear that these morphological changes come not quite from strict evolution but from phenotypic plasticity. The quantitative genetic analyses they used show that the genetic components of these traits did not significantly change over their study period. This suggests that “cryptic genetic variation,” e.g. preexisting variation in the population that is only made visible phenotypically by some novel environmental condition, is primarily behind the rapid shift. Researchers point to the increased overall growth rate in kite nestlings in areas with higher invasive snail availability as an indication that this richer food source may be triggering a shift to longer beaks and talons somewhat incidentally. They also highlight that these traits show a high degree of “additive genetic variance,” which is the genetic heritability component that is uncluttered by epistatic and dominance effects and otherwise known as “narrow-sense heritability.” This is good news for the kites since it means that real evolution in the form of genetic change may be easier/faster to generate in these traits, compared to a situation where more complicated dominance and epistatic hierarchies among alleles were involved and slowing down the adaptive process.  

Even though phenotypic plasticity is not technically evolution, this rapid morphological change is an incredible example of how the complex web of interactions between genes, phenotypes, and the environment is always changing and bringing about unique forms of life. Human beings are challenging wildlife in ways that go beyond habitat degradation and destruction and include the reshuffling of wildlife interactions via the promulgation of invasive species, and native wildlife must be resilient. In many cases these changes only bring devastation, but it is nice from time to time to read about a positive case, especially when it involves a species as unique and wonderful as the Snail Kite.

Citations:

Cattau, C.E., Fletcher Jr, R.J., Kimball, R.T. et al. Rapid morphological change of a top predator with the invasion of a novel prey. Nat Ecol Evol 2, 108–115 (2018).

Darby, Philip C., David J. Mellow, and Miranda L. Watford. Food-handling difficulties for Snail Kites capturing non-native apple snails. Florida Field Naturalist 35.3 (2007): 2.

 

 

The Limits of Delimitation

Those of us who aren’t so well-versed in speciation research--or perhaps even biologists from a few decades ago who were in their day but are just now waking up from some kind of cryogenic chamber--may be puzzled by the question of why relatively easy access to full genome data has not cleanly solved the problem of how to define species. We live in a modern world where genomes of numerous species have been fully mapped out, but determining what constitutes enough genomic divergence to define a species unit remains a surprisingly nuanced issue. Even a complete understanding of a species’ genome may not always be a skeleton’s key. A recent review article by Singhal et al., titled “A Genomic Perspective on Species Delimitation,” provides a succinct summary of many of those lingering difficulties.

I remember a few years ago being amused by a (now paywalled) episode of the Big Biology podcast with entomologist Michael Sharkey, about a paper he produced that used genetic barcoding techniques to identify over 400 new species of South American wasps in one fell swoop. Despite the waves of backlash he received from concerned taxonomists claiming a simple “cut and paste” barcoding method provided nowhere near enough information to accurately identify “real” species, Sharkey maintained in a fairly convincing way that in the face of our ever-growing ecological crisis where so many unidentified species are in danger of extinction, his methodology is the only feasible way to immortalize a good share before it’s too late. Hymenoptera, the most diverse order on earth, contains over 150,000 identified species, with an estimated 1 million more remaining unidentified. Sharkey argued that without high-volume identification methods, an inestimable number of these species will go extinct before being recognized by science. He goes on to mention that his article contained supplementary material pointing to the museum specimens he collected, as well as detailed methodology that anyone can follow to develop his work. It is an interesting philosophy that highlights the diverse assortment of practical challenges biologists face these days, but it clashes somewhat with the gist of this review article.

Singhal et al. begin with a nice summary of genomic delimitation techniques, followed by several brief case studies, one of which focused on redpolls. They highlight research that led to the collapse of three existing redpoll species into one (and ruthlessly took a hard-earned Hoary Redpoll from my life list, damn them!). In this case, RADseq and Multispecies Coalescent data revealed no significant difference between the three putative species, yet populations showed clear differentiation in certain genomic regions that were associated with the subtle morphological differences that previously formed the basis of their species status. “Whole-genome resequencing data” revealed the culprit: a large inversion in an area of the genome coding for plumage and beak morphology. Inversions tend to be relatively immune to recombination, so it’s natural that even in a freely breeding population, an inverted chunk of genes could persist as a separately diverging region. Without a careful understanding of these and related structural factors, such divergence can be interpreted as meeting an arbitrary threshold of difference that could justify species demarcation, but with no effect on reproductive isolation, this would be a mistake.

I took this picture of one of the hundreds of Common Redpolls that were eating seeds in this winter field in Vermont a few years ago. Birders had reported one or two Hoary Redpolls in this large flock, so I spent a few hours sifting through trying to convince myself that one was paler with a smaller bill. I finally did and even though my lifer was ripped away from me a few months later when the species were lumped, it was not a bad way to spend a morning! 

Genomic data is significantly more accurate than single-locus barcoding techniques, and even more accurate than purely morphological approaches. It is becoming cheaper and more effective to use, but that doesn’t mean it offers a perfect solution. As the redpoll example illustrates, and as Singhal et al. go on to hammer home, it is still only a proxy for understanding what is really happening at the ecological level. The researchers recommend thinking of genomic data not as a neutral and objective sequence of nucleotides, but as one (albeit important) factor in a broader context of ecological and phenotypic relationships. Sequence divergence is not always enough to characterize real world reproductive barriers. As a quick aside, they also briefly highlight the utility of relating bacterial symbiont divergence to host species divergence, a fascinating topic which reminded me of this very cool paper from Marcella Baiz et al. on wood warblers (Baiz 2022).

Looking back at De Quieroz’s seminal 2007 “Species Concepts and Species Delimitation” paper, it bears asking, how much progress have we made on this front? At the time, De Quieroz tried to reassure us that a lot of the acrimony in the field stemmed from confusing “species delimitation” with “species conceptualization.” Maybe it’s wise to leave these controversial questions to more experienced researchers than myself, but perhaps we can agree that by defining species simply as a “separately evolving metapopulation lineage,” De Quieroz mostly solved the species conceptualization side of the problem (or at least neatly cleaned it up), giving us the freedom to toil entirely over the delimitation side. As genomic techniques have exponentially developed since 2007, I think it’s fair to say researchers have made huge leaps on this more technical rather than philosophical side, though as Singhal et al. illuminates, there are still many subtleties and pitfalls to watch out for.

 

Works Cited:

Singhal, Sonal, et al. “A Genomic Perspective on Species Delimitation.” Annual Review of Ecology Evolution and Systematics, 3 Sept. 2025, https://doi.org/10.1146/annurev-ecolsys-102723-055311.

“Hymenoptera - an Overview | ScienceDirect Topics.” Www.sciencedirect.com, www.sciencedirect.com/topics/agricultural-and-biological-sciences/hymenoptera.

Baiz, Marcella D, et al. “Gut Microbiome Composition Better Reflects Host Phylogeny than Diet Diversity in Breeding Wood‐Warblers.” Molecular Ecology, vol. 32, no. 2, 20 Nov. 2022, pp. 518–536, https://doi.org/10.1111/mec.16762.

De Queiroz, Kevin. “Species Concepts and Species Delimitation.” Systematic Biology, vol. 56, no. 6, 1 Dec. 2007, pp. 879–886, https://doi.org/10.1080/10635150701701083. Accessed 13 Oct. 2019.

  

Birding Needs Gatekeeping

Every once in a while a phenomenon in birding transcends its niche and achieves its highest possible level of notoriety: your non-birder relatives start texting you about it. The most recent topic to reach this status is the Reiser brother’s big year documentary Listers: A Glimpse into Extreme Birdwatching. It’s a well-made, creatively-edited, and largely well-researched documentary, and as of writing this it sits at nearly two million views on youtube. The film’s reception has been overwhelmingly positive from birders and nonbirders alike, but it makes some serious mistakes that threaten the integrity of birding culture. 

Even before he speaks, Quinten Reiser’s first appearance in the film tells us this is NOT your grandma’s birding documentary. To remain committed to the mullet and mustache look in 2025 is in a way more impressive than to have jumped ship a few years ago when the zeitgeist began to sour on it. Rocking the style these days announces, “I know I look like a TikTok caricature but I don’t care,” which is significantly braver than moving on to the more up-to-date matcha and tote bag look, and almost as brave as ignoring such trends in the first place. Add to Quinten’s picture a timbre and cadence of speech that is so laidback it borders on vocal fry, and what we end up with is unequivocal: birding is finally getting the trendy, gen-z, ironically-detached treatment it’s been waiting for. Our little hobby will be pried from the cold dead grip it’s been languishing in and launched headlong into youthful freedom. But is that really such a good thing? 

The cracks begin to show early in the film when they introduce Ebird, a popular website for reporting and viewing bird sightings from around the world. Their first experience with an Ebird reviewer, a volunteer who monitors the Ebird database for mistakes, comes after they misreport a Red-shouldered Hawk as a Golden Eagle. The reviewer who politely corrects them is referred to dismissively as an “Ebird cop.” Later on when their erroneous Gray Flycatcher sighting is corrected by a different reviewer, Quinten announces sarcastically, “thank you guys for being so on our fucking dick about bird ID’s, I super appreciate it,” as if it’s such a pain to be held to even the most basic standard of accuracy. At another point, he refers to the well-established convention of including as much detail in the reporting of rare bird sightings as possible with smug disbelief: “You’ll see people describing a bird with sometimes full paragraphs, they describe it like it’s a huge scientific finding.” 

Their biggest sin comes around the 35 minute mark, when they introduce “swallow-gate,” North American birding’s most infamous cheating scandal. For the uninitiated, the story centers on a talented young birder from North Carolina who started reporting dubious sightings, many of which went uncorroborated. After reporting a state first Violet-Green Swallow and attaching a picture as proof, other birders became suspicious, checked the photo’s metadata, and realized it had been taken months earlier in the west coast, where the bird is common. Just about as cut and dry as it gets when it comes to cheating. It’s a story that has been told to death, but the Reiser brother’s novelty was that they managed to interview the perpetrator. They introduce him anonymously and boldly with the claim, “he’s actually kind of the man, and I’m convinced he really saw the bird.” Incredible investigative journalism is on display as the cheater proceeds to trot out some utter nonsense in defense of his behavior without any pushback from Owen. He laments that the high-pressure world of birding was too much for him, and he was at once so excited about finding this massive rarity and anxiety-ridden about having to prove it that he caved to the pressure and used a picture of a different bird. On its face an unlikely story, besides the obvious point that it’s still unethical, but apparently it was enough to convince the Reiser’s (Woodward and Bernstein, I like to call them). To add insult to injury, in an interview in the following scene, Owen has the nerve to ask a record committee member who presided over the ordeal, “is it a funny story to you yet? It’s all kind of ridiculous, right?” To which the committee member, with much more composure than I would’ve mustered in his place, gives a measured and obvious response about how without the honor system, the whole hobby falls apart. 

The flippancy that defines their approach to these issues is based on a naive and superficial “us-vs-them” mentality. It’s clear the Reiser’s haven’t given it much thought, but the position they’ve landed on is that they are young and hip and know that birding is nothing more than a bit of a joke. In their minds, they are righteously taking on birding’s overly serious gatekeepers. I wouldn’t be shocked if they were so quick to lump the swallow-gate cheater into their in-group simply because he is also young and tuned into their Zoom interview with a “badass” profile picture holding a big venomous snake. The record committee member on the other hand, with his gray hair, glasses, and monotone voice, interviewed with a professorial backdrop of a bookshelf-lined study, and didn’t stand a chance. This is the fundamental and shallow dichotomy that makes Listers so frustrating to watch. 

Of course there are many aspects of birding that are pretty silly, and there is nothing inherently wrong with making fun of them. The problem is aside from the elements of silliness, birding is also fundamentally a giant citizen science project, deserving of all the rigors and guardrails that come naturally to the rest of science. Ebird is not, as the film is so eager to make it out to be, just a place for neurotic, hyper-obsessed listers to fan their egos, it is also a massively valuable resource fueling research on bird movements, population trends, and conservation. The many reviewers and record committee members who volunteer tirelessly all over the world to maintain the integrity of this resource do so not out of some self-satisfying impulse to nag, but in order to uphold scientific values. To treat their hard work with such adolescent disdain is not only quite rude, it also risks tainting the whole enterprise. 

Besides these concrete issues, a subtler one remains. Birding has always been a rare bastion of sincerity in this increasingly detached world. There is very little clout to chase in birding, since most of the time, no one really cares what we’re up to. The only reason to get involved is to pursue a real connection with nature. The Reiser’s stale, ironic approach in this film may have attracted a lot of trendy interest, but in my view it is antithetical to the heart and soul of the hobby. Birding has always been about the “doing” rather than the “being seen,” and Listers is an attempt to invert that and drag our hobby into the vacuum of middle-of-the-road, easily-accessible mediocrity that so much of modern culture has already succumbed to. “Gatekeeping” is almost always interpreted negatively, but birding absolutely needs *some* gatekeeping. At the very least we need the kind that will maintain birding’s scientific integrity, and is it really too much, on top of that, to expect that to call oneself a “birder,” one should at least be curious about the difference between a Golden Eagle and a Red-shouldered Hawk? Birding is for everyone, and focusing on welcoming as many people as possible into the fold is admirable, but not if it undermines the integrity and sincerity that make the hobby so special.