This post was contributed by Ethan Gyllenhaal, co-author of a paper in a recent issue of the The Wilson Journal of Ornithology documenting the unusual warbler hybrid described here.

Before I started down the academic path of ornithology, I was a birder with an appetite for difficult bird IDs. I loved the challenge of identifying the gulls that wintered along the shores of Lake Michigan, where I grew up. With so much variation within gull species and such subtle differences between species, each individual was a new puzzle. One complication is that many species of gull regularly mate with each other, producing intermediate hybrids. The strict definition of a species I learned in high school, which said different species never interbred, was clearly an oversimplification. I regularly saw rarer hybrids between distinct species (e.g., Herring x Great Black-backed Gull) and challenging individuals that were the products of hybrids swarms (i.e., Iceland Gulls). My fascination with the complexity of this phenomenon led to my desire to track down hybrids I’d never seen before, as I would any other new life bird.

Hybrids in nature tend to fall into two general categories: Those found in zones where two similar species overlap and regularly interbreed, such as the Iceland Gulls; and those between species that rarely hybridize, either due to geographic isolation or nearly perfect barriers to interbreeding, such as Herring and Great Black-backed Gulls. Of particular interest are those rare hybrids where the strong isolating barriers break down between dramatically different parent species that overlap in range extensively. These hybrids are quite enigmatic and can teach us a lot about how traits are inherited.

Rare hybrids are more common in some groups of birds. Ducks are a classic example, where species in different genera, such as Gadwalls and Mallards, hybridize with some degree of regularity. It can be difficult to discover and identify rare hybrids because there are so few examples of any one pairing; however, with help from community scientists, we are learning about many different combinations of rare hybrids that can occur. One group with an increasing number of documented rare hybrids the warblers, which also have regularly interbreeding taxa that have provided new insights into how plumage traits are inherited.

Despite extensive work on warbler hybridization, most studies have focused on pairs that hybridize regularly, which are more easily identified in the field and facilitate genomic analyses. Those pairs allow for high sample sizes and easier genomic analyses. However, rare warbler hybrids (several examples of which have been published in The Wilson Journal of Ornithology — see here and here) present a unique opportunity to study complex evolutionary processes. There are also passionate communities of birders and other community scientists interested in the identification of hybrids, such as the members of the Facebook group Bird Hybrids of North America. Together, these resources provide a broad set of hybrid pairings and their offspring for researchers to study.

The two major aspects of warbler hybrid pairings — which ones have occurred, and what their phenotypes are — lend themselves to two different research questions. The first question is whether specific traits make certain rare pairings more likely to happen. That question was tackled by a group of researchers led by Dr. Pamela Willis in a 2014 paper. Among other factors, they found that more related species, species with more range overlap, and species with more similar songs hybridized more often.

The second question is how plumage patterns are inherited when the parent species look dramatically different — a question we had an ideal opportunity to investigate after a chance encounter with a remarkable hybrid. On September 17^th, 2020, one of my co-authors (who is also my advisor), Dr. Michael Andersen, was birding on the University of New Mexico campus. When observing a mixed warbler flock, he noted a bizarre, almost ghost-like bird that looked strikingly like a Cerulean Warbler. He quickly got the word out to our campus rare bird alert, which caught my attention and that of my co-authors. While observing the bird, we speculated on many possible identities: Townsend’s Warbler without dark pigments? Yellow Warbler without any yellow? A hybrid we’d never heard of? The bird disappeared before we could get a sample for genetic analysis, but we were determined to identify its parents. Fortunately, the bird had two unique features: a patch of white in the flight feathers, and extensive white in almost all of its tail feathers. It also had a feature uncommon in similar warblers, no well-defined wing bars, one more or less unique to hybrids, faint, splotchy traces of yellow on the underparts. That evening, we reached a tentative conclusion: our bird was most likely a Yellow x Black-throated Blue Warbler hybrid. However, we needed to build a body of evidence to support that exceptional ID.

Our desire to identify this hybrid led us to research the inheritance of yellow plumage and wing bars in hybrid warblers. My co-first-author Jessie Williamson and I dug through the literature to identify as many warbler hybrids as possible, using diverse sources including academic literature, community science databases such as eBird, and input from birders fascinated with the identification of hybrids. We found evidence for 61 hybrid pairings between 44 Nearctic warbler species (93% of the 47 species reviewed). Once the list of hybrids was compiled, we used it to compare strength of difference in wing bar extent and yellow coloration in hybrids versus ­­­­­­­­­­­parents. We found that wing bars were generally intermediate between the parents, and never more or less extreme than either parent. This meant that a hybrid lacking wing bars, for example, must be the product of parents that lacked wing bars. We also found that yellow plumage was intermediate, but the extent and saturation of yellow could vary dramatically. One pattern we observed was that some hybrids that had one parent with yellow and one without yellow had their own yellow coloration almost completely “cancelled out” and limited to faint patches. This finding suggested near-dominance of non-yellow pigmentation in a few cases. However, other examples were less clear, reflecting the variable genetic basis for plumage coloration. We drew three conclusions about our focal hybrid from our synthesis: the parents lacked distinct wing bars, one parent lacked yellow, and the other parent had extensive carotenoid pigmenta­­­­tion on its underparts. This was consistent with our initial conclusion, as Black-throated Blue Warbler and Yellow Warbler is the only pair in the genus Setophaga that match those traits.

We then used two methods to verify our hybrid identification using the distinct plumage features and the results of our meta-analysis, facilitated by examination of specimens at the Museum of Southwestern Biology. One method followed the contradictory character approach, where unique features are used to eliminate potential parents. With this, we that determined one parent had a pale patch restricted to its outer primaries, meaning it was either an American Restart or Black-throated Blue Warbler. The other parent had to have extensive pale color in all outer rectrices, which could only be Yellow Warbler, with the yellow “canceled out” by a parent without yellow in the tail (ruling out American Redstart). This approach provided further confirmation of the exceptionally odd pairing of Yellow x Black-throated Blue Warbler.

This Yellow x Black-throated Blue hybrid was a very rare find and a fascinating case study in hybridization. Our hybrid represented the first photo-documented record of this pairing, with only one previous sighting from 1992. This bird also demonstrated how dramatically different taxa can fail to recognize each other as invalid mates. Yellow and Black-throated Blue warblers differ strongly in pigmentation, song, call note, and even habitat preference. Any one of these traits could — and theoretically, should — prevent them from interbreeding, and yet… we found this hybrid. We hypothesized that the hybrid was the product of a Black-throated Blue Warbler straying west of its normal range, failing to find a suitable mate, and mating with a Yellow Warbler. This matches the theory that hybrids are more likely to occur when one of the parents is rare and in the range of a more common species for it to mate with.

Our synthesis aided in identification of an unusual hybrid and helped us better understand how inheritance of warbler plumage traits works, representing a great tool for identifying other rare warbler hybrids in the future. Our paper also demonstrated that plumage trait inheritance is complex, and likely has different causes in different species. Despite the broadly predictable patterns, we often cannot predict with certainty what a hypothetical hybrid will look like before we see it. Finally, we hope that our compilation of hybrid pairings will be of use for future researchers who, like us, are fascinated by bizarre hybrid pairings. We’re grateful to the Wilson Ornithological Society for the opportunity to build on the rich body of research on warbler hybrids that have been published in The Wilson Journal of Ornithology and Wilson Bulletin over the past decades.