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Open Mic: Shedding Light on the Mysterious Mangrove Cuckoo

At the Mic: John Lloyd

I’ve been studying Mangrove Cuckoos in the field in Florida, and dead ones in museum collections across North America, for the past several years. Mangrove Cuckoos are fairly uncommon in Florida, which is the only part of North America where they nest and occur regularly. When I started studying these birds, I was interested in helping conserve them by learning more about their ecology and natural history. I had written a paper earlier showing steep declines in the number of Mangrove Cuckoos counted in southwest Florida, but no one, including me, knew why they were faring so poorly. Could it be a problem on the breeding grounds, something awry in the mangrove forests of Florida? What about a change in wintering habitat, whatever that might be? Our knowledge of this bird at the time was so limited that no one was quite sure whether they were year-round residents of the state or whether some migrated into the West Indies during winter. To start understanding why Mangrove Cuckoos might be in decline, I needed to isolate the problem by learning more about where they spent their time.

MangroveCuckooFlorida

Tracking individual birds throughout their annual cycle remains a challenge, especially for those species not large enough to carry a satellite transmitter. As I puzzled over this problem, learning more about the species, I was struck by the writings of the early naturalists that had encountered this bird. Descriptions of Mangrove Cuckoos noted a remarkable amount of variability in their appearance: some individuals had thick, deep bills and others were quite slender-billed; some individuals had underparts tawny orange in color whereas the breast and belly of others were nearly pure white.

Screen Shot 2016-04-11 at 9.59.07 AMInformation sharing in the early 20th century being what it was – no Internet, no widespread use of photography, no easy access to museum collections – and with a fully modern definition of a biological species still decades away, nearly every collection of Mangrove Cuckoos from a new locale was described as a new subspecies and given its own scientific name. Simply put, an ornithologist collecting a Mangrove Cuckoo on Grand Cayman in 1919 had no easy way to tell exactly how different his specimens were from those collected a decade earlier by someone else on Grenada. By 1940, James Lee Peters’ “Check-list of Birds of the World” recognized 13 different subspecies of Mangrove Cuckoo (keep this in mind when you’re puzzling over the eBird taxonomy of Dark-eyed Junco). Presumably, based on appearance alone, one could tell whether a Mangrove Cuckoo came from Jamaica or Florida or the Pacific slope of Mexico or any number of other locales.

Without much fanfare, this complicated taxonomy was retired on the advice of noted ornithologist Richard Banks and his colleague Robert Hole, Jr., who argued that none of these distinctions stood up under close scrutiny. As a consequence, you won’t find any mention of subspecies of Mangrove Cuckoo in current avian taxonomies. Nonetheless I wondered whether early insights about the nature of variation in Mangrove Cuckoos might offer a clue about whether birds from Florida migrated south in the winter. That is, if Florida birds looked different from individuals of other populations, then perhaps I could sift through museum collections to see whether Florida-type birds appeared regularly on any of the West Indian islands during the winter. If they did, it would suggest that perhaps at least some Mangrove Cuckoos migrated out of Florida each winter and, if they did, where they might be headed.

Screen Shot 2016-04-11 at 9.57.42 AMThe first step was to determine whether early ornithologists had it right: could I tell where a bird was from based on its appearance? This led me to natural-history museums around the country. I looked at and measured hundreds of Mangrove Cuckoos from every part of the West Indies, Mexico, Central America, and even a few from South America. I was struck by the range of colors and sizes, but when I plotted these differences on a map it was quite clear that the odds of guessing an individual’s provenance based on its looks were poor indeed.  Birds from Florida were, on average, relatively small and pale, but so too were birds from the Bahamas, Hispaniola, Jamaica, and every other island in the Greater Antilles. Birds from the Lesser Antilles tended to be big and colorful, but not noticeably more so than birds from South America, Central America, or Mexico. Overall, the picture was not one of discrete groups of different-looking birds, but rather a subtler pattern of variation. In other words, although noticeable extremes of size and appearance existed, they were linked by a long and muddled middle ground.

A dead-end for using differences in appearance – the variation that led to recognition of all of those subspecies in the first place – to track migration, for sure, but two positive outcomes emerged from this research. First, it spurred my colleagues and I to find other ways of answering the question of whether Mangrove Cuckoos migrated. After four years of following radio-tagged birds around the mangroves of southwest Florida, we are now confident that although Mangrove Cuckoos wander widely, they do not migrate in the traditional sense. Second, studying variation in the size and appearance of Mangrove Cuckoos also raised a number of interesting questions that I hope someone can answer someday. Why are Mangrove Cuckoos on islands like Montserrat so thick-billed and dark in plumage, when birds only a few hundred miles away on Tortola are relatively pale and slender of bill? Is it chance alone, or have environmental differences favored unique, local adaptations reflected in these different appearances? Categorizing Mangrove Cuckoo populations into subspecies based on appearance may not make much sense, but we still have much to learn about the nature of variation in this species. And that, for a scientist, can be a pretty satisfying conclusion itself.

MangroveCuckooFlorida2

For birders, the upshot of my research is that you don’t need to spend a whole lot of time puzzling over the origin of the next Mangrove Cuckoo that shows up in Texas. The old subspecies boundaries and their accompanying descriptions of plumage and size don’t offer much guidance. Better to find the two nearest breeding populations, flip a coin, and then get back to enjoying your chance to observe these mysterious beauties.

For the full details on how Mangrove Cuckoos look throughout their range, including maps and photographs, you can read – free of charge – my paper on this research in the open-access scientific journal PLOS ONE.

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John Lloyd is an ornithologist and Director of Science at Vermont Center for Ecostudies. He has studied birds across the US and throughout the Caribbean, but his current work focuses on the study and conservation of Bicknell’s Thrush in Vermont and the Dominican Republic.