Stabilizing natural selection acts on white tail spot size to maximize both foraging success and long-term survival in a wild songbird
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A lovely new study of a migratory songbird finds that individuals with average-sized white tail spots live longer than those with larger white tail patches. Previous research has revealed that these white tail spots are critical to successful foraging as well as to longevity.
This newest study continues a body of work that focuses on the hooded warbler, Setophaga citrina, a small colorful migratory songbird that breeds in forests in eastern North America and winters in southern Mexico, Central America and the Caribbean. Hooded warblers earn their living by using an unusual feeding strategy known as flush-pursuit foraging. This is where they continuously flick open their tails to reveal outer white tail feathers that flash and startle winged insects from hiding, so the birds can then hunt them down and capture them in flight.
“We’ve known for some time that the white tail spots are important in hooded warbler foraging,” the study author, animal behavioral ecologist Ron Mumme, a Professor Emeritus of biology at Allegheny College, said in a statement, adding: “If you temporarily darken a bird’s tail spots, foraging success drops substantially.”
So if no white is bad, but a little bit of white is good, then a lot of white is better, right? Well, maybe: in fact, this was the question that Professor Mumme investigated in this study.
“What we didn’t previously know is whether natural variation in the extent of white in the tail affects long-term survival,” Professor Mumme explained. “That’s the question I wanted to answer.”
To explore this question, Professor Mumme analyzed a 14-year database of hooded warblers at Hemlock Hill Field Station in northwest Pennsylvania. From 2010 through 2023, Professor Mumme had captured, color-banded, and measured the tail patterns of 625 individual hooded warblers that settled and bred at Hemlock Hill (Figure 1) during the nesting season, which is between May and August. He then monitored the long-term survival of the color-banded birds that returned — or not — to the study site in the following years.
Professor Mumme’s analysis of the database revealed three things. First, he found that hooded warblers show considerable individual variation in the extent of white in the tail, but these white markings are highly consistent across annual feather moults that occur during the late summer: analysis of these data show that 75% of the variation in the extent of white is consistent amongst individuals, with only 25% attributable to annual changes within individuals. Could these individual plumage variations be the result of slight differences in how key feather coloration genes are regulated from one season to the next?
“That’s a very strong hint that the variation in tail pattern likely has an underlying genetic basis,” Professor Mumme replied.
Second, Professor Mumme discovered that most hooded warblers had poor long-term survival and showed up at Hemlock Hill for only 1–2 breeding seasons, whereas a few individuals were considerably more successful, surviving and returning to breed at Hemlock Hill for as many as nine consecutive seasons. This differential long-term survival allowed for greater breeding success (more chicks overall) for some individuals, thereby providing natural selection the opportunity to act on population traits — specifically, white tail spot size — that could potentially affect the birds’ survival.
Finally, Professor Mumme found that long-term survival was not random. In short, birds with an intermediate extent of white in their tails — those that were “average” — lived longer than individuals with more white in their tails (Figure 2). These results strongly suggest that the extent of white in this species’ tail is fine-tuned by stabilizing natural selection to maximize both foraging success and long-term survival.
Professor Mumme’s analysis found that stabilizing selection was especially strong in males, an observation that is probably related to pronounced sexual habitat segregation on the wintering range: it’s unlikely that this life history trait could have been discovered any other way. Further, these results illustrate how stabilizing selection can act on avian plumage traits outside of sexual and social signalling contexts.
This study adds more evidence to previous studies from other species of flush-pursuit foragers that the extent of white in the tail is fine-tuned by natural selection; atypical tail patterns that depart from the population average are less effective at startling insect prey and lead to reduced foraging success.
“It’s a great example of how stabilizing selection can act on slight plumage variations that at first glance seem completely inconsequential.”
Source:
Ronald L. Mumme (2023). Stabilizing selection on a plumage-based foraging adaptation: hooded warblers with average-sized white tail spots live longer, Proceedings of the Royal Society B: Biological Sciences 290(2011) | doi:10.1098/rspb.2023.1752
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