Migratory songbirds’ balancing act between foraging and longevity

Stabilizing natural selection acts on white tail patch size to maximize both foraging success and long-term survival in a wild songbird

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A wonderful new study of a migratory songbird finds that individuals with medium-sized white tail patches live longer than those with larger white tail patches. Previous research has revealed that these white spots on the tail are critical for successful foraging as well as longevity.

This most recent study continues a body of work focused on the hooded fox, 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 woodpeckers make their living using an unusual feeding strategy known as flush-pursuit foraging. Here they constantly open their tails to reveal the outer white tail feathers which flash and scare the winged insects out of hiding so that the birds can hunt them down and capture them in flight.

“We’ve known for some time that white spots on the tail are important for foraging for hooded warblers,” study author animal behavioral ecologist Ron Mumme, professor emeritus of biology at Allegheny Collegehe said in a statement, adding: “If you temporarily darken a bird’s tail spots, its foraging success is significantly reduced.”

So if no white is bad, but a little white is good, then a lot of white is better, right? Well, maybe: in fact, that was the question Professor Mumme investigated in this study.

“What we didn’t know before 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 investigate this question, Professor Mumme analyzed a 14-year database of hooded warblers at the Hemlock Hill Field Station in northwestern Pennsylvania. From 2010 to 2023, Professor Mumme had captured, colored and measured the tail patterns of 625 individual hooded warblers that settled and bred at Hemlock Hill (Figure 1) during the egg-laying season, which is between May and August . He then tracked the long-term survival of color-banded birds that returned — or did not — to the study area in subsequent 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 on the tail, but that these white markings are very consistent in annual feathers emerging in late summer: analysis of these data shows that 75% of the variation in White extent is consistent among individuals, with only 25% attributable to annual variation within individuals. Could these individual plumage variations be the result of slight differences in the way key feather coloration genes are regulated from one season to the next?

“This is a very strong indication that variation in tail pattern likely has an underlying genetic basis,” Professor Mumme replied.

Second, Professor Mumme found that most hooded warblers had poor long-term survival and appeared at Hemlock Hill for only 1-2 breeding seasons, while some individuals were much 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 reproductive success (more chicks overall) for some individuals, thereby providing natural selection with the opportunity to act on population traits—specifically, white tail spot size—that could potentially affect survival of the birds.

Finally, Professor Mumme found that long-term survival was no accident. In short, birds with an intermediate amount of tail white – those that were “moderate” – lived longer than individuals with more tail white (Figure 2). These results strongly suggest that the extent of white on the tail of this species 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 particularly strong in males, an observation possibly related to the sharp segregation of sexual habitats in the wintering area: it is unlikely that this life history feature would have been discovered otherwise. Furthermore, these results show how stabilizing selection can act on plumage traits in birds outside of sexual and social signaling contexts.

This study adds more evidence to previous studies from other flush-seeking forager species that the extent of white on the tail is fine-tuned by natural selection. Atypical tail patterns that deviate from the population mean are less effective at scaring away insect prey and lead to reduced foraging success.

“It’s an excellent example of how stabilizing selection can act on small variations in plumage that at first glance seem completely insignificant.”

Source:

Ronald L. Mumme (2023). Stabilizing selection on plumage-based foraging adaptation: hooded warblers with medium-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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