For many birds, molting is an awkward phase. Months of sun, rain, and general wear cause their feathers to deteriorate, so they refresh their plumage at least once a year. That swap usually falls between breeding and fall migration, when it’s easy to lay low and still find plenty of food. The birds stop singing from perches, preferring instead to melt into the browns and greys of dense thickets until their feathers are replaced—a period that lasts a month or more.
For a long time, it was assumed that North American land birds molt close to where they nest. But as shown in a new paper in The Auk, many of these fair-weather visitors may be long gone come changing time.
The study, by Peter Pyle, James Saracco, and David DeSante of the Institute for Bird Populations in California, examined data from more than 760,000 individuals caught at 936 banding stations across the United States, Mexico, and Canada. The researchers sought to better understand “molt-migration,” which occurs when birds leave their breeding areas to grow fresh feathers in a geographically distinct spot, prior to flying south for the winter. Molt-migration is well known in some aquatic species like ducks, and has also been documented in terrestrial species. For example, various Western songbirds migrate to southeastern Arizona in late summer for a plumage upgrade, taking advantage of lush green conditions after monsoons. But to date such movements have been considered an exception rather than a rule.
But by comparing records of breeding and molting birds across the continent and estimating the probability of capturing the same individual during both stages, the study authors assumed that most of the 140 species in the analysis didn't feather-swap at their summering sites. What's more, the distances these birds traveled to molt seemed to vary by species and geography. In general, Western species moved farther than Eastern ones, though some like the Swainson's Thrush were found hundreds of miles away from their nearest breeding grounds. Even non-migrants such as Northern Cardinals and eastern Carolina Wrens roamed a little.
Nashville Warblers exhibited some curious patterns as well. Most Nashvilles breeding west of the Rockies tended to nest at lower elevations than where they molted, whereas Nashvilles east of the Rockies generally showed the opposite trend. But there were also inconsistencies in molt-migration among individuals. This, lead researcher Peter Pyle says, may be because seasoned birds snap up high-quality patches and remain in place to molt, while less-experienced birds, failed breeders, or breeders in undesirable habitats must move elsewhere to find suitable conditions.
While these findings may feel subtle, they could be critical for preserving migratory birds. To properly safeguard a species, much of the habitat it relies on between its breeding and wintering grounds needs to be protected as well. Molt-migrations add more complexity to the approach. “I might rank these areas as pretty close to breeding grounds in conservation importance, and above migratory stopover locations and winter grounds,” Pyle says. “These locations are critical to a bird's annual survival.”
Erik Johnson, director of bird conservation at Audubon Louisiana, says that Pyle’s results are even more crucial because they cover “familiar birds like American Goldfinch, Song Sparrow, and Gray Catbird,” in addition to the rare and cryptic ones. “This highlights just how little we know about molt in even common, widespread species,” he says.
Now the question is, what draws birds to a molt site? “[While] individuals may not return to the same location annually, there are definitely specific habitats in general locations needed for molts,” Pyle says. “We can identify important characteristics, just as we do for stopover locations for migratory species.”
Some of this work will fall to the dedicated banders of North America. “Banding stations are the primary way that we can unravel this molt-migration mystery,” says Sarah Milligan, who manages four sites at Bandelier National Monument in New Mexico. In addition to documenting timing and abundance of breeding and molting birds, the stations allow for easy deployment of technologies such as geolocators or GPS trackers, which can pinpoint key destinations for molt-migration.
Avian enthusiasts can also pitch in by making birds more comfortable as they endure the indignities of molting. Some ideas, Johnson says, include providing native shrubs and trees for food and cover, along with clean water for hydration and climate control.
Gathering intel from thousands of birds over thousands of miles is no easy task. But given how much we've uncovered about molt-migration from this one study, it seems there are plenty more surprises to discover.