Traditionally, migratory songbirds are thought to follow a fairly standard life history. Breed in the United States and Canada during the spring and summer and then migrate to Central and South America for the winter to avoid the harsh weather conditions and food scarcity found across much of North America. However, new scientific evidence may turn that on its head, at least for a few species.
Researchers from the University of Washington in Seattle have presented evidence that at least five species, Yellow-billed Cuckoo, Cassin’s Vireo, Yellow-breasted Chat, Hooded Oriole, and Orchard Oriole, may not follow this pattern exactly. Their evidence points to a second breeding season, where at least some portion of the population migrates to North America and breeds, then departs for an early southbound migration in midsummer. However, instead of going all the way to the tropics, they stop in western Mexico to breed a second time in the thorn forest there. This second breeding season corresponds with an influx of rain and pulse of insect life.
Sievert Rohwer and his colleagues not only documented breeding of each of these species but, using both physiological observation and stable isotope analysis, demonstrated that many of these individuals had bred previously that year outside of western Mexico. Females displayed worn brood patches, featherless areas on the belly of the bird used for incubating eggs, suggesting that these birds had previously bred earlier in the season. Stable isotope analysis demonstrated through chemical signatures that these individual birds had spent time farther north and not spent the entire spring in the thorn forest.
Breeding patterns like this have never been recorded in the New World and only twice in the Old World. It may have major conservation implications as, while we don’t know the percentage of individuals from these five species that follow this migratory double breeding pattern, the thorn forests of western Mexico may prove crucial to supporting the entire species populations.
The researchers also raised numerous other life history questions. How do young birds from the first breeding season and the young birds from the second both find their way to similar winter grounds despite being born thousands of kilometers apart? How do these birds regulate their physiological condition altering between migratory condition (northbound), breeding condition (first season), migratory (southbound), breeding condition (second season), and migratory again?
This fascinating research has uncovered a phenomenon in several species and only through further study can we begin to understand how important this second breeding season may be to these and other birds.
Source: Rohwer, Sievert, Keith A. Hobson, and Vanya G. Rohwer. “Migratory double breeding in Neotropical migrant birds.” Proceedings of National Academy of Sciences 106.45 (2009): 19050-19055.