Migratory double breeding of five North American songbirds

For the first time, biologists have documented a second breeding season for five species of North American songbirds as they head south towards their wintering habitat in Central and South America. After initially breeding in temperate regions of Canada and the United States, the birds stop over in western Mexico and breed again, having implications for conservation planning.

This is the first time that biologists have recorded “migratory double breeding” for New World birds, and the first instance of southbound double breeding in the world. Some Old World species have been known to double breed, but do so as they move north in the spring.

It has been long known that yellow-billed cuckoo, yellow-breasted chat, orchard oriole, Cassin’s vireo and hooded oriole travel to northwest Mexico following breeding in the north to molt and make use of the seasonal monsoonal rains in the area. Now, a subpopulation of these species clearly breeds again in Mexico before moving further south to winter.

These results give evidence that breeding populations of these five species are potentially subsidized by areas outside their suspected American and Canadian breeding range, and in areas of northwestern Mexico that are currently undergoing large-scale agricultural development. 

Researchers say the conversion of coastal thorn forests to industrial agriculture could be responsible for population declines in migratory double breeders and in the populations of molting birds that also use the region as a stop over.

For example, the yellow-billed cuckoo could be particularly susceptible to landscape changes in western Mexico because it may require the second breeding season to keep the species viable. The bird has almost disappeared from most of its breeding range in the United States and Canada, and its overall population size is down.

This study challenges the long-held assumption that birds are too limited by natural hormonal and reproductive systems to add additional stages of life history into their annual cycle.

Using stable isotope methods to predict the approximate latitude where tissues were formed, researchers from the University of Washington and Environment Canada investigated feathers, muscle, and reproductive tissues of birds breeding and molting in late summer in western Mexico. It was shown that some birds were local residents, others moved to this area to molt after breeding in western North America, and another distinct group of birds initiated breeding there clearly after an earlier attempt in Canada or the United States.

This research underlines the need to establish migratory connectivity for populations of migratory birds currently protected by the Migratory Birds Convention Act. Researchers urge programs and policy to consider connectivity in general and migratory double breeding in particular when developing demographic models, population studies, and conservation plans.

For more information

• Keith Hobson's S&T Expert profile
• Rohwer, S., K.A. Hobson and V. Rohwer. 2009. Migratory double breeding in neotropical migrant birds. Proceedings of the National Academy of Science 106: 19050-19055.