Sources, sinks, and drains in double breeding bird species?

The recognition of a second breeding season in some migratory bird species by Rohwer et al. (1) creates an interesting scenario for conserving migratory bird populations. Previously, conservation biologists assessed sources as large undisturbed habitats and sinks as degraded breeding grounds (2), and treated fragmented and deforested habitats in nonbreeding “wintering” areas as drains on the population. In light of a second, and perhaps vital, breeding season in midmigration, a dichotomous source–sink model is no longer applicable.

The prevailing view of postbreeding habitats for Neotropical migrant birds as a place where only population loss occurs (drain) has focused a great deal of conservation resources on protecting habitat in Central and South America, even though research has shown that there were larger problems in breeding failures in the United States (3). Furthermore, aside from the southernmost parts of Mexico, most conservation efforts to protect North American breeders after their southbound migration focused on Central and South America rather than Mexico.

Although Rohwer et al. modestly touch upon the idea that the Mexican breeding of species such as yellow-billed cuckoos may be important for maintenance of some of the western US populations of this species, the full impact of this is not emphasized enough in the paper. Their revelation that yellow-billed cuckoos are breeding in high numbers in Mexico after some of them have bred in the United States may suggest that the declining US populations are a sink for the Mexican-bred birds.

From a conservation perspective, the discovery of Rohwer et al. borders on revolutionary. The prevailing view of high-population vulnerabilities in their southern “wintering” ranges, and treatment of these as a drain, may need revision. Additionally, conservation “ownership” of these species may no longer belong in the United States. It will be exciting to see new species added to the list of double breeders, and important to revise plans for protecting these species.

What will we do in the future when Mexican birdwatchers decry the United States’ diversion of water in the west as leading to the loss of one of their valuable breeding bird species, and demand that we protect our water resources so as to protect their population of yellow-billed cuckoos? The truly groundbreaking research of Rohwer et al. has opened up this question as legitimate, and its impacts must quickly be considered in bird conservation.