For the Birds Radio Program: Double Breeding
Some birds don’t only nest in the north: recent studies show that five different species nest a second time, in Mexico.
My favorite birding magazine, Birder’s World (now BirdWatching), just reported that biologists have made a discovery about something totally unheard of—a discovery that makes my brand new book obsolete before it even arrives in bookstores.
People have long asked me whether birds nest while migrating or while they’re at their wintering grounds, and the answer has always been a clear and absolute no. They breed in the north and replenish themselves in the south. But it turns out that five species, the Yellow-billed Cuckoo, Orchard and Hooded Orioles, Yellow-breasted Chat, and Cassin’s Vireo, which all breed primarily in the United States and Canada, make a stopover in northern Mexico during migration, where they apparently raise another brood of babies.
The lead researcher, Sievert Rohwer, Professor Emeritus at the University of Washington, said, “It’s pretty much unheard of to have a nocturnal migrant with a second breeding season. We saw these birds breeding and we were completely surprised.” Rohwer and his team were studying the lower thorn forests of coastal Sinaloa and Baja California Sur to survey and collect songbirds that had raised their young in the United States and Canada, and were migrating through Mexico. The researchers were expecting to find the birds in the middle of molting. Instead, during July and August in 2005, 2006, and 2007, the researchers found individuals that were breeding rather than molting. Active nests were found for the two orioles, and males of all five species were singing and defending territories or guarding females, behaviors associated with breeding. In addition, isotopic analysis of the birds’ tissues showed that many had recently arrived in western Mexico from temperate areas farther north.
The team noted that Orchard Orioles apparently raise a first brood in the Midwestern and South Central United States, and a second on Mexico’s western coast, yet both sets of offspring find the same wintering area in Central America. They don’t yet know how both groups find the right wintering place, since they must travel in different directions.
The Yellow-billed Cuckoo was once common in the Western United States, but is now seldom seen along the West Coast. Disappearing habitat in the United States is usually cited as the reason, but Rohwer now believes the problem could be the transformation of thorn forests of southern Sonora and Sinaloa in northwestern Mexico into irrigated industrial farms. That loss of habitat could mean, he said, not enough young are produced in the second breeding season to sustain the populations previously seen in the West. He said, “It turns out that many of those migrants, both molt migrants and the migratory double breeders, are dependent on the low altitude thorn forests that become very productive during the monsoon.”
Those thorn forests lie in an arid scrubland that springs to life with the monsoon, which lasts from June through August. The monsoon brings virtually all the area’s annual rainfall. The small trees leaf out and insects become abundant, making an ideal stopover for migrating songbirds. But biting insects, temperatures often over a hundred, and humidity hovering near a hundred percent make it a difficult place to work, so there’s been little previous documentation of life in the thorn forests. The new findings are expected to spur more work there. You can read more about it on birdersworld.com. This also means that my Bird Watching Answer Book is already due for a revision. Let’s hope it makes it into a second edition so I can fix it.