Biologists studying songbirds that breed in North America and then migrate to Mexico have discovered something totally unheard of in the New World — a second breeding season.
Five species — Yellow-billed Cuckoo (right), Orchard Oriole, Hooded Oriole, Yellow-breasted Chat, and Cassin's Vireo — breed primarily in the United States and Canada. Then they squeeze in a second breeding season during a stopover in western Mexico on their southward migration.
"It's pretty much unheard of to have a nocturnal migrant with a second breeding season. It's a pretty special observation," said Sievert Rohwer, lead author of a paper describing the discovery. "We saw these birds breeding, and we were completely surprised."
Migratory double-breeding has been observed in two Old World bird species — European Quail and Dotterel — on their northward migration, but this is the first documented observation of "migratory double breeders" in the New World, and the first anywhere for the southward migration, Rohwer said.
Rohwer and his fellow scientists traveled to the lowland 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 then immediately migrated to Mexico to molt, or shed and replace, their feathers.
But during July and August in three consecutive summers, 2005-2007, the researchers found individuals that were breeding rather than molting.
They found evidence that the birds had, in fact, bred earlier that year. Females of all five species had dry and featherless brood patches, indicating they had bred earlier that summer. (To more efficiently transfer heat to eggs, the abdominal brood patch becomes featherless and thickened with fluid when females are incubating, but as the young mature, it dries out and remains featherless.) "During hundreds of hours of netting and observation in July, we found no recently fledged juveniles of these species, suggesting that they had not bred in west Mexico earlier in the same summer," the researchers write.
Active nests were found for the two orioles. (Orchard Oriole eggs collected by the researchers in 2006 are pictured at left.) 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.
Rohwer published the paper this week in the online edition of the Proceedings of the National Academy of Sciences. He is a University of Washington professor emeritus of biology, curator emeritus of birds at the Burke Museum of Natural History and Culture, and the 2006 recipient of the Elliott Coues Award from the American Ornithologists' Union. The abstract is online here; the full article is available to PNAS subscribers only.
Coauthors are Keith Hobson, a world-renowned research scientist with Environment Canada and a leader in the study of stable isotopes in birds, and Vanya Rohwer, Sievert Rohwer's son and a graduate student at Queen's University in Kingston, Ontario.
The percentage of northern breeding populations that attempt to breed again in Mexico is unknown, they write. "Hooded Orioles are extraordinarily common as late summer breeders in Baja California Sur, and Orchard Orioles, Yellow-breasted Chats, and Yellow-billed Cuckoos breed abundantly in the coastal lowlands of extreme southern Sonora and much of coastal Sinaloa. Thus, migratory double breeding may be common in at least some North American breeding populations of these species."
The observation is much more than an oddity in bird behavior, Sievert Rohwer said. He noted that Orchard Orioles might raise a first brood in the Midwestern and south-central U.S. and a second on Mexico's western coast, yet both sets of offspring find the same wintering area in Central America. The question is how both groups find the right place, since they must travel in different directions.
The Yellow-billed Cuckoo was once commonly seen throughout the western United States and as far north as the Seattle area, but now it's seldom seen along the West Coast. Disappearing habitat in the U.S. is usually cited as the reason.
But Rohwer believes the real problem could be the transformation of thorn forests of southern Sonora and Sinaloa in northwestern Mexico, into irrigated industrial farms. That loss of habitat, he said, could mean not enough young are produced in the second breeding season to sustain the populations previously seen on the U.S. west coast.
"It turns out that many of those migrants, both molt migrants and the newly discovered migratory double breeders, are dependent on the low-altitude thorn forests that become very productive during the monsoon," Rohwer said.
The thorn forests lie in an arid and forbidding scrubland that springs to life with the monsoon lasting from June through August. The monsoon brings virtually all of the area's annual rainfall. The small trees leaf out and insects become abundant, making an ideal stopover for migrating songbirds.
However, with plenty of biting insects, temperatures often at 100 degrees Fahrenheit, and humidity hovering near 100 percent, it is a difficult place for researchers to work, so there has been little previous documentation of life in the thorn forest. The new findings could spur more work there.
"For western North America, the conservation implications are pretty serious," Rohwer said. "Biologists know theoretically that they should pay attention to these migration stopover sites, but they've been largely ignored for their conservation implications." -- M.M., with thanks to Vince Stricherz, University of Washington.
Photos: Top, Yellow-billed Cuckoo by Seabrooke Leckie,
Middle, four Orchard Oriole eggs collected by researchers in Sinaloa, Mexico, in 2006. University of Washington/Burke Museum.
Bottom: Recently fledged Orchard Oriole, photographed by Bernie Monette at Tommy Thompson Park in Toronto in 2008. Might its parents have migrated to Mexico and raised a second brood?
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