The birds continue to surprise us. We think we know their routines, their travel schedules, and then someone comes up with evidence that at least five North American species—yellow-billed cuckoo, Cassin’s vireo, yellow-breasted chat, and hooded and orchard orioles—have been leading double lives. These birds, according to newly published research by Sievert Rohwer at the University of Washington, rear one brood in the United States, then fly to western Mexico and produce a second brood. Some, at least, continue on to South American wintering grounds. This is a bit like learning that your Uncle Henry, the Amtrak conductor, has a second family at the end of his route in Sacramento.
Such behavior, which has been called “itinerant breeding,” had been thought to be rare in birds. It has only been documented in a few species, like the European quail, the dotterel, and the notorious red-billed quelea of Africa. Phainopeplas may be itinerant breeders, but the jury still seems to be out on that one. It’s known that these odd crested mistletoe-eating birds breed in late winter in the Sonora and Colorado deserts, then in spring in the California foothills. Whether the desert phainopeplas fly north to nest again has not been determined.
More typical neotropical migrant birds breed just once (or twice, if the season is unusually favorable), then head south. Until Rohwer’s field studies in Baja California Sur and Sinaloa, no one suspected that some of them were stopping for a second nesting bout along the way.
There’s no question that the birds Rohwer and his colleagues observed in Mexico over three consecutive summers were nesting. “We found many active nests for orchard orioles and hooded orioles, and males of all five species were singing and defending territories or guarding females,” they write in Proceedings of the National Academy of Sciences. Other males that were collected had enlarged gonads, indicative of breeding condition. The females had dry and featherless brood patches, suggesting that they had already nested earlier in the year.
How did they know the birds weren’t Mexican residents? One species, the Cassin’s vireo, had not previously been known to nest on the Sinaloan coast. Besides that, there’s persuasive biochemical evidence. Stable isotopes—how did we ever manage without them?
Biologists have learned that birds accumulate variant forms of hydrogen (the isotope deuterium), carbon, and nitrogen in their feathers and flesh. The ratio of deuterium to plain old hydrogen varies by latitude and can indicate how far north the bird was when its tissues grew. By that criterion, many of the migrants caught in the act in Sinaloa had come from farther north. One subtle refinement: the muscle tissue that had powered their migratory flight had northern deuterium signatures, but their reproductive tissues had been built in Mexico.
The northern and southern nesting habitats of these double-breeding birds can be radically different. In California, what’s left of the yellow-billed cuckoo population nests in lush riparian growth. In Sinaloa, cuckoos nest in coastal thorn forest. Raising a second brood in Mexico during the monsoon season at a time when dry summer conditions are limiting potential insect prey in the north may be a form of bet-hedging. That has major conservation implications for both the cuckoo and the yellow-breasted chat, since the Mexican thorn forest is being converted to industrial farmland: both northern and southern breeding environments need to be protected or restored.
There’s plenty of room for follow-up studies here. Do double-breeding migrants behave differently in Mexico? Phainopeplas—assuming we’re talking about the same phainopeplas—are fiercely territorial when they nest in the desert, but sociable, even semi-colonial in the California hills. It may be a matter of how food resources, berries in this instance, are distributed. The orioles, chats, vireos and cuckoos may go through comparable behavioral changes.
We don’t know whether pairs of these birds migrate and rear a new brood together, or whether they find new partners for the southern season. We don’t know whether or how nest spacing, clutch size, or hatchling survival differs between the two nesting habitats. We don’t know what predators and nest parasites the birds encounter in Mexico. We don’t know when or where double-breeding birds undergo their annual molt.
And the mysteries of migration are only compounded by the discovery. “How do the offspring of cuckoos and orioles hatched in eastern North America orient southwest in their migration, whereas offspring hatched from the same parents in west Mexico orient southeast in their migration toward a presumably common winter range?” ask Rohwer and his co-authors. How indeed? Remember that first-year neotropical migrants travel on their own, without parental guidance. Their itinerary must be innate. But how can the same parental genes produce one set of offspring programmed to travel in one direction and a second set programmed for the opposite direction? And how do the Mexican hatchlings get back to their parents’ northern starting point?