Prothonotary warblers, songbirds as yellow as sliced mangoes, breed in U.S. wetlands during warm months. Their plumage clashes against dark forests so starkly that a flock of them appears like “butter dripping from the trees,” Ohio State University biologist Christopher M. Tonra said.
Such scenes are increasingly rare. The U.S. Fish and Wildlife Service considers prothonotary warblers to be a species of concern, and in Canada, the warblers are endangered. Tonra and his colleagues recently tracked the declining birds. The goal, as the researchers reported in the journal the Condor: Ornithological Applications, was to discover where the animals spend the winter.
Scientists knew that the animals, each of which weighs no more than a pair of nickels, migrated thousands of miles south each autumn and returned north in the spring. Previous research suggested that the birds sought shelter in coastal mangrove forests. If so, the loss of these habitats could explain the shrinking warbler populations.
The warblers are too small for conventional satellite tags. Only in the past decade has technological miniaturization allowed researchers to track small birds. These geolocators sense sunlight, measuring the time from sunrise and sunset, which allows scientists to estimate a bird’s location each day. The authors of the study, conservationists in states from Wisconsin south to Louisiana and east to Virginia, tagged nearly 150 warblers with the tiny devices.
Data recovered from 34 birds contained several surprises, Tonra said.
Nine in every 10 birds spent the wintertime in a relatively small region in Colombia. “That was regardless of whether they were tagged in Wisconsin or they were tagged in Louisiana,” Tonra said.
The warblers clustered around the Magdalena River Valley in inland Colombia, not the coastal jungles. This valley “more mimics the kinds of habitats they breed in, in the temperate zone, than mangroves do,” Tonra said.
Jared Wolfe, a wildlife biologist at Michigan Technological University who was not involved with this research, said the warbler study was “precedent-setting” because of its geographic span and the number of birds tracked.
“Identifying important winter areas represents a critical step towards conserving birds throughout their entire annual cycle,” Wolfe said.
“It challenges our ideas about where we think these birds go,” said Kenneth Rosenberg, a conservation scientist at the Cornell Lab of Ornithology who was not a member of the research team. Rosenberg applauded the thoroughness of the study but cautioned the trackers, affixed to birds that roost in dark and dense forests, may not sense the full amounts of sunlight. If that happened, it’s possible the birds were actually living in forests closer to the equatorial coast, he said.
The research team has partnered with Selva, a nonprofit conservation organization in Colombia. Focusing on this region in Colombia would produce “the biggest bang for our conservation buck,” Tonra said, for protecting winter habitats.
Tonra and his colleagues also discovered that the birds dawdled on their way south. Instead of stopping just for a quick refuel, some birds spent as many as three weeks in Central American tropics before arriving in Colombia for the winter. Other migratory bird species, scientists are learning, make long stopovers, too.
“This paper, and others, have really highlighted this blind spot,” Tonra said.
For these birds, migration appears to be a series of long-haul jumps punctuated by rest periods, rather than a steady trickle from backyard to backyard to, finally, jungle. During the stopovers, birds “need to put on fat, basically, to gain energy,” Rosenberg said. In the warblers’ case, that means feasting on snails and insects.
Conservationists frequently point out the “ecosystem services” that animals provide for humans. Prothonotary warblers, for instance, hunt pests that damage coffee crops. But Tonra spoke, too, of this migration — a 3,000-mile journey completed by a creature that weighs less than most pocket change — as an ecological marvel.
On fall and spring nights, warblers join “billions of animals flying above our heads,” Tonra said. “And the more we can do to help them get there and complete that journey, since we’ve made it more difficult for them in many ways, is a really important thing.”