Birds of a feather don't always flock together. In the world of birds, migratory habits are common. There are, at a minimum, 9,000 to 10,000 living avian species. About 4,000 of them have been observed making regular, large-scale journeys in pursuit of food, nesting sites or other resources.
Yet no species is a monolith. Animals are individuals, and two members of the same species might behave in very different ways. The same is true of creatures who live side by side for part of the year. Often, a local population of birds, fish or hoofed mammals will be split between individuals who migrate on a regular basis and others who stay put all year round. Scientists have a name for this phenomenon: "partial migration."
"Lots of bird species are partial migrants, especially at higher latitudes," says Hamilton College ecologist Andrea K. Townsend in an email exchange. "More than a third of birds that breed in Europe for example have been classified as partially migratory. Some of our most familiar birds like blue jays, American robins, and Killdeer are partial migrants."
Our feathered friends don't have a monopoly on the practice. "There are many other partial migratory species, ranging from killer whales and Chinook salmon to field voles and field crickets," adds Townsend. Moose, red deer and white perch fish also have displayed an affinity for this individual-oriented migration style. So have Hawaiian tiger sharks.
And How About Crows?
Pervasive as it is, we still have much to learn about partial migration. Townsend's the lead author of a new study that explores the topic through a crow-shaped lens.
Published by The Auk: Ornithological Advances on Aug. 8, the paper is all about one of the Western Hemisphere's best-known birds: the American crow (Corvus brachyrhynchos). For their investigation, Townsend and her three co-authors put satellite tracking devices on 18 wild crows. Eleven of the birds had been caught in Davis, California, while the remaining seven were tagged in Utica, New York.
Townsend's group started keeping tabs on their crows in 2014. Some birds were tracked for as little as 54 days, but the scientists monitored others for up to 1,305 days. The team also took feather and blood samples from their animals in order to analyze them on a genetic and isotopic level.
Earlier this year, the surveillance period ended. We've known for some time that the American crow is partially migratory — a point reaffirmed by the new tracking study. Eight of the Californian crows and six of their New York counterparts (or 77.8 percent of the birds) went on annual migrations north in order to breed. The average distance covered in these trips was 571 kilometers (354 miles).
Those corvids who migrated did so in every year. Likewise, the ones that abstained never experimented with migrations. So, in this respect, the crows in both camps remained set in their ways. But the biologists did find some variability among the migrators. Although they kept coming back to the same breeding areas year after year, the birds were liable to seek out new winter roosts from season to season.
"Their faithfulness to their breeding grounds is probably linked to territoriality," notes Townsend. "Crow families defend their breeding territories against intruders. If they didn't return to the same breeding territory year after year, they would need to establish a new [one] somewhere else, which can be difficult and dangerous." Crows may fight to the death when intruders try to settle on their land.
On the flip side, fidelity to winter habitats is rather weak. "In the winter, migratory crows don't defend territories. They hang out in communal flocks, usually around huge, indefensible food sources like community compost piles, animal feedlots, or dumps," Townsend says. She adds that this may give the birds "the flexibility ... to move among overwinter sites until they find one that suits them."
Making New Flight Plans
Scientists have plenty of ideas about why partial migration is so common in vertebrate animals. In some cases, it seems to be linked to social dominance. One researcher studied a population of dark-eyed junco sparrows and found that the more submissive birds within the group were more inclined to migrate. Predation troubles, lack of access to mates and limited resources also may drive some individuals to make regular long-distance trips while their peers stay put.
Going forward, Townsend and her co-authors think global warming could fan the flames of partial migration, making the practice even more widespread than it already is. "Recent work," they write in their new paper, "suggests that partial migration might buffer some species against a warming climate."
"Their behavior could actually benefit them," Townsend tells us in her email. "Instead of being locked into overwinter sites that are no longer suitable, species with flexible migration strategies might be able to find and settle in sites that are still appropriate as conditions change. They might also be able to shorten their migratory journey, which would save them energy." All this goes to show that fortune favors the adaptable.
If you're a birdwatcher, you'll want to keep track of your favorite species' migration patterns. Townsend predicts that we "may see changes" in winter bird communities around the world. "When it comes to crows, for example, we might start seeing larger overwinter roosts at the more northern end of their range."