Birds that show up outside of their normal range—vagrants or accidentals, as they are known, have long fascinated birdwatchers. Wildlife tours of the Alaskan archipelago, for example, lure customers with the possibility of sighting exotic vagrants like the Eurasian common cuckoo. Now scientists are beginning to consider the possibility that these misplaced birds might be more than curiosities, exploring the question of whether vagrants could provide clues about future bird distributions as climate shifts.
Vagrants have traditionally been perceived as unlucky birds blown off course by severe weather during migration, or dud birds with faulty internal GPS. In autumn, North American species are sometimes found as vagrants in Europe, often in association with storms. Other vagrants apparently veer way the wrong way, or overshoot their target due to navigation-impeding genetic mutations. In truth, vagrants may not necessarily be the oddballs that observers have assumed them to be. According to ecologist Richard Veit of College of Staten Island at the City University of New York, that notion that all vagrants are “messed up, and unable to navigate,” has led people to ignore their potential importance in understanding animal distributions and how they change over time. He and his student Lucinda Zawadzki are now mining predictive hints from vagrancy data for the ash-throated flycatcher.
Veit frames vagrancy as part of the overall spectrum of bird dispersal. “In any population there are always a small proportion of individuals that go much farther than the others,” he says. Veit has described vagrants as the expanding fringe of a growing population.
Frédéric Jiguet of the Center for Ecology and Conservation Sciences* in Paris, France, has been examining links between climate change, range shifts and Siberian birds found as vagrants in Europe. In a 2013 study, he found that as populations expand, so, too, do the number of vagrants detected far from home. Conversely, his data suggest that shrinking populations produce fewer vagrants.
“Vagrancy is condition-dependent, which can also be linked to climate change,” Jiguet says. As meteorological conditions change over the oceans, birds that are crossing the Gulf of Mexico may be increasingly likely to get caught by the winds, and “deported to the Atlantic,” with the few survivors ending up in Europe, he explains. That raises an important point about vagrants. Unlike “normal” migrants, they are very often on a one-way trip.
Records of three such wanderers recently prompted animal behaviorists Vladimir Dinets of the University of Tennessee, Mark Hauber of Hunter College and their colleagues to investigate the possible future impacts of such cross-oceanic invasions. Dinets and Hauber study brood parasites, species that foist their eggs on unwitting foster species by sneaking them into a host’s nest, saving themselves the effort of rearing chicks and reducing breeding success of the hosts they dupe in the process. These egg sneaks include the Eurasian common cuckoo, the Oriental cuckoo, and the brown-headed cowbird. All have begun showing up far from home.
Although there are signs that common cuckoo populations are declining in Europe, the common and Oriental cuckoo species, native to temperate Eurasia, appear to be expanding their ranges northward. Having now colonized northeastern Siberia, both cuckoo species now breed within 300 kilometers of the Bering Strait, according to Russian museum records examined by Dinets and his colleagues. Vagrant cuckoo sightings have been recorded in mainland Alaska and its archipelagos, including one common cuckoo pair observed in courtship, plus multiple specimens collected locally by the University of Alaska’s Museum of the North. In 2012 one common cuckoo overwintered in California. At the same time, North American brown-headed cowbird males and females have been reported with increased frequency in Western Europe, often in association with storm events.
Dinets and his colleagues wondered how naïve North American birds would respond to brood parasitism by common cuckoos, and how Eurasian birds would respond to eggs from brown-headed cowbirds. In the summer of 2014 they conducted two experiments. In Southern Finland, common cuckoos often parasitize common redstarts with deceptive color-matched eggs. So the researchers fashioned fake eggs out of plaster, making some that looked like cuckoo mimics of the redstart’s own eggs, and others that were plain creamy-white or speckled like cowbird eggs. They then added eggs to the redstart’s nest to test the bird’s reaction. Over six days they watched to see if the added egg was accepted or rejected. Similarly, in California they tested the reaction of American robins to eggs modeled after their own, plus those resembling redstart or thrush-mimicking cuckoo eggs. They discovered that Finnish redstarts accepted about half of the cowbird-like eggs. The American robins were even less discerning, rarely rejecting the blue cuckoo-like eggs.
The findings suggest that if these brood parasites do invade new continents in significant numbers as climate shifts, the naïve bird species that would become their hosts could face “potentially a very serious problem,” Dinets says. Cowbirds, in particular, have hammered naïve North American host species as they have rapidly expanded their range across the continent. Soil tilling has fostered the bird’s spread, because the species thrives in agricultural and grassland habitats.
The idea that the small number of vagrant birds on record represents the beginnings of a new wave of colonization remains wildly speculative. Indeed there are more questions than answers when it comes to vagrant cuckoos, cowbirds and their possible future breeding ranges. On new continents, where would cuckoos or cowbirds spend the winter? What routes would they use to get there? And could they find hosts with breeding behaviors and timing compatible for exploitation? “All this stuff has to be worked out… to figure out the probability of this [transoceanic invasion of cowbirds/cuckoos] being an issue," says Liana Zanette, an ecologist at Western University in Canada who has studied cowbirds.
Nevertheless, if mixed-sex groups of individuals survive in their unfamiliar surroundings, vagrancy can be a precursor to colonization. That is presumably how finches accidentally colonized the Galapagos Islands, how honeycreepers colonized Hawaii, and, more recently, how cattle egrets expanded from Africa to South America in the 1800s before moving into North America by the early 1950s.
Vagrants have been tricky to study, for all of the reasons that make them intriguing to birdwatchers: they appear in small numbers, infrequently, in unexpected places. But the crowdsourcing of bird-sighting data through programs such as ebird is making this research increasingly feasible. Vagrants may thus provide a potentially rich window on where animals will be found in the future, “particularly when combined with climate models” and the promise of ever-improving tracking technology, in Veit’s view. Vagrants, he says, may be “very biologically important.”
*Editor's note (8/18/2016): Jiguet's affiliation was corrected from a previous version of this article.