Home advantage or pastures new? Shorebirds in the high Arctic rarely return to the same breeding ground

Shorebirds are capable of some of the longest migrations on Earth, yet most return year after year to the same area to breed. Coming back has distinct advantages, including knowing the best spots to feed or roost, having information about where predators lurk, and - hopefully - finding last year's partner again. But not all of these mostly ground-nesting birds stick with what they know: some species, such as pectoral sandpipers and long-billed dowitchers, rarely return to the same area at all, while even among reliable returners like oystercatchers, some individuals choose to settle somewhere new. This raises a key question: how does a shorebird’s previous geographic breeding location influence its decision to return or settle elsewhere?

A new study in Ecography by a team of researchers led by Bart Kempenaers, head of the Department of Ornithology at the Max Planck Institute for Biological Intelligence (MPI-BI) has shed new light on this question, drawing on decades of published field data from shorebird populations across the northern hemisphere. The team found that shorebirds breeding in the Arctic tundra return far less often than birds breeding in forested areas, grasslands and arid regions – and that within the tundra, the pattern intensifies the further north they go. In every other habitat, how far north an individual bred made little difference to the likelihood it returned to the same area.

“Several factors are known to affect whether a bird returns to the same site to breed, such as how a species divides parental duties, and an individual’s sex and body size, but how much geography matters had not been studied systematically,” says Peter Santema, a postdoctoral researcher at MPI-BI and first author of the study. “At first, the data seemed to show a simple pattern: the further north birds breed, the fewer that come back. But that impression turned out to be misleading, because tundra birds happen to breed at the highest latitudes and return least often, which accounted entirely for the apparent relationship between latitude and return rates. Once we compared habitats separately, it was clear the effect belongs to the tundra alone.”

Digging into the numbers

Across the vast expanse of the Arctic tundra, conditions can vary greatly from one year to the next. A site that may have offered perfect conditions in June of one year could be buried in snow a year later, or be under far greater threat from predators than in the previous breeding season. With only a relatively narrow window in which to breed, shorebirds have little time to spare waiting for conditions to improve. To find out whether shorebirds breeding in tundra environs really are less likely to return than those in other habitats, the researchers turned to a large body of existing data.

Co-author Eunbi Kwon, a staff scientist at MPI-BI, had spent years compiling estimates of return rates for 120 populations of 55 shorebird species from across the northern hemisphere. The data came from published field studies and records shared by researchers. An earlier analysis of these records showed that species sharing parental duties return to breeding sites more reliably than those where only one parent incubates the eggs and cares for the young. However, the role of geography remained an open question. Working through the records, Santema noticed that many of the species with the lowest return rates bred in tundra, prompting the team to look at whether habitat, rather than latitude alone, might explain the pattern.

The researchers first examined how return rates varied with latitude, which showed that birds seemed less likely to return the further north they had bred. However, when Mihai Valcu, another staff scientist and co-author from the research group, broke down the analysis by habitat, a different picture emerged: only in tundra were shorebirds markedly less likely to return to the same breeding area, with latitude appearing to make little difference in every other habitat.

Within the tundra, return rates ranged from roughly half of the individuals returning near the southern edge of the biome, which is at about the latitude of Oslo, to about only one in ten returning at its northernmost limits in the high Arctic. As a comparison, in forest, grassland and arid biomes, researchers recorded that around two in three birds returned to their previous breeding site. Notably, the pattern held within species too: among those breeding in both tundra and other habitats, it was consistently in tundra populations that individuals returned less often.

“Finding that one habitat drives so much of this variation opens up fascinating questions,” says Kempenaers. “Do other unpredictable habitats that are not included in the study, such as ephemeral wetlands, also favour nomadic breeders? What leads an individual bird to return or move on? And as breeding habitats come under growing pressure worldwide, does a readiness to relocate help these birds maintain stable populations? Clearly, where a bird breeds from one year to the next affects how widely genes mix between populations, and how readily disease travels with them. Combining broad comparative studies with tracking of individual birds has the potential to reveal a great deal more about these processes.”