Arctic tundra shorebirds rarely return to the same breeding spot - and the farther north they nest, the less likely they are to come back.
Shorebirds nesting on the Arctic tundra break that pattern far more than birds nesting anywhere else.
Splitting birds by habitatWorking through the records, Santema noticed that many of the species with the lowest return rates all bred in the tundra.
Only in the tundra were shorebirds markedly less likely to return to their old breeding ground.
It’s one shaped by conditions that can shift as fast as an Arctic food web does, year to year.
Arctic tundra shorebirds rarely return to the same breeding spot - and the farther north they nest, the less likely they are to come back.
Shorebirds fly some of the longest migration routes on the planet. Many still return to the very same patch of ground to nest, year after year, and it usually pays off.
A returning bird already knows where the food is. It knows where predators tend to strike. It may even find its old mate waiting.
Shorebirds nesting on the Arctic tundra break that pattern far more than birds nesting anywhere else. The farther north they go, the less likely they are to come back at all.
Pectoral sandpipers and long-billed dowitchers rarely return to the same ground.
Oystercatchers usually do come back, yet even among them, some individuals settle somewhere new each year.
Bart Kempenaers, who heads the Department of Ornithology at the Max Planck Institute for Biological Intelligence, led a team that dug into why.
Peter Santema, a postdoctoral researcher in Kempenaers’ lab, pulled together decades of published field data on shorebird populations across the Northern Hemisphere.
Latitude looked like the answer
Some things were already known to affect whether a shorebird returns to breed in the same spot: how a species splits parenting duties, and an individual’s sex and body size.
An earlier analysis of these same records had already shown something else. Species that share parental duties return to their breeding sites more reliably than species where only one parent incubates and raises the young.
Whether raw geography mattered had never been tested directly, so Santema went looking.
The first pass through the numbers pointed at latitude. Birds that bred farther north seemed to come back less often, in a fairly clean line. It looked like a simple story.
“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,” Santema said.
Splitting birds by habitat
Working through the records, Santema noticed that many of the species with the lowest return rates all bred in the tundra.
That pushed the team to check habitat instead of latitude alone.
Mihai Valcu, a staff scientist on the team, split the same data by habitat. A different picture emerged. Only in the tundra were shorebirds markedly less likely to return to their old breeding ground.
Birds in forest, grassland, and arid habitats returned about two-thirds of the time. How far north those birds had bred made almost no difference.
The comparison drew on a large body of existing records.
Eunbi Kwon, another staff scientist on the team, had spent years compiling return-rate estimates.
Her tally covered 120 populations across 55 shorebird species, gathered from published field studies and records that other researchers shared directly.
Farther north in the tundra
Even within the tundra alone, the pattern kept sharpening. Near the biome’s southern edge, roughly at the latitude of Oslo, about half of the birds came back.
At the tundra’s northernmost limits, in the high Arctic, only about one in ten did.
The same split showed up within single species that breed in both the tundra and other habitats. Individuals of those species returned less often after nesting in the tundra than after nesting elsewhere.
That argues against one simple alternative: that differences among species were driving the entire result.
Snow and predators can flip a site
Arctic conditions don’t stay put from one summer to the next. A site with good snow-free ground and few predators in one June can turn hostile the next year.
Late snow can bury the same spot. More foxes or jaegers can move in and start raiding nests that went untouched the year before. Shorebirds don’t have the luxury of waiting things out.
Their breeding window at these latitudes is short, so a bird gains little by gambling on a site that failed it once. Moving on might land it somewhere better this time around.
The bigger picture is still coming together
Kempenaers said the pattern raises questions well beyond shorebirds. Other unpredictable habitats, such as ephemeral wetlands, might favor similarly nomadic breeders.
Growing pressure on breeding grounds worldwide could make a readiness to relocate valuable for keeping populations stable.
Where a bird chooses to breed from one year to the next also shapes how genes mix between populations, and how easily 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,” Kempenaers said.
What determines whether any single tundra-nesting bird comes back or moves on remains an open question. It’s one shaped by conditions that can shift as fast as an Arctic food web does, year to year.
The full study was published in the journal Ecography.
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