Doing It For the Kids: The Evolution of Migration

tags: evolutionary biology, behavioral ecology, life history, migration, long-distance migration,birds,ornithology,researchblogging.org,peer-reviewed research, peer-reviewed paper

I recently told you about research that used new microtechnology to document the incredible journey of Arctic Terns, a small bird species that annually migrates from its wintering area in Antarctica to its breeding colonies in Arctic and sub-Arctic regions of Europe, Asia, and North America. But Arctic Terns are not the only birds that migrate extremely long distances: many birds, particularly shorebirds, regularly travel tens of thousands of kilometers between their wintering and breeding territories every year.

The costs of migration are many, including the metabolic and energetic demands of flight, high mortality risk, and exposure to extreme weather events. Further, for birds that breed in the Arctic and sub-Arctic, safe arrival on their breeding territories does not mark the end of the challenges that they face. Severe weather events, a frequent occurrence in the Arctic, can lead to poor body condition and cause breeding failures, sometimes forcing the birds to abandon their nests and leave early, or even killing them.

Considering the tremendous sacrifices demanded by migration, why did it evolve as a life history strategy? And why do some long-distance migrants travel so far north? If these birds stop and breed farther south, then surely they could avoid some of the costs associated with traveling to and nesting in the Arctic.

According to a new report published by a Canadian team of researchers, shorebirds migrate over great distances at least in part because their eggs are less likely to be eaten in the Far North.

"These birds are flying thousands of kilometers to reach their breeding grounds in the north." Why they don't just stop in Hudson Bay has always been the perplexing question, according to Grant Gilchrist, a biologist with Environment Canada and Carleton University in Ottawa, a member of the research team. "They're flying over suitable habitat and spending enormous energy to get to northern regions."

This work builds on previously published research that found that nest predation risk is a driving force in the evolution of the life histories of birds, influencing nest site selection and the observed latitudinal variations in the average clutch size of passerines (DOI: 10.1126/science.287.5457.1482).

Based on that information, the researchers hypothesized that the risk of nest predation might play a major role in balancing the costs of long-distance migration for shorebirds as well. If their hypothesis was correct, they predicted they would find a negative relationship between nest predation risk and latitude in arctic ground-nesting shorebirds.

To test this hypothesis, for two or more summers, the researchers constructed and monitored 1555 artificial nests containing four Japanese quail eggs at seven Canadian shorebird breeding sites.

"Quail eggs resemble those of shorebirds in coloration and size and the depression made is similar to the simple nest scrapes used by shorebirds," says lead author Laura McKinnon, a PhD student in the biology department at the University of Quebec at Rimouski.

The researchers also chose to make artificial nests rather than observing natural nests because some species are better at hiding or defending their nests than others.

"Artificial nests permitted us to measure predation risk while controlling for these factors, so we could compare the same measure of predation risk across sites," explains Ms McKinnon.

The researchers took GPS readings for each camouflaged nest and returned nine days later to count the number of eggs that had not been eaten by foxes and other predators.

These artificial nests spanned a latitudinal gradient of 29° (~3350 km or 2081 miles) from the sub-Arctic (Akimiski Island in James Bay) to the high-Arctic (Alert at the northern tip of Ellesmere Island) regions in Canada (Figure 1).

Even though a previous study of neotropical migrants failed to detect increased predation risks (DOI: 10.2307/2937160), this study found a striking correlation between latitude and predation risk: For each 1° increase in latitude, the relative risk of predation declined by nearly four percent (Figure 2). Stated another way, the eggs in Alert were 65 percent less likely to be eaten than those on Akimiski Island.

Comparing survival rates for artificial nests in the south versus those in the north makes it possible to gain a more precise understanding of the trade-offs involved between stopping to breed farther south (and thereby avoiding the added costs of migrating yet further north) versus the higher losses of eggs and nestlings and increased competition for food. This knowledge can provide insight into how extreme behaviors, such as long-distance migration, evolved in the first place.

"Understanding the mechanisms leading to such extreme behavior help us understanding species distribution and biodiversity," says ecologist Joël Bêty, McKinnon's supervisor and co-author of the paper.

This research can also provide important clues for conservation. Many shorebirds' numbers are in steep decline, but no one knows why. Some people think that loss of habitat either along their migratory routes or in their wintering areas in South America is the cause, while others blame the loss of insects and crustacean eggs that they consume.

"These globe-spanning migrations, and a number of other traits, make shorebirds atypically sensitive to environmental change. Shorebird populations appear to be in a widespread state of decline, and efforts to monitor and understand these declines are mounting," says Dr Bêty.

Nearly half of Canada's 47 shorebird species nest in the Arctic.

Source:

McKinnon, L., Smith, P., Nol, E., Martin, J., Doyle, F., Abraham, K., Gilchrist, H., Morrison, R., & Bety, J. (2010). Lower Predation Risk for Migratory Birds at High Latitudes. Science, 327 (5963), 326-327 DOI: 10.1126/science.1183010