Why migrate?

Growing up in New Hampshire, I took the yearly migration of ducks, geese, and (less obviously, but more impressively) Monarch butterflies pretty much for granted. It never really occurred to me to ask why all of these animals migrate. If I had been asked, I probably would have made the (common) anthropomorphic fallacy and said that they migrate because they "want to," or because they "like it better" at their ultimate destination.

Think about it: migration--especially very long-distance migration, such as that performed by some shorebirds, which can migrate from the southern tip of Africa all the way to the Arctic--is incredibly resource-intensive. It takes a long time, requires enormous amounts of energy, and is pretty dangerous. For a behavior like that to survive and develop in a population, it must provide significant survival or reproductive benefits. There are three main hypothesis about what those benefits might be:

1. Increases in food resources. For example, migrating south might allow birds to avoid competing for the limited food available in New England in the winter.
2. Reduction in parasite load. For example, migrating out of an area during a parasite's main breeding season might allow a bird to avoid infestation by the parasite.
3. Reduction in predation pressure. For example, migrating to different areas during different times of year might allow birds (or their eggs or hatchlings) to avoid attacks by predators that are common during those times.

It's possible to test these different hypotheses by looking at exactly where different populations migrate to. For example, consider those shorebirds I just mentioned. They migrate to high northern latitudes from the southern parts of Africa. The range of latitudes to which they migrate is wide; some stop just below the Arctic circle, but others keep going nearly to the North Pole. Previous studies have shown increased food availability and reduced parasite loads at these high latitudes. However, until now, there have been few studies that produced quantitative data on how migration site affects predation risk. In the 15 January issue of Science, Gilg and Yoccoz and McKinnon et al offer substantial evidence to support the idea that migrating to high latitudes offers significant benefits in terms of avoiding predators.

McKinnon et al placed more than 1500 artificial shorebird nests at various locations in northern Canada, over a latitude range of about 3350 km. They monitored the nests for two or more summers and recorded how well they survived predation.

The result? For every one degree further northward a nest was placed, the risk of predation on the nest decreased by about 3.6%. Over the range of latitudes they studied, that translates to 65% lower predation on the northernmost sites than on the southernmost sites. This is a significant reduction, and suggests that predation may indeed play an important role in driving bird migration.

However, other studies of predation risk at different latitudes didn't show such a clear trend. McKinnon et al suggest that this might be because those other studies used real nests, which vary in size, health, etc. In other words, previous trials were not as well controlled as was this investigation. By using artificial nests, the researchers reduced other potentially confounding factors.

Gilg and Yoccoz add to the story by suggesting that an important factor influencing predation on the nests is the distribution of another common prey species, the lemming. Lemmings and shorebirds (and shorebird eggs) are common prey for the Arctic fox. By comparing the distributions of the lemmings and the shorebirds, Gilg and Yoccoz show that the shorebirds most commonly hunted by foxes are typically common only where lemmings also occur. They hypothesize that, in areas without lemming populations, the predation pressure on the shorebirds is too high for their populations to survive.

Together, these two articles indicate that there are many more factors influencing bird migration patterns than simply "because the birds like it better there."

Gilg, O., & Yoccoz, N. (2010). Explaining Bird Migration Science, 327 (5963), 276-277 DOI: 10.1126/science.1184964

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