Why do birds migrate?

November 16, 2011 7:46 PM

Why do birds migrate to the Arctic and Antarctic?

I've been watching Frozen Planet and during the summer there is this huge explosion of life and all sorts of sea birds and what not migrate to the poles. Truly spectacular in HD, but I can't help but wonder: Given how much work it takes just to get there, well, why do birds bother?

Or to make my question sound more science-y: what is/was the selective pressure that drove various sorts of sea birds to begin these large migratory patterns in the first place?

posted by selenized to science & nature (11 answers total)

They travel for the summer because there is shit tons of food to support them but the strongest pressures likely come from few young eating predators and less competition. They've got to make the round trip because it doesn't stay pleasant.
posted by Blasdelb at 7:51 PM on November 16, 2011 [1 favorite]

There's no simple one obvious answer (well there is, and it is that those birds that migrate are successful in reproducing). Nature abhors a vacuum. If there's a resource, it'll be exploited. If it's easier to do that on a FIFO basis than an overwintering one, well the successful birds get to breed.
posted by wilful at 7:52 PM on November 16, 2011 [1 favorite]

Very few predators. Mostly just foxes and the occasional bear. Not one snake. And even then, the long days and flat terrain mean predators are dead easy to spot and shoo away long before they can do much damage to the nest site.

Also, the long days mean steady temperatures -- slightly warmer than room temperature, usually -- generally sunny skies, and no trees to shade the place. Perfect conditions for eggs, chicks, and birds in general.

Aside from the huge clouds of mosquitoes, which are a fairly recent thing, it's a bird paradise, basically.
posted by Sys Rq at 8:13 PM on November 16, 2011

Some species of birds nest near where they were born.

As far as the Arctic in particular... I wonder what it has to do with the fact that we're in an inter-glacial period. Birds living/evolving in the last ice age would have been dealing with a much colder global temperature and a much smaller area of habitable land (the white shows glaciers). (Note: this is a guess, I don't have any actual bird-specific knowledge.)
posted by DoubleLune at 8:15 PM on November 16, 2011

Oxygen disolves in cold water better than in warm water. Therefore there is lots of life in the arctic and antarctic oceans. As a result there's lots of food. Migratory organisms can make use of this resource, and go and breed somewhere else during the cold months.
posted by singingfish at 8:43 PM on November 16, 2011

Another way to think about it is that they're antarctic birds who go on vacation in the winter. Six of one, half a dozen of the other.
posted by Rinku at 9:18 PM on November 16, 2011 [1 favorite]

(This is seabird (gulls, auks, penguins probably, guillemots, fulmars) specific and only kind of related to things like sandpipers, or geese, or eiders)

First, there's literally tons of food up there. The Arctic spring is short but the productivity is amazing. Second, there isn't as much competition. There are very few species using the resources up there so if you can get up there, you're pretty much set. Plus seabirds in particular are used to pretty harsh conditions. The murres that breed in the high Arctic spend their winters off Newfoundland so their summer conditions are pretty sweet.

As for how this evolved? I would think that birds just slowly moved north as land was freed up from ice. There's a conflict between a big enough colony (see below) and a too big colony. When resources (either space or food) were low, some individuals could start flying farther a field. Prospecting occurs early in the season before the bird settle into their colonies and before individuals are old enough to breed. A lot of seabird species take 2, 3, 5 years to reach sexual maturity but they will show up at the colony and check things out (and engage in breeding behaviour like picking up nesting material and brooding someone else's chick). It is hard to establish a breeding seabird colony but not impossible (obviously).

And there are predators up there - other birds, foxes, bears, and even walruses (and in the Antarctic even more (excluding some of the biggest penguins who breed really far inland). Because seabirds breed in colonies the predators actually do pretty well. At the colonies I was at there were at least a pair of ravens every 1 km of cliff and a family of foxes. But because there are so many birds and their breeding schedule is so synchronized, they can swamp the predators. For example, predators pick off fledging seabird chicks from the ocean surface after they jumped from their nests on the cliffs. But the timing was so precise that the whole colony of chicks would jump over a couple days. So even if 10% of the chicks got eaten, the predators would be full and couldn't eat all of the year's chicks.
posted by hydrobatidae at 9:25 PM on November 16, 2011 [1 favorite]

Oxygen disolves in cold water better than in warm water.

True, but

Therefore there is lots of life in the arctic and antarctic oceans.

No, not at all. It's nothing to do with the cold or the oxygen. Combination of factors: deep mixing, dark winters, some other things - means that there are high nutrients in the water column during the brief spring and summer, supporting impressive amounts of primary productivity. The higher trophic levels follow along as fast as they can.
posted by zomg at 9:38 PM on November 16, 2011

Another way to think about it is that they're antarctic birds who go on vacation in the winter. Six of one, half a dozen of the other.

According to my ornithologist ecology prof, the thinking is that the common ancestor of birds evolved in the tropics, and they've all radiated from there. (Although saying all birds are tropical is like saying that all humans are Africans.)

zomg and hydrobatidae are spot on.
posted by momus_window at 10:21 PM on November 16, 2011

There are many birds that migrate to the Arctic to nest. Many Arctic islands have few if any natural predators.
posted by Foam Pants at 11:31 PM on November 16, 2011

The answers above have it pretty well; I was just going to say, simply, "The Ocean." In the summer, algae and plankton bloom, and then you get an EXPLOSION of life that cascades up the food chain. The birds are going to exploit that. As the summer winds down, the food dies off or leaves. So the birds do, too. - aj
posted by Alaska Jack at 5:13 PM on November 17, 2011