ethology - Where do migratory birds go when it rains?

I was reading a comic from XKCD which inspired me to ask this question: "well, where do they go when it rains?" I know that when it rains you usually don't see any birds flying through the sky.

I imagine they normally hide in trees or under other kinds of cover. But for migratory birds, I don't think it is possible to hiding under cover until the rain passes. There might not be any cover (trekking across seas) or it might stay rainy for a very long time (and therefore cold...).

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I did some research, but the answers are conflicting. Some people say they do stay in trees, others say they AVOID trees (because trees can snap in a storm, plus if there's lightning then sitting in a tree will net fried birds). Then there's people saying that birds have a waterproof coating... but that's a bit contradictory with me not seeing birds when it rains.

So where do birds go when it rains?

I assume that different bird species and groups of birds will behave in different ways. I am interested in the case of migratory birds because they are likely to encounter different climates and have perhaps devised strategies to be safe. I would appreciate any well-referenced examples.

Answer

Obviously, as you point out, some birds, such as waterbirds (ducks, puffins, auks, sandpipers) are perfectly fine in the rain, as they spend much of their time in the water anyhow and have oil glands that waterproof their feathers.

For other birds, rain can increase heat loss. For instance, rain increases metabolism by up to 22% in eagles (Stalmaster & Gessaman, 1984), although the actual impact on average daily metabolism was 'negligible'. However, eagles were able to feed their offspring only half as much, and therefore had only half as many eaglets, when it rained (Elliott et al., 2005). (On the other hand, the authors point out that in the Tropics, eagles actually do better when it rains because there is more food available).

Migrating birds stop migrating, hide where they can, and wait until it dries up before they continue (Erdi et al, 2002); those that don't, often end up far away from where they should be and, eventually, dead. In short, some birds don't mind the rain (e.g. seabirds), others sit it out in misery (e.g. nesting eagles) and others hunker down wherever they can find shelter (e.g. migrating birds).