I’ve been meaning to write about this topic for a while—now xkcd has beaten me to it:
Oh, well. Since the comic doesn’t actually answer the question, I’m hoping you’re all still interested! (Also, at the end there will be a bonus discussion of ant rain. Yes, ant rain. You won’t find that on xkcd!)
You’ll notice I’ve changed the question a bit from the xkcd comic: where Randall Munroe asks “Where do birds go?” I’d like to answer “What do birds do?”—because birds don’t necessarily go anywhere when it rains.
Rain poses two major possible dangers to a bird. The biggest one is hypothermia: birds stay warm by trapping tiny pockets of air under their feathers, and if those pockets fill with water instead of air, that bird is going to get cold very quickly. (This is why down jackets don’t work when wet.) American Kestrels exposed to rain increase their metabolic rate, presumably to compensate for heat loss (Wilson et al. 2004).
The smaller the bird, the bigger a problem this is, since smaller birds have higher surface-area-to-volume ratios, meaning they lose heat more quickly, and they generally have smaller energy reserves.
Already you can see how avian responses to rain might depend on the bird: small birds might seek out shelter more readily than large birds, to avoid getting wet and cold. Indeed, while smaller birds sought shelter during one torrential rain, a flock of Turkey Vultures was seen perched at the tops of tree with wings spread, rain-bathing (Hume 1986).
Birds can and do take shelter from rain: in bushes, in reeds, under eaves, in nesting cavities. But this is necessarily a short-term solution, because birds also need to eat. This is the second possible danger of rain: starvation. You can hide from an afternoon shower, but a week-long downpour is not something you can wait out, especially if you’re a small bird without much stored energy. You’re going to have to eat, and to do that, you’ll have to get wet.
(If you haven’t already, check out the videos of hummingbirds flying in heavy rain at the end of this older post.)
I’ve seen this in the field: birds may briefly disappear when rain first starts, but if it lasts any length of time, the birds start to reappear. They can’t afford to stop foraging—especially if they have chicks to feed.
Length isn’t the only aspect of rain that matters—intensity makes a difference too. A sufficiently light rain probably won’t be a problem. Most bird feathers are somewhat water resistant, and in light rain, you may see birds fluffed up, just as they would in a dry cold.
Heavy rain calls for a different tactic. Fluffing up your feathers keeps you warm, but if the rain is heavy enough, water will get in between your fluffed-up feathers and chill you faster. Instead, in heavy rains birds sleek down their feathers to reduce their wettability.
The classic bird-in-heavy-rain posture, as described by Hume (1986), is “head withdrawn, bill pointed towards the rain, body rather upright and feathers sleeked,” a pose which combines staying warm (“head withdrawn” is a heat-conserving position), minimum exposure of the bird to rain, and maximum opportunity for raindrops to slide off the feathers rather than being absorbed. Birds may take up this posture when they don’t have access to shelter. Shorebirds in the rain have been observed huddled close together, all in this posture (Hume 1986).
So where do birds go in the rain? To shelter, or not. Out to lunch, or not. In to huddle with some buddies, or not. It all depends on the bird, the rain, and the environment.
And now, in a special bonus part of our birds-and-rain-themed post, we bring you: birds and a completely different kind of rain.
Specifically, ant rain.
Ant rain is a phenomenon in the tropics where ants climb up trees to forage and then, while most of them climb back down, some of them jump or fall down, forming “ant rain.” Haemig (1997) reports that this “continuous rain of free-falling ants” is affected by birds: when birds forage in a tree full of ants, the ants try to escape by jumping, and the ant rain increases.
Next time there’s an awkward lull in conversation, you can whip out that factoid and impress everybody. You’re welcome.
Haemig P. 1997. Effects of birds on the intensity of ant rain: a terrestrial form of invertebrate drift. Animal Behaviour 54:89-97.
Hume R. 1986. Reactions of birds to heavy rain. British Birds 79:326-329.
Wilson G, Cooper S, Gessaman J. 2004. The effects of temperature and artificial rain on the metabolism of American kestrels (Falco sparverius). Comparative Biochemistry and Physiology, Part A. 139(3):389-394.
*Photos obtained from Flickr and used via Creative Commons. Many thanks to these photographers for using Creative Commons!