Little birds in crevices

Having just gotten yet more consultancy work for a children’s book out of the way, and with all notions of meeting the end-January deadline for the British dinosaurs manuscript, and for the Crato turtles manuscript, now out of the window, I may as well give up and resort to writing more nonsense.

I’ve been thinking a lot about the sleep behaviour of small birds. As Will and I walked to school one cold morning a few weeks ago, we stopped and looked at the two Wood pigeons Columba palumba sat together on their nest. It was January, so they weren't nesting. Could it be, I wondered, that they sleep on the nest – as in, sit there during the night? I don’t think they do, but at least some passerines do. A recent report on nesting behaviour in phoebes (Sayornis) showed that they started sleeping on the nest some time before egg-laying started – pretty unexpected given that most other passerines only start spending the night on the nest the night before they begin laying. Other passerines – some sparrows – are reported to make special ‘sleep’ nests. That’s pretty weird, what about other little birds?

Given that (so far as we know) all animals have to sleep, the diversity of sleep behaviours is interesting, and many animals do things that we might not predict. We all know, for example, that many large ungulates sleep standing up, but less well known is that at least some of these species have to lie down for a short while during the night. Elephants do, and giraffes even get down on the ground and sleep deeply for a few minutes at a time, legs folded up and head lying on the ground. So where do small birds go to sleep? They can’t just sit tight out on branches, surely: they’d get chilled and would be highly vulnerable to nocturnal predators. Some social species, such as swallows and long-tailed tits, do this, and huddle closely together, and starlings, social finches, some thrushes and so many others form huge communal roosts.

But for small passerine species that roost individually, the answer is that, like giraffes and elephants, they do something quite unexpected (well, unexpected to me): they climb deep into tangled undergrowth, or tuck themselves away into cracks and crevices. I don’t know why, but I find the idea of passerines secreting themselves away into crevices to be rather odd. It’s documented in the literature, and, what’s more, I’ve observed it myself. Months ago Colin Lacey and I were exploring local woodlands late at night (and there were honest zoological reasons for this nocturnal activity, I promise you). Projecting from a small cavity in the bark of an oak tree, we noticed a few little blue feathers sticking out. It was a Blue tit, stuffed deep into a crack in the bark. This behaviour is indeed well known (Perrins 1992), but it was still news to me (incidentally, Blue tits are no longer in the genus Parus: parid taxonomy was recently revised).

There are many other interesting facets to this area, but I won’t cover them now. Do certain birds (frigatebirds, swifts and migrating passerines among them) really sleep on the wing? Are migratory birds actually adapted for going for long periods without sleep? (Rattenborg et al. 2004). How much, or how little, sleep do certain species need? (Some seabirds, for example, seem to hardly sleep at all – and when they do sleep they do so for only a few seconds at a time). And why do animals sleep at all? I dunno, it’s all more stuff to think about. Just what I need.

At top, it’s me with Dawkins. I’ve been reading that book for about a year now….

Refs - -

Perrins, C. 1992. Bird Life: An Introduction to the World of Birds. Magna Books (Liecester [sic]), pp. 160.

Rattenborg, N. C., Mandt, B. H., Obermeyer, W. H., Winsauer, P. J., Huber, R., Wikelski, M. & Benca, R. M. 2004. Migratory sleeplessness in the white-crowned sparrow (Zonotrichia leucophrys gambelii). PloS Biology 2, 924-936.