Here is a mystery I’d really like to know the answer to. On the way to school this morning, Will discovered a dead baby bird. Here it is: the photo (which I took on my phone) is atrocious, so there’s little point in showing it at larger size. Clearly, this is an altricial, nidicolous passerine chick, probably of a Blackbird Turdus merula. The question is: how does an altricial nestling like this – barely able to walk in its own nest, least of all out of it – get to be dead in the middle of the pavement? No, it didn’t fall out of its nest, because there were no overhanging trees or bushes where a nest might be.
The idea that a predator – like a corvid or domestic cat – pulled it out of the nest and then dropped it is, of course, possible, and there are certainly magpies, carrion crows and cats in the area. However, over the years I’ve seen dead passerine nestlings out in the open – on pavements, roads and such – on so many occasions that I’ve wondered if something else might be going on. Surely predators can’t be dropping their spoils on such regular occasions? We know that cuckoldry is common in passerines like starlings, and that some species (like House sparrows Passer domesticus and Barn swallows Hirundo rustica) sometimes practise infanticide (e.g., Møller 1988, Hansson et al. 1997, Veiga 1990, 2003, 2004). Incidentally, while typically associated with males, infanticide is commonly practised by females in some species, like the House sparrow, and also in those most evil of mammals, rodents and primates. In species with polygynous males, females might practise infanticide so that the male ends up allocating more of his time towards the infanticidal individual’s brood. Clever.
But perhaps most interesting is the fact that some passerines destroy the eggs, nestlings and even nests of other birds: this is mostly an intraspecific behaviour, but – in cavity-nesting wrens, at least – members of some species will even destroy the families and nests of other species (Belles-Isles & Picman 1986 and references within). It’s generally thought that this behaviour reduces competition for resources, and that makes sense. How do these passerines kill the eggs and nestlings of other birds? Eggs are broken open by pecking, but I’m not sure what’s done with nestlings. Are they smacked on the head, or just pulled out of the nest? Once this happens, the nestlings would die quickly from exposure. And is this sort of behaviour more widespread in passerines than indicated by the literature? Might it occur intraspecifically within species like the Blackbird, for example? We already know that these birds are fiercely territorial and hence in constant competition with their neighbours, so it doesn’t seem much of a stretch to imagine that the birds might kill the nestlings of their competitors if the chance arises. However, you’d think that this sort of thing would have been observed in such a well-studied species: if it is, I can’t find a mention of it in the literature.
Incidentally, I did once keep and skeletonise a dead passerine nestling I found on the pavement. Unfortunately it’s not with me here, but at Portsmouth University, so no photo.
Refs – –
Belles-Isles, J.-C. & Picman, J. 1986. House wren nest-destroying behaviour. The Condor 88, 190-193.
Hansson, B., Bensch, S. & Hasselquist, D. 1997. Infanticide in great reed warblers: secondary females destroy eggs of primary females. Animal Behaviour 54, 297-304.
Møller, A. P. 1988. Infanticidal and anti-infanticidal strategies in the swallow Hirundo rustica. Behavioural Ecology and Social Biology 22, 365-371.
Veiga, J. P. 1990. Infanticide by male and female house sparrows. Animal Behaviour 39, 496-502.
– . 2003. Infanticide by male house sparrows: gaining time or manipulating females? Proceedings of the Royal Society B 270 (Suppl 1), S87-S89.
– . 2004. Replacement female house sparrows regularly commit infanticide: gaining time or signaling status? Behavioral Ecology 15, 219-222.