In May 2008, Adrian Lim a.k.a. wmw998 documented a pair of juvenile Asian Glossy Starlings (Aplonis panayensis) copulating, or so it seems (below).
“Are they actually copulating or just experimenting as with most juveniles?” he wonders.
Birds learn from observing the adults, and this includes copulation. So obviously, the juveniles were imitating the adults when the older juvenile mounted the younger. The age difference can be seen in the eye colour.
There are actually reports of juvenile birds playing at copulation or practicing at it. It is doubtful whether at that age these birds can differentiate the sexes, let alone being sexually matured.
The colour of the eye comes from the iris that surrounds the black pupil. In some species, eye colour changes as the bird ages. In others, the sexes differ in eye colour. In yet other birds, the eye colour of the juvenile is duller that that of the adult. In bulbuls, the eye colour varies with the species.
How eye colour affects social behaviour has not been extensively studied. But some ornithologists believe that this can reflect different social positions and, like plumage, colour may play a role in social interaction.
In Asian Glossy Starling, however, the juvenile has a different plumage from that of the adult. So the social status of adult and juvenile can be easily distinguished. However, among the juveniles, the eye colour of the younger juvenile is brownish, distinctly different from that of the older juvenile (above) and adult. Here again, there is obviously a distinction between young and old juveniles.
All images by Adrian Lim.
Clark, G. A. Jr. (2004). [‘Form and function: The external bird.’]. Pp. 3.1-3.70 in Podulka, S., Rohrbaugh, R.W. Jr & Bonney, R. (eds.) Handbook of bird biology. Ithaca, NY: The Cornell Lab of Ornithology.
This post is a cooperative effort between www.naturepixels.org and BESG to bring the study of bird behaviour through photography to a wider audience.