Asian Glossy Starling: Juvenile sex

on 12th June 2008

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 and BESG to bring the study of bird behaviour through photography to a wider audience.

If you like this post please tap on the Like button at the left bottom of page. Any views and opinions expressed in this article are solely those of the authors/contributors, and are not endorsed by the Lee Kong Chian Natural History Museum (LKCNHM, NUS) or its affiliated institutions. Readers are encouraged to use their discretion before making any decisions or judgements based on the information presented.

YC Wee

Dr Wee played a significant role as a green advocate in Singapore through his extensive involvement in various organizations and committees: as Secretary and Chairman for the Malayan Nature Society (Singapore Branch), and with the Nature Society (Singapore) as founding President (1978-1995). He has also served in the Nature Reserve Board (1987-1989), Nature Reserves Committee (1990-1996), National Council on the Environment/Singapore Environment Council (1992-1996), Work-Group on Nature Conservation (1992) and Inter-Varsity Council on the Environment (1995-1997). He is Patron of the Singapore Gardening Society and was appointed Honorary Museum Associate of the Lee Kong Chian Natural History Museum (LKCNHM) in 2012. In 2005, Dr Wee started the Bird Ecology Study Group. With more than 6,000 entries, the website has become a valuable resource consulted by students, birdwatchers and researchers locally and internationally. The views and opinions expressed in this article are his own, and do not represent those of LKCNHM, the National University of Singapore or its affiliated institutions.

Other posts by YC Wee

7 responses

  1. So here’s a question – do birds also use sex as a form of social bond besides for reproduction?

  2. At the risk of simplifying the facts,I would think the act of mounting another individual bird of a sub-standard age i.e a juvenile, shows how birds or any other species make their present felt in the social hierarchy. This happens often in many primate groups and big cats like lions as they are very sociable as compared to most cats which are solitary by nature. It has been observed that usually the same sex specimens will mount each other.This behaviour is a an act of subordination. That the one mounting is of a higher hierarchy and wishes this fact be known to the rest who is of lesser social standing within the group.I doubt there are any penetration in the act itself and even so,not fertile enough to produce offsprings.But being able to pin another individual puts the one pinning in a higher position within the group.
    My thoughts =)

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