Egg-dumping by Asian Koels
Egg-dumping is the term used by birders to refer to nest parasitism. This is where a bird lays its eggs in the nest of other birds. It can be a bird of the same species (intraspecific parasitism) or of another species (interspecific parasitism). The former is fairly widespread but seldom noticed. However, this can be detected when there are two eggs seen in a day as few birds lay more than one a day. The latter is seen in the Asian Koel (Eudynamys scolopacea) where the koel lays its eggs in the nests of other birds. This is because the koel never builds its own nest.
In Singapore these koels parasitise the nests of House Crows (Corvus splendens) mainly. We had a number of earlier postings by Angie Ng (1, 2, 3, 4, 5) and by Hung Bun Tang (1, 2, 3). Below are some engaging discussions on the subject.
Ong Kiem Sian reported that she saw a cuckoo nesting in a fantail nest. The female cuckoo pecked the egg of the fantail and immediately deposited her egg in the nest. This was completed within a few seconds when the host was not around. She wondered if the cuckoos were desperate and cannot find a host nest, would they then become aggressive, like the koels reported in the postings? She added: “And if the female bird still cannot find and cannot tahan (ie desperate to lay her egg), maybe she will drop her egg on to the ground.”
Yap Kim Fatt countered: “I would have thought the koel chabohs (Hokkien for females) would deposit their eggs in the crow's nest by stealth rather than by the gangster-ish method as witnessed by Angie Ng (see links above). I would expect a sort of a hit-and-run ova-parturition when the adult crows are not around the nests.”
Jeremy Lee is of the opinion that “If they are nest parasites, I believe they would have to evolve a quick hit-and-run method of depositing the egg in the nest. I have seen documentaries on TV showing cuckoos caught in the act. And it is surprisingly quick to drop the eggs in the nest.”
At the talk in the National Library by Prof NS Sodhi and Ilsa Sharp to launch their book, Winged Invaders – Pest Birds of the Asian Pacific (Singapore: SNP References, 2006) on 10th March 2006, I nearly got my answer to the above. It would appear that the male Asian Koel will seek out an active House Crow’s nest after which he will call out for his mate. Once the female koel appears, the male will approach the crow’s nest whereby the incubating crow will immediately chase it away. At that split second, the female koel will sneak into the nest and lay her egg. The female will then call to signal to her mate that the mission has been accomplished.
But I was unable to elicit a direct response from Prof Sodhi on whether the female koel will drop her egg on the ground if she is not able to immediately find a nest to lay her egg. He believes that in all probability the crow will leave its nest to chase off the male koel, giving the female an opportunity to lay her egg.
Geoffrey Davison has this to say about fertilisation and egg laying: “I had a look at what few books I have at home, but didn't find anything specific on the time taken from fertilisation of the ovum to laying of the egg by birds. But for all birds, fertilisation has to take place at the top end of the oviduct, before the fertilised ovum is surrounded by albumen, two membranes, and the shell. Since eggs of poultry and many other birds are laid at about one-day intervals, this implies about 24 hours for the egg to proceed down the oviduct.
“Smaller birds lay eggs at shorter intervals, but seldom less than 15 hours or so, and big birds like ostriches would lay eggs at intervals of several days - again, that implies several days for each egg to travel down the oviduct being wrapped in albumen, membranes and shell after fertilisation.
“...It's also possible that copulation is performed shortly before the laying of an egg that was fertilised by an earlier copulation.”
Input by Ong Kiem Sian, Yap Kim Fatt, Jeremy Lee, Prof NS Sodhi and YC; additional comment by Geoffrey Davison; and images of a female Asian Koel sneaking into the nest of the House Crow by YC