Less House Crows… more Asian Koels…

13 Jan 2013   in Brood parasitism, Crows, Intraspecific, Nesting No Comments »
Contributed by YC Wee

The Asian Koel (Eudynamys scolopacea) is a brood parasite that sneaks into the nests of House Crow (Corvus splendens) to lay her eggs (above). The bird is a resident species but during the northern winter months, the local population is supplemented by migratory species. It has a sharp, shrill call LINK that some locals find irritating – especially when made during the early hours of the morning.

The number of koels in Singapore has been increasing slowly through the years. And since 1987 breeding has become widespread (Wang & Hails, 2007). Isolated House Crow nests can be parasitised every time the crows are in the breeding mode and this can be once every three months, as observed by Wee (2005).

A concerted culling programme, initiated since the early 2000s followed by efforts to deprive the crows of their traditional food sources at refuse bins and open-air food outlets have seen a definite decline in the number of House Crows. Also, trees that were popular with crows were “managed” to discourage their nesting. Another measure observed was the tipping of crow nests whenever located LINK.

This decline in crow number was confirmed by Kwek et al. (2012) when the 30 randomly selected transects surveyed 10 years earlier were subjected to a repeat survey in 2010-11. And contrary to expectation, the decline in House Crows did not see a corresponding decline in Asian Koels. In fact the number of koels increased. The reason why was revealed in a study in Bangladesh (Brgum et al., 2011). Colonial nesting in crows acts as a deterrent against brood parasitism. With nests close by, there will always be more pairs of eyes to keep watch on approaching koels. Thus the risk of parasitism increases as the distance between host nests increases.

Another factor may be the possibility of the Asian Koel laying her eggs in the nests of other host species, like that of the Black-naped Oriole (Oriolus chinensis) (Lowther, 2011). Birdwatchers should be more vigilant in looking out for this possibility.

Acknowledgement:
Thanks to Chong Kwek Yan LINK who kindly sent me a PDF of the paper by Begun et al. (2011).

References:
1.
Begum, S., A. Moksnes, E. Røskaft, B. G. Stokke, 2011. Factors influencing host nest use by brood parasitic Asian Koel (Eudynamys scolopacea). Journal of Ornithology, 152: 793-800.
2. Chong, K. Y., S. Teo, Kurukulasuriya, Y. F. Chung & S. Rajathurai, 2012. Decadal changes in urban bird abundance in Singapore. The Raffles Bulletin of Zoology. Supplement No. 25: 189-196.
3. Lowther, P. E., 2011. Host List of Avian Brood Parasites 2 –– Cuculiformes. Version 12 Sep.2011). Available online: http:// fm1.eldmuseum.org/aa/Files/lowther/OWcList.pdf.
4. Wang, L.K. & C. J. Hails, 2007. An annotated checklist of birds of Singapore. Raffles Bulletin of Zoology, Supplement 15: 1-179.
5. Wee, Y.C., 2005. Look, what came out of the crow’s nests. Nature Watch 13(1): 22-25.




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