Why do birds roost in some trees but not others?

posted in: Roosting | 0

In a study on pest birds by Sodhi & Sharp (2006), the authors listed a number of tree species favoured by some of the more common birds as roosting trees. By late evenings, you could hear the loud, shrill cries around these trees as the birds swarm around preparing to roost in the trees. The noise continues for some time until the birds settled for the night. Early next morning the cacaphony of calls would start again until they fly off to their feeding grounds. The ground below these trees would be littered with their droppings and any cars parked below would be literally discoloured.

According to the authors, House Crows (Corvus splendens) favour roosting in tall trees with large dense crowns like Angsana (Pterocarpus indicus). These trees are usually surrounded by tall buildings that provide shelter against the weather and protection against predators.

Javan Mynas (Acridotheres javanicus) similarly prefer trees with dense crowns like those of Sea Apple (Syzygium grande) and Angsana. These trees should preferably be close to food centres and surrounded by vegetation. Again, these trees should provide shelter from the weather and protection from predators. These mynas either gather in large communal roosts or join mixed-species roosts that include Common Mynas (Acridotheres tristis), House Crows, Asian Glossy Starling (Aplonis panayensis) and Purple-backed Starlings (Sturnus sturninus).

So it is not just the species of trees that attract roosting birds but where they are planted. But then how many people are aware of this? Very few, and not even experienced birdwatchers from the Nature Society, who have been leading newbie’s on bird walks LINK.

I had the opportunity to make observations on the roosting of the Pink-necked Green-pigeons (Treron vernans) when they roosted in the Golden Penda (Xanthostemon chrysanthus) lining the road outside my house LINK. Admittedly the number of green-pigeons was not large but they confirmed the findings made by Sodhi & Sharp (2006).

The two trees they roosted in (above, arrowed) have dense crowns. On the side away from the road, tall vegetation provides shelter from the weather and protection from predators to these two trees. Another view of these two trees is shown in the image below (left). Compare these trees to the other image below (right), where the crown is just as dense but the tree is standing alone, with no vegetation around it. Thus no roosting pigeons.

Anyway, the roosting pigeons are now gone because of the noisy construction work to enlarge and cover the drains on both sides of the road, that went on for weeks on end.

YC Wee
Singapore
September 2014

Reference:
Sodhi, N.S. & I. Sharp, 2006. Winged invaders: Pest birds in the Asia Pacific. SNP Reference, Singapore. 184 pp.

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