Small home high off the ground: Nesting ecology of the Grey-rumped Treeswift.

I work in the Singapore Botanic Gardens and the Grey-rumped Treeswifts (Hemiprocne longipennis) do well here. Outside the Visitor Centre they perch in the big trees, they always select the tallest branches, often dead exposed ones. From there they sally for insects, and I presume they nest, although I have never had a chance to study it. But in 2006 I got the chance. A friend who comes in our shop told me about a nesting pair she had near her home, a high rise development nearby off North Buona Vista Road.

The treeswifts form a small family, Hemiprocnidae in the Apodiformes order which includes three families: treeswifts, swifts and (maybe a bit surprisingly) hummingbirds. Hemiprocnidae is only in the Oriental region and only has one genus with four species. Treeswifts differ from true swift in that they can and often do perch on branches, they are arboreal birds. True swifts form a much larger family (92 species world wide); they spend almost all their time in the air, they even sleep and mate on the wing. They cannot perch on a branch or on the ground, they can only grab a vertical surface with their small, weak feet; therefore they only land when they have to nest, which they do in caves or under cliffs, or under man-made structures like buildings and bridges (a few species fly into tree holes), using the only building material available to them: their own saliva and feathers.

The Grey-rumped Treeswift forms a superspecies and was previously considered conspicific with the Crested Treeswift (H. coronata). The former occurs in the Sunda subregion plus Sulawesi, the latter replaces it in Thailand and Indochina into India. Incredibly, for these widespread and fairly numerous two species there are big gaps in our knowledge of their nesting biology, neither incubation nor fledging periods have ever been recorded (Handbook of Birds of the World, Vol. 5, p. 465). The North Buona Vista pair built a small nest in some dead branches in a tall tree, some 20 meters off the ground, it was clearly visible from the balcony of a nearby building. The nest was a tiny cup made of hardened saliva mixed with minute pieces of what appeared to be bark or moss (above, chicks in nest). When the adult sat on the nest the whole structure was invisible, covered by the bird! We know that the egg was laid somewhere between 7th -11th May 2006, it hatched 3rd of June. I visited the nesting site with my son Adam on 5th June. He took some photos with his digital compact camera - the chick was then 2 days old. Both female and male (recognisable by the rufous ear coverts) took turn attending to the chick and feeding it with a regurgitated substance, presumably somewhat pre-digested insect matter. I visited the site again on 16th June. The chick was bigger of course but still covered in down, still fed regularly and still sheltered completely by one of the parents between feeds. Later on it started to develop feathers, it moved away from the tiny nest and perched on the branch on its own, feeding became less regular. On 1st of July 2006 it left the nesting branch, almost the same hour in the afternoon that it hatched. We now know the fledging period of the Grey-rumped Treeswift: it is exactly 4 weeks! (above, male tending to young; below pair tending to young) So, how is the Grey-rumped Treeswift doing? Well, none of the four treeswifts are globally threatened with extinction. However, the HBW states that ‘Pesticides are suspected to be behind recent population declines in Singapore’. David Wells states something similar: ‘Declining in some suburban areas, including on Singapore main island where the largest party of non-breeders recently recorded was of only five birds’ (The Birds of the Thai-Malay Peninsula Vol. 1, p. 473).

Luckily, in the Singapore Botanic Gardens we still have many more treeswifts than that. The treeswifts like to interact and fly around high together while calling, often late in the day; it is not unusual to see 10-12 birds at one time during those occasions. One evening during ‘winter’ a few years back I was at the exercise ground around 6 pm when a Japanese Sparrow-Hawk (Accipiter gularis) flew quickly by towards the rainforest area, and out of the blue congregated a large flock of treeswifts to mob it. I managed to positively count 35 individuals in the air at one time, but there must have been more, maybe 40. They swerved around excitedly for a few minutes after the hawk had disappeared before they gradually dispersed, many settling around the edges of the rainforest.

Text by Morten Strange; images by Adam Strange.