Juicy caterpillars, beachfront real estate, a mild, coastal climate and an accompanying band of paparazzi – all the ingredients of a perfect summer vacation! As birders and photographers rushed to document the largest known non-breeding concentration of Horsfield’s Bronze Cuckoos on vacation in Singapore, questions began to arise – Why are they here? Why would Australian birds, which enjoy comparatively mild winters, even need to migrate thousands of kilometres to spend the austral winter? In this article, we summarise available knowledge on the phenomenon of intratropical migration with reference to Australasian birds and shed some light on this poorly known aspect of avian migration.
Australasia, a region comprising Australia, New Zealand, New Guinea and surrounding islands like New Caledonia, is generally regarded as a self-contained system with regard to avian migration. A study of bird migration in this region noted that only 30 shorebird and 10 landbird species which breed in the Northern Hemisphere regularly winter in the region (Dingle, 2004). However, there is significant avian movement within the region, involving species which birders might be familiar with.
In the south, globally threatened parrot species such as Swift and Orange-bellied Parrots migrate hundreds of kilometres from their breeding grounds in Tasmania to winter along southeast Australia. Further north, forest jewels such as the Buff-breasted Paradise Kingfisher, Noisy and Red-bellied Pittas and Rufous Fantail migrate from Queensland to spend the austral or southern winter (March – October) on the island of New Guinea (Pratt and Beehler, 2014). Similarly, large numbers of waterbirds make the journey north as well, wintering as far west as the Indonesian islands of Timor and Flores. It is believed that the number of migrating birds and distance they travel is related to the onset of winter temperatures in Australia as well as rainfall patterns during this period.
Until recently, it was thought that no Australasian breeding birds enter Asia beyond Wallace’s Line, a boundary separating the faunal regions of Asia and Australia (Dingle, 2009). However, with an increase in observer coverage, we now know that a small number of Australasian birds regularly visit Asia during the austral winter. In recent years, Sacred Kingfisher has been recorded from Borneo with some regularity, while further west Horsfield’s Bronze Cuckoo has now been recorded annually in Singapore, with small numbers reaching as far north as Penang Island in the current irruption.
The lengthy migrations undertaken by these two species are particularly surprising given that both are habitat generalists, able to survive in a wide variety of environments ranging from coastal scrub to urban gardens. As such, one would expect there to be sufficient areas of suitable habitat within Australasia for them to spend the winter, so why the Malay Peninsula?
One possibility is the cold snap that was experienced throughout most of eastern Australia in the first week of June. Many areas experienced subzero overnight temperatures and it is possible that the sudden onset of cold weather might have triggered eastern populations of the Horsfield’s Bronze Cuckoo to migrate further north than they usually do. In view of the territoriality demonstrated by birds on wintering grounds, it is plausible that given the potentially large number of birds involved, immatures and older adults might have been forced to winter at the limits of their wintering range along the Malay Peninsula, with suitable areas closer to Australia occupied by territorial adults in their prime.
Interestingly, these cuckoos do not stay for more than a week or two from the time of their first sighting in Singapore. It is recommended that birders continuing to document the cuckoos at Punggol Barat make notes of the dates on which they observed the birds as well as the number observed. On a regional scale, with the advent of geolocator technology (a kind of miniature tag to monitor the movement of animals), long distance austral migrants like the Horsfield’s Bronze Cuckoo could serve as an excellent study species for mapping the migratory routes of Australasian birds during the austral winter, thereby unravelling the mysteries of this poorly understood aspect of avian migration.
Dingle, H. (2004). The Australo-Papuan bird migration system: another consequence of Wallace’s Line. Emu, 104(2), 95-108.
Dingle, H. (2009). ROWLEY REVIEW. Bird migration in the southern hemisphere: a review comparing continents. Emu, 108(4), 341-359.
Pratt, T. K., & Beehler, B. M. (2014). Birds of New Guinea. Princeton University Press.
Author: Albert Low