Ardea
Official journal of the Netherlands Ornithologists' Union

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Bijlsma R.G. & van den Brink B. (2005) A Barn Swallow Hirundo rustica roost under attack: timing and risks in the presence of African Hobbies Falco cuvieri. ARDEA 93 (1): 37-48
A large Barn Swallow Hirundo rustica roost in Pennisetum-covered clearings amid rainforest in SE Nigeria attracted at least eleven species of avian predators. Departure and arrival of Swallows at the roost was highly predictable and synchronised: >90% of up to 1.5 million birds ascended and descended within 10 min around sunrise and sunset respectively, in spectacularly fast and dense masses. African Hobbies Falco cuvieri timed their presence and behaviour in conjunction with the Swallows, arriving 214 min before departure or arrival of the first Swallows. Overall success rate of hunts was 38%, and independent of hunting height, hunting mode and time relative to sunrise or sunset. Hunting success showed a strong quadratic relationship with flock size, and was highest in attacks on small flocks of 150 Swallows. The hunting window, i.e. the period that Swallows were present and airborne, amounted to 2040 min per twilight cycle. Swallows reduced predation risks by (1) roosting in huge numbers, (2) timing arrival and departure in a short period around sunset and sunrise, (3) making a fast and synchronised descent and ascent (swamping aerial predators) and (4) possibly keeping their weights low until the very end of the moulting cycle (when body mass increases in anticipation of migration). Assuming the seven local African Hobbies succeeded in capturing a Swallow each twilight period, we estimate the number of Swallows taken per wintering period at 2500. This is a small proportion in comparison with the depredation by local people that have been reported to collect more than 100 000 Swallows each year.


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