Dan Reiff wrote:

Hello Jon and all,
Thank you for sending the great research articles, Jon.
Please note that my comments about bats were about C. Poorwill, not C. Nighthawks.


Ha! Thanks, Dan - that'll teach me to pay better attention to what I read!!

I looked up the reference by "CW" in the Birds of North America account that " Poorwills will sometimes leave foraging areas when bats are present ", but couldn't find much in the way of published papers on the subject in a short search just now.

There was one paper that stated that the poorwills would leave an area when bats are present, but does not suggest a reason or mechanism.



See: "The Avian Enigma: 'Hibernation' by Common Poorwills (Phalaenoptilus nuttalli)" by Christopher P Woods and R. Mark Brigham, in Barnes BM, Carey HV, eds (2004) Life in the Cold: Evolution, Mechanisms, Adaptation, and Application. Twelfth International Hibernation Symposium. Biological Papers of the University of Alaska, number 27. Fairbanks, Alaska, USA: Institute of Arctic Biology, University of Alaska.



"In North America, migratory poorwills presumably winter within the year-round range of resident birds in the southern United States and Mexico, and also overlap in southern portions of that range with other caprimulgids. Migrants must therefore compete with resident birds for limited food resources in northern portions of their winter range, and with other resident and migratory caprimulgids for those same resources farther south. Further competition may occur with migratory and resident insectivorous owls and bats, and owing to the geography of North and Central America, these animals are concentrated in a region during winter that is geographically much smaller than the region over which they breed.



"Perhaps owing to similar foraging strategies, evidence for competitive displacement has been observed within North American caprimulgids and between them and bats (Bjorklund and Bjorklund, 1983/1984; Boyce, 1980; Brigham and Fenton, 1991; Caccamise, 1974; Shields and Bildstein, 1979; Stevenson et al., 1983). Poorwills are the smallest North American caprimulgid, and timid birds as well, and we occasionally observed poorwills leave foraging areas when bats arrived. It is thus plausible that they would be displaced in interactions with larger or more aggressive species, and poorwills probably moderate this competition by wintering north of the winter range of those species, where torpor use is necessary to balance long-term energy budgets."

I still wonder whether the bats are being aggressive or otherwise out-competing the birds, or whether some sort of prey selection differences might explain why the Poorwills & Bats separate themselves? If you have some references, I'd be keen to read them.

Again - many thanks!
- Jon. Anderson
OlyWA
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