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.
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On May 31, 2018, at 9:47 AM, festuca at comcast.net wrote:
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Hi folks,
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I was intrigued by Dan's statement that "They don't like bats and will temporarily leave an area if bats compete for food."
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I thought about the times that I'd seen bats and nighthawks foraging together and couldn't remember times that I saw negative interactions. As a young boy in Albany, Oregon in the 1960s, when & where the nighthawks used to nest on the town's rooftops, I would often watch the nighthawks and bats come out and concurrently forage late into the summer evenings and then at dark under and around the street lamps.
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My first thought on the matter is that bats and nighthawks would not necessarily be 'competing' for the same species of insects, due to their different body sizes and nutritional requirements, as well as their foraging strategies and abilities.
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I did a quick 'google' of bats & nighthawks, and was surprised to find this was one of the first scientific papers that came up on the subject:
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"Bird Versus Bats: Behavioral Interactions at a Localized Food Source" by William M. Shields and Keith L. Bildstein, in Ecology Volume 60, No. 3 (Jun., 1979), pp. 468-474
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Abstract: During June and July 1976, we investigated the foraging behavior and interactions of bats and common nighthawks (Chordeiles minor) at a localized food source in Columbus, Ohio. The patch consisted of insects attracted to a cone of light produced by six large (1000 W) spotlights illuminating a sign. While foraging in the light cone members of both taxa engaged in both intra- and interspecific chases. Seventy-five percent of the bat-initiated and 94% of the nighthawk-initiated chases had nighthawk targets. The smaller bats appeared to dominate the larger birds in all individual aggressive encounters. Members of both taxa foraged differently while using the patch. When either birds or bats foraged in the absence of members of the other taxon, they foraged low in the light cone. When birds and bats were present simultaneously, the aggressive interactions conditioned their foraging behavior. When together, bats remained in the lower zone of the light cone, while some nighthawks rose into the upper zones. When foraging in these upper zones, nighthawks suffered a decrease in foraging efficiency based on energetic considerations. They missed more prey per attempted capture, and either increased the time between captures or increased their foraging speed in the upper zones. In any case, more energy was expended by nighthawks foraging in the presence of bats than when they were alone. The expanded pattern of patch used by the larger yet socially subordinate nighthawks in the presence of bats supports Morse's (1974) hypothesis. Morse predicted that social dominance would be more important than body size in determining resource use in communities where interspecific aggression and dominance is important in resource partitioning.
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On the other hand, the next article that came up was "Observation of Common Nighthawks (Chordeiles minor) and Bats (Chiroptera) Feeding Concurrently", by Gabriel J. Foley and Lyndsie S. Wszola in the Northeastern Naturalist Jun 2017 : Vol. 24, Issue 2, pages N26- N28.
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Abstract: Records of bats and birds concurrently exploiting the same food source are rare in the literature. We observed an instance of bats and Chordeiles minor (Common Nighthawk) foraging in artificial light around the Washington Monument. Our observation corroborates earlier evidence that bats and Common Nighthawks both exploit the foraging opportunity created by artificial lights. Because the monument provided spatial perspective, we were also able to observe that bats and Common Nighthawks foraged at different heights, suggesting that they partitioned the available foraging space vertically.
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Foley and Wszola's conclusions make more sense to me than the idea that aggressive bats would chase off the nighthawks. I could (easily) be wrong, but perhaps what Dan observed was merely that the nighthawks were going for different species of nocturnal insects than were the bats, so they were in different places? (Bug) food for thought.
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Best,
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- Jon. Anderson
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OlyWA
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