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Optimal Foraging Questions from Lab Group Discussion of Popa-Lisseanu et al. 2007

Posted by matina1 on 29 Sep 2008 at 17:01 GMT

We questioned the use of optimal foraging theory to explain the use of migrating songbirds by N. lasiopterus in the context of prey abundance as opposed to prey profitability. Is there any evidence, from time-budget analysis or other sources, that migrating songbirds are a more profitable prey item than the insects known to be part of the diet? Is there any evidence from the energy balance of this unusually large bat that they can get away without consuming songbirds? This was especially interesting given presumed heights at which these predation events were taking place and the presumed arial-hawking nature of the predation events.

RE: Optimal Foraging Questions from Lab Group Discussion of Popa-Lisseanu et al. 2007

anapopa replied to matina1 on 25 Oct 2008 at 16:05 GMT

We have not done any analysis to prove that migrating songbirds are a more profitable prey than insects, and unfortunately, we are not aware of any study comparing energetic value per mass of insects and birds. But in terms of biomass obtained per successful hunting event, a bird would be clearly more profitable than an insect. A successful bird predation event early in the night would probably be enough to fulfill the daily energetic requirements of the bat, which would otherwise need to spend large part of the night hunting in order to get the same amount of food out of small insects. Whether a large aerial-feeding bat such as the giant noctule can get away without birds is indeed one of the top questions. As we already mention in our paper, size in aerial-hawking bats seems to be limited by echolocation parameters, big bats not being able to detect small prey and thus being limited by the availability of large insects, which is generally not very high. The giant noctule is among the largest aerial-hawking bats, and the largest in Europe. The second largest European aerial-feeding bat is Tadarida teniotis, a moth specialist, which compared to all other aerial insectivorous bats has to spend an unusually long time foraging (Marques et al., 2004, Acta Chiropt. 6: 99–110). It thus seems that large aerial hawking bats face more difficulties in fulfilling their energetic requirements and have to find some solutions. The other large aerial-hawking bats of the world similar in size or larger then N. lasiopterus are very poorly studied. It would not be surprising if it turns out that they also have developed a similar foraging strategy. In fact, Ibáñez et al. 2001 suggested the Asian bat Ia io as another candidate for bird predation because of its size, wing morphology and echolocation characteristics (similar to those of N. lasiopterus), and recently Thabha et al. 2007 found feathers in its fecal pellets and suggested aerial preying on migrating birds. In the case of N. lasiopterus, we already point out in our paper that its Mediterranean distribution might be a consequence of a dependence on bird migration. The distribution in Iberia is intermittent; the species is locally abundant at certain regions but is not found across large part of the territory. Thus, we may speculate that the distribution in Iberia could match the areas with higher concentrations of passing-by migratory birds. However, Nyctalus aviator living in Japan and China (formerly considered the subspecies Nyctalus lasiopterus aviator, now a separate species but most closely related to N. lasiopterus and only slightly smaller) is unlikely to depend on migratory birds, as Japan, where it has mostly been studied, is not located in any main bird migratory passage. If so, this species manages to survive without a bird diet. There is also no evidence yet of feathers in the fecal pellets of Slovakian N. lasiopterus, although the numbers of bats captured are very low (Uhrin et al. 2006. Vespertilio 9–10:183–192). Thus, as an alternative explanation we could think of bird predation as a local, or Iberian, foraging specialization, this means, distinctive of Iberian giant noctule populations, which could then have gained substantial fitness benefits and increase population numbers relative to other areas. Behavioral specializations in certain populations are not so rare, for example eastern N. lasiopterus populations seem to be migratory while western (Iberian) populations are sedentary. However, giant noctules captured in Switzerland in mountain passes and in Italy had feathers in their feces. In summary, although we are personally inclined to think so, we cannot say for sure that giant noctules cannot survive without the bird resource. What we can say is that the species is very near the size limit for aerial insectivorous bats, and it undoubtedly benefits from the bird resource, which might be the cause of the high success of the species in the Iberian Peninsula. The species could eventually also survive in other areas without, or with low, bird migration, but most probably with the cost of low fitness and at very low population densities.