Greater noctules: specialist predators of migrating passerines

Yesterday I assisted my brother in sorting through old piles of bricks at the back of his garden. They’d become home to a million spiders, including some of the largest Tegenaria I’ve ever seen, and as I picked up and gently squeezed a Dysdera between my (gloved) fingers I watched it bite into the material with its massive, tremendously powerful fangs (it ordinarily eats woodlice, hence those big fangs). And as I sat in the evening in his garden, I watched pipistrelle bats flying overhead, the first time I’ve seen bats in the wild in many months. It’s funny how things work out.

In the previous post I discussed bird predation in the megadermatids (the false vampires or yellow-winged bats). Here, we’re going to look at the bird predation recently documented in another bat group, the vespertilionids (vesper bats). To those of us living in the Northern Hemisphere, vesper bats are usually the most common and familiar of bats, and because they’re generally small, inoffensive, insectivorous bats, the idea that some of them might be bird predators may seem pretty radical.

Noctules (Nyctalus) are a group of Eurasian vesper bats closely related to little yellow bats (Rhogeesa), big-eared brown bats (Histiotus) and little brown bats (Myotis) (Jones et al. 2002). Six species are currently recognized, although a few additional species are included by some experts in this genus as well (Nowak 1999). Mostly dwelling in forests, they are large (body lengths 50-100 mm, forearm lengths 40-70 mm) compared to pipistrelles and other typical vesper bats, and with sleek fur that ranges in colour from golden to dark brown. Some species are migratory and make journeys of over 2300 km. They eat relatively large prey, being particularly fond of beetles, and Nowak (1999) mentions a remarkable case where a Eurasian noctule (N. noctula) was observed to catch and eat mice. The species we’re interested in here, the Greater noctule N. lasiopterus, is mostly western European but also occurs as far east as the Urals, and as far south as Libya and Morocco. It isn’t well known, and is also rare.

To better understand the ecology and behaviour of this species, Ibáñez et al. (2001) netted individuals and recorded their wing morphology, and also recorded echolocation calls from the field. Greater noctule wing morphology indicates fast flight in open areas, as they have high wing loading and high aspect ratios. This is quite different from what’s seen in the megadermatids and nycterids I discussed in the previous post where ground- and foliage-gleaning are employed to find and catch prey, and where prey are often hunted from perches. These bats use low wing loadings and low aspect ratios to practice slow, manoeuvrable flight as they glean for mostly terrestrial prey in cluttered habitats.

The echolocation calls of the Greater noctule, being long in terms of pulse duration with a low frequency and narrow bandwidth, are suited for long-range target detection in open air. This technique is quite different from the prey detection style usually employed by vesper bats: a technique suited for short-range prey that are detected in cluttered habitats.

So, both wing morphology and echolocation style indicate that Greater noctules chase and catch flying prey in the open. But what are they chasing and catching? Ibáñez et al. (2001) found that feathers made up a significant component of the bat’s droppings during March-May and again during August-November: those parts of the year when migrating birds pass through the Spanish study area. Both the proportion of feathers in the droppings, and the proportion of captured Greater noctules that produced feather-filled droppings, showed that Greater noctules must capture and eat large numbers of migrating passerines. Bird predation was first documented in this species by Dondini & Vergari (2000), and judging from later comments in the literature it doesn’t seem that they got the credit they deserve for this discovery.

Unlike those of megadermatids (see previous post), Greater noctule roosts are never littered with bird remains (which partly explains why this behaviour was overlooked for so long). Ibáñez et al. (2001) did find fresh, recently cut passerine wings (of a Robin Erithacus rubecula and Wood warbler Phylloscopus sibilatrix) underneath the bats they were netting, and they also found Robin feathers adhering to the claws of one of the captured bats. It seems that Greater noctules catch, overpower and eat the passerines during flight, just as other vesper bats do with flying insects. Greater noctules are clearly large enough and powerful enough to do this: they weigh in at about 50 g and have a wingspan of 45 cm (making them the largest of Europe’s bats). For comparison, a European robin weighs c. 20 g (though this goes down to c. 15 g after migration) and has a wingspan of 20-22 cm.

Unsurprisingly, this idea has been regarded as controversial and doubtful by some. Bontadina & Arlettaz (2003) argued that the passerine-catching idea was so unlikely that it couldn’t be regarded as correct (excellent use of logic there). They also noted that other noctule species prey on insects and not birds, and that, suspiciously, Greater noctule droppings lack bird bone fragments. None of these arguments count for much. Bird bone fragments in fact are present in Greater noctule droppings, as was reported by Dondini & Vergari (2000), who even subjected the bone fragments to SEM observation to determine conclusively their avian origin (strangely, Ibáñez et al. (2001) did not report the discovery of bone fragments, and Bontadina & Arlettaz (2003) didn’t cite Dondini & Vergari’s (2000) discovery of them). The fact that other noctules don’t hunt passerines means nothing.

Anyway, how did Bontadina & Arlettaz (2003) account for the presence of all those feathers in the Greater noctule droppings? They proposed that the bats regularly eat falling feathers, mistaking them for flying insects! That’s pretty incredible, and arguably more amazing than the idea of predation on passerines. Two responses to Bontadina & Arlettaz’s article were published (Ibáñez et al. 2003, Dondini & Vergari 2004), and both showed that the scepticism was unfounded. The case for passerine predation in Greater noctules is pretty compelling. Note to wildlife camera-people reading this: someone should try and film this behaviour, though for obvious reasons no-one’s even observed it yet (to my knowledge).

So Greater noctules are specialist predators that exploit nocturnally migrating passerines, and to date they are the only animals known to do so. There are diurnal raptors (notably Eleonora’s falcon Falco eleonorae and Sooty falcons F. concolor) that specialize on migrating passerines, but nothing else that makes a point of catching those passerines that migrate at night. Dondini & Vergari’s paper ‘Carnivory in the greater noctule bat (Nyctalus lasiopterus) in Italy’ and Ibáñez et al.’s paper ‘Bat predation on nocturnally migrating birds’ therefore have to be two of the most remarkable recent publications in the annals of bat science, and this is a field where lactation in males (Francis et al. 1994), ultraviolet vision (Winter et al. 2003) and the re-evolution of running (Riskin & Hermanson 2005) have recently been reported.

Of course bats don’t have it all their own way against birds. Raptors and owls are important predators of bats and some studies suggest that owls may account for as much as 10% of annual bat mortality (Altringham 2003). Some raptors are bat-killing specialists, like the Bat kite (or Bat hawk) Machaerhamphus alcinus of tropical Africa, SE Asia and New Guinea, and the South American Bat falcon Falco rufigularis. In fact raptor predation seems to be so important to bats that it appears to explain why bats don’t fly more during the daytime (Speakman 1991) and it may also explain why bats became nocturnal in the first place (Rydell & Speakman 1995). On the subject of raptors vs bats I was amused to see, in John Altringham’s Bats: Biology and Behaviour, a drawing of a Harrier hawk Polyboroides typus using its flexible ankle joint to probe into a cavity where some molossid bats are roosting (p. 221) (go here for more on Polyboroides).

Finally, if you're interested in bats please check out my post on vampires.

Coming soon: the domestication of dogs, more on plethodontids, and early tyrant dinosaurs.

The excellent Greater noctule photo used above is from the Slovak Academy of Science site.

Refs - -

Altringham, J. D. 2003. British Bats. HarperCollins, London.

Bontadina F. and Arlettaz R. 2003. A heap of feathers does not make a bat’s diet. Functional Ecology 17, 141-142.

Dondini, G. & Vergari, S. 2000. Carnivory in the greater noctule bat (Nyctalus lasiopterus) in Italy. Journal of Zoology 251, 233-236.

- . & Vergari, S. 2004. Bats: bird-eaters or feather-eaters? A contribution to debate on Great noctule carnivory. Hystix 15, 86-88.

Francis, C. M., Anthony, E. L. P., Brunton, J. A. & Kunz, T. H. 1994. Lactation in male fruit bats. Nature 367, 691-692.

Ibáñez, C., Juste, J., García-Mudarra, J. L. & Agirre-Mendi, P. T. 2001. Bat predation on nocturnally migrating birds. Proceedings of the National Academy of Sciences 98, 9700-9702.

- ., Juste J., García-Mudarra J.L. & Agirre-Mendi P.T. 2003. Feathers as indicator of a bat’s diet: a reply to Bontadina & Arlettaz. Functional Ecology 17, 143-145.

Jones, K. E., Purvis, A., MacLarnon, A., Bininda-Emonds, O. R. P. & Simmons, N. B. 2002. A phylogenetic supertree of the bats (Mammalia: Chiroptera). Biological Reviews 77, 223-259.

Nowak, R. M. 1999. Walker’s Mammals of the World, Sixth Edition. The Johns Hopkins University Press, Baltimore and London.

Riskin, D. K. & Hermanson, J. W. 2005. Independent evolution of running in vampire bats. Nature 434, 292.

Rydell, J. & Speakman, J. R. 1995. Evolution of nocturnality in bats: potential competitors and predators during their early history. Biological Journal of the Linnean Society 54, 183-191.

Speakman, J. R. 1991. Why do insectivorous bats in Britain not fly in daylight more frequently? Functional Ecology 5, 518-525.

Winter, Y., López, J. & von Helversen, O. 2003. Ultraviolet vision in a bat. Nature 425, 612-614.