Death from below, 2013

Bats. You know what bats are right? The little weird creatures that live in your roof, and fly around at night? Harmless beings, that just feed on mosquitoes, moths and other invertebrates that people generally (sadly) don’t like.

Well, that’s not strictly true. Latin America’s vampire bats are likely the most infamous exception, landing on mammals and birds, and then sucking the sleeping animals’ blood. Awesome, fascinating creatures; but certainly a little creepy.

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Some species of bats manage to fish. Using echolocation they are able to detect ripples being formed on the water’s surface to alert them to their prey’s presence, before swooping down and catching them with a specialised pouch between their legs. And so a creature from an entirely different habitat is caught unawares, and dragged into the air to die.

More pleasant are the fruit bats, or ‘flying foxes’, which can have wingspans of up to 1.7m wide. Rather than catching prey they feed instead on fruit and nectar, flying around forests pollinating and dispersing seeds. In some old-world forests, they are considered vital for the overall ecosystem’s health through offering these services.

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Perhaps less known is the Greater Noctule, a rare bat species found around the Mediterranean. Quite large for a bat species found outside of the tropics; adults have wingspans of up to around 45cm across-considerably larger than the ones commonly seen flitting about in British summers. The wing’s physiology suggests that they hunt by chasing down prey on the wing.

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Unusually, it seems that this prey is not invertebrates, but birds. There are some bat species that specialise in catching their prey (mostly invertebrates) by ‘gleaning’-pulling them off of leaves. Some of the larger gleaning species have been found to occasionally take small birds when they sleep in sufficiently exposed positions.

But the Greater Noctule is different. Found in areas which act as corridors for migratory birds, they receive a food bonanza in spring and autumn every year. Huge quantities of small birds fly over their habitats at night, and no other nocturnal predators have the necessary adaptations to catch them.Image

It seems that they feed primarily on birds; particularly Robins and Wood Warblers whilst on the wing, by chasing them down. Similarly when feeding on invertebrates, the bats don’t even need to land to feed on their feathered prey but eat it whilst in flight; all the better to catch more.

As the bats hunt at night and their prey are species that depend on sight as their primary sense, they are thus able to ambush them with ease, bursting out of the gloom. The sound the bats emit in their echolocation is outside of the birds’ hearing range, so it is likely they have no knowledge of their danger until they are caught.

During migratory periods birds may make up 78% of the bat’s diet, a phenomenal switch from the summer periods when there are no migratory birds, leaving them to feed on invertebrates instead.

This may sound an exotic phenomenon, but whilst the bats themselves live outside of the UK, the birds affected do not. One of the main species found to be affected, the Wood Warbler, migrates to the UK from northern Africa every spring, and then returning in autumn. Birds that we see in the summer may have run a gauntlet of carnivorous bats. Britain’s Robins are not migratory, but ones from Scandinavia do migrate, and so the cousins of the Robins you see in your back garden may meet a bizarre end in the jaws of a bat.

Let it not be said that nature doesn’t have a sense of irony, however. As with other bat species, many Greater Noctules in turn meet their end at the talons of a Tawny Owl.

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Follow me on twitter @goldenmole

References :

Dondini, G. & Vergari, S., 2004. Bats: Bird eaters or feather eaters? A contribution to debate on Great Noctule carnivory. Hystrix It. J. Mamm, 15(2), pp. 86-88.

Ibáñez, C., Juste, J., García-Mudarra, J. L. & Agirre-Mendi, P. T., 2001. Bat predation on nocturnally migrating birds. PNAS, 98(17), p. 9700–9702.

Popa-Lisseanu, A. G. et al., 2007. Bats’ Conquest of a Formidable Foraging Niche: The Myriads of Nocturnally Migrating Songbirds. PLoS One, Volume 2.

Photos (in order) by myself, littleREDelf, two by Wikimedia commons, and DarrelBirkett

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About hammerheadbat

A conservation biology PhD, I spend my days studying tropical deforestation, bats, and wider ecological questions.

Posted on April 17, 2013, in Uncategorized and tagged , , , , , . Bookmark the permalink. Leave a comment.

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