Sunday, 26 June 2016

Bats v. Hawks

Bat hawk
There are not many animals that eat bats. Or, at least, that's often been the received wisdom. Bats only come out at night, fly through the air, where it's difficult to catch them, and many spend the day sheltering in caves that are difficult to access, and that rapidly become filled with so much bat poo that not much else is willing to venture within. But they don't all live in caves, they aren't the only things that come out at night, and, anyway, there's just an awful lot of them, so surely something must, in fact, eat them?

And, of course, the answer is "yes". In fact, there are actually quite a lot of animals that eat bats from time to time. In Brazil, for instance, snakes, especially tree-dwelling constrictors, have been reported to prey on bats. So do other vertebrates, from fish to mammals, and even a few invertebrates, such as, yes, really - giant centipedes. But, considering all that flying they do, it's perhaps unsurprising that the main predators of bats are, in fact, birds. And, among the birds, it's doubtless also not a great shock to discover that owls are the primary culprits.

For instance, it has been estimated that in Britain, at least 11% of all bat deaths (and probably more than that) are caused by birds. Around three quarters of those attacks are by tawny owls (Strix aluco), and 90% of all predation incidents on bats in Britain are due to owls of some kind. With 5% of predation incidents having nothing to do with birds at all, it nonetheless follows that a further 5% must be due to birds that aren't owls... and, of course, this is just Britain, where (for example) tree-roosting bats are quite rare.

So, granted, if bats were worried about their own mortality, being eaten probably wouldn't be top of their list of existential fears. (If they're North American, they should probably be more worried about catching white nose syndrome, and if they're European, the possibility of having the pressure wave from a wind turbine make their lungs explode ought to be somewhat concerning). Nonetheless, the need to avoid predators does seem to shape their behaviour, at least in the tropics, and we can't exclude the possibility that it's a large part of why they're nocturnal in the first place.

So which birds other than owls attack bats, and how often do they do it? A recent review of the literature uncovered over 1,500 reports of such attacks, across 109 different countries, from northern Norway to southern Argentina. Clearly, then, these attacks do happen, even if few birds go so far as to make bats a regular part of their diet. The majority of the predatory bird species belonged to the hawk and falcon families, along with the one and only species in the osprey family. Of course, these are pretty much the only families of bird that hunt large land-dwelling animals (as opposed to invertebrates or fish) during the daytime, so, at this level, it's likely what you'd expect.

Among the falcon family, most birds, such as kestrels and hobbies, attack the smaller bats, where in some places, they can form a significant part of their diet. Merlins, for example, have been seen regularly preying on Mexican free-tailed bats (Tadarida brasiliensis) in the town of Abilene, Texas, even though they would more normally eat small birds like pipits or larks. However, the larger members of the family, such as peregrine falcons, will attack bats the size of flying foxes if they get the chance.

The pattern is broadly similar among members of the hawk family, with most preying on small bats, while larger birds - most of them loosely describable as "eagles" - also attack flying foxes. On the other hand, vultures don't normally seem to bother, although the African white-backed vulture has apparently been seen feeding on straw-coloured fruit bats, which have a wingspan of about 75 cm (2'6"). The hawk family also includes a number of specialists, adapted to eating things like fish, which, understandably, we can also count out.

With, however, one notable exception: the bat hawk (Macheiramphus alcinus). This is quite possibly the only species of animal - of any kind - that feeds primarily on bats. (There is, incidentally, such a thing as a "bat falcon", too, but it doesn't seem to be anywhere near as specialised). Bat hawks live in central and southern Africa, southeast Asia, and the Indonesian islands, and, while not truly nocturnal, they do hunt primarily at dusk, when most other daytime raptors have returned to their roosts because of the poor visibility.

As one might expect, their low-light vision is pretty good, but an even more significant bat-eating adaptation is the size of their gape. Bat hawks can open their mouths wider than any other raptor, to the point that they start to resemble birds like swallows in this respect. And, just like swallows, they catch their food on the wing, gulping the entire bat down in one go.

Outside of the raptors, a few other birds, such as gulls and crows, have been seen to prey on bats, but in most cases, seemingly because they just got lucky while looking for something else. There is, however, at least one report of a shrike grabbing a bat from a crevice in which it had been resting, which implies at least some degree of active searching.

Indeed, this is the main tactic applied by typical falcons and hawks when hunting bats: hang about somewhere close to a cave or other roosting site, and wait for the bats to come out in the evening. The fact that quite so many bats are concentrated in one area, and that they tend to come out at more or less the same time each day, likely makes this quite an effective tactic, and it is employed not just at caves, but near bat roosts in urban areas or deep in forests.

The question remains as to what effect this really has on bats. It clearly isn't a massive cause of mortality for them, and bat hawks aside, attacks by diurnal raptors on bats are a relatively unusual thing - certainly compared with attacks by owls. As noted above, though, this could be a result of the bats' own success; that flying at night keeps them relatively safe. It's hard to find clear evidence of this, since it suggests an evolutionary pressure that must have developed over a considerable period of time, and likely very early on in bat evolution.

The classic example that might support this theory is the Azores noctule (Nyctalus azoreum), an endangered species of bat living only on a small island chain far out in the North Atlantic. The islands are so remote that they don't have any native hawks, falcons, or, indeed, owls, and, apparently given complete freedom as to what time of day they come out, a significant portion of the bats choose to hunt for insects in the sunlight. Similarly, the Samoan flying fox flies during the day - albeit avoiding the hottest parts - and it, too, mostly lives in areas where daytime predators are rare or absent.

But, otherwise, if you're a bat, it seems you may have very good reason for keeping to the shadows.

[Photo by Johan van Rensburg, at Wikimedia Commons].


  1. "Bat hawks live in central and southern Africa, southeast Asia, and the Indonesian islands"

    That sounds like a slightly odd distribution. Is it known if they used to live in intervening places like East Africa or India?

    1. Quite right - I should have been more specific! They're found in most of Sub-Saharan Africa, except for the far south (Namibia, South Africa) and some areas around the Horn of Africa (although they apparently live in Ethiopia). They're also found in Madagascar - but not in India. It is an odd distribution, I agree, with the entire Indian Ocean being in the middle of it, but birds can sometimes be odd like that.