I Ain't Afraid of No Owls

Northern bat

There is no particular reason why bats should be nocturnal. True, nocturnality has many advantages, but so does daytime activity; the real question is why so few bats fly during the day. I looked at this last year, where I mentioned that one of the main theories is that since birds evolved flight before bats did, the bats originally flew at night so that daytime predators, such as hawks and eagles, didn't try to eat them.

If so, it may be an effective strategy, since there isn't very much that eats bats on a regular basis. That isn't to say that there isn't anything, however. The bat hawk is, as its name implies, probably the single most specialised bat predator, but studies in Africa have shown that hobbies (which overwinter there), Wahlberg's eagles, and African goshawks also attack bats with some frequency, and they're probably not alone. In this part of the world, bats, as one might expect, take measures to reduce their risk of attack. Flying in large flocks may help, but it's also known, for example, that they avoid flying on moonlit nights, and, when they have to, they don't fly above the trees as they normally would, keeping themselves out of view.

The story is different when we move away from the tropics. In colder climates, predators just don't seem to be a major threat to bats. This is probably just as well, since during the summer, the nights may just not be long enough to find enough food to stay healthy. Take, for example, the northern bat (Eptesicus nilssonii). In addition to living in central and eastern Europe and across southern Russia as far as the Pacific, this also lives throughout Scandinavia. That includes areas north of the Arctic Circle, where the sun never sets in the middle of summer. The northern bats living here are far more tolerant of light than members of the same species just to the south... although, even then, they prefer to come out when the sun gets close enough to the horizon to dip below a hill. 

However, even if there are no predators that regularly feed on bats outside the tropics, there are still some that will give it a go when the opportunity arises. The main culprits here, perhaps unsurprisingly, are owls. Even so, owls eat far fewer bats than one might think, given that they are themselves nocturnal predators. Generally speaking, bats constitute no more than 2% of the diet of most owls, and often considerably less. But the odds of being eaten are still vastly higher than a human's chance of dying in a traffic accident (0.003% per year in the UK, 0.013% in the US) and we still do our best to avoid that.

Only coming out at night may protect against attacks by hawks or falcons, but it won't have much effect against owls. So, if bats do take sensible measures to protect themselves, what might they be? 

From the perspective of an owl that wants to eat bats, the best way of doing this is to hang around the entrance to their caves in the evening, and catch them as they emerge. Some individual owls do seem to do this (as do some falcons), consuming far more than the 2% figure that most members of their species do. Bats could counter this by avoiding coming out of their roosts if they hear an owl hooting; they will have to do so eventually, but even a delay in emerging might make all the difference if the owl gets bored and goes away. 

There are, however, plenty of other reasons why they might stay in their caves for longer than usual - for instance, if the weather is really bad. (If nothing else, it's harder to fly when it's chucking down with rain). So, to see whether this is due to a perceived risk of predators, we need to test for that specifically.

Studies to try and answer this question have tended to be equivocal. This, in itself, may show that bats in temperate environments show less anti-predator behaviour than other similarly-sized mammals - and, indeed, less than tropical bats. Perhaps they are, if not actually reckless, sufficiently safe that they have not evolved the sort of clear measures we might otherwise expect. It may also depend where in the world the studies are conducted, if there is genuinely a different predation risk in some parts of the world than others.

The latest such study was conducted near the south coast of Finland. The site was picked because it is known to include several roosting sites of the tawny owl (Strix aluco). This lives throughout almost the whole of Europe, where it feeds mainly on small rodents such as voles. Southern Finland, however, is near the northern edge of its range, where voles are not always easy to come by, causing the owls to find alternative food sources - and these are known to include bats. So bats probably should be nervous of them - but just how much does this affect their behaviour?

To find out, the researchers placed miniature audio loggers near known bat caves and in areas where bats were known to forage for insects. The latter were divided into those areas where tawny owls are also thought to hunt, and to those where, according to BirdLife Finland, no owls had been seen in the last three years. And then they played the hoots of tawny owls on portable MP3 players, to see what the bats did in response.

There are, in fact, six different species of bat living in this part of the world, three of which - the northern bat, Nathusius' bat (Pipistrellus nathusii), and the brown long-eared bat (Plecotus auritus) have distinctive calls that allow them to be distinguished from the other three (Myotis spp.), all of which sound much the same as each other. 

One question that had arisen from previous studies of this kind was whether or not the bats avoided areas with strange sounds coming from them, regardless of whether or not they recognised them as belonging to predators. So, in addition to the owl sounds, the researchers also tested to see what happened when those sounds were replaced... by the music of Ozzy Osbourne.

The research paper concedes that Osbourne could be thought of as an opportunistic bat predator... but it's unlikely the bats know that.

The end results were, as with so many previous studies of temperate-zone bats, mixed. There were, indeed nights when the bats significantly delayed their emergence from their caves compared with the usual. But this had little to do with what sounds they could hear outside. What worries a bat in this part of the world it seems, is cold weather and high winds. Not so much fear, as just not fancying going out.

There was, admittedly, a slight tendency for the bats to emerge from their caves about a quarter of an hour later if they could hear what sounded like adult owls outside. But there were so many occasions where they simply ignored the sounds that this could just be a fluke; in scientific terms, it's statistically insignificant, drowned out by the noise from all the times it didn't happen. 

Having said which, the bats seem to care even less when all they could hear were fledgling owls, which logically should have implied the presence of unheard adults. But, again, there isn't enough data here to prove anything. Probably, hearing predators does make the bats a little reluctant to exit their caves, but evidently not all that much. Being hungry may just be a bigger motivator when most owls aren't going to eat you anyway.

However, it turned out that, once they were already outside, the bats did avoid the areas where they could hear hooting owls. Moreover, they only did so in areas where owls really were known to be present. This sounds more significant... until you realise that they reacted exactly the same way to Osbourne's music.

As well they might, but it didn't worry them when the same music was played outside their caves. So it's probably not an aversion to heavy metal, and given that they didn't obviously react when they heard owls where they didn't expect them, they don't seem to find those noises particularly frightening either. The obvious explanation is that they just don't like sound, and, indeed, we know from previous studies that at least some bat species stay away from areas with traffic noise. 

So what may be going on here is that when they forage in areas known to have owls, they are more cautious than usual, carefully listening out for anything out of the ordinary and being more easily spooked. At the end of the day, though, it seems that temperate bats just go about their business regardless, making the most of the short summer nights in Finland. 

But that doesn't mean they're entirely reckless.

[Photo by "Mnolf" from Wikimedia Commons.]