Sunday 23 June 2024

Bats in the Daytime

One of the things that we can say about almost all species of bat is that they are nocturnal. Regardless of whether they roost in caves or under the branches of trees, bats sleep through the day, wake up around dusk, fly out to get their food, and return home when the sun rises. But, when you stop to think about it, why should that be?

Consider: the great majority of bats eat insects and, sure there are plenty of insects around at night. But there's hardly a shortage during the day, either. Swallows, robins, and thrushes are all flying vertebrates that eat insects, and you see plenty of those around during the day. While for fruit bats and other herbivorous species, it's quite obviously not going to make a difference. If birds are perfectly happy flying around during the day, why not bats? It's not even as if there are only a small number of bat species that happen to occupy some narrow niche; there are well over a thousand of them.

Perhaps the most obvious explanation is that birds already have this covered. According to this hypothesis, by flying at night, bats avoid birds and get the chance to feed on insects that the birds can't, or can feed on fruit without being interrupted by frugivorous birds, such as parrots. Although this seems logical, and may well be part of the story, there isn't much evidence for it specifically.

A related possibility is that flying at night helps bats avoid birds that might eat them. Here, there is solid evidence that birds such as falcons and hawks will attack bats if they get the chance, often around dusk and dawn. While this provides a good reason to only come out at night when falcons are asleep and probably couldn't see bats well even if they weren't, it's worth noting that owls also attack bats. Granted, in most cases, bats are a very small part of the diet of most owls, but, given the right circumstances, some of the larger species, such as barn owls, may be a significant threat.

The third possible explanation that has been offered is that the bats are trying to avoid overheating. Wings made of skin absorb heat from the sun more effectively than those covered in feathers and this is even more so if the wings are black - as most bat wings are, probably to make it harder for predatory owls to spot them. Here, there is some experimental evidence that bats are indeed, prone to overheating if they try to fly during the day, although this makes more of a difference at moderate latitudes than at the extremes.

It may well be that the real explanation is a combination of all of these, together with the vagaries of evolution given that modern-type birds have been around slightly longer than bats have, allowing the former to get a head start. But the reality is that, despite being nocturnal, bats are sometimes seen during the day. So, having attempted to explain why they don't, we now have to try and explain why, occasionally, they do.

One possibility is that they have been disturbed, forced to fly away from something when they'd really prefer not to. It's inevitable that this must happen from time to time, and there is, for example, at least one study showing how raccoons and woodpeckers can disturb tree-roosting bats trying to get a good day's sleep. Bad, or exceptionally cold, weather at nighttime might also make it difficult for bats to feed, leaving them hungry enough to face the uncomfortable sunlight.

Some bats, however, do seem to do this sort of thing more often than others. This can be a local thing; bats have been observed regularly flying during the day where they live on islands that, for whatever reason, have few predatory birds to bother them, or where the vegetation cover is thick enough to both hide them and provide them with shade. But it's unlikely that this explains all of the daytime observations, especially away from remote islands, so other factors may also be at play.

In Europe, the bat most often seen flying during the daytime is the common noctule (Nyctalus noctula). This is a vesper bat, a member of the largest of the various bat families, and it's fairly large, if lightly built, with a wingspan of up to 40 cm (16 inches) and weighing up to 30 g (one ounce). It's found throughout most of Europe apart from Spain, Portugal, Ireland, and central to northern Scandinavia, with some populations as far east as Central Asia. 

As such, it may be that it's more often seen during the day than other bats, simply because it's relatively common and large enough to be clearly visible. It has also been suggested that, in the northern parts of its range, it flies on summer days because the nights are too short for it to get enough food otherwise. Even if daytime flying is dangerous or uncomfortable, hunger can be a powerful motivator.

But there may be more to it than this. 

There are many reports of bats being seen flying during the day but they are sporadic and not, generally speaking, the sort of thing you would find in scientific research papers. So where to find enough data to see if there's a pattern to the sightings? One answer is to look at citizen science projects, where sightings of whatever you are interested in have been sent for inclusion in some central database. In the UK, for example, these have been used to monitor the distribution of hedgehogs or the plants and wildlife along our shorelines. 

There isn't one specifically for daytime sightings of bats... but there are some for birds. Among these is the AVIF project for monitoring birdlife in the Czech Republic. Although technically operating since 1986, this really took off in 2010, and since then there have been many thousands of reports added to its database. And, while it may not be what the project is for, if you're looking for daytime birds and happen to see bats instead, they'll take the report.

In total, the database contained 542 daytime bat sightings as of 2019, with an average of 31 bats per sighting, and sometimes huge flocks of several hundred. A recent study analysed those reports to see if there was any pattern. 

For one thing, this confirmed that common noctules are the species most commonly seen. Of the 52 reports with photographs good enough to identify the bat species, 48 were of common noctules, with pipistrelles coming a distant second. Furthermore, the bats were almost always seen flying in open areas, something that only noctules normally do in Europe, with pipistrelles, for instance, staying close to trees. (Although there are two other kinds of noctule on the continent, which we can't exclude on this basis alone). They were often seen moving together in a single direction, and in just six cases were being attacked by birds of prey - with only one kestrel being successful.

But what's useful is when they were spotted. Bats were typically seen flying in the afternoon, suggesting that they had at least got some daytime sleep before heading out much earlier than we might expect and were far more likely to be seen in March, April, September, or October, than in other months.

That tends to count out the "short nights make bats hungry" hypothesis, at least for the majority of sightings. It could be that warmer weather discourages them, but there is a more likely explanation. Because, although this is a relatively recent discovery, it turns out that common noctules, like many insect-eating birds, migrate south for the winter.

They're not travelling all the way to Africa, or anything like that, but they do still travel significant distances from northern to southern Europe. In fact, it's only the females that migrate, with the males largely milling around at various points along the migration route in the hope of finding a passing mate during the autumn breeding season. Presumably, there's a balance here - the further north the male is, the colder the weather he will have to deal with, but the more likely he is to find a mate before she sees anyone else.

The females, however, head north in the spring, returning to the place of their birth to have their own children and raise their young in maternity roosts. As the weather cools in the autumn, they, and their young daughters, head south, mating along the way and then hibernating through the winter. The purpose of this, most likely, is to reduce the length of the hibernation period while avoiding the heat of a Mediterranean summer.

Of course, they mostly migrate at night, because... well, they're bats. But supporting the idea that these daytime bats were migrating females is the fact that, in the autumn, the reports of them flying overwhelmingly said that they were seen heading in a southerly direction. 

As to why they would be more likely to be out in the day when migrating than at other times, this is probably due to keep up their energy. Bats don't tend to accumulate fat reserves before they migrate in the way that some birds do, forcing them to spend a lot more time feeding along the way. This may allow them to catch evening swarms of flies and midges while saving the cool nights for the exhausting long-distance flights. That they're often seen heading south in autumn, but just searching for food during daytime spring sightings, may suggest that there's a mix of such behaviours involved.

The bats were most commonly seen in the lowlands, flying along major rivers such as the Elbe and the Morava. That would suggest that these are important migration corridors for the animals, something that would be of great interest to identify in our rapidly evolving knowledge of bat migration, something that has only been widely recognised in the last three decades or so.

[Photo by Kamran Safi, from Wikimedia Commons.]

No comments:

Post a Comment