Why don't sleeping birds fall out of trees?

Animal expert Sophie Mowles from ARU took on this question ...

Sophie - OK. So birds when they sleep are very vulnerable, so they've got to find somewhere very nice and safe for them to spend the night. Typically if they're a diurnal, or day-active bird, they do tend to tuck themselves up in trees, for the most part. And often they'll aggregate, so they'll join together in a big flock that will roost in a particular place overnight. And this is what we see in programs such as Autumnwatch with the big murmurations of starlings that are swooping around: they are in the process of finding a place to roost overnight. And that's typically in reed banks, so if you're in Cambridgeshire you might go out to Wicken Fen in the evening and see these flocks which will then drop into the reeds.

Another really nice example are pied wagtails, which we tend to see in ones and twos around towns, but at night they'll form big flocks and they'll roost together in safety, usually in trees that are in quite safe places. So supermarket car parks; there's a big roost at Addenbrookes hospital; and weirdly where I am - when I grew up, where I went to school - there was a roost that happened at night. And I noticed this when I was a kid, I was there late one evening - maybe for a play or something - and noticed the trees were full of these pied wagtails. They all come together. Even tree hollows, cavities, nest boxes that we put up to allow birds to build their nests and raise their young; they're a really nice, warm, safe place that birds can spend the night.

They don't fall out of the tree because of the way that their foot muscles relax. They're actually the opposite way round to how we would imagine; when bird muscles in their feet relax, they grip. So that's how they stay in the tree - they're relaxed, so they're holding on.

Chris - Do bats do something similar? Don't bats have an anatomical arrangement so that when the bat dangles upside down, as it applies force downwards on its claws, it actually encourages the foot to grip so they can hang upside down?

Sophie - I believe it's the same mechanism.

Chris - Because the one disease that bats are really terrified of is diarrhoea, isn't it, when they're in that situation.

Sophie - Of course...