Have birds got a built-in cooling system?

THERE’S a large elder tree on my patio and a pair of wood pigeons have nested in it.

The foliage on the tree is very dense and there’s also a lot of ivy growing up it. This means that I can’t actually see the nest.

But I can hear what’s going on up there because I like to sit under the tree on fine evenings and keep an eye on what’s happening in the natural world. And I’ve been hearing something rather strange.