September 10th

Most birds finished breeding two months ago and only the very big species like gannets are still to completely finish. But you can still see some late fledglings, particularly house sparrows, swallow and house martins chasing their parents. The giveaway for a fledgeling songbird – apart from the general dopey, fluffy look – is a yellow gape. This is an enlarged and conspicuous side of the mouth so that when a chick begs in the nest it offers the biggest brightest target for the parent. You can imagine that those chicks with the most conspicuous mouths get fed more often and are more likely to survive until fledging – a good example of natural selection.

A young swallow in my garden over the weekend with its yellow gape conspicuous. This has less than a month to feed up before it needs to head to Africa (WC)