There are still young house martins looking out of their mud nests with their little bull-like heads. House martins go on breeding later in the summer than almost any other bird, though most of these last youngsters will be out on the wing in the next week or so.

However, their parents have to lure them out. They fly to and fro in front of the nest under the eaves, and hover by the entrance, until finally the young birds come out and join them. One of the parents may fly about with a young bird when it first takes to the air, but normally it can fly perfectly well from the moment it launches itself.

It will sit on a roof with its siblings and go on begging for food for a day or two, but will soon be wheeling and side-stepping in the air in pursuit of its own flies. These new arrivals in the sky can sometimes be picked out by their browner plumage — not the sparkling black and white of their parents.

In most species of bird, once the young leave the nest they do not return to it. But with house martins, the whole family go back into the nest to roost at night. A few forked tails may be seen sticking out when the nest is packed.

Not long after that, the time for departure looms. The young go off and fly about with the other local juveniles, and sometimes apparently lose their way, for there is a record of a young house martin trying to roost in a sand martin's hole in a quarry. Then the adults gather, and by mid-September the whole local colony has started assembling in lines on telephone wires.

Then one morning the telephone wires are bare. But where have the house martins gone? Flocks of them are often seen setting out over the water from places such as Sandwich Bay in Kent, and in due course flocks are seen crossing the Mediterranean. They are also known to cross the Sahara. After that, they disappear.

No one knows where in southern Africa they spend the winter. They are glimpsed here and there over the forests, but it seems that most of the time they feed so high in the sky that they are invisible to the human eye. Nor is it known where they roost at night. Perhaps in Africa they sleep on the wing.

It is a strange transition from the bird that in summer lives so close to human beings to the bird that in winter vanishes from human ken.

derwent.may@thetimes.co.uk