Swifts are now feeding their young in their nests under roofs. Few people ever see the nestlings, but one can watch the parents sweeping up to the nest entrance, their throats bulging with insects, and pausing for a moment before they go in. One can also sometimes hear a thin scream of hunger coming from a nestling. All day long the parents are up in the sky, searching for food on their narrow, curved-back wings. In the morning they come dashing low over parks and gardens. Later, as warm air starts carrying the insects up, they fly higher and higher, until sometimes they pass out of sight.
If there are a number of them criss-crossing overhead, as there often are in the evening, it is almost impossible to count them. They fly so fast that you cannot be sure if the two birds that have just come in from the right are not the same two birds that just shot off to the left.
Later in the evening they often form parties that fly, screaming wildly, round and round the buildings that have nests in them. No one knows why they do this, but it would seem to give an opportunity for competitive self-assertion by the males.
The wet weather we had in June did not help British swifts this year. They obviously prefer to forage near their nest, but if rain and chill have driven the insects down, they will fly a long way in search of sunshine or at least dry air. Even then, they may not find many aerial insects.
Fortunately, when the nestlings are deprived of food in bad weather, they have a strategy for survival. They can go into a torpor — a kind of hibernation, like animals. Two ornithologists from Sheffield Hallam University, Mark Walker and Ian Rotherham, measured the temperature of some swift nestlings on cold days, in nests in a bridge over a German reservoir. They found that the chicks could go from a normal body temperature of about 36C down to as little as 15C without any ill effect.
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When we were shivering on those wet June days, perhaps our baby British swifts, instead, were doing just that.
Derwent May