Common Swift

In the past the Common Swift Apus apus was known as the Devil’s bitch or Devil bird but no one really knows why. With its long slim swept-back wings, slender body and tail and its almost completely black plumage one could imagine it being Satan’s messenger. In my mind I always associate it with the arrival of summer and listen out for its shrill screaming call in early May. They are often confused with the House Martin and Swallow although they are not related.

Until 1947 when David and Elizabeth Lack undertook their important study of the species in Britain very little was known about the habits of this enigmatic bird. For instance they discovered that the chick’s have the capacity to enter a state of torpor with their body temperature and metabolism reduced when the parent birds are finding it difficult to catch insects in poor weather conditions. They are able to survive in this state for 48 hours, a truly amazing feat for a nestling.

The Common Swift feeds almost exclusively on flying insects and small to moderate sized airborne spiders. Stinging insects are avoided but quite how they discern a hoverfly from a wasp intrigues me. The birds may undertake phenomenal journeys to collect enough insects for their young when they are faced with inclement weather conditions. They are known to fly right round a depression and English birds have been recorded feeding over Germany. British Swifts have been known to fly towards unstable air behind occluded fronts where there can be an estimated 3.5 billion insects within a square kilometre. The adult birds accumulate the insects at the back of their throats and bind them together with a salivary substance to produce a ball. There are an estimated 300 insects in a ball.

The bird’s capacity for long distance flight is staggering. It has been suggested that they average 500 miles each day. Some birds have been known to survive for 20 years, which means they could have flown more than 3.65 million miles in their lifetime!

Young birds are known to roost on the wing circling for hours at high altitude. It is thought that immatures, which may not start breeding until their fourth year, may remain aloft for the whole of this time. However on migration birds have been observed using a variety of roost sites including hanging upside down from wires or hanging from the pendulous branch of a tree.

There is a common assumption that Swifts are unable to land on the ground as their legs and feet are too weak. People thought that the birds had no feet. Their scientific name apus means “footless”. In fact adult birds are capable of taking off from a flat surface. Interestingly the bird is used in heraldry as a symbol for the fourth son of the house, a hapless creature unable to plant his feet on the land.

The function of the characteristic social screaming parties when adults and newly fledged young fly in close formation constantly screeching is not totally understood. It is thought that it influences social cohesion. Prior to migration birds from several colonies often unite in this display then circle up to a great height before departing.

When this happens our skies become silent again.

Alwyn Jackson

Tags: birding, bird watching, bird watching club