The call of the swift, recorded at Oxford. One of the common sites of summer is to see a gang of swifts calling with a loud and harsh 'screeee' scream as they chase each other. Although dark brown in colour, the swift often appears black as it is seen silhouetted against the sky with its rapid, flickering flight followed by long glides. It is more at home in the air than any other species, and when a young bird leaves the nest it may not land again for months or even years. It is well-adapted to sleeping on the wing, as well as feeding, drinking, gathering nest material, and even mating while airborne. The swift feeds almost entirely on flying insects such as beetles, aphids, flies, and moths, and groups of swifts can be seen hunting over almost any habitat during the summer months. When feeding young, a swift may catch up to 10,000 insects a day. The swift traditionally nested on cliffs and in rocky crevices although most birds now nest in the roof spaces of older buildings. Despite the population being relatively stable, there is concern that modern buildings tend to lack sufficient access to roof spaces. It is currently estimated that there are around 80,000 pairs of swifts nesting in Britain.