Swifts | Apodidae

Gierzwaluwen | Vencejos

Almost everything happens in the air. Insects are sieved from the airstream with the mouth held wide open, water is scooped up in a low pass over a lake, and even the nest material — feathers, plant down, dry twigs — is caught in flight or snapped off a branch on the wing. In the breeding season the salivary glands swell, and with that sticky saliva the twigs are glued into a half-cup and fixed to a vertical surface inside a chimney, a hollow tree or under an overhanging rock. There the birds cling with four toes that can all be turned forward, braced by stiff tail feathers whose shafts project as spines. In long spells of cold, wet weather, when no insects are flying, the nestlings' metabolism drops away and they survive for days without food, so that fledging may be delayed by weeks. Around sunset hundreds of birds may funnel down into a chimney in a spiral; the voice is a dry, rapid chittering.