A gregarious species, the common swift forages in large flocks for a variety of small flying insects. These feeding groups may be frequently heard ‘screaming’, in a display that probably serves to advertise the presence of the group to other swifts. By encouraging more swifts to join the group, there will be larger numbers of birds seeking out the best feeding areas, thereby benefiting all the individuals (6). It is its dependency on insects that explains why the common swift must migrate, as during the northern winter insects become scarce, whereas they are in abundance at this time in the tropics (8).
Breeding common swifts return to the same colonies to mate each year, usually around the start of May. Pairs may breed together for many years, but if one partner dies, its place is immediately taken by another. A clutch of two or three eggs is laid in a cup-shaped nest, made of feathers and vegetation glued together with saliva. The site of the nest is usually chosen by the male swift. Incubation duties are shared equally by the male and female and last for 19 or 20 days. Both adults feed the young for the 42 or so days that they are in the nest, with the adults flying as far as 560 miles each day whilst foraging (2) (3).
While nesting, the common swift can be exposed to sustained periods of cold weather. However, a layer of fat insulates the body and its slow metabolism prevents excessive energy use, enabling the swift to survive such conditions. Furthermore, swift chicks can survive long periods of cold weather and starvation by entering a state of torpor (8), a coma-like condition in which its metabolism slows to almost nothing. For a bird of its size, the common swift can live for a remarkably long time, with some individuals known to live for as long as 21 years (3).