before the arrival of european colonists into north america, the chimney swift nested in hollow trees; now, it uses human-built structures almost exclusively. while the occasional nest is still built in a hollow tree (or, exceptionally, in an abandoned woodpecker nest), most are now found in chimneys, with smaller numbers in airshafts, the dark corners of lightly used buildings, cisterns, or wells. the nest is a shallow bracket made of sticks, which the birds gather in flight, breaking them off trees. the sticks are glued together (and the nest to a vertical surface) with copious amounts of the bird's saliva. during the breeding season, each adult's salivary glands more than double in size, from in the non-breeding season to during the breeding season.