Eighteen pairs of common terns (Sterna hirundo ) at three different colonies laid second clutches while still feeding young from their first broods. Seven clutches were laid before the chicks from the first brood fledged, and 11 were laid after the chicks from the first brood fledged. In each case, parents alternately fed chicks from the first brood and incubated the second clutch. Sixteen of the 18 clutches disappeared, were addled, or were deserted. Young hatched from the two other second clutches, but all chicks died or disappeared 2-4 days after hatching. The authors suggest that a second clutch, laid before chicks from the first brood fledged, probably results from a physiological miscue associated with chick loss from the first brood and stimulated by an unusual surplus of food. Conversely, when laid after chicks from the first brood fledge, a second clutch might function as insurance, permitting a pair to raise young late in the season if chicks from the first brood are lost. In either case, parents must partition care between eggs and chicks from the second clutch and fledged chicks from the first brood. Accordingly, the successful fledging of chicks from two broods in a single season is unlikely unless exceptionally favorable conditions occur.
Additional publication details
Occurrence and timing of second clutches in common terns