Wiggins, D. A., Morris, R. D., Nisbet, I. C. T., and Custer, T. W., 1984, Occurrence
and timing of second clutches in common terns: The Auk , v. 101, no. 2, p.
281-287.

Abstract:

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.

Keywords:

reproduction, population-dynamics, sexual-reproduction, Sterna-hirundo