Seasonal variation in the incidence of double broods:The date hypothesis fits better than the quality hypothesis

1. In three great tit (Parus major) populations the probability that a pair starts a second clutch, a clutch produced after a successful first brood, varied between years and areas but generally declined through the breeding season.2. By exchanging first clutches between early and late breeding pairs during incubation we tested experimentally whether this seasonal decline was related to quality differences between early and late breeding pairs (quality hypothesis) or hatching date of the first clutch (date hypothesis).3. Body mass of first brood fledglings increased through the season and was only determined by hatching date of the first clutch.4. Advanced pairs were more likely and delayed pairs less likely to lay a second clutch compared to control pairs. As predicted from the date hypothesis, individual birds did not differ in their ability to produce a second clutch, but only those birds that bred early did so.5. The interbrood interval and possibly also the probability of a second clutch was related to the number of first brood fledglings and their body mass.