The quality and the timing hypotheses evaluated using data on great reed warblers

The seasonal decline in reproductive success observed in many animal specie s may be caused by timing per se (timing hypothesis) or by variation in phe notypic quality between early and late breeding females (quality hypothesis ). To distinguish between these two hypotheses, several studies of birds ha ve used clutch removal experiments to manipulate breeding date. However; re moval experiments also increase the females' previous reproductive effort d ue to the production of an extra clutch and a longer incubation period. Acc ording to life-history theory an increase in reproductive effort lowers fut ure reproduction. Hence, life-history theory predicts lowered success of re placement broods for other reasons than expected from the timing hypothesis . Female great reed warblers, Acrocephalus ni arundinaceus, studied in Swed en are frequently exposed to nest predation, after which many lay replaceme nt clutches. In order to examine possible effects of previous reproductive effort on different fitness components, we analysed the re-laying frequency and the reproductive success of replacement broods in relation to time of the season and previous reproductive effort (measured as the length of the previous breeding attempt, LPB). In clutches of re-laying females both the number of fledglings and the proportion of recruits were negatively correla ted with LPB, whereas re-laying frequency and clutch size were not related to LPB. We expect such relationships to be present also among other species . Consequently, the use of replacement clutches, as for example in clutch r emoval experiments, in evaluations of the cause of the often observed seaso nal decline in various fitness components, might exaggerate the importance of the timing hypothesis over the quality hypothesis.