Seasonal decline in clutch size in European starlings: a novel randomization test to distinguish between the timing and quality hypotheses

1. The seasonal decline in clutch size observed in many avian populations may be due to (a) a causal effect of the timing of breeding or (b) variation in individual quality (i.e. lower quality individuals lay fewer eggs and lay later in the season 2. To distinguish between these two hypotheses, we manipulated the timing of breeding of European starlings (Sturnus vulgaris L.) by removing first clutches to induce the production of replacement clutches. This experiment was performed at two sites over three breeding seasons. 3. To test whether the within-individual trend in clutch size differed from the seasonal decline in clutch size observed in the population, we devised a novel randomization test that overcomes some of the potential limitations of previously used analyses. 4. We observed no within-female decline in clutch size in any year at either site. Therefore, clutch removal did not appear to manipulate female quality (due to the costs of egg production), a potentially confounding factor in such experiments: only the timing of breeding was manipulated. 5. The within-individual seasonal trend in clutch size differed significantly from the population decline in four of six cases (year-site combinations); there is good reason to believe that the lack of a significant difference in the two other cases was due to low statistical power. Thus, our data support the hypothesis that the season decline in clutch size in this species is due to variation in female quality. 6. The between-year repeatability of clutch size was 0.47 and significantly different from zero,whereas that of relative laying date was much lower (-0.23) and non-significant. The repeatability data are thus consistent with the quality hypothesis: clutch size is characteristic of individuals regardless of their timing of breeding. 7. Female body mass at the beginning of incubation was not related consistently to clutch size or laying date, and therefore does not appear to be an indicator of female quality in this context.