TIMING OF REPRODUCTION AND FLEDGING SUCCESS IN THE COOT FULICA-ATRA - EVIDENCE FOR A CAUSAL RELATIONSHIP

1. We investigated the relationship between hatching date and fledging success in the European coot (Fulica atra). 2. The production of fledglings per brood increased in the first half of the season and decreased in the second half, independent of clutch size or egg size. We tested experimentally whether this convex seasonal pattern is causally related to date. 3. The timing of parental care was manipulated by exchanging complete first clutches that differed in stage of incubation. Our experiments tested whether the natural variation in fledging success was due to (i) factors related to date (date hypothesis), (ii) factors related to parental and/or territory quality (parental quality hypothesis), or to a combination of both factors. 4. In the first half of the season, an experimental advance of the timing of parental care reduced fledging success, while an experimental delay raised it. In the second half of the season the opposite was found. Fledgling production by experimental pairs did not differ systematically from that of control pairs raising young at the same time. 5. Thus, the results were consistent only with the date hypothesis, and we conclude that timing of breeding and fledging success are causally related in the coot. 6. Independent of date, age of male parents was positively correlated with fledging success, suggesting that aspects of parental quality play an additional role.