Seasonal Decline in Body Size of Coot Chicks

Abstract

I investigated the seasonal variation in growth rates among the semi-precocial chicks of European Coots Fulica atra. Mean body size and nutritional condition at four weeks of age showed a distinct seasonal decline among first broods, whereas variation in growth was independent of hatching date among later broods. Body dimensions differed between years, but there was no significant between-year variation in the seasonal patterns in chick growth. The seasonal pattern in mean body size was not explained by differences in mean egg mass, brood size or parental age between early and late breeding pairs. Independently of hatching date, chick body size was positively related to egg mass and negatively to brood size one week after hatching. To ascertain whether the seasonal decline in chick growth among first broods was due to a difference in the quality of early and late pairs (parental quality hypothesis), the brood care period of individual pairs was experimentally advanced or delayed by exchanging clutches that differed in laying date. In the first half of the season, the mean body size and nutritional condition of broods of experimental pairs were associated with the original hatching date, and not with the assigned actual (advanced or delayed) hatching date. Hence, the higher growth rate in the earliest broods was consistent with the parental quality hypothesis. In the second half of the season the mean body size and nutritional condition of broods fostered by experimentally advanced or delayed pairs were associated with the actual hatching date, and not with the original one. Thus, the mean body size and condition of chicks were causally related to hatching date in the second half of the season, either through a hatching date-linked deterioration of the environmental conditions for chick growth (date hypothesis) or through a decline in the quality of chicks with progressive hatching date (chick quality hypothesis). Analysis of individual variation in body size among chicks which were sexed upon recapture as adults, revealed a seasonal decline only among males. Body dimensions of female chicks were independent of hatching date. Finally, chicks of either sex that grew poorly were significantly smaller as adults. Therefore, variation in juvenile growth may represent a major factor in determining the reproductive value of coot chicks, as breeding success may depend on phenotypic differences in adult size.