Aims: The effects of food availability, habitat structure, laying date and laying order on clutch size and egg mass variation in the Purple Swamphen (Porphyrio porphyrio) are analyzed.
Location: Data were obtained from a Purple Swamphen population in Alto Guadalquivir Natural Park (Jaén, Southern Spain) in two consequtive yeas (1990-1991).
Methods: General and generalised linear models (GLM, GLZ) were used to account for relationships among variables and variance partitioning. Models included nested effects (eggs within nests), fixed effects (clutch size) and covariates (food availability, habitat structure, laying date and laying order, where appropriate). Clutch size and egg mass were used as dependent variables. Repeated measures ANOVA's were used for detailed analyses on the effect of laying order on egg mass variation.
Results: Food availability around the nest was the most important determinant of clutch size and egg mass variation. A significant negative relationship between clutch size and egg mass was found, with larger clutches consisting of smaller eggs. Egg mass was also negatively affected by laying order. The slightest eggs in the clutch were more likely to fail hatching than earlier, larger ones, and they usually produced smaller chicks.
Conclusions: Clutch size and egg mass variation seem to be highly influenced by environmental traits, particularly food sources, which may be a limiting factor in this wetland area compared to others in southern Spain.
