Variation in egg-size traits of the European Roller (Coracias garrulus) in eastern Poland

Abstract

Life-history theory predicts that allocation of resources into offspring size and number should be traded-off. The trade-off may be masked in good environmental conditions and be expressed only in some reproductive attempts, when conditions are poor. In this paper we analyse variation in size-related traits of eggs in relation to clutch size in a marginal population of the European Roller (Coracias garrulus) in eastern Poland, declining as a result of agriculture intensification and habitat loss. Because large-bodied insects that constitute the main kind of food of Rollers are greatly affected by the weather in spring, we assumed that their abundance may differ from year to year in association with differences in weather conditions. We predicted that egg size-clutch size relationship and egg traits should differ between years in correspondence with environmental conditions prior to the time of egg laying. We also predicted that variation in egg size and shape should have some fitness-related consequences. We found that year and clutch size interacted in their influence on egg size: in one year egg size was positively and in another year negatively related to clutch size. We also found that egg shape (sphericity) was positively associated with clutch size and negatively related with the date of laying. Both egg size and spheric-ity in shape positively affected hatching success, thus confirming the existence of fitness-consequences of variation in the traits of Roller eggs. Our results are in general consistent with theoretical expectations, but we failed to find any specific links between egg size variation and the decline of the study population.