SEASONAL DECLINE IN COLLARED FLYCATCHER FICEDULA-ALBICOLLIS REPRODUCTIVE SUCCESS - AN EXPERIMENTAL APPROACH

To assess the relationship between parental quality, seasonal environm ental changes, and seasonal reproductive success, we manipulated both hatching time and clutch size among collared flycatchers (Ficedula alb icollis) breeding on the island of Gotland, Sweden. Our experimental h atching delays of one week had no significant effect on the body condi tion of experimental females, relative to that of controls. In the abs ence of simultaneous clutch size reductions, experimental delays in ha tching date caused significant decreases in fledging success relative to controls. However, when coupled with a 1-egg decrease in clutch siz e, experimental delays had no effect on fledging success. The results suggest that both parental/territory quality and environmental changes over the season affect the seasonal decline in reproductive success o f birds. The magnitude of the observed relationships also appeared to be affected by selection for small clutch sizes during a relatively po or (e.g., weather, food supply) breeding season.