THE FREQUENCY AND TIMING OF LAYING GAPS

The frequency of laying gaps was studied in a nestbox breeding populat ion of Blue Tits Parus caeruleus during the breeding season of 1990. I n as many as 27% of the 55 clutches studied, laying was interrupted fo r at least one day. It is concluded that the interruption of egg layin g depends on food availability since the frequency of laying gaps decr eased in clutches laid by females experimentally provided with extra f ood Most gaps occurred after the laying of the first or second egg, in dicating that each female started egg laying on the verge of what is p ossible for uninterrupted laying. The occurrence of laying gaps was no t connected with certain dates, female age, clutch size or mean egg ma ss of the clutch, instead each female seemed to individually optimize the start of egg laying in relation to the risk of interrupted laying. To some extent the female is able to save energy for egg production b ecause an egg laid after a gap was heavier than corresponding eggs lai d in clutches without gaps. Furthermore, eggs laid immediately before a gap were lighter than equivalent eggs in other clutches, indicating that females, to some extent, are willing to trade egg quality against uninterrupted laying.