Egg weight variation in Collared Flycatchers Ficedula albicollis

Inter-and intra-clutch egg size variation in Collared Flycatchers (Ficedula albicollis) were studied during 1994-1995 on the Baltic Sea island of Gotland (Sweden). The main question was whether Collared Flycatchers increased egg size in laying order and employed a brood survival strategy, or decreased egg size and employed a brood reduction strategy. The relationship between egg weight and clutch size, and between egg weight and female and male characteristics was also investigated. In 1994 the study was performed in two distinct periods, but only in one period in 1995. Nest boxes were inspected daily during nest construction and during egg laying, and eggs were weighed the day of laying. Basic breeding data were collected and measurements of adults and young were taken. In general, egg weight increased versus laying order and in 1994 the increase was significant only among late breeders. Mean egg weight positively correlated with female body size (both years) and negatively with male body size (1994 only). Eggs in clutches of 6 were heavier than those from clutches of 7 in 1995 but not in 1994. This indicates a trade-off between egg size and egg number. It is concluded that the Collared Flycatcher possibly employs a brood survival strategy to prevent competition between nestlings.