THE EFFECT OF FOOD ON LAYING DATE AND CLUTCH-SIZE IN TENGMALMS OWL AEGOLIUS-FUNEREUS

The breeding of Tengmalm's Owl Aegolius funereus was studied at Umeå, Sweden, during the 1984–85. Mean clutch‐size was one egg larger in 1984 than in 1985 despite the later laying in 1984. The difference in clutch‐size was related to a better food supply in 1984. Daily weight increase of females during the prelaying period showed a high negative correlation with laying date in 1985, and a high positive correlation with clutch‐size independently of laying date in 1984–85. This suggested that food eaten before and during laying had a great and direct influence on both laying date and clutch‐size. Many females increased in weight during laying and most others decreased only moderately (relative to egg weight), suggesting that body reserves were not a main source for egg production. Late breeding females were provided with extra food during the prelaying and laying periods in 1985. Fed females weighed more, bred eight days earlier and laid one more egg than controls. At the same laying dates in mid season, and after heavy snow‐fall, clutch‐size and female weight were larger in the fed birds than in controls, but this was not so near the end of the laying season. Although the earliest of the fed late breeders weighed more, and probably were less restricted by food availability just before or during laying, they did not lay more eggs than did early breeders. This result suggested some limitation on clutch‐size that could not be overcome by the supplementary feeding. Weights of females during laying did not show any consistent relationship with clutch‐size during successive laying date intervals, suggesting that clutch‐size was not directly related to body condition. Copyright © 1990, Wiley Blackwell. All rights reserved