Reproductive Effort, as Expressed in Body and Organ Weights, in the Pied Flycatcher

Abstract

In the male Pied Flycatcher maximum body weight was detected during spring migration. A significant decrease was noticed during the pre-nesting period. A second significant decrease in body weight occurred between the incubation period and the nestling period. The body weight of the female Pied Flycatcher reached a maximum during the egg-laying period. No decrease occurred during the incubation period, but there was a significant drop around the hatching time. For both sexes, the spleen weight and the spleen-somatic index (SSI) increased after the birds arrived in Sweden. These variables continued to increase between the nest-building and the egg-laying periods and remained high for the remainder of the breeding season. There was a transitory increase in haematocrit levels in males during the nest-building and egg-laying periods, while in females a transitory decrease was found during the egg-laying period. The highest liver weights were recorded in males during the nestling period and in females during the egg-laying period. For both sexes liver-somatic index (LSI) was lowest in spring migrants and highest in birds from the nestling period.