1999 Volume 48 Issue 2 Pages 127-133
Gull (Laridae) species usually lay two or three eggs. We tested the hypothesis that clutch size is determined by incubation ability of parents in Slaty-backed Gulls (Larus schistisagus) whose normal clutch size is three on Teuri Island. Their clutch sizes were manipulated to be two-egg clutches where one egg was removed, three-egg clutches with no manipulation (control) and four-egg clutches where one egg was added. Hatching success and hatchling masses were not different between experimental and control groups. The mean incubation period of the four-egg clutches (29.2± 1.9 days) was one day longer than that of the two-egg clutches (28.1± 1.4days) and control (28.2 ± 1.6 days). The mean egg temperature of four-egg clutches (33.0 ± 2.8 °C) was lower than that of two-egg clutches (36.2 ± 1.8 °C) and control (35.9 ± 1.8 °C). These results suggest that the parents can not incubate experimental enlarged clutches effectively but this does not affect offspring production.