Is Clutch Size of Slaty-backed Gulls Determined by Incubation Cost

Abstract

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.