Does incubation capacity limits clutch size in south polar skuas Catharacta maccormicki

Abstract

Incubation ability has been suggested as an factor determining clutch size in birds. This hypothesis proposes that the number of brood patch could limit the maximum clutch size. To test this hypothesis, we measured egg temperature in the manipulated clutches (2 or 3-egg) of south polar skuas Catharacta maccormicki in a breeding site Antarctic region. South polar skuas had two brood patches. Clutch-size of South polar skua is 1-2 eggs normally. Four abnormal clutches of 3 eggs were found in Barton peninsula, King George Island, Antarctica during the three consecutive breeding seasons (2004-07), but the eggs of 3-egg clutches were not hatched at all. To look at the difference in the egg temperature between 2 and 3-egg clutches during the incubation period, we replaced all original egg from each clutches to two or three experimental eggs loaded dataloggers. Mean temperature of eggs in 2-egg clutches was significantly higher than that of 3-egg clutches. Fluctuations of egg temperature of 2-egg clutches in same clutch showed synchronous pattern with those of other egg. In 3-egg clutches, temperatures of two eggs tended to change synchronously and were higher compared to other single egg. Egg temperature of other single egg changed inversely with two eggs within same duration. However egg compositions showing synchronous egg temperature fluctuation were shifted randomly. The accumulated duration that egg temperature decreased to below 30 °C in 3-egg clutch was longer than in 2-egg clutch about eight times. Mean nest temperature when skuas incubated two eggs were higher than incubated three eggs. When one egg was added into 2-egg clutch, nest temperature decreased, compared with that of original clutches. The above results indicate that the clutch size in south polar skuas breeding in cold region is likely to be restricted by incubation capacity.