Observations and experiments on egg-laying in the black-headed gull (Larus ridibundus L.)

Abstract

Two hundred and ninety-five nests of a Black-headed gull colony were inspected for eggs once, twice or three times daily from 14th April till 6th May, and at irregular intervals later. In 196 nests first clutches appeared over a period of more than three weeks. In the first week only 13 per cent. of the birds started laying. Darling's (1938) suggestion that discrete parts of a big colony behave as he thought isolated colonies did, could not be verified. Slightly more a-, b- and c-eggs are laid during the day than during the night but the differences are not significant. No significant difference was found for the number of eggs laid in the afternoon and in the morning. Of 184 clutches 70 per cent. consisted of 3 eggs. This figure is almost certainly too small, because it does not include the clutches which had been robbed before they were inspected. The most common interval between the laying of successive eggs of a clutch is 1 1/2–2 days. The mean interval between the laying of the a- and b-eggs is not significantly different from the mean interval between the b- and c-eggs. The Black-headed gull lays its clutch in a shorter time than do Common and Herring gulls.

If the a-egg was removed within 12 hours after it was laid the ♂ laid a d-egg 2 days after the c-egg. If both a- and b-eggs or all 3 eggs were removed immediately after each was laid protracted laying occurred in a number of cases. In one nest 7 eggs were obtained from one ♂. Therefore, the Black-headed gull also is an indeterminate layer as Salomonsen (1939) and Paludan (1951) proved for the Herring gull and the Lesser Black-backed gull. Reducing the number of eggs had no effect on egg-laying if one egg was left in the nest. It is thus not the size of the clutch but the gull's brooding which causes the cessation of egg-laying. Removing both eggs immediately after the b-egg was laid caused protracted laying in a number of cases. If, however, all eggs were removed after the c-egg was found no further egg was laid immediately but a full second clutch appeared after 8–13 days.

By adding eggs several days before the gulls laid it was possible to suppress egg-laying completely or partially: 13 pairs never laid though many brooded our wooden eggs for 2 months. In 4 nests one egg was laid after 5–7 days' sitting, in 13 nests we found 2 eggs and in 4 nests where the birds had only sat for one day all 3 eggs appeared.

Five ovaries were inspected at various times during the egg-laying cycle. Around the time the first egg is laid, the second and third are successively smaller and a tiny fourth follicle was found, apart from the still smaller reserve eggs. In two birds killed after the b-egg had been laid this d-egg was degenerating. It is concluded, as it was for the Herring gull (Paludan, 1951), that incubating has an inhibitory influence on the growth of follicles and causes them to degenerate as long as they have not reached a certain stage of maturity. This limits the number of eggs to be laid after the gull has begun to sit.