Incubation and Nest-Building Behaviour of Black-Headed Gulls III : the Pre-Laying Period

Abstract

Abstract(I) Elements of incubation behaviour and nest-building behaviour that are performed by Black-headed Gulls in the pre-laying period are described as they appear in the natural situation and when a model egg is placed in the nest. (2) Quantitative material is presented and shows the following:- (a) Time spent on the territory, time spent sitting in the nest, the frequency of settling, the proportion of complete settlings, the proportion of relatively long combinations of pre-settling movements, and the frequency of sideways-building all increase steadily as the date of laying draws closer. This is not true for time spent by partners together on the territory or the frequency of collecting trips. (b) Time spent on the territory, time spent sitting in the nest, the frequency of settling, and the frequency of sideways-building vary together to significant extents; this is at least partly a result of common correlation with the passing of time. Settling and sideways-building, however, remain highly significantly correlated after the effects of common correlation with all other variables are eliminated. (c) Performances of settling and sideways-building tend to occur in close temporal proximity to each other. (d) Comparison with the behaviour in the natural situation shows that presence of an egg in the pre-laying period depresses the amount of time spent on the territory somewhat but increases the amount of time spent sitting in the nest, the frequency of settling, and the frequency of sideways-building. Frequency of collecting trips and amounts of time partners were together on the territory were not affected by presence of the model egg. Presence of an egg in the nest is almost a necessary condition for performance of shifting and quivering in the pre-laying period. (e) Most collecting trips are performed when the partners are together on the territory. This is not true for settling or sideways-building. (f) Males perform more settlings, more sideways-building movements, more collecting trips, and spend more time sitting in the nest than females but the partners spend more or less equal amounts of time on the territory. (g) The evidence suggests that sideways-building has more causal affinity with settling and sitting in the nest than with collecting trips. (3) A sample of brood-patch and gonad measurements from birds found at different stages of the pre-laying period indicate that, in both males and females, defeathering of the brood patches begins some time before eggs are laid, and that changes in these structures may develop parallel with, and be implicated in, the changes in sitting and nesting behaviour in the pre-laying period.