Ecology of Laridae under conditions of unstable hydrological regime: colony sizes and synchronization of reproduction

The problem of criteria for distinguishing colonies from similar socio-demographic structures (mainly in terms of nesting density) is highly relevant and has remained in the focus of attention of ornithologists for a long time. The synchronization of reproduction in a colony is one of the criteria which require special development. Based on particular works (1972–2005), I present synchronization of the reproduction of gulls in colonies of different sizes. In contrast to previous studies, this paper uses a specially developed Index of Synchronization of Bird Breeding (Isr) to study this phenomenon, making it relatively easy to determine its level. The index distinguishes between different species of birds of this group: 75.7% (white-winged black tern) and 97.6% (black-headed gull) of the total variability of synchronization of breeding birds in colonies. Frequent failure of nesting attempts often causes repeated (compensatory) reproduction, which in the case of a mass manifestation significantly reduces the synchronization of the nesting period in colonies and thus significantly reduces this indicator. It is proved that a higher synchronization of reproduction characterizes small colonies (up to 50 nests). In all species of gulls, the beginning of reproduction in different colonies differs in terms of the appearance of the first eggs by 1–10 days and at the beginning of mass egg-laying – by 1–18 days. To the same extent, they differ in the timing of the hatching of eggs. In small colonies, the total egg-laying period is shorter by 34.9–49.7% compared to larger colonies. My observations show that large colonies are formed by the nesting of several small colonies on one plot. This phenomenon is noticeable during periods of mass re-nesting of birds after a high loss of nests (up to 69.5% or more) because of severe flooding. Differences in the breeding periods of colonies that differ in size appear when several small colonies with different breeding periods of birds are combined into one larger colony. This phenomenon is well detected in the formation of several sub-colonies and in the differences in the timing of reproduction of different parts of a large colony.