Colonial Larids (Charadriiformes): Breeding Synchrony and Determination of its Level

Based on many years of research (1972–2018) on the biology, ecology, and behavior of gull birds, whose membership in colonial species is not in doubt, the features of the specific phenomenon of the colony – high reproduction synchronization – are examined. It consists in a very short period of mass egg laying (in small colonies 2-3, and very rarely 4 days), but at this time the main number of birds forming the colony begins to nest. It was shown that this trait varies quite significantly depending on its size and is most strongly and clearly expressed in small colonies. The latter is due to the fact that large colonies are formed from small ones, often differing in terms of reproduction. As a result, the seasonal breeding cycle of a large colony often covers the entire nesting period characteristic of a particular species. The features of synchronizing the breeding of birds in colonies as one of its main characteristics are considered in detail. The factors that most determine the degree of synchronization of reproduction of birds in colonies of different sizes were identified: a) the size of the colony and the total duration of egg laying, b) the size of the colony and the duration of mass egg laying, c) the size of the colony and the proportion of birds that formed clutches during the period of mass reproduction, d) the duration of the mass egg laying and the proportion of birds that formed the clutches during this period. The correlation relationships between all these signs of the colony are calculated. Of all the factors considered by us, the two most characteristic for the period of mass nesting of colonial birds are detected immediately and very simply. All colonies differ well in the length of the period of mass breeding and in the proportion of birds that started to nest at that time. These are interrelated factors, but the degree of their correlation is relatively small and they determine the level of synchronization of breeding birds in the colony. With an increase in the proportion of birds that formed clutches during the period of mass reproduction and a decrease in time of this period (mass egg laying), the synchronization of reproduction of birds in the colony increases. An ideal relationship between the level of synchronization and these factors - the entire colony is formed in one day. The case is quite rare, but, nevertheless, constantly found in small colonies of all species of gull birds. Since the index is always a relative indicator, and almost all of the signs that we have examined are related to the size of the colony, it is precisely it that should be included in the developed comprehensive indicator. Based on them, a special index has been developed and proposed for use, which allows us to assess the level of synchronization of bird breeding in colonies, nesting clusters and aggregations – Isr (breeding synchronization index). As a result of our work, this index has the following form: Isr = √(n / l) / N, 0 ≤ Isr ≤ 1, where: n is the number of clutches formed during the period of mass breeding of birds, in item (by the date of laying the first egg in the nest); l – the duration of the period of mass egg-laying, day and night (accurate to tenths); N is the size of the colony (number of nests or pairs); Isr is the synchronization index. The essence of this indicator can be formulated as follows: the index of reproduction synchronization, as a relative indicator indicates how many nests in the average are formed during one day of mass egg laying, depending on its duration and the size of a particular colony. Extraction of the square root increases the obtained value, sometimes very small (in very large colonies), which greatly facilitates the use of this index and improves its perception. The maximum estimate of the level of synchronization of breeding birds in the colony tends to 1.0 and sometimes reaches this value. The minimum value of this indicator tends to 0, but is unlikely to reach it. In any case, our minimum estimates of the level of synchronization of the entire breeding season of birds in a very large colony (about 5.0 thousand nests) did not fall below 0.05. Comparison of the results of his calculation with actual observational data shows that he clearly responds to changes in colony parameters used to determine the level of egg laying synchronization and its other reproductive indicators. In addition, according to the results of statistical analysis, its high relationship with the main reproductive parameters of the colonies was established, which allows us to consider the synchronization index as one of the most important and promising relative indicators. Its use allows us to discover new, still very poorly studied relationships of various parameters of the reproductive processes of colonial bird species with environmental factors.