Reduction of variability of individual colonization programs in black-headed gulls (Larus ridibundus) leads to diminished reproductive success

Abstract

The study is devoted to the problem of adaptive significance of territorial antagonism in colonial birds. Many of them are characterized by a high intensity and diversity of territorial demonstrations, along with tolerance of each other and a tendency to aggregate. Here we consider the effects of territorial behavior on the viability of a breeding colony. We analyze the breeding of black-headed gull in two colonies formed under the impact of a short-term temperature fall in 2008, which resulted in a 7-day shift in the timing of egg laying. Compared with the colonies at the same reservoirs, one in 2007 and one in 2009, the number of nests and their density in 2008 were much smaller, whereas the duration of settling, nest density growth, and average size of breeding territories did not change. The average body weight of adult birds during incubation was stable throughout all years; however, the correlations typical for 2007 and 2009 (negative between the female mass and date of clutching start and positive between masses of parents) were absent, suggesting that assortative mating by body mass and territory size was not the case in 2008. The average clutch, egg, and hatching sizes were on average smaller in 2008, while mortality due to attacks of aerial predators was much higher than in other years. In 2007 and 2009, birds that started egg-laying in the first 5 days after the first egg appeared in the colony (pioneer settlers) were larger than others (the so-called followers) and produced a larger offspring. These differences were not observed in 2008. According to our data, due to the impact of cold weather, some birds abandoned their nesting sites and others were sick, but they mostly behaved as pioneer settlers and formed underpopulated and sparse colonies. We assume that the formation of a viable colony requires the interaction of highly territorial “pioneers” and a certain number of less competitive followers. The withdrawal of the latter results in a general reproductive failure of the colony. Thus, the pattern of colony formation in black-headed gull turns natural selection towards a maintenance of the variety of individual programs of territorial competition.