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

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