THE EFFECT OF AGE AND DENSITY OF BREEDING BIRDS ON THE TIME OF BREEDING OF THE KITTIWAKE RISSA TRIDACTYLA
Authors: Coulson J. C. , White E. 
Published: April 2008

In 1938, Fraser Darling put forward the concept of “social stimulation” This hypothesis postulated that colonial‐nesting birds received stimulation from their neighbours which resulted in earlier breeding, a shorter spread of the nesting period in the colony and greater breeding success.

Darling's hypothesis has been criticized from a number of points of view. His original data were not sufficient to show significant differences between the behaviour of large and small colonies of the Herring Gull Larus argentatus and of the Lesser Blackbacked Gull L. fuscus. Furthermore, the present authors have shown that large colonies of the Kittiwake Rissa tridactyla have a longer nesting period than small colonies. While these and other facts cast doubt on the importance of the “Fraser Darling effect” they do not disprove the possibility that neighbouring birds in a colony may stimulate each other. Studies have therefore been undertaken on colonies of the Kittiwake on the Durham, Northumberland and East Lothian coasts to test as thoroughly as possible whether or not social stimulation is of importance to colonial‐nesting birds.

The colonies studied in greatest detail were close together and it is possible to exclude the possible effects of the physical environment affecting the colonies in a different manner. Differences of up to 21 days were observed between the mean time of breeding of nine colonies, and since the differences were greatest in the youngest colonies, the effect of differences in the age composition of the colonies was considered. A study of colourringed Kittiwakes showed that females which were breeding for at least the fourth time bred 9.8 days earlier than females which were breeding for the first time. When, however, the age composition of the colonies was determined, it was always found that less than 20% of the observed differences in the time of breeding could be accounted for by differences in the age composition.

A study of the nest densities within individual colonies showed that the time of breeding was related to the density of nests. Thus, at an average density of nine nests within a radius of 5 ft. of each nest, the time of breeding was about 18 days earlier than on sites where the equivalent density was about two nests.

It was observed that the colonies with the highest density also had the greatest range of nest densities. The colonies differed from each other in the mean density and also the maximum density attained, but were similar in that they all had some nests which had no other nests within a 5 ft. radius. Since breeding is earlier at higher densities, these observations can explain the earlier onset of breeding and greater spread of breeding in the denser colonies and also that the last birds to breed in each colony do so at the same time.

Observations show that the time of return to the colonies is also related to nest‐density and, together with other evidence, suggest that the effect on the birds of the density of their breeding neighhours is carried over from one year to the next.

It follows from these studies that social stimulation has a distinct effect on the breeding condition of birds within Kittiwake colonies.