THE EFFECT OF AGE AND DENSITY OF BREEDING BIRDS ON THE TIME OF BREEDING OF THE KimWAKE RISSA TRIDACTYLA

Abstract

SUMMARY1. Differences were recorded in the time of breeding of colonies of the Kittiwake at Marsden, Co. Durham, between 1951 and 1958 inclusive. During 1958, observations were extended to other colonies in northeast England and southeast Scotland.2. Females breeding for the first time bred 9–8 days later than those breeding for at least the fourth time.3. The age composition of the Marsden Kittiwake colonies has been estimated. The old, established, colonies always had less than 25% of the birds breeding for the first time, while the younger colonies had much higher proportions of young birds.4. The age‐compositions of the colonies and the time of breeding of birds according to previous breeding experience did not account for the differences found in the time of breeding of the colonies.5. The time of breeding was directly related to the density of breeding birds, particularly those within a radius of 5 feet of the nest.6. The maximum nest‐density was usually related to the rock structure, but in one colony the maximum density was probably determined by the minimum distance at which Kirtiwakes can tolerate their neighbours.7. The time of return to the colonies also appeared to be related to density and this suggested that the effects produced by the density of breeding birds was carried over from one year to the next.8. All the Kittiwake colonies studied completed breeding at about the same time but differed in the time that breeding commenced. The spread of breeding was apparently related to the range of densities within the colony concerned.9. It is suggested that the onset, spread and completion of breeding within a colony can be explained by the maximum density, the range in density and the presence of low‐density areas in all colonies.10. The “socia‘ stimulation” apparently operating in the Kittiwake colonies differed from that envisaged by Darling; the effective distance appeared to be short (5 feet) and tended to produce a longer breeding season.