Dispersion asymmetry within a Feral Pigeon Columba livia population

Abstract

The aim of the study was to determine the pattern of dispersion in Feral Pigeons, as well as the factors influencing the degree of dispersion. Aside from studying variation in dispersion among the bird colonies, the direction and distance of dispersion were also analysed. The results of the study point unequivocally to strong dispersion asymmetry in the population, which is mainly age-biased. There were great differences in dispersion between adult (reproducing) individuals and young individuals that had not yet joined the breeding population. Each year, young individuals which had permanently left their natal colonies accounted for 20-30% of the young birds that ultimately joined the breeding population. The insignificant degree of dispersion among adult birds (from 0 to 0.8% per year) was due to their strong philopatry towards their breeding sites. A lack of reproductive success did not have any effect on dispersion in the case of the breeding pairs studied. A factor conducive to a bird's departure from the natal colony was the high density of breeding pairs present in the colony. It was found that the direction of dispersion was from a colony with a higher density to a colony with a lower density of pigeons. The Feral Pigeons did not emigrate to join colonies of domestic pigeons kept on the outskirts of the city, nor did they emigrate to other towns in the neighbourhood of the study area (Słupsk, NW Poland). The time when young birds left the natal colony did not influence the degree of their dispersion. Young birds that left their natal colony experienced significantly higher breeding success in their new colony, compared to those young birds that remained in the natal colony, where the density of breeding pairs was high. Young females dispersed more often than young males, although this difference was not statistically significant. This article also discusses the dispersion mechanism in the case of young pigeons.