Natal and Breeding Dispersal in Mute Swans<i>Cygnus olor</i>: Influence of Sex, Mate Switching and Reproductive Success

Abstract

Dispersive behaviour in birds is shaped by a set of different external factors (climate, human pressure, territory quality), as well as by internal constraints (reproductive strategy, sex, age, competitive abilities). Natal and breeding dispersal was studied in the migratory population of the Mute Swan Cygnus olor in Poland. The species showed male-biased natal dispersal with mean dispersal distance of 99.7 ± 20.0 km for males and 17.7 ± 5.4 km for females. Consistently, larger proportion of female swans showed natal philopatry, when compared with males (36.4% vs. 9.1%). The dispersal distance has shortened over the last three decades. This tendency prevailed in males and was associated with reduction in annual migration distance due to milder winters and adaptation for wintering in urban areas. Analysis of breeding dispersal showed that only 12.3% of breeding attempts were associated with changing territories (N = 945). Breeding dispersal distance was female-biased (0.66 ± 0.19 km for females vs. 0.35 ± 0.14 for males; N = 316 individuals). Occurrence of mate-switching and poor reproductive success were positively associated with probability of dispersal, but moving to a new territory did not increase breeding output in a season following dispersal event. Dispersive behaviour in the Mute Swan may be perceived as a part of species-specific ecological flexibility, allowing individuals to adjust to the changing conditions at the breeding grounds.