Dispersal and recruitment during population growth in a colonial bird, the great cormorant Phalacrocorax carbo sinensis

While the factors influencing reproduction and survival in colonial populations are relatively well studied, factors involved in dispersal and settlement decisions are not well understood. The present study investigated exchanges of great cormorants Phalacrocorax carbo sinensis among six breeding colonies over a 13-year period when the breeding population in Denmark increased from 2800 to 36 400 nests. We used a multistate capture-recapture model that combined multisite resightings and recoveries to examine simultaneously recruitment, natal dispersal, breeding dispersal and annual survival of first-year, immature and breeding great cormorants. Mean survival of first-year birds (0.50 +/- 0.09, range=0.42-0.66 among colonies) was lower than survival of breeders (0.90 +/- 0.06, range=0.81-0.97). Mean survival of immature birds over the study period was 0.87 +/- 0.08. Dispersal from a colony increased with decreasing mean brood size in the colony in both first-time and experienced breeders. The choice of the settlement colony in first-time breeders was affected by conditions in the natal colony and in the colonies prospected during the pre-breeding years. In particular, first-time breeders recruited to colonies where they could expect better breeding success. Experienced breeders relied mainly on cues present early in the season and on their own breeding experience to choose a new breeding colony. Newly established colonies resulted mainly from the immigration of first-time breeders originating from denser colonies. Dispersal was distance-dependent and first-time breeders dispersed longer distances than breeders. We suggest that the prospecting behaviour allows first-time breeders to recruit in nearby as well as more distant potential breeding colonies. Dispersing breeders preferred to settle in neighbouring colonies likely to benefit from their experience with foraging areas. We discuss the importance of these movements for growth and expansion of the breeding population.