Life history decisions in a changing environment : A long-term study of a temperate barnacle goose population

Abstract: This thesis shows that climate and increasing population density have profound effects on survival, age at first reproduction, and natal dispersal in a rapidly increasing temperate barnacle goose Branta leucopsis population on Gotland, Sweden.Colony size was found to be the most important fitness-determinant because of strong density-dependent effects on reproduction. The amount of rain during the breeding season influenced the food supply of growing geese, resulting in considerable differences in weight and structural body size among birth-cohorts. These differences, in turn, affected pre-breeding survival Thus, members of some cohorts had a much higher probability of surviving to their first breeding attempt than members of other cohorts. Structural body size of cohorts declined rapidly during the initial phases of colony growth, and subsequent recruitment to the breeding population of these cohorts caused the mean body size of breeding birds to decline slowly but constantly during the study. Since large females produced more young, and body size was heritable, an increase in mean body size was expected, but density-dependent effects on growth and final structural body size prevented such a microevolutionary response to selection.Age at first reproduction increased with increasing colony size, especially in females. This was caused by an increased competition for space, making it harder for young individuals to establish a territory, and not by deteriorating body condition due to-increased competition for food. Male natal dispersal rate increased, but female natal dispersal rates remained very low throughout the study, suggesting high benefits of philopatry in females. Large males were more likely to disperse than small ones, because only large males coulddeal with the costs that were associated with settling in an unfamiliar environment. Barnacle geese sampled potential mates by engaging in temporary trial liaisons. One-year-old birds had higher status when accompanied by a trial partner, and females that were often in the company of older trial partners early in life produced more young. Finally, birds that found their final partner early also started breeding early in life.