Pre-breeding survival of barnacle geese Branta leucopsis in relation to fledgling characteristics.

Abstract

Pre-breeding survival, i.e. survival from fledging up to the third winter, of barnacle geese (Branta leucopsis) was analysed by using more than 45000 resightings of 1302 individually marked birds. Since observations from the wintering areas only were used, the survival estimates obtained were not confounded by natal dispersal. Post-fledging survival, i.e. survival from fledging to the start of the first winter, differed significantly among the 10 cohorts analysed. These differences were related to the mean body weight and mean tarsus length of cohorts just before fledging. In further analyses, when data from all cohorts were combined, body weight and tarsus length of individuals just before fledging were found to be positively related to Post-fledging survival. Post-fledging survival rates were, on average, lower than survival rates after the first winter. This age effect was largest in cohorts with low mean body weight at capture and was absent in cohorts with high mean body weight at capture. The age effect on survival could only partly be explained by individuals with low body weight having lower survival and successively disappearing from the cohort. It was therefore concluded that other factors must have contributed to the age effect as well. It is hypothesized that lightweight birds are more vulnerable to, for example, diseases, parasites and predation during the first months of their life. Once they have survived this critical period, the effects of characters related to low body weight at capture seem to disappear. Body size of juveniles just before fledging has previously been found to be greatly affected by weather factors and the availability of high quality food during the growth period. Hence, a considerable part of the observed variation in Pre-breeding survival appears to be outside the individual's control.