ProjectPre-fledging survival of temperate-nesting Canada geese: the relative importance of density-dependent effects and individual covariates
Several bird species have taken advantage of human modified habitats, resulting in population increases. Determining how and which demographic parameters can be affected by these anthropogenic modifications is essential to understand population dynamics. Among these parameters, unbiased estimates of pre-fledging survival remain relatively difficult to obtain for precocial birds. Furthermore this parameter can vary from year to year and find the causes of these variations is another challenge for ecologists. In this study, we developed an accurate method to estimate pre-fledging survival in a population of temperate-nesting Canada geese (Branta canadensis maxima) that has undergone an exponential increase during the last two decades. We also examined whether pre-fledging survival was affected by density-dependent effects and individual covariates. Between 2005 and 2014, 8,679 goslings were marked with web tags when hatching at two adjacent sites that support different brood densities. A total of 3,922 of these birds were recaptured and banded before fledging while 338 were recaptured and banded in subsequent years as after-hatching year birds. Multistate models with joint live and dead encounters were used to estimate survival and to look at the effects of rearing sites, hatching date, initial brood size, mother age, and weather conditions at hatching. During the 10 years of the study, survival varied between 0.45 (95% CI: 0.40–0.50) and 0.76 (0.62–0.84) according to years and sites with an overall mean of 0.62 (0.55–0.69). Survival rates varied little between the two sites and declined over time at the site with the highest density. Pre-fledging survival generally declined for birds hatched after median hatching date and tended to increase with initial brood size and mother age. Weather conditions at time of hatching did not affect gosling survival. This study provides the first estimate of true pre-fledging survival for a temperate-nesting population of Canada geese using an improved and robust method. Density dependent effects that were shown to influence nesting success of this population also appear to affect gosling survival.