Sources of variation in estimating breeding success of migratory birds from autumn counts

Abstract Understanding drivers of change in population sizes requires estimation of demographic rates such as survival and productivity. In migratory geese, productivity or breeding success is typically assessed at the autumn staging and wintering grounds by observing the number of young versus adults in flocks of geese—also called age counts. Such age counts are, however, likely to be affected by a number of factors as we are compelled to sample from an open population, in which the temporal and spatial age composition can vary due to differential migration, mortality and flocking behaviour. In this study we seek to provide guidance for the design of age counts, by identifying which factors need to be taken into account when collecting data. Identification of these factors will facilitate a more targeted data collection and enable better conservation and management recommendations. We use the long‐term age count dataset for the Svalbard population of the pink‐footed goose and focus on the following factors: May thaw days on Svalbard, region of sampling, flock size, time and cumulative harvest, calculated as the amount of shot individuals up to each observation of juveniles. We find that the temporal trend in goose productivity based on raw data will be affected by variation in how (which flock sizes), where (which region) and when (which Julian day) the data have been collected. The sources of variation in observations of goose productivity confound inference at the population level. Thus, if the goal is to follow changes in annual productivity, pooled totals of age counts are only useful if data are collected following a random design. We recommend adjusting for these effects using a demographic population model, particularly for hunted species where age counts are conducted during the hunting season and where annual population assessment work is used to recommend an optimal harvest strategy and allocation of hunting quotas. These considerations not only apply to geese, but more generally to migratory birds where breeding and non‐breeding segments or age groups partially segregate in time and space in autumn.