Use of distance sampling to improve estimates of national population sizes for common and widespread breeding birds in the UK

1. Population estimates are of fundamental importance for setting conservation priorities and for numerous aspects of conservation biology. 2. Distance sampling, which takes undetected individuals into account, is one of the most widely used methods for generating population estimates. We use this method to generate estimates of the national population size for all common and widespread non-marine breeding birds in the UK using Breeding Bird Survey data. 3. There is a strong positive relationship between our distance-sampled estimates and estimates generated using other methods. This implies that most methods get the broad picture right, even if one of them is less precise for individual species. 4. For some species, detectability may vary sufficiently between males and females to generate biases in population size estimates if these differences are not taken into account. We found a slight tendency for population estimates from distance sampling to be lower than existing estimates for species with marked sex biases in detectability. This may be a wider problem than is currently acknowledged in distance sampling. 5. Distance sampling provides a method for estimating total population size. Other bird population survey methods, such as intensive territory mapping, aim to count the number of breeding pairs, and thus exclude non-breeding individuals. Not surprisingly, we found that distance-sampled estimates tended to be higher for species with a large proportion of non-breeders. Both approaches are valid, but when calculating, reporting and using population estimates attention needs to be paid to which variable is of most interest. 6. The new estimates that we present are significantly larger than existing ones for species whose preferred habitat types were not previously well surveyed. This highlights the importance of sampling across all main habitat types. 7. Synthesis and applications. This study reviews alternative methods used for producing estimates of population size for common and widespread breeding birds in the UK. We assess a number of factors affecting population estimates generated by distance sampling.