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The population of wintering Barnacle Goose Branta leucopsis in Northwestern Europe has increased enormously during the last decades. A large proportion of these occur in Denmark during winter. These birds are part of the Russian/Northwest European population, which is the largest and fastest growing population with an increase from 20,000 individuals in the1950’ies to estimated 1.2 million individuals in 2014. DOFbasen (www.dofbasen.dk) is the main database on casual bird observations in Denmark and provides the data for a description of the population changes in this study. Despite the unsystematic nature of the data, the very high number of records of birds (1.5 million annually, 22 million in total) from all over the country makes these data highly useful to describe changes of distribution and phenology inthe Danish nature. This knowledge is relevant for the management of a species like the Barnacle Goose. During winter, the species often forage on winter crops. Damage to agricultural crops has been registered all over the wintering range, with grassland as the most affected crop followed by winter and spring cereal. This have led to increasing conflicts with farmers due to the negative impact the foraging has on production and thereby the economy. Here we present recent changes in the abundance and distribution in the Danish Barnacle Goose winter population.

Avian Introduced Alien Species (IAS) constitute a threat to the integrity of native biodiversity, the economy and human health, so here we briefly review some of the problems posed by such species around the world in relation to such bird species in Denmark. A new European Union Regulation on Invasive Alien Species implemented in January 2015 establishes a framework for actions to combat alien species, which requires Member States to prevent the spread of alien species, provide early warning and rapid responses to their presence and management of established alien species where they occur. We show the importance of mechanisms such as DOF’s (Dansk Ornitologisk Forening, BirdLife Denmark) Atlas project, Common Bird Census (breeding and wintering species) and DOFbasen to contribute data on the current geographical and numerical distribution of the few serious alien avian species already present in Denmark. We review the status, abundance and distribution of seven critical IAS that do, or have, occurred in Denmark in the last 10 years and conclude that none of these pose a major threat as things stand at the present, although breeding Egyptian Geese Alopochen aegyptiaca and Canada Geese Branta canadensis potentially give cause for future concern. We underline the need for continued surveillance of all avian IAS through data collection via DOF’s monitoring programmes and Aarhus University’s mid-winter waterbird census, hunting bag and wing surveys. These programmes are essential if we are to continue to effectively monitor the extent and nature of the problems constituted by IAS in support of the Danish Nature Agency in their direct management of alien species problems in this country.