Data from: Where do wintering cormorants come from? Long-term changes in the geographical origin of a migratory bird on a continental scale

1. Populations of migratory birds often mix to a considerable extent in
their wintering areas. Knowledge about the composition of wintering
populations is highly relevant in relation to management, not least for
species, such as the great cormorant Phalacrocorax carbo sinensis, prone
to conflicts with human interests. However, few studies have been able to
estimate long-term changes in winter population composition. 2. We use 30
years of ringing and recovery data (1983-2013) from all major breeding
populations of cormorants in continental Europe (except the Black Sea
region) to estimate partitioning probabilities (i.e. the probabilities of
moving to specific wintering areas) using a Bayesian capture-mark-recovery
model. Combining these results with information on breeding numbers and
reproductive output in a population model, we estimate the size and
composition of wintering populations in Europe and North Africa. 3.
Partitioning probabilities showed some variation over time, but were
similar for first-winter and older birds. Cormorants from the western part
of the breeding range tended to winter progressively further west over
time. This may be a density-dependent response to the recent growth of
more easterly breeding populations. 4. All wintering populations grew
rapidly over the study period, and their composition showed pronounced
changes. All wintering populations were composed of birds from many
different breeding populations, but the proportion of cormorants of more
easterly origin increased markedly over time in most wintering areas. 5.
Policy implications. Cormorant wintering populations in Europe consist of
mixtures of birds of different breeding origin, and these mixtures are
highly variable over time. This reduces the chances of successfully
limiting conflicts in a specific wintering area through e.g. regulation of
breeding numbers in one breeding area. The dynamic nature of cormorant
winter populations means that conflicts are best addressed when and where
the conflict occurs, or on the scale of the entire continental population.
It is unlikely that the latter will be cost-effective and politically
realistic.09-Jan-2018