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

Abstract

Populations of migratory birds often mix to a considerable extent in their wintering areas. Knowledge about the composition of wintering populations is highly relevant 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. 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. 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. 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. Policy implications. Cormorant wintering populations in Europe consist of mixtures of birds of different breeding origins. These mixtures are also highly variable over time. These factors reduce the chances of successfully limiting conflicts in specific wintering areas through, for example, 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.

Consequences of past and present harvest management in a declining flyway population of common eidersSomateria mollissima

“…Step 2 94, delta AIC: 93). According to the best model, the probability for hunters to recover rings of shot birds was estimated as 0.36 (95% CI, 0.25-0.49), whereas the recovery probability of birds dead due to other causes fell linearly from 0.32 to 0.04 between 1970 and 2015; similar declines have been reported for other species (Frederiksen, Korner-Nievergelt, Marion, & Bregnballe, 2018;Robinson et al, 2009). Furthermore, the probability that recoveries were correctly assigned to hunting (δ h ) and other causes (δ o ) was estimated as 0.94 and 0.95, respectively.…”

Genetic diversity of Contracaecum rudolphii sp. A (Nematoda: Anisakidae) parasitizing the European Shag Phalacrocorax aristotelis desmarestii from the Spanish Mediterranean coast

“…A from Sardinia in comparison with those conspecific ones from the Catalan and Tyrrhenian areas ( Table 2 ). This fish-eating bird presents its high migration pattern along the brackish and freshwater environments of Europe, inhabiting, among the others, the Catalan and the Tyrrhenian Mediterranean waters during its non-breeding season ( 51 , 52 ). Although the low number of samples from the Sardinian Sea herein studied ( n = 33) should be considered, the existence of a population of Ph.…”

Northern range shift may be due to increased competition induced by protection of species rather than to climate change alone

“…The decrease in wintering cormorants along the Atlantic coast after 1992 (Figure ), although in an optimal area for the species, participated to the NNE shift range of the national population and was probably due to competition between the two marine and inland subspecies. Danish cormorants initially migrated mainly through eastern routes in the 1980s and 1990s, but migrated further west afterwards (Frederiksen et al., ; Marion, ). Maybe there where under the pressure of competition with new northeastern populations migrating for the first time, which could also partly explain the decreasing numbers of cormorants in Tunisia.…”

“…France is the main country for wintering and migrating cormorants in Europe. They mainly come from northern Europe (Frederiksen, Korner‐Nievergelt, Marion, & Bregnballe, ; Marion, ), and France is the only European country where their populations have been monitored by exhaustive and regular national winter counts using the same method (this is essential to describe changes in bird populations according to Elmberg et al., ). As such, France is particularly adapted for a long‐term study (33 years) of the rapid changes in cormorant distribution and numbers.…”