Site fidelity of wintering cormorants Phalacrocorax carbo sinensis in Europe

Abstract

The breeding population of cormorants Phalacrocorax carbo sinensis has expanded more than twenty-fold in continental Europe since 1970, and these large piscivorous birds cause conflicts with human fishery interests in large parts of Europe, including areas outside their breeding range. Culling aimed at reducing local population size takes place in some wintering areas, particularly in France and Switzerland. The effectiveness of such management actions is unknown and depends on how high cormorant site fidelity is. We use winter resightings from the period 1980-2000 of 24,000 cormorants colour-ringed in Denmark and The Netherlands to estimate site fidelity. Year-to-year fidelity to single sites (typically lakes) was 85-90%. For cormorants that changed site mean distances moved from year to year were typically between 10 and 50 km, depending on geographical location. However, long-distance movements (maximum observed was 1,891 km) also occurred regularly. For larger parts of the winter range, exemplified by France, about 3% of all birds left the area from year to year. A mathematical model showed that the observed levels of movement among wintering areas were sufficiently high to sustain a wintering population in France at levels of culling which would drive it to extinction in the absence of immigration. Although cormorants are not nomadic outside the breeding season, as previously believed, site fidelity is sufficiently low and movement among wintering areas sufficiently high that it must be taken into account in management programmes.