Integrating genetic data into the study of the significance of the North Sea and Skagerrak for wintering auks

Seabirds are long-lived organisms with high adult survival from one year to the next. Most adult mortality occurs during the winter, when food availability may be lower and weather conditions harsher.

Seabird mass mortality events - which can be caused by starvation, oil spills, and severe storms, amongst other possible mechanisms - are infrequent but can have serious consequences for adult survival rates and, ultimately, population trends. Such events are detected through dead or dying birds washing ashore, usually during the winter. Numbers vary but can reach tens of thousands of wrecked birds. As most birds die offshore, the numbers wrecked often only represent a fraction of the total fatalities.

In the North Sea, there have been a number of mass mortality events recorded over the last century, which mostly seem to affect auks, such as common guillemots Uria aalge and razorbills Alca torda. The frequency of these events appears to be increasing but their consequences for North Sea seabird populations remain poorly understood because the origin of the birds, except a few ringed birds, is currently unknown.