From
https://ec.europa.eu/environment/nature/cormorants/faq.htm
"
Q: Surely, cormorants are considered seabirds – why are they found inland?
A: It is true that the Atlantic race (
carbo sub-species) has commonly been regarded as a seabird. This sub-species breeds almost exclusively on the rocky coasts of Norway, Great Britain and western France, and some birds moved inland recently to over winter and feed on freshwater habitats. In addition, some birds of this sub-species now choose to breed at inland sites where they can mix with birds of the
sinensis sub-species.
The continental race (
sinensis sub-species), in contrast, has historically always utilised inland as well as coastal sites. Even when numbers were relatively low in the past, there were some breeding colonies on inland lakes and large, lowland river systems. In winter many individuals from coastal
sinensis colonies would cross over inland regions on their flyway south, resting and feeding on inland waters. Since the recent large increase in cormorant numbers across Europe, many more birds of this sub-species now occur at inland sites during as well as outside the breeding season. 'Surplus birds' from breeding colonies have emigrated and founded new colonies at inland waters.
The reason that more birds of both sub-species now occur at inland sites reflects, in part at least, the increase in their numbers and the availability of food and suitable habitat. Whereas most major European rivers have experienced a decline in fish abundance, a number of sites like open reservoirs, man-made gravel pits and a large number of fish farms provide good foraging opportunities for cormorants.
Overall the vast majority of cormorants present in Europe are found near to large waterbodies and wetlands both during and outside the breeding season. Moreover, the current legal protection afforded to cormorants make it possible for the birds to breed, roost and feed much closer to densely populated areas than they would have been allowed to in the past. At present, both coastal areas and inland waters provide favourable conditions for cormorants, and the birds are able to adapt their behaviour to exploit the feeding and breeding opportunities available to them."
The map in
this link shows quite a few breeding colonies, albeit mostly small, throughout most of Germany.
