BB
In some areas of the UK it would be almost impossible to avoid concentrations of birds at any time of year, more so during the predictable migration periods. Although the coastlines are more obvious features for some species, quite a few also congregate inland - some resident, some migratory. The birds don't know what is a nature reserve and what is not, given that the adjacent (non-NR) land is often exactly the same, providing the same preferred habit for security, shelter and food source.
Have alook at this site and you will get a good idea of where large species birds could be encountered in UK - just about everywhere:
The RSPB: Birds by family: Swans, ducks and geese
So requiring crews to avoid specified known areas of bird concentrations is no guarantee of not meeting them elsewhere as part of a normal operational hazard. That's the way it is in parts of the country, it just happens that coastal areas are more popular for some (large) species.
Mister B