Surprised to hear piping oystercatchers fly over (why here?, they are estuary/coastal birds)

I was extremely surprised to hear oystercatchers flying around. Their normal habitats are tidal coasts and estuaries, which are kilometers away. What are they doing here, so far from the sea? My only guess is that, like gulls, they come to check out possible food around the slurry pit. Otherwise the tractor work continues and the pigs are rather active.