Ross,
I think the main reason the birds thrived with the hovercraft being there was that there was a large area created by the craft transversing regularly that the lugwormers, etc had to stay clear of.
That's what I was told.
Alas at Stanford Hall where we used to race the major problem was swans.
On a regular basis the starts were held up by the shout "Swans on the course" and it was quite a problem clearing them off. The Colonel ( Owner) was not amused when I suggest shooting them. No sense of humour.
Kip