As has been said its finding them and getting a run at them, there is probably between 40-50 in each of the two fields 2 minutes from me, but everyone walks their dog there, so as soon as lamp on they are off mind they only sitting 10 - 15 yards off of their holes,
The holes are on a railway too so ferreting them is quite difficult, and on the other field there's holes either side of the path which is walked quite often,