I think it's a reasonably well known that birds are generally considered a very good indicator of what's 'going on'.
If you're drifting on lochs and in particular reservoirs (and very much the case at Menteith Bewl, Rutland, Grafham, Ravensthorpe etc etc) watch what the seagulls are up to....... and generally head for the piece of water the birds are 'mobbing' as there's a good chance they're hitting shoals of fry.
As as example, the last time I fished Bewl (probably 15+yrs ago now
) we were having a quiet day then I saw the gulls hitting the water hard in a concentrated area.... very similar to what I'd seen at home (albeit on a lesser scale) so off we went and my boat partner and I 'bagged' up on 'bows and browns that were hammering the fry.
Without the indication provided by the gulls, wouldn't have known the fry were there as they weren't 'boiling' on the surface as they sometimes do when being hit by a shoal of bows and the occasional brownie.
This is equally applicable on big lochs, it's not always fry the gulls are after, but if you're having a quiet day wander over to where the gulls are and chances are you'll have some sport