In many cases this may be down to taste, feral pigeon may be more to there liking rather than Black Headed Gull for instance, the Gull is no doubt easier to take, so it could possibly be taste. Also the feral pigeon is just the right size to carry, I know that some Tiercels fly over a mile to a favoured hunting area, carrying the larger Gull back this far is hard work for the male, especially at dawn when there is no lift. I have recorded Kestrel as prey along with 2 male Sparrowhawks and know of other Kestrels taken as prey elsewhere. The smaller Kestrel will certainly go up and mob the Peregrine, I have seen this, some of this comes down to knowing when you can get away with it, and when not.
On all of these sites where the 2 coincide I have noticed that the Kestrels give the Peregrines a wide berth at dawn onwards when they are 100% hunting, they fly extremely low to and from the nest site, and will not go up high. When the Peregrines have killed, fed and then layed up, the Kestrels will go up higher and even thermal. The Peregrines, although extremely aggressive and territorial do not perceive them as a threat to eggs/young unlike Carrion Crows, these are attacked with relish, laying up or not.
|Kestrel in flight|
|Hopefully the Peregrines will stay with there normal prey|
It is good to see the 2 species together, on one site both species have 3 juveniles apiece, that gives the possibility of 10 Falcons in the air, should be quite a sight.