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.
The Kestrels may just see the Peregrines as an occupational hazard, similar to Carrion Crows, they have a territory and a nest site, they are certainly not going to move site due to the presence of a higher authority, no matter how dangerous it could be.
|Hopefully the Peregrines will keep to there normal prey|