The hind quarter of a cottontail rabbit?! Really?! Are you kidding us, TPC? I thought PF's didn't chow down on furry mammals. Sure hope it wasn't the Easter Bunny! 
Nope, not kidding. I thought the timing was ironic too.
Some populations of peregrines include mammals in their diet - tundra peregrines most certainly do because there are often more mammals than birds to eat in an area. Some populations in arid areas will include snakes for the same reason. Peregrines who nest near bat hibernacula will prey on them because of proximity and the large number of bats rather than lack of birds - but that is not a population, those are individual birds.
Anatum peregrines on the Prairies and US Midwest are a population, and as a subspecies and as a population they are bird killers - waterfowl and water birds primarily, which makes sense because this area is the continental breeding grounds for a huge number of these species - proximity and an exceeding large number of prey species defines what they prey upon. But like the individual birds that hunt bats - there are always outliers who will not be exclusive and it looks like Smiley is one of those.
It may be that Smiley spent time where small mammals were more abundant than birds and he got good at hunting for them. Hunting for small mammals in an urban environment is not an easy or safe thing to do for a peregrine - getting close to the ground at the speeds they hunt at is more than a little bit dangers. It has occurred to me that perhaps hatching/growing up in Grand Forks gave him many more opportunities to hunt "low" safely. Would be interesting to know if other Grand Forks offspring do the same, but not many of his sibs have survived to nest elsewhere and I'm not sure if any of them are at a nest with a camera - I most certainly wouldn't have known what he can hunt if I hadn't been able to watch on-camera.
Smiley may be our most interesting hunter, but other pairs do/have had personal preferences as well - Madame wouldn't eat pigeon during the nesting season, she'd feed her kids pigeon but she wouldn't eat it, I think because she was here year-round and in the winter had to eat them. When we used to have peregrines nesting in south Winnipeg near St Norbert, the largest part of their diet was seagull off the river. When we had a pair nesting at the Provincial Legislature (only happened once, they couldn't handle the stress from the Radisson) Blue-winged Teal off the river were their primary prey. And both Beau & Jules and Beau & Beatrix have been dining heavily on blackbirds in the last two years. And RCF can attest that sora rails and pied-billed grebes are very popular at McKenzie Seeds - more so than at other sites in the province - they are hunting them off the floodplain and the nearby marshes (i.e., Douglas Marsh)
