Maybe they stuff the pigeon with the mice?
Seriously I guess vegetarian geese weren't fed the usual stuff. You know how cows are supposed to be vegetarians too, right? In the past they were sometims fed grain with offal mixed in, in fact becoming somewhat cannibalistic.
I laugh when I see adverts for "Grain-fed western beef". Cattle aren't supposed to eat grain in the first place, they can't digest it, which is why we have to shoot them full of drugs and hormones.
Cattle should eat grass.
And geese should eat...whatever it is they eat.