Who actually chose the date? I went looking, because it isn't Francis in the thirteenth century arranging anything: it's Heinrich Zimmermann, a German editor, who held the first animal day in Berlin in March 1925, and only a few years later shifted it to 4 October. By 1929 it had settled on Francis's feast, and that's where it stayed.
Which turns out to suit a great many people at once — churches that bless dogs and cats and the occasional patient horse on the steps, shelters, vets, groups with no church in them at all. As for the sermon to the birds, the sources disagree: Celano and Bonaventure and the later Fioretti don't tell it quite the same way, and the place shifts about too. The painted copy of the fresco has him bending towards them with open hands, one bird still in the air, a second friar hanging back.
Miriam, I've stood at the back of two of those blessings. Mostly dogs, a lot of them barking at each other, one cat in a carrier with firm opinions about the whole business, and a tortoise in a shoebox lined with newspaper. The horse behaved best.
In the Upper Church at Assisi the birds are banked low on the left, a few on the ground in a dark cluster and one or two still up by the tree, so your eye runs down Francis's bent back and lands on them. The companion stands straight and does nothing, which I've always liked.