I noted elsewhere that it's
forbidden to feed an animal on שבת, unless that animal is dependent for food upon the one feeding it. For this reason, מג״א writes that the custom of feeding stray birds on שבת שירה is improper. עה״ש justifies the custom, writing that we do not feed the birds for their own sake, but, rather, for ours, as we wish to remember the joy of the crossing of the sea.... Indeed, the entire reason we may not feed stray animals on שבת is that it is too much labor (טירחא) for us; if we do it for our own sake, not the animals', or, especially, if we do it in fulfillment of a custom, then there is no problem. So goes the argument. Nonetheless, שו״ע הרב, קש״ע, and מ״ב — a formidable list of latter-day פוסקים — all write, as מג״א does, that the practice is forbidden, and שש״כ accepts this as הלכה. However, שש״כ quotes אשל אברהם as writing that since the reason for the prohibition is merely טירחא, it is permissible to allow one's children to feed animals, and it is appropriate to do so on שבת שירה. Further, if one is going to shake off his tablecloth anyway, then there is no greater טירחא in doing so out of doors (where there is an עֵרוב), in the direction of birds. Finally, אשל אברהם also writes the obvious היתר that one can feed pet birds, if he has any — birds that are dependent on him for sustenance.