87:18 On Shabbos, one may feed animals and birds that are raised at his home and that depend on him for their food. However, one may not exert the effort to feed animals that are not raised at his home and that don't depend on him for their food. One may not even throw food on the ground in front of them. Therefore, one may not feeds doves because they can go and eat from the fields. One may put food out for dogs - even for stray dogs. This is permitted because we see that G-d has mercy on them from the fact that they have little food and He made their digestive process slow. (One is not permitted, however, to feed a vicious dog - Mishnah Brurah 324:31.) There is a custom to feed the birds grain on Shabbos Shira but this custom is not appropriate since we are not responsible for their food.
87:19 One may invite a non-Jew to his home for a meal on Shabbos. This is not the case on yom tov because of the concern that one may end up cooking extra on the holiday for his non-Jewish guest but this is not a concern on Shabbos (since we're not permitted to cook on Shabbos in any event). One may even serve him alone even though he doesn't depend on the Jew for his food. Since we have an obligation to support the needy - Jewish and non-Jewish alike - it's as if one actually is responsible for his food.