Question
Why is it forbidden on Shabbat to feed animals (or birds, or fish) that don’t depend on you for their food?
Answer
This is forbidden because it is regarded as “unnecessary exertion” (Tircha Yeseira). We forbid such things in order to encourage the proper atmosphere and spirit on Shabbos. Since the animal is quite capable of foraging for food for itself, feeding it is considered unnecessary. Handling Muktzah and unnecessary exercise are also forbidden for this reason.