Truth is, yes, birds can eat cat food. Most often the only birds that eat such dry crunchy food, are the larger ones. Black Billed Magpies, American Crow, Common Raven. Those such birds. If you have any further questions on this topic, I would gladly answer them.
Magpie's do not eat anything in runescape. They are a summoning familiar that is used to collect "shiny objects." They have a chance of randomly finding jems and rings throughout the period that they are summoned.