Why is mackerel kosher, and not birds of prey? Both eat treif?
The Torah’s classifications of animals as kosher or not kosher does not depend on the foods eaten by the animals. If we were to feed an animal non-kosher food, the animal would remain kosher, and the ingested food would not render the animal prohibitied for consumption. As a general rule, it is true that birds of prey are not kosher. Commentaries explain that this is because of the negative disposition of the bird, and not because of the kashrus or otherwise of the food it its.