This depends entirely upon the species.
Some are carnivores, such as eagles, hawks, vultures and fish-eating birds such as kingfishers and kookaburras.
Some are purely herbivores, such as parrots, lorikeets and finches.
Many more are omnivores: Emus and crows, for instance, eat smaller animals as well as grains (bread) and vegetation; some of the smaller, nectar-eating birds also eat insects; water birds and waders eat both water animals and plant matter. All of these are examples of birds being omnivorous.