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Red-tailed hawks are generally preying on mammals, from rabbits and hares down to voles and mice. They are also opportunistic predators that hunting whatever prey is abundant and available to them.
Some red-tailed hawks also prey regularly on birds. This is a little surprising considering how these hawks hunt. They are not incredibly fast in level flight and so don't general chase birds down in the air. They do hunt mostly by ambush, either from a perch or from high up in the air while soaring (or occasionally hovering).
Red-tailed hawks that more regularly hunt birds are also treated differently by other birds. Although birds will alarm to some extent at every red-tailed hawk that is hunting, they will alarm much more dramatically at red-tails that hunt and kill birds. For example, you might witness some being mobbed very dramatically by crows, woodpeckers, jays and others. While a few miles down the road another red-tailed hawk might be half-heartedly pestered by the same crows later that same day.
It is likely that red-tailed hawks earn a reputation as a bird-killer or not, as other birds witness its daily hunts and successes or failures. This certainly seems to be implied in the bird behavior around various hawks. I have witnessed this in other bird-eating predators, especially house cats.
This particular hawk was witnessed taking and feeding on a pin-tailed duck along the edge of a wetland area in Washington state.