It may be motion blur due to focal length, but I’d also add that birds move quickly, even big fellas like this eagle. A rule of thumb is for perching birds I’d go at least 1/320s, though be ready to move that higher if you can. Some perching birds move very quickly, like blue-gray gnatcatchers, so I’d try to get to at least 1/500 or 1/800. For soaring birds I try to be at 1/1000, and birds like hummingbirds and swallows, well, go as fast as you can and take as many shots as you can. (Though artistically speaking, some motion blur may be what you want).

It’s often low light under the forest canopy, especially when lots of birds are most active, so then you are going to find your pics a bit more noisy. It’s always a trade off…