Have you tried to photograph a tree swallow in flight? They feed on small, aerial insects that they catch in their mouths during acrobatic flight. And that kind of flight makes getting good shots of them difficult. I was pleased to capture this little bullet as it flew about feeding. I am not sure of the top speed a tree swallow can attain, but it isn't so much speed that makes it difficult to capture them in flight as it is their acrobatic maneuvering.