The mass found under a hawk's perch, rounded on one end and grey, is likely to be what?

The mass found under a hawk's perch, rounded on one end and grey, is likely to be referred to as the so-called mute. This term specifically describes the droppings of birds of prey, particularly raptors, and their appearance can typically include the characteristics mentioned: rounded and often greyish in color.

The term "mute" emphasizes the fact that this is a normal biological byproduct of a bird's diet and digestive process, distinct from other waste forms. While it is important to note that raptors do produce pellets that contain the indigestible parts of their prey, these pellets are more often found in a different context—specifically associated with the regurgitation process, which is a separate behavior from what is represented by a mute. The mute reflects the combination of digested material and other waste that is expelled from the body.

The clarity of the description given in the question suggests this specific type of mass is more aligned with the regular droppings of a hawk rather than the pellets or the other waste forms produced by the bird. Thus, the identification of it as the mute aligns well with both its physical characteristics and the behavioral aspects of raptors in their natural activities.