During April the day length triggers a hormonal response in all birds, including grouse. The males get highly terriorial and aggressive, and are probably attracted by the motion of the 4-wheeler, and possible the low frequency motor noise as well (similar to low frequency drumming makes?).
Out West here, blue grouse can become very aggressive, slapping at boots which they might mistake for something about the same size and shape as a grouse, particularly if you also "hoot" by blowing in the top of a pop bottle, which is similar to their call. Expect both species to fight with their reflection in a window if they come across that as well.
Spring has sprung!
Ben
