Here is a link to an article in the Dec. 05 issue of Transmission and Distribution zine. http://tdworld.com/mag/power_birds_power_lines/
They are both perches and guards. As a perch it keeps the bird further away from the energized equipment, and as a guard, at least in the pic above, on the lower arm, it seems to discourage a bird from landing on the lower arm.
From the zine article mentioned above
[This message has been edited by rmiell (edited 02-08-2006).]