Simpy put, the eyebrows of birds are called Supercilium. These eyebrows can be subtle in color and form, or bold, bright, and dazzling.Superciliums can help identify birds by their colour, shape and other characteristics.
The White-browed Bulbul is an endemic bird of the South India. Largely olive coloured, its pale supercilium is its distinguishing characteristic.
Wikipedia definition - The supercilium is a plumage feature found on the heads of some bird species. It is a stripe which runs from the base of the bird's beak above its eye, finishing somewhere towards the rear of the bird's head. Also known as an "eyebrow",it is distinct from the eyestripe, which is a line that runs across the lores, and continues behind the eye.