Nightjars have an enormous gape. These birds are capable of opening their mouths both vertically and horizontally due to the presence of a specialised spreading mechanism in the lower jaw. The above image shows the gape of the Large-tailed Nightjar (Caprimulgus macrurus).
The gape in most species is surrounded by very long and tactile rictal bristles (left). These bristles are strong, barbless contour feathers that are connected to special muscles in the skin. It has been suggested that they help funnel insects into the large gape when the nightjar is feeding on the wing. Rictal bristles also enable the bird to detect insect movements held in the bill, just like the whiskers in some mammals.
The gape and bristles are obviously nocturnal feeding adaptations.
Images by G Sreedharan (gape) and Dr Jonathan Cheah Weng Kwong (head).
This post is a cooperative effort between NaturePixels.org and BESG to bring the study of bird behaviour through photography to a wider audience.