Adult Brown Pelicans are gray-brown birds with yellow heads and white necks.
In breeding plumage, the back and sides of the neck turn a rich, dark reddish-brown.
Immatures have a gray-brown head and neck with a pale whitish belly and breast (4th photo).

First 4 photos: Santa Marta area, Colombia, 24 October 2014
Last photo: Los Flamencos Sanctuary, La Guajira, Colombia, 1 November 2014