The Cayenne Jay is mostly black, blue and white with a short black frontal crest. It has a black forehead, sides of the head, throat and chest are black, with a white moustachial streak. The rear crown and nape are prominently white, as are the lower breast and belly and the tip to the tail. The wings and tail are bluish-purple and the back is brownish-purple.
The Cayenne Jay is the only jay found in most of its range (Ridgely and Tudor 1989); there is some geographic overlap with the Violaceous Jay (Cyanocorax violaceus), but that species is mostly blue, with no white in the plumage. The Cayenne Jay is superficially similar to the White-tailed Jay (Cyanocorax mystacalis) of the Pacific coast of Ecuador and Peru, but the White-tailed Jay is much bluer above and has white outer rectrices (in addition to white tips to the tail feathers). The White-naped Jay (Cyanocorax cyanopogon), of northeastern South America south of the Amazon, also is similar. The White-naped Jay has a more extensive black crown and a smaller white nape spot, a blue (not white) moustachial streak, and less white in the tail.
The following description is based in Haverschmidt (1968) and Madge and Burn (1994):
Adult: Forecrown (including short frontal crest), sides of head, throat, and upper breast black. Small bluish white or whitish spot above and below eye; malar stripe whitish. Crown, rear crown and nape white; the white of the nape diffuses into the violet-brown or purplish-brown back and scapulars. Wings and tail bluish purple, tail slightly graduated and broadly tipped white. Lower breast, belly, and thighs creamy white.
Juvenile: Plumage duller. Spots above and below the eye smaller or lacking; if present, are bluer than in the adult, as is the malar stripe. White band at tip of tail is suffused with violet-fawn, especially on the central rectrices.
Data from Haverschmidt (1968):
Total length: 33 cm (Ridgely and Tudor 1989, Madge and Burn 1994).
Wing length, males mean 159.5 mm (range 152-168 mm, n = 6; Haverschmidt and Mees 1994)
Wing length, females mean 159.8 mm (range 147-167 mm, n = 4; Haverschmidt and Mees 1994)
Tail length: 140 mm (sample size?; Madge and Burn 1994)
Bill length: 36 mm (sample size?; Madge and Burn 1994)
Tarsus length: 46 mm (sample size?; Madge and Burn 1994)
Males: mean 174.8 g (range 165-196 g, n = 5; Haverschmidt and Mees 1994)
Females: mean 174.6 g (range 147-192 g, n = 8; Haverschmidt and Mees 1994)
Combined sample (properly, a sample of birds with sex undetermined): mean 190.6 g (SD ± 40 g, n = 5; Bosque and Molina 2002)