Great Frigatebird

I previously posted on the magnificent frigatebirds (June 16, 2022) we saw in the Panama Canal and in the Galapagos Islands. We also saw some recently at Caye Caulker in Belize. They are truly "magnificent" and as much fun to watch as any bird I've seen. One of the frigatebirds I saw in the Galapagos Islands, on the north shore of Santa Cruz Island at Bachas Beach, was posted on iNaturalist as a great frigatebird and one identifier has confirmed it. The magnificent frigatebird and great frigatebird are difficult to differentiate and the Galapagos Islands are one of the few places in the world where they overlap. 

In differentiating the two: (a) the great is a little smaller than the magnificent; (b) the male great has a greenish sheen (which is a surface gloss that reflects light uniformly) in sunlight and the male magnificent has a violet or purple iridescence (which is a structural phenomenon where the perceived color shifts depending on the viewing angle) in sunlight; (c) the female great has a white chest and neck extending all the way to the chin and the female magnificent has a white chest with a black throat patch that forms a triangle and looks like a white "M" shape from below; and (d) the female great has a reddish-pink or dull eye-ring and the female magnificent has a bluish-gray eye-ring. 

Illustration of a female from Bird of the World. 

Illustration of a juvenile from Birds of the World. 

Illustration of a male from Birds of the World.

Illustration of a dorsal view of a male from Birds of the World. 

In trying to differentiate, in this case, I believe the key is the back. See above and then see the illustration below of a magnificent frigatebird from Birds of the World. The great has a more brown stripe on the back of the wing than the magnificent. 

Range from Birds of the World.