EDITOR: No species info was provided by the photographer. The White-chested Emerald and Plain-bellied Emerald are both found in French Guiana, and are hard to differentiate. Because of the solid black bill and solid bronze-green tail and lower back resemble the illustration in 'Birds of Venezuela', I am going with the White-chested, but it could very easily be a Plain-bellied Emerald, especially since they were also photographed here. The tail colors are really not clear.
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Not a 'chlorostilbon' emerald hummingbird, which are smaller and greener, but an 'amazilia', a large genus of medium-sized 'ordinary' hummingbirds. The White-chested Emerald has a black bill, and has a gold rump and tail (compared to [Plain-bellied Emerald]). Male and female similar.