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Polioptila plumbea
It is a small active insectivorous songbird, which is a resident species throughout a large part of the Neotropics. Males of the nominate group have a contrasting black cap that reaches just below the eye, while this is lacking in females, which instead have a grey cap (same colour as back).
They are found in a wide range of wooded habitats, ranging from the arid woodland and scrub to humid forest.
So these two birds perching together, but I'm not sure if they are the same species. // For what I read, I think they are male and female.