Adults of the nominate subspecies have a brown head and neck, dark upperparts, bright chestnut belly, pale blue facial skin, and red eyes.

C. g. sanctaemarthae's upper breast, throat, and cheeks are redder than the nominate's and its vent area a darker red.

C. g. rufiventris has gray edges on its neck feathers that give a scaly appearance; it is also a paler olive above and less chestnut on the underparts than tschudii.

[5] The sickle-winged guan's breeding season in the Colombian Andes spans from January to June and may extend beyond that in other areas.

The nest is a platform of thin branches, moss, and green and dead vegetation, usually placed in a tree fork or on a bromeliad.

The fagini subspecies of sickle-winged guan gives an " extremely thin, high-pitched...whistle" and a "keeeeeee-uk!"