The absence of sexually dimorphic characteristics in organisms. Males and females of a species that exhibit monomorphism do not vary in outward appearance. Animals that show monomorphism must use alternate means to attract a mate. Vocalizations are one way for males to woo females in the absence of bright, showy feathers or larger size.
Examples of Monomorphic Organisms
Penguins (Family Spheniscidae). Males are similar in plumage and size to females and can only be distinguished through behavioral cues (eoearth.org)
House Wren (Troglodytes aedon). No discernible difference between males and females (birdvancouver.com)