Monochromatism can be characterized as the opposite of dichromatism in species; i.e. males and females appear identical as opposed to expressing different colors. Different selective pressures, especially sexual selection, result in dichromatism in most species. Theoretical studies suggest that monochromatism results from species in which both parents take equal turns caring for young, or in species where one parent cares for the young in a safe cavity.
Black-legged Kittiwakes and Atlantic Puffins have both been found to be monochromatic
Doutrelant, C., A. Grégoire, D. Gomez, V. Staszewski, E. Arnoux. T. Tveraa, B. Faivre and T. Boulinier. 2013. Colouration in Atlantic puffins and blacklegged kittiwakes: monochromatism and links to body condition in both sexes. Journal of Avian Biology. 44:01-10.