(1) It is suggested that among the selection pressures contributing to the assumption of a neutral or dull plumage in the off-season (seasonal dimorphism) by males of some migratory North American species a strong component is that for prevention or reduction of intraspecific, inter-individual hostility. This is considered as an extension of Moynihan's views (1960) on the factors promoting gregariousness in birds: that both morphological and behavioral characters are available to natural selection in controlling hostility. (2) Ten functions which have been attributed to species-specific plumage characters of birds are listed, and the yet unresolved problem of their respective validities is acknowledged. (3) Speculations are made on the relation of the "proximate" to the "ultimate" causes of seasonal dimorphism and/or sexual dimorphism in some migratory, North American species of the higher latitudes having sedentary counterparts or relatives in the lower latitudes. An empirical observation, the value of which as a generalization is still uncertain, notes certain biological characters associated with tropical-to-temperate, sedentary-to-migratory trends: (a) in tropical latitudes (stable environments), an association between reduced seasonal or sexual dimorphism and relatively long periods for maintenance of pair-bond; and (b) in temperate latitudes (unstable environments), an association between increased seasonal and/or sexual dimorphism and relatively reduced periods for maintenance of pairbonds. From this observation, attention is directed to the possibility of detecting comparable, even if less pronounced, trends for comparisons between related members of groups adapted to various stable or unstable environments within the tropical latitudes of either the New World or the Old World. On a qualitative basis, such comparisons are made within and between the Ploceus complex of Africa and the Parulidae complex of the New World.