The Avicultural Society

South American tropics (32%) > South American nontropics (27%) = North America (23%) > Europe (10%)

Birds of the South American lowland tropics are therefore more frequently colourful than those of North America or the South American nontropics; a consideration of degree of colourfulness or combination of colour would have emphasised the difference. European birds are relatively dull in colour. Colourfulness is not particularly associated with a tendency towards dichromatism, for North American and South American tropical birds have a higher proportion of sexually dichromatic forms than the others; Europe and the South American nontropical avifaunas tend toward sexual monomorphism.

In almost all cases the incidence of sexual dichromatism and colourfulness in open habitats is similar to their incidence in the entire regional avifauna. An exception is found in the South American non tropics, when colourfulness in open habitats is significantly (χ², P < 0.05) less frequent than in the whole avifauna. Given that a species lives in an open habitat then, it usually has the same probability of being colourful as any species taken from that avifauna.

Although in all four regions the percentage of colourful and sexually dichromatic species that live in the "open" habitats is the markedly less than the percent that live in wooded habitats, the relative number of species living in the open is also less than that of wooded areas. Neither kind of habitat, then, is relatively more conducive to the development of dichromatism or colourfulness.

Certain families clearly contain relatively more dichromatic or colourful species than other families. In addition, for each region, certain families are the largest contributors of species to the dichromatic and colourful portions of the avifauna. However, seldom is an entire family dichromatic or colourful, which argues to some extent against genetic fixation of such a trait within a phylogenetic lineage, in most cases.

Seasonal sexual dichromatism in Nectariniidae, Parulidae, Icteridae (Hamilton 1961, Hamilton & Barth 1962, Skutch 1957), Central American Fringillidae and Thraupidae (Skutch 1940), and in African Ploceidae (Moreau 1960) was associated with duration and nature of the pair bonds, social behaviour in the non breeding season, degree of migratoriness, and, for some, habitat. For comparisons of regional avifaunas, however, the explanations do not seem to be the general ones. For instance, dichromatism (seasonal and permanent) is least developed in the European and South American nontropical avifaunas, but we expect that long distance migration or wandering is probably more common there than in the South American lowland tropics.

Perhaps a general explanation is unavailable. Hamilton and Barth (op. cit.) list several kinds of selective factors that influence the evolution of species-specific plumage patterns. These, and perhaps others, probably vary in their importance amongst species. Colourfulness is equally difficult to assess, especially in view of our ignorance of the possible cryptic nature of the bright plumages amidst brightly coloured flowers, fruits and leaves, and of our lack of information concerning relative predation rates.

Summary

Tropical birds of South America are more frequently colourful than those of North America or nontropical areas of South America; European birds are the least colourful of all. Assessment of degree of colourfulness or colour combinations would likely increase most of these differences. Sexual dichromatism is most common amongst members of the North American and South American tropical avifauna, and least in Europe and the South American nontropics. Habitat association of these characters are not generally evident, and taxonomic associations are not very helpful. At this time we seem to lack a general explanation for the evolution of the observed tendencies.

Literature Cited

• HAMILTON, T.H. 1961 On the functions and causes of sexual dimorphism in breeding plumage character of North American species of warblers and orioles. Am. Nat. 95 : 121-123
• HAMILTON, T.H. and BARTH, R.H. 1962 The biological significance of season change in male plumage appearance in some New World migratory bird species. Am. Nat. 96 : 129-144
• MEYER DE SCHAUENSEE, R. 1970 A Guide to the Birds of South America. Livingston Publ. Co., Wynnewood, Pa.
• MOREAU, R.E. 1960 Conspectus and classification of the Ploceine weaverbirds. Ibis 102 : 298-321
• PETERSON, R.T., MOUNTFORT, G. and HOLLOM, P.A.D. 1966 A Field Guide to the Birds of Britain and Europe. H.M. Co., Riverside Press, Cambridge, Mass.
• ROBBINS, C.S., BRUUN, B., and ZIM, H.S. 1966 A Guide to the Field Identification. Birds of North America. Golden Press, N.Y.
• SKUTCH, A.F. 1940 Some aspects of Central American birdlife. Sci. Monthly, 51 : 409-418; 500-511
• SKUTCH, A.F. 1957 The resident wood warblers of Central America. In Griscom, L., Sprunt, A., et al. The Warblers of America. Devin-Adair, N.Y. : 275-285

APPENDIX 1

Numbers of sexually dichromatic and colourful species in each family. Winter visitors from North America are included in the South American tallies because many of these live for several months in lower latitudes. Accidental and exotic species and occasional visitors are omitted. The systematic classification of each source book are retained, and brackets used to indicate correspondence between regions.