Patterns of Morphological Evolution in Bird Genera of New World and Old World Peatlands

Abstract

Morphological convergence is defined in evolutionary ecology as evolution toward similar dimensions of traits in unrelated animals that live in the same or similar environments. Because the genus is a taxonomic unit in which differences among species represent directional changes in morphology from a common ancestor, I look for evidence of morphological convergence by comparing radiations in sets of congeneric species of birds that occur in peatland habitats of northern Minnesota and eastern Finland. Through discriminant function analysis, I determined that habitats selected by species in peatland shrub areas of Minnesota were similar to habitats selected by species in peatland shrub areas Finland. This pattern held for the species in peatland forests, also, but not for those found in open peatlands. Open peatlands in Finland had more water pools and less vegetation than those in Minnesota. The morphological analysis used 15 characters: size and 14 shape variables (skeletal measurements adjusted for size) pertaining to three functional complexes (bill, flight apparatus, and hind limb). Special emphasis was placed on four genera in Minnesota, Zonotrichia, Spizella, Geothlypis, Dendroica, and one genus in Finland, Emberiza, because these genera have species that occur both in shrub and coniferous forest peatland habitats. In all five genera, species in shrub vegetation are consistently smaller, have relatively longer pelves and legs, and have wider and shallower sterna than their respective congeners forests. Furthermore, with the exception of Emberiza and Spizella, the bill, sternum, and humerus are relatively longer in the shrub—dwelling peatland species than in their respective congeners in coniferous forest. The most abundant species in the shrub vegetation in Minnesota (Cistothorus platensis) and in Finland (Acrocephalus schoenobaenus) are among the most extreme in this respect; they are relatively small, with relatively wide and shallow sterna, relatively long pelves and legs. Also, warblers breeding in shrub peatlands have relatively deeper and wider bills, but sparrows have relatively shallower and narrower bills than their respective congeners in coniferous forests. These morphological relationships indicated significant (P = .03, sign test) directional patterns of differentiation among distantly related genera (warbleres and sparrows) for morphology associated with movement through the habitat (e.g., hind limb structure, size, and sternal shape). Morphological differences associated with feeding, however, showed trends at the family level that probably reflect differences in diet and behavior between sparrows and warblers. The consistency of these patterns among the genera studied emphasizes (1) the sensitivity of species adaptations in morphology to habitat structure, and (2) the likelihood of a common underlying mechanisms for these results.