Correlation of Variation in the Wrentit with Environmental Gradients

Abstract

Correlations between environment and animal size or color have long aroused the interest of biologists and have stimulated the formulations of ecogeographical rules and the evaluation of the validity of these principles in many different animals. The questions that arose concerning the mechanisms governing correspondence between environment and the animal led to early, unsuccessful experiments to establish direct effects of humidity, temperature, and color of surroundings (Beebe, 1907 ; Ogle, 1934; Sumner, 1934; and many others) on captive animals raised under artificially controlled conditions. The geographical principle concerned with high levels of melanism found in vertebrates inhabiting humid regions, namely Gloger' s rule, has been described in general terms many times. However, there is a paucity of evidence causally relating humidity and other environmental factors with color expression. It is even stated in a recent paper (Hamilton, 1958:336) that the adaptive significance of increase in pigment deposition with increases in environmental humidity is unknown. The work of Sumner (1932) on Peromyscus maniculatus and Peromyscus polionotus indicates clearly that differences of pelage color in these mammals are under genetic control. Similar genetic analyses of wild species and races of birds have not been made in so far as I know, but there seems to be no reason to suppose that similar results would not be obtained in birds, and the genetic control of variation of a species such as the Wrentit (Ckamaea fasciata) is therefore assumed. The best evidence for adaptive value of presumably genetic factors of coloration is indirect in birds and comes from field investigations of the larks in which it has been found by Meinertzhagen (1951) and Bates (1936)) especially for Ammomanes deserti of Asia, that the plumage coloration of various races of this species agree very closely with soil colors on which the races exist when the environments otherwise are similar. Thus dark birds are found on dark soils and light birds on light sandy soils. Behle (1942) has described the same close association between plumage colors and soil colors in the Horned Larks (Eremopkila alpestris) of western North America. Both these birds are prey species for several predatory birds, and since they are open-ground dwellers, the need for concealing coloration, in this case correspondence with local soil color, is evident. In the races of the Wrentit in northwestern California, there is variation of plumage color that presents a rather consistent clinal pattern. The darkest birds, ascribable to the race Ckamaea fasciata rufula, exist in the coastal humid belt from San Francisco Bay northward. The lightest birds of the race C. f. kenskati occur from the southern tip of the Vaca Mountains in Solano County northward and northwestward. In between the two, the race C. f. intermedia is found (see Grinnell and Miller, 1944). This race has been thus named because the coloration of the breast and belly regions is somewhere between that of the other two. The aim of the present work is not to review the rather artificial racial entities but to consider precise correlations in a series of populations which grade sharply in color from the coast interiorward. After collecting 267 specimens in fresh fall plumage, I noticed that there were also variations in several dimensions of the Wrentits. In addition to a study of the color variation, then, an analysis of some of the body parts was undertaken. The geographic area of California under consideration, as shown in figure 1, consists of all of Marin, Sonoma, and Napa counties and the adjoining parts of Solano, Yolo, and Lake counties. Wrentits are inhabitants of chaparral and scrubby vegetation border