Seasonal Changes in the Color of Starling Bills

Abstract

EARLY in the breeding season the bills of both sexes of the Starling (Sturnus vulgaris) become yellow, and upon completion of the breeding season the bills become black. Although other studies (Hicks, 1934; Nichols, 1945) have described the changes in color of the bill, no extensive study on the rate of change is available. The purposes of this study were to determine quantitatively the rate at which the yellow color progresses prior to the breeding season and to determine whether differences, according to age and sex, occur in the rate of color change. I estimated the length of time required for complete color change, and compared the times of the year when the color change occurs in two areas (Baltimore, Maryland, and State College, Pennsylvania). Some information about the time and rate of change from yellow back to black was also obtained. In addition, the rate of growth of the bill was measured in captive birds to compare the rate of color change with the growth. Bill color and color-control mechanism.-Witschi (1961) reviewed our present knowledge of bill color. In general, two types of pigments, melanins and carotenoids, are found in bills. The melanins, produced in the usual rhizoid-shaped melanophores, become injected into epidermal cells that are moving out of the proliferating layer and then become compressed into thin flakes that remain embedded in the cornified cells composing the sheath of the bill. The various yellow, orange, and red carotenoids are not synthesized by the bird but are absorbed along with other foodstuffs. The mechanism by which the carotenoids are selectively acquired by certain structures, such as the bill, is not known. Extractions of carotenoids in the Starling show that they are present the year around, although yields are highest during the breeding season. By castration and hormone injections, it has been shown that the yellow bill color is a specific indicator of the presence of androgens (Witschi and Miller, 1938). The amount of androgen necessary to cause the bill to become yellow in the Starling is not known. However, Hilton (1958) reports that, in male Starlings, enough androgen (testosterone) has been secreted by the time the testes weigh only 30 mg to change the color of the bill completely. Not all Starlings have yellow bills during the breeding season. For example, Hicks (1934) reported that two to four per cent of the Starling population that he studied in Ohio in the breeding season did not have yellow bills. Hicks claimed that these birds were primarily non-breeding birds, diseased or immature, and usually females. 1 Authorized for publication on 14 May 1963 as paper No. 2777 in the journal series of the Pennsylvania Agricultural Experiment Station.