Annual Testicular Cycle and Bill Color Change in the Eastern American Goldfinch

Abstract

THE avian testicular cycle of seasonally-breeding birds consists of three phases: a regeneration phase when the tissues of the regressed gonad are reorganized, an acceleration phase characterized by development and growth of the testis, and a culmination phase in which the birds are responsive to certain environmental end stimuli that result in their nest building and gamete deposition (Marshall, 1961). Among wild populations of north-temperate passerines such a cycle has been demonstrated by histological examination of the testis in only a few early nesting species (e.g. Blanchard, 1941; Bullough, 1942; Blanchard and Erickson, 1949; Threadgold, 1956a, 1956b; Marshall and Coombs, 1957). Comparative study of the cycles of more species, particularly late nesters, is desirable. An analysis of seasonal changes in external characters that reflect seasonal changes in the gonad may provide an indirect method of determining the testis cycle in other species quickly. Bill color and postnuptial molt are well-suited for such an analysis as, in certain species, they are readily measurable from museum skins. Both characters have been correlated with changes in the titer of one or more of the gonadal hormones (Witschi and Fugo, 1940; Kobayashi, 1954a, 1954b, 1954c, 1958; Witschi, 1961; Engels, 1962), and Marshall (1961) associates the onset of the postnuptial molt with early regeneration phase and the termination of postnuptial molt with the onset of acceleration phase. The Eastern American Goldfinch, Spinus t. tristis, exhibits both a seasonal change in bill color and a late breeding season, and is principally single-brooded. Throughout the northern part of its range this goldfinch generally does not nest until July and later (Walkinshaw, 1938; Stokes, 1950; Nickell, 1951; Berger, 1968), and breeding behavior occurs seasonally later in this species than in other passerines nesting in northeastern North America. Field experiments conducted in Ithaca, New York revealed that not until late June or early July do female goldfinches begin gathering cotton artificially supplied them in their nesting habitat since early June, and they do not begin collecting thistle papus, a major natural nesting material, until July or, more rarely, in the last week of June (Mundinger, 1968). This correlates temporally with the onset of nest construction. In a study field in Ithaca only about 6 percent (2 of 35 nests) of all nests were initiated as early as the period mid-June to 1 July in 1963-65. Likewise the males do not engage in the defense of a geographically fixed area with flight song display and other territorial behaviors until nest construction begins or, in some cases, only a few days