Variation in Black Bib of the Eurasian Siskin (Carduelis spinus) and Its Role as a Reliable Badge of Dominance

Abstract

Rohwer (1975, 1982) proposed that the variation and extent of color patches in the plumage of wintering birds could work as badges of social status. The major advantage of these signals would be that individuals of unequal status competing for limited resources would not need to risk accidental injury or waste energy assessing the relative fighting ability of potential opponents (Rohwer 1982). The status-signalling hypothesis has been tested in several species with variable plumage (reviewed by Whitfield 1987; see also Amat 1986, Watt 1986a, b, Fugle and Rothstein 1987, Moller 1987a, b,1989, Jackson et al. 1988). These studies have, however, produced contradictory results. For plumage differences to act as true badges of dominance, they should correlate within and not just between sex and age classes. This has been demonstrated in only a few species (Great Tit [Parus major], Jarvi and Bakken 1984; Yellow Warbler [Dendroica petechia], Studd and Robertson 1985; House Sparrow [Passer domesticus], Meller 1987a; Whitfield 1987; see however Ritchison 1985, Poysa 1988, Wilson 1992). The Eurasian Siskin (Carduelis spinus) shows great variability in the extent of blackish plumage in the bib (Newton 1972). This chin patch, as in House Sparrows (Moller 1987a), is present only in males, which are dominant over females. Dominance, however, is unaffected by age (Senar 1985). This lack of a relationship between age and dominance, and the absence of the badge in females simplifies analysis of the significance of the chin patch in status signalling. Other features in the Eurasian Siskin plumage also could be used for dominance signalling. For instance, yellow wing stripes have been found to work as a badge of status in its American counterpart, the Pine Siskin (Carduelis pinus; Balph and Balph unpubl. paper); they could have an important role. However, since wings are displayed in only 25% of the agonistic displays used by the Eurasian Siskin (Senar 1990), the chin is always visible, and most of the encounters between unfamiliar birds in wild flocks are between males (i.e. the badged individuals; Senar et al. 1990b), we restricted this analysis to the black bib. We describe variation in the size of the chin patch in male Eurasian Siskins and test (both in captivity and in the field) whether this variation is related to dominance status. Methods.-Plumage variability and dominance relations in the field were studied in a suburban area of Barcelona (NE Spain) during the 1990-1991 winter. Birds were trapped at baited feeders using platform traps and clap nets, and were marked with numbered aluminium bands. Trapping was carried out at least twice weekly, but the food was available continually so that Eurasian Siskins used the area heavily. For each male siskin we recorded, whenever possible, age and the maximum length and breadth of the badge. Badge size was measured by tilting the bird's head back in line with the body. Following the procedure of Moller (1987a), we determined the relationship between badge area, length (L, mm) and breadth (B, mm) from 12 museum skins. Thus, badge area (A, mm) was calculated as: