Why Are Waxwings "Waxy"? Delayed Plumage Maturation in the Cedar Waxwing

Cedar Waxwings (Bombycilla cedrorum) often have their secondaries tipped with red waxlike appendages, but a plausible hypothesis for the function of these tips has not been presented. Both males and females may have such tips, but second-year birds normally have none or only a few. This appears to be the first described instance of a passerine species to show distinctive delayed plumage maturation in both sexes. We found that waxwings mated assortatively with respect to the number of tips on the secondaries of the birds. Pairs of older birds (with high tip index scores) tended to nest earlier than younger (low tip index) birds. Older birds also had larger clutch/brood sizes and fledged more young. We suggest that the waxlike tips function as signals of age and status and that some combination of mate choice and competition for mates results in the observed assortative mating. Of the hypotheses proposed to explain the adaptive significance of delayed plumage maturation, two (the female-mimicry and cryptic hypotheses) are not confirmed in the case of the Cedar Waxwing, while the status-signaling hypothesis appears to offer the best explanation for the occurrence of delayed plumage maturation in this species.