Plumage polymorphism in Terpsiphone and the rule of island drabness

Abstract

Plompen, W. 2000. Plumage polymorphism in Terpsiphone and the rule of island drabness. Ostrich 71 (1 & 2): 316. Many related bird species differ through their conspicuous plumage coloration characteristics. On islands, birds often have a drabber plumage coloration than their mainland counterpart. This rule is regarded as the main evidence for the species recognition hypothesis, because on islands mostly only one species per genus is present. In the absence of congeners there may be no benefit from a species specific badge, with drabness as a result. However drabness as such is not predicted by this species recognition hypothesis. In fact the only selective pressure caused by a species recognition mechanism, works in sympatry. Therefore on an island with no related species present, the elaborate coloration that originated in the mainland situation, is allowed to evolve in any sense, or may even not evolve at all. Taking this all together, if under the. species recognition hypothesis island taxa may evolve at random with respect to colouration, and island drabness is a well known phenomenon, then an additional mechanism inherent to the island situation is needed, that causes the degradation of any bright plumage colouration. As a consequence, island drabness in itself does not provide any information at all on the original mechanism that caused plumage elaboration on the mainland.