HISTORICAL EXAMINATION OF DELAYED PLUMAGE MATURATION IN THE SHOREBIRDS (AVES: CHARADRIIFORMES)

Delayed plumage maturation refers to the presence of nonadultlike immature plumages (juvenal plumage excluded). It is usually considered the result of selection for distinctive first-winter or first-summer appearance. In the present study, evolution of delayed plumage maturation is examined in the shorebirds: the sandpipers, plovers, gulls, and their allies. Nine plumage-maturation characters were identified, and their states were superimposed onto topologies generated during two recent investigations of shorebird relationships (Sibley and Ahlquist; revised Strauch). The characters were then optimized so as to assign character states to interior nodes of the trees in the most parsimonious way. Reconstructions of character evolution on six of the shortest revised Strauch trees were ambiguous with respect to delayed plumage maturation in the hypothetical ancestral shorebird. If plumage maturation was not delayed in the shorebird ancestor, optimization indicated that delay appeared when nonadultlike juvenal feathers were acquired. In contrast, on the single Sibley and Ahlquist tree, absence of delayed plumage maturation in the shorebird ancestor was indicated unambiguously, with three evolutionary novelties (nonadultlike juvenal feathers, seasonal plumage change, and a reduced first-spring molt) implicated in its acquisition. Optimization indicated that delayed plumage maturation in shorebirds can be explained plausibly without invoking selection for distinctive first-winter or first-summer appearance. Two of the novel conditions generating delayed plumage maturation (modified juvenal feathers and seasonal plumage change) did so only because they were acquired in a taxon possessing restricted first-year molts, which are primitive. Given these observations, it seems simplest to explain the delay in plumage maturation as an incidental consequence of the phylogenetic inertia of shorebird molts. The third novelty that generates delayed plumage maturation, a reduced first-spring molt, may have been acquired to reduce molt-associated energetic demands in young birds.