Historical examination of plumage maturation in the shorebirds (Aves: Charadriiformes), with special reference to the gulls (Charadriiformes: Larinae).

Abstract

Delayed plumage maturation refers to the delayed acquisition of a "mature" or adultlike plumage. It is usually considered to be the result of selection for distinctive appearance in post-juvenile young birds. In the present study I examined the evolution of delayed plumage maturation in the shorebirds (Aves: Charadriiformes). Nine plumage maturation characters were identified and their states optimized on topologies generated during two recent investigations of shorebird relationships. Optimization indicated that delayed plumage maturation in shorebirds can be plausibly explained without invoking selection for distinctive appearance in post-juvenile subadults. I was interested in a more detailed examination of delayed plumage maturation in one assemblage of shorebird taxa, the gulls (Charadriiformes: Larinae). Accordingly, I used 169 skeletal and plumage characters to test gull monophyly and estimate gull phylogeny. Character data were analyzed cladistically using the computer program PAUP 3.0s; states within the 44-taxon ingroup (33 of them gulls, and all of them from the taxon Lari) were polarized using 19 outgroups chosen from among the plover-like shorebirds. Fifteen shortest trees were found (l = 2354, CI = 0.180). Monophyly of the Larinae was supported unambiguously. Other major conclusions included the following: (1) plumage characters are less homoplasious than skeletal characters; (2) hoodedness is primitive in gulls, and is thus a poor indicator of relationship; and (3) the genus Larus is paraphyletic. To investigate delayed maturation of plumage in the Larinae I used a three-step procedure. First, plumage characters were optimized on each of the fifteen shortest trees generated above; second, the optimized characters were used to characterize ontogenetic trajectories at each interior node; and third, the characterized ontogenies were compared on each tree. Modifications to the rate or timing of plumage ontogeny were found in all major gull lineages. Some involved all regions of plumage, while others involved the primaries or tail alone; however, only modifications to the primaries and tail generated paedomorphic or peramorphic plumage features. The functional significance of modifications to plumage development remains elusive.