Evolution. 48(21. 1994. pp. 327-350 HISTORICAL EXAMINATION OF DELAYED PLUMAGE MATURAnON IN THE SHOREBIRDS (AVES: CHARADRIIFORMES) PH1uPc. CHu Museum ofZoology and Department ofBiology, University ofMichigan, Ann Arbor. Michigan 48109 Abstract.- Delayed plumage maturation refers to the presence ofnon adultlike 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 ofdelayed plumage maturation is examined in the shorebirds: the sandpipers, plovers, gulls, and their allies. Nine plumage­ maturation characters were identified, and theirstates were superimposed onto topologies generated during two recent investigations ofshorebird 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. Reconstructionsofcharacterevolution on six ofthe shortest revisedStrauchtrees 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 ofdelayed plumage maturation in the shorebird ancestor was indicated unambigu­ ously, with three evolutionary novelties (nonadultlike juvenal feathers, seasonal plumage change, and a reduced first-spring molt) implicated in its acquisition. Optimizationindicated thatdelayed plumage maturation in shorebirdscan be explained plausibly without invoking selection for distinctive first-winter orfirst-summer appearance. Two ofthe novel conditionsgeneratingdelayed plumage maturation (modifiedjuvenal 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 ofthe phylogenetic inertia ofshorebird 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. Key words.-Character evolution, delayed maturation, molt, plumage, shorebirds. Received June 10, 1991. Accepted November 10,1992. In their first year, birds typically pass through juvenal plumage is perceived as a specialization four plumages, a downy one followed by three for fledglings; even when it looks different from that include contour feathers. The latter are, in any adult plumage (e.g., fig. 1), it is not consid­ order of appearance, juvenal, first-winter, and ered part of the delayed-plumage-maturation first-summer plumages. In subsequent years, each phenomenon. individual has only two plumages per year, a The immature plumages associated with de­ winter one and a summer one. Both of these layed plumage maturation may be evident for include contour feathers as well. one year or for many. Most shorebirds and pas­ The color and pattern ofcontour feathers may serine birds with immature plumages take about be invariant across all age and sex classes in a 1 yr to acquire adult plumage; large gulls take 4 species. In many species, however, feather ap­ yr, and large tubenoses like the wandering al­ pearance varies with time of year, sex, age, or batross (Diomedea exulans) take 20 to 30 yr some combination of the three. (Cramp and Simmons 1977). For the sake of When a bird's appearance varies with age, it simplicity, this paper will use delayed plumage is said to exhibit delayed plumage maturation; maturation to refer only to postjuvenal, non­ that is, it shows delayed, rather than immediate, adultlike plumages worn during the first year, acquisition ofa "mature" or adultlike plumage. that is, nonadultlike first-winter and first-sum­ The exception to this rule is juvenal plumage. mer plumages. Juvenal feathers differ from their adult counter­ Students ofdelayed plumage maturation ask the parts in fine structure (e.g., Gohringer 1951) and question, "Why not look like an adult?" The hy­ sometimes in shape as well. In addition, many potheses proposed to answer this question suggest ofthem are worn only briefly. As a consequence, that the delayed assumption of adult plumage is 327 © 1994 The Society for the Study of Evolution. All rights reserved. 328 PHILIP C. CHU the result ofselection for distinctive first-winter or 1988). Their retention into the following breed­ first-summer appearance, with distinctive appear­ ing season could then be explained in terms of ance being an adaptation for the special conditions molt constraints; Rohwer et al. (1983) and Roh­ that first-year birds are thought to face. The hy­ wer and Butcher speculated that an extensive potheses fall into three broad classes: cryptic hy­ molt from winter to summer plumage may have potheses, mimetic hypotheses, and the winter-ad­ been difficult to evolve, or that it may require a aptation hypothesis. prohibitively large energy expenditure. The cryptic and mimetic hypotheses propose Previous comparative investigations of de­ that delayed plumage maturation is an adapta­ layed plumage maturation used the hypothesized tion related to breeding. Both assume that first­ functions ofimmature plumages to generate pre­ year birds lack experience and are thus handi­ dictions, and then asked whether the predictions capped when trying to breed. Both also recognize are better satisfied in species with immature thatbirds less than I-yr old sometimes do breed plumages than in species without them (e.g., successfully. Rohwer et al. 1980; Studd and Robertson 1985; The cryptic hypotheses propose that the in­ Lyon and Montgomerie 1986). This approach is experience of first-year birds makes breeding a useful; however, the investigations themselves costly, low-return endeavor for them. They may were designed without considering phylogeny. breed ifa suitably low-cost opportunity is avail­ Phylogenies are a widely used tool for exam­ able; if not, however, they forgo breeding and ining evolutionary problems (e.g., Sillen-Tull­ maximize early survival, thereby possibly in­ berg 1988; Donoghue 1989; Lauder 1989; Brooks creasing theirlifetime reproductive success (Lack and McLennan 1991; McKitrick 1992). Phylo­ 1954, 1968; Wittenberger 1979; Studd and Rob­ genetic hypotheses are important because they ertson 1985). estimate patterns ofancestry and descent, there­ The maximization of early survival has been by providing a historical framework that can be linked to delayed plumage maturation in two used to structure questions about character evo­ ways. One is direct: Procter-Gray and Holmes lution (e.g., Ridley 1983; Felsenstein 1985; Cod­ (1981) suggested that early survival is maxi­ dington 1988; O'Hara 1988; Pagel and Harvey mized by wearing a cryptic plumage that helps 1988; Burghardt and Gittleman 1990). For ex­ young birds escape detection by predators and ample, phylogenetic trees can be used to assess potentially aggressive adults. The second is in­ the polarity ofa particular trait; polarity, in turn, direct and hinges on the enhanced survivorship determines the direction from which the trait is of young birds that do not expend energy in a examined (Ridley 1983; Coddington 1988). If breeding attempt. Selander (1965), Ficken and delayed plumage maturation is derived within a Ficken (1967), and Wiley (1974) argued that se­ lineage, the question, "Why was plumage mat­ lection against early breeders results in delayed uration delayed?" is useful, but ifit is primitive, gonadal development, which in turn leads to re­ the more appropriate question is, "Why was duced plasma sex-hormone levels in young birds plumage maturation accelerated?" Previous con­ and a delay in the assumption ofadult plumage. tributors to the delayed-plumage-maturation lit­ The mimetic hypotheses suggest that first-year erature assumed that delay is the derived con­ birds wear a plumage mimicking an age or sex dition, but did not test the assumption. class that older birds treat less aggressively In addition, a hypothesis of historical rela­ (Rohwer et al. 1980; Foster 1987). By tricking tionships permits the identification of control older birds into reduced aggression, the deceitful groups for testing ideas about function. Earlier plumage signals are hypothesized to compensate studies ofdelayed plumage maturation selected somewhat for the inexperience of young birds, species for analysis on the basis of their mem­ thereby improving their chances for breeding bership in a subset ofa particular avifauna, for successfully. example, North American passerine birds. How­ Unlike the cryptic and mimetic hypotheses, ever, the appropriate taxa to select are those be­ the winter-adaptation hypothesis (Rohwer et al. longing to lineages that arose immediately before 1983; Rohwer and Butcher 1988) proposes that and after the point in history at which delayed nonadultlike immature plumages are important plumage maturation arose (Coddington 1988). duringthe winter, for reasons that are notdirectly Thus, ifa delay in plumage maturation is thought related to breeding, for example, avoiding pre­ to have arisen in the most recent common an­ dation or signaling status (Rohwer and Butcher cestor ofa particular ingroup, comparison ofin- HISTORICAL EXAMINATION OF DELAYED PLUMAGE MATURATION 329 group members versus several proximate out­ monly, birds