Iridescent polychromatism in a female hummingbird: is it related to feeding strategies?

-Conspicuous coloration may evolve through aggressive competition either for mates, in sexual selection, or for any limiting resource, in social competition. Most species of hummingbirds (Trochilidae) have polygynous (promiscuous) mating systems, bright male plumage, and conspicuous sexual dichromatism. These patterns have led to the view that their iridescent plumage has evolved by sexual selection. I report an iridescent polychromatism in females of the Andean hummingbird Heliangelus exortis exortis that suggests the evolution of iridescent plumage via aggressive feeding behavior. Female H. e. exortis vary within and among populations in the development of the large iridescent gorget characteristic of males. I document a regular north-south pattern of geographic variation in the polychromatism. In samples from northern Colombia, female gorgets vary from a noniridescent (femalelike) to a fully iridescent (malelike) condition, whereas females from southern Colombia have only the discrete noniridescent and fully iridescent morphs. Samples of females from Ecuador are comprised predominantly of the noniridescent morph. The color forms are not due to seasonal plumage differences or ontogenetic changes. A significant negative correlation between bill length and degree of iridescence in the largest geographic sample suggests that females with iridescent gorgets are more aggressive territorial foragers than less iridescent females. Received 16 July 1984, accepted 8 April 1985. NUMEROUS factors have been proposed to explain the evolution of conspicuous coloration in animals. These include predator warning and avoidance (Curio 1976, Baker and Parker 1979, Endler 1979), thermoregulation (Watt 1968, Clark 1974, Walsberg et al. 1978), and behavioral communication as influenced by the biotic (Nobel and Vogt 1935, Tinbergen 1951, Murray 1981) or physical (Burtt and Gatz 1982) environment. Evaluation of the relative importance of these potential influences presents a major challenge to evolutionists. Within the category of behavioral communication, the effects of nonsexual social competition are particularly difficult to distinguish from those of sexual selection. Darwin (1871) proposed that conspicuous sexually dimorphic features evolve via sexually selected competition for mates, through the advantage these features confer either in mate choice (epigamic selection) or in aggressive interactions among members of one sex that determine access to the other (intrasexual selection). The theory of I Present address: Department of Ornithology, American Museum of Natural History, New York, New York 10024 USA. social competition (Crook 1972; West-Eberhard 1979, 1984) recognizes that analogous interactions may extend to competition over limiting resources other than mates, and promote the elaboration of conspicuous signals used in nonsexual contexts. The effects of sexual selection and of nonsexual social competition are difficult to distinguish in practice because both phenomena lead to the prediction that increased dominance and aggressive behaviors can lead to the elaboration of conspicuous signals such as bright coloration. Moreover, the effects of sexual selection are difficult to control in sexually reproducing animals. Specifically, variation in male coloration (e.g. Rohwer and Niles 1979, Dominey 1980, Ewald and Rohwer 1980, Gross and Charnov 1980, Rohwer et al. 1980, ProcterGrey and Holmes 1981, Edwards 1982, Flood 1984) often fails to distinguish these hypotheses because male dominance and territoriality invariably are tied to breeding activities (Wilson 1975, O'Donald 1977). This ubiquitous correlation makes it difficult to determine whether nonsexual social competition is a sufficient explanation for the evolution of conspicuous coloration in males. The occurrence of intraspecific variation in 701 The Auk 102: 701-713. October 1985 This content downloaded from 207.46.13.144 on Mon, 13 Jun 2016 07:02:15 UTC All use subject to http://about.jstor.org/terms 702 ROBERT BLEIWEISS [Auk, Vol. 102 female coloration provides a natural control of this difficulty that has gone unappreciated. In many species, especially those in which males mate promiscuously, females are less aggressive sexually, thereby providing some control for the influence of sexual selection. Variation in female plumage is particularly relevant for determining the evolutionary significance of iridescent plumage in the largely promiscuous hummingbirds (Trochilidae), which exhibit some anomalous female plumages that are difficult to explain by sexual selection. Male hummingbirds employ their iridescent plumage in a variety of contexts, including nuptial displays, aggressive sexual displays, and aggressive displays associated with nectar-centered feeding territoriality (Stiles 1973, 1982). Because promiscuous breeding systems are thought to be associated with high levels of sexual selection (Payne 1984), the bright male plumage and sexual dichromatism characteristic of most hummingbirds are consistent with sexual selection theory (Selander 1972). However, sexual selection does not explain the evolution of "bright monomorphic" species characterized by a brilliant female plumage similar to that of the male (e.g. in the genera Panterpe, Eulampis, Amazilia). Bright monomorphic species do not show a consistent reduction in the intensity of sexual selection as inferred from their breeding systems (Wolf and Stiles 1970, Wolf 1975a). Rather, both sexes in bright monomorphic species exhibit nonbreeding nectar-centered feeding territoriality (Wolf 1969, 1975a, b; Ingles 1976). Feeding territoriality is usually lacking in the dull-colored females of most sexually dichromatic species (Wolf 1969, 1975a, b; Linhart 1973). Therefore, the correlation between bright female plumage and feeding territoriality strongly suggests that conspicuous iridescence may evolve solely for its importance as a nonsexual aggressive signal (Pitelka 1942, Wolf and Stiles 1970, Stiles 1973). In this paper I address a previously unanalyzed color pattern that involves the variable expression of iridescent malelike features in females of an Andean hummingbird, the Tourmaline Sunangel (Heliangelus exortis exortis). Subtle examples of this "polychromatic" variation in hummingbirds have been noted in the temperate species Calypte anna (Williamson 1956) and in several tropical genera, including Heliangelus (Berlioz 1944, 1949), but distinctive female variants in iridescent male plumage are recorded only for the tropical White-necked Jacobin (Florisuga mellivora; Zimmer 1950b, Monroe 1968, Elgar 1978). Although H. e. exortis is considered to be sexually dichromatic, with a highly iridescent gular patch or "gorget" in males and a white gorget in females (e.g. Meyer de Schauensee 1964), Chapman (1917) noted that some females of H. e. exortis from the Western Cordillera of Colombia had fully iridescent gorgets like those of males. I show here that female H. e. exortis vary dramatically both within and among populations in the malelike iridescence on their gorgets, and I argue that the polychromatic variation supports the origin of iridescent coloration in hummingbirds through selection for aggressive signals associated with foraging behavior. MATERIALS AND METHODS In Heliangelus exortis, the violet-chinned and rosethroated nominate form, H. e. exortis, occurs throughout the three Andean Cordilleras of Colombia and through the Ecuadorian Andes, primarily on the eastern slope. Two orange-throated forms, H. e. micraster and H. e. cutervensis, occur in the Andes of southwestern Ecuador and northern Peru, respectively (Peters 1945, Zimmer 1951), but are rare in collections and are not treated here. I examined 275 individuals from the entire range of H. e. exortis in this study. Sample size and geographic distribution of specimens are given in Table 1. A more complete description of the geographic relationships among localities can be found in Bleiweiss (1985). Determination of age and sex. -Only two specimens, both immatures, had gonad data noted on their labels. Therefore, alternative methods had to be used to determine age and sex. To distinguish immatures from adults, I relied on the presence of corrugations on the culmens of immature birds (Ortiz-Crespo 1972, Stiles and Wolf 1974). Specimens were examined under a Bausch and Lomb zoom dissecting microscope (7-30x) to look for the presence of corrugations. A specimen was considered adult if the bill was perfectly smooth outside the nasal depression. The inverse relationship between the number of bill corrugations and age of immature birds permits one to study plumage ontogeny (Ortiz-Crespo 1972, Ewald and Rohwer 1980). To determine the relative age of immatures, bill corrugations were counted with the aid of a Wild microscope with camera lucida attachment. Wing and tail measurements are considerably larger in males of many except the smallest hummingbird species (Zimmer 1950a, 1952; Stiles 1983; Payne 1984). I have verified this relationship for Heliangelus with large series of H. amethysticollis for which gonadal data were available (Bleiweiss in prep.). To determine the sex of H. e. exortis specimens, I emThis content downloaded from 207.46.13.144 on Mon, 13 Jun 2016 07:02:15 UTC All use subject to http://about.jstor.org/terms October 1985] Female Variation in Iridescence 703 TABLE 1. Distribution and sample size for Heliangelus exortis exortis specimens examined.