Behavioral and ecological correlates of interference competition among some Hawaiian drepanidinae

-Interference interactions were studied on the island of Maui among four species of drepanidines that fed on canopy flowers of the ohia tree (Metrosideros collina). The birds had a size-related dominance hierarchy, with ranking (most to least dominant, by location in dominance matrix) as follows: Crested Honeycreeper (Palmeria dolei), Iiwi (Vestiaria coccinea), Apapane (Himatione sanguinea), and Common Amakihi (Hemignathus virens). Immatures were subordinate to conspecific adults. Crested Honeycreepers and liwis defended treecanopy territories, Apapanes were nomadic flock foragers, and Common Amakihis were secretive foragers. Dominance by Iiwis was indicated by a lack of reversals in the dominance matrix. For the top-ranked Crested Honeycreeper dominance was also reflected in greater chase or greater resulting retreat distances or both. Age-related differences in interference behavior by dominants appeared to be due to inexperience, as indicated by shorter chase distances for immatures. Among subordinates both experience and plumage could be responsible for age differences in behavior. Immatures were chased equal distances (despite closer spacing) but less often than adults, and they chose safer retreat sites. Apapanes fed in flocks in a dominant's tree. This increased a dominant's territorial costs, reduced chase frequencies, increased Apapane foraging times, and let Apapanes forage in trees from which they otherwise were excluded. Comparisons with the assemblage on the island of Hawaii suggested similar structure among the three species the two islands have in common. On Maui the bottom-ranked Common Amakihi may be more affected by interference competition, perhaps undergoing a niche shift toward greater insectivory because of an additional dominant. Received 22 July 1985, accepted 17 February 1986. ALTHOUGH interference (vs. exploitation) is a major interaction that occurs among competing organisms (Schoener 1982, 1983), the few detailed studies of interspecific behavioral encounters in birds have dealt primarily with multispecies flocks (e.g. Morse 1970, 1976). Interference behavior has been invoked to explain parapatric distributions or joint occupancy of a single habitat through mutually exclusive territories (Orians and Willson 1964) or shifts in niche and foraging role (e.g. Murray 1971, 1981; Davis 1973; Feinsinger and Colwell 1978; Williams and Batzli 1979). Nectar-feeding birds are known for their intraand interspecific aggressiveness (Pitelka 1951, Carpenter 1978, Wolf 1978, Murray 1981), but details of the effectiveness and consequences of these agonistic behaviors are poorly known. This is particularly true for immature birds, especially if they occupy different habitats or utilize different food resources. When immatures form a major proportion of a population, however, their role in social dynamics and interspecific interactions can be important. I studied interference interactions among three species of nectar-feeding Hawaiian drepanidines (Fringillidae) from the island of Maui: the Crested Honeycreeper (Palmeria dolei), the Apapane (Himatione sanguinea), and the liwi (Vestiaria coccinea). A fourth drepanidine, the Common Amakihi (Hemignathus virens), is discussed where data are available. The latter three species have been studied extensively on the island of Hawaii (e.g. Baldwin 1953; Carpenter 1976; Carpenter and MacMillen 1976a, b, 1980; van Riper et al. 1978; MacMillen and Carpenter 1980; Pimm and Pimm 1982; Scott et al. 1984; van Riper 1984; Mountainspring and Scott 1985). In contrast, there are few ecological accounts of those on Maui (Carothers 1982, 1986; Carothers et al. 1983). I determined the basic social structures of the birds and the behavioral correlates of dominance relationships among them. Because im564 The Auk 103: 564-574. July 1986 This content downloaded from 207.46.13.145 on Sun, 03 Jul 2016 05:10:46 UTC All use subject to http://about.jstor.org/terms July 1986] Hawaiian Honeycreeper Behavior 565 : ~~~~~~~~~~~~~~~~~~H I Ves/ilarta. coccir/Ze4 "zazoze san9ujneaczd. ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ a Fig. Adta mabnerz dohet X s Fig. 1. Adult and immature plumages for the three main species of Hawaiian honeycreeper in this study. matures sometimes outnumbered adults, agerelated differences in behavior were of special concern. I attempted to establish whether birds of higher dominance rank, as determined by location in a dominance matrix, induced greater retreat responses and greater interindividual distances (herein termed spacing) in subordinates. This could result from increased chase frequency or increased chase distance by the more dominant birds. Alternatively, greater intimidation of subordinates also might result solely from plumage or size differences. Age should influence dominance status and general behavior because immatures are perhaps less aggressive and certainly less experienced than adults. Chase distances and the resulting retreat responses and spacing by subordinates were expected to be lower for immature than for adult dominants. For low-ranking immature subordinates (which should be more intimidated than conspecific adults), relatively farther retreats to safer retreat sites were expected. Their inexperience at avoiding aggression, however, might result in closer, not greater, spacing from dominants. Because other studies (review in Murray 1981) showed that dominants alter resource use by subordinates and that subordinates attempt to ameliorate such effects, I investigated the behavioral and ecological consequences of joint resource use on interactions among the birds. The results of these behavioral and ecological studies of the Maui species were then compared with those on Hawaii. Because the Maui assemblage contains an extra species, I investigated whether the assemblage structure and the behavior of Maui birds differ much with the addition of an extra species. Such a comparison may give insight into the nature of the drepanidine assemblage before the extinction of part of its avifauna.