Diversity in adversity: the behaviors of two subordinate antbirds

The main subordinate ant-following ant birds south of the eastern Amazon (White-backed Fire-Eyes, Pyriglena leuconota) and north of the eastern Amazon (White-plumed Antbirds, Pithys albifrons) differ in many behavioral aspects despite their similar niches. They and other subordinate ant-followers studied previously are more diverse in behavior than are the dominant antbirds of the same guilds. The main foraging adaptation of White-plumeds is rapid flying, infiltrating among larger antbirds by being nonsocial and speedy. The main foraging adaptation of Fire-Eyes is rapid hopping around the periphery of dominant birds. Flying is most useful between vertical perches low inside forests, where White-plumeds are common; hopping is most useful where horizontal vines and twigs occur near the ground, as in the second-growth, swampy or dry forests where Fire-Eyes are common. White-plumeds therefore forage near the centers of forest swarms, darting to the ground for prey and fleeing instantly to escape attacks of dominant birds Fire-Eyes forage more diversely, capturing food on the ground or on foliage, either high or peripherally, around woodland swarms. Fire-Eyes also forage away from ants" whenever colonies are inactive. Voice and aggressive behaviors are minimal in White-plumeds, probably so as to be inconsp ous when infiltrating among larger birds; the facial plumes may signal to other White-plumeds instead. Individuals wander widely between ant swarms and show little sign of territoriality, probably because their small foraging niches at a given ant swarm are easily closed off by dominant birds. Fire-Eyes are very vocal and aggressive, and pairs maintain dominance in their territories even though trespassing individuals are grudgingly permitted to stay. Probably the necessity of foraging away from ants makes it advantageous for Fire-Eyes to retain territories in which other foragers will not reduce food supplies irregularly. Territories plus relative safety from dominant birds at swarm margins favor aggressive behavior, in retaining foraging sites. Both White-plumeds and Fire-Eyes, like the subordinate antbirds previously studied, have little intraspecific submissive behavior. Poor etiquette may be characteristic of subordinate species because they cannot as-sociate in family groups without attracting dominant birds, and hence have little kin selection for intraspecific appeasement behavior. Tendencies for subordinate antbirds to seldom use "keening" alarm calls may also result from lack of kin selection. When a subordinate species emphasizes chipping alarm to hawks or mobbing to mammals, it may be gaining in individual selection by scaring dominant competitors or by disturbing the possible predators. Fire-Eyes do not mob mammals as much as do most subordinate antbirds, perhaps because mammals are too dangerous in the cluttered undergrowth where Fire-Eyes normally live. Courtship in subordinate antbirds depends less on courtship feeding of females and more on mutual grooming and special displays than in dominant antbirds, which probably get food for females more easily. An unusual wingwaving-zeesinging display in White-plumeds may be sexual, as may be a draping display in Fire-Eyes. Hypertrophy of courtship patterns may occur because pair bonds are somewhat unstable in Fire-Eyes and very unstable in White-plumeds. Fire-Eye pairs stayed together except when foraging at ant swarms, but separated readily when their woodland territories were cut. White-plumed pairs stayed together only for brief nestings, and females left males feeding single young to move quickly to new males for new nestings. Because they show high divorce rates or repeat nestings throughout'most of the year, Fire-Eyes and White-plumeds are relatively "r-seiected"for high reproductive rates. The Spotted Antbirds previously studied, however, have undergone a demographic transition to short (6-month) breeding seasons despite subordinate status. Apparently long breeding seasons are linked with high adult mortality rates in Wandering Fire-Eyes and White-plumeds, short seasons with low adult mortality rates in territorial Spot teds. Nest-success rates are low (10%) for Spotteds, moderate (20%) for Fire-Eyes, and higher in White-plumeds. It may thus be unusually expensive for Spotted Antbirds to rear young, contributing to deemphasis of repeated breeding as a path to genetic survival. Sexual dimorphism is the rule in subordinate ant-following antbirds, perhaps because individuals forage far apart at swarm borders and females need not bluff males. Females may, by adopting dull plumage, reduce likelihood of male attack if birds are widely separated and attack therefore expensive. Relative lack of dimorphism in White-plumeds, as in most centrally foraging dominant antbirds, may be linked with tendencies to crowd together between dominant antbirds and hence greater female need or success in bluffing the relatively unaggressive males. Reduced dimorphism in some subspecies of Fire-Eyes may be linked with their tendencies to forage close together in certain environments. Fire-Eyes have subspeciated repeatedly, while White-plumeds and Spotteds tend to show speciation instead. A ttaining speciation may indicate more rapid evolution than does subspeciation, and seems associated with high-diversity habitats like the rain forest. Taxon flow of rapidly evolving forms from high-diversity habitats replaces species in lower-diversity habitats. Diverse form and behavior evolve in subordinate ant followers despite competitive adversity rather than because of it. Moderate diversity oj opportunity, not the negative factors of competition or extreme diversity, seem to explain high rates of diversification.