Mercedes S. Foster

Hotshots, Hotspots, and Female Preference in the Organization of Lek Mating Systems

We critically review the female-preference and hotspot models, the two most widely accepted recent explanations of lek organization. On the basis of what we believe are the inadequacies of these models-too great a reliance on the presumed acuity of female discrimination, the assumption that females have full freedom of choice within the lek, and insufficient recognition of the importance of male-male interactions-we develop an alternative set of hypotheses, which we call the hotshot model, to explain the development and maintenance of lek behavior. Our model attributes strong male mating skew to the interaction between (1) simplified and conservative mating rules of females and (2) social dominance among males. We demonstrate the importance of male-male dominance relationships in lek and non-lek court mating systems. We then argue that a strong mating skew among males forces novice males entering a population to adopt a long-term mating strategy that involves delayed breeding (floating) and subordinate lek behavior. The structure of leks is created by a complex of malemale interactions, with conflict between hotshots (who attempt to control lek mating) and subordinates, who may benefit from disrupting lek activities. Explanations for the number of males in an arena and inter-arena distances are based on modifications of the hotspot and female-preference models. We suggest specific field tests to help distinguish which hypothesis best models the behavioral interactions that produce lek mating.

Dispersal of Stemmadenia donnell-smithii (Apocynaceae) by Birds

The dispersal ecology of Stemmadenia donnell-smithii was studied in the tropical dry forest zone of northwestern Costa Rica. Fruit was most abundant late in the dry season and was eaten by 22 species of birds, many of which are primarily insectivorous. Stomach-content analyses substantiated our observations. Total crop size per tree and rate of fruit opening were greatest in pasture-edge trees and smallest in forest trees. Rates and percent of seed germination were highest for seeds from which the surrounding aril was removed, scarified seeds, and seeds which had passed through the digestive tract of a bird. The mean compositions of the fruit tissues were, aril: 7.9% ash, 63.9% lipid, 10.9% protein, 16.8% carbohydrate; seed: 3.4% ash, 31.4% lipid, 10.9% protein, 54.2% carbohydrate; husk: 17.0% ash, 24.0% lipid, 11.2% protein, 47.6% carbohydrate. The foraging behavior of birds taking S. donnell-smithii included hovering, and perching and reaching. Rates of pulp utilization in each habitat were relatively constant; all pulp plus seeds were removed in only a few hours. Calculations of daily energy expenditures suggest that Stemmadenia may provide up to 25 percent of the total energy requirement of individuals of several bird species. Interspecific displacements of birds at fruit were rare. Characteristics of S. donnell-smithii that enhance dispersal include peak fruit availability in dry season, slow rate of opening, seed protection until maturation by husk, bright color of arils, relative accessibility and ease of separation of seed from husk and aril from seed, and high nutritive value of aril. Energetically the plant expends the most calories for protection (husk), followed by expenditures for germination (seed) and dispersal (aril). However, on a calorieper-gram, ash-free, dry-weight basis, the plant puts the greatest amount of energy into aril for dispersal, followed by seed for germination, and then husk for protection. Percent nutrient composition and caloric content of seeds and husk were relatively uniform, whereas lipid, protein, and caloric content of aril tissue varied among samples from different fruits, habitats, and years. This variation may be important in allowing the plant to maintain protection and seed quality while maximizing seed production under varying environmental conditions.

The Overlap of Molting and Breeding in Some Tropical Birds

Studies of temperate and tropical land birds reveal that the resource-demanding activities of the life cycle generally are mutually exclusive in an individual (Miller 1963, Farner 1964, Fogden 1972). Presumably food (calories, specific organic or inorganic nutrients, or the time to gather them) is limiting, and temporal spacing has evolved in response to the high requirements of each event. Thus an activity is timed to occur when environmental conditions are favorable and when interference from other activities is minimal.

Disruption, dispersion, and dominance in lek-breeding birds

Disruption is any behavior that interferes with male-female courtship interactions or copulation and leads to a decrease in the number of copulations completed by a male, or an increase in the time and energy required to complete them. Here I examine the origin and maintenance of disruption, its effects on the fitness of the individuals involved, and its influence on dominance relationships and spacing of males at leks. Disruptive behavior is most common among lek-breeding birds, probably because its frequency is proportional to the amount of contact between members of the same sex. Amount of contact, in turn, is inversely proportional to distance between males, so that clustering of males on leks may result in increased disruption. Disruption usually is performed by a rival male who, as a result of his actions, may be able to mate with the female disrupted. Such behavior should be highly advantageous if, on average, the disruptor is able to decrease his age of first reproduction or to obtain more copulations. On the other hand, it is disadvantageous to the disrupted pair. Disruption also may occur as a by-product of high levels of male aggressiveness that are favored by selection because they increase the ability of a lek male to obtain and hold a high quality court or a high rank in a dominance hierarchy and to attract and excite females for copulation. Selection should operate to minimize the amount of disruption at leks through the evolution of strict dominance hierarchies among lek males, or by increasing the separation of lek males which may result in the formation of exploded leks.

ODD COUPLES IN MANAKINS: A STUDY OF SOCIAL ORGANIZATION AND COOPERATIVE BREEDING IN CHIROXIPHIA LINEARIS

Male long-tailed manakins (Chiroxiphia linearis) perform highly specialized communal or cooperative displays to advertise their presence and to excite females for copulation. For the purpose of performing these displays, males form persistent associations (pairs or trios) analogous to those between members of heterosexual monogamous pairs. Associations persist through an entire breeding season or from year to year. One male is dominant in the association and responsible for all copulations. Evidence argues against a close kin relationship between males, and the behavior of the subordinate individual does not appear to be altruistic. The subordinate male probably receives a delayed benefit if he outlives his dominant partner. It is suggested that on re-pairing, he will assume the dominant position in the new partnership because of the psychological advantage gained as a result of his familiarity with and prior possession of the court where the displays are performed. Ultimately, his mating success will be greater than that of a solitary male who cannot perform alone, and, therefore, is incapable of attracting and exciting females. Long-tailed manakin social organization, male-male bonding, and communal courtship behavior can best be explained on the basis of selection operating on the individual.

Factors influencing bird foraging preferences among conspecific fruit trees

The rates at which birds visit fruiting individuals of Allophylus edulis (Sapindaceae) differ substantially among trees. Such avian feeding preferences are well-known, but usually involve fruits and trees of different species. Factors controlling avian preferences for particular trees in a population of conspecifics are generally undocumented. To address this issue, I attempted to correlate rates at which individuals birds and species fed in trees of A1ZophyZu.s with 27 fruit or plant characteristics. Birds that swallow fruits whole were considered separately from those that feed in other ways. Plant characters were selected on the basis of their potential influence on feeding efficiency or predation risk, assuming that birds would select feeding trees so as to maximize the net rate of energy or nutrient intake and to minimize predation. Correlations were found between feeding visits by some groups of birds and percent water in the pulp, milligrams of mineral ash in the pulp, and crop size. No character was correlated with feeding visits by all groups of birds in both years of the study. The correlations with water and mineral ash are unexplained and may be artifacts. The correlation with crop size may represent a tactic to minimize predation.

Feeding methods and efficiencies of selected frugivorous birds

I report on handling methods and efficiencies of 26 species of Paraguayan birds feeding on fruits of Allophyllus edulis (Sapindaceae). A bird may swallow fruits whole (Type I: pluck and swallow feeders), hold a fruit and cut the pulp from the seed with the edge of the bill, swallowing the pulp but not the seed (Type II: cut or mash feeders), or take bites of pulp from a fruit that hangs from the tree or that is held and manipulated against a branch (Type III: push and bite feeders). In terms of absolute amount of pulp obtained from a fruit, and amount obtained per unit time, Type I species are far more efficient than Type II and III species. Bill morphology influences feeding methods but is not the only important factor. Diet breadth does not appear to be significant. Consideration of feeding efficiency relative to the needs of the birds indicates that these species need to spend relatively little time feeding to meet their estimated energetic needs, and that handling time has a relatively trivial effect on the time/energy budgets of the bird species observed.

Rain, Feeding Behavior, and Clutch Size in Tropical Birds

THAT avian clutch size generally decreases with latitude has been demonstrated repeatedly (Moreau 1944; Lack 1947, 1948; Skutch 1949, 1967; Lack and Moreau 1965). Several theories have been put forth to explain this. Two of the most diametrically opposed are those of Lack (1947, 1966) and Skutch (1949, 1967). Lack proposed that clutch size represents the average number of offspring that parents can nourish adequately so that the maximum number will survive to reproduce. In the tropics, presumably, parents can feed fewer young than can parents in temperate regions because of decreased daylength and increased competition and predation. Hence, they have lower clutch sizes. Adults still produce as many young as they can, and mortality is adjusted to balance natality. In view of this, the remarks of several ornithologists that tropical birds appear to have more time available for feeding young than they use are of interest (Skutch 1949, 1967; Wagner 1957; Miller and Miller 1968), particularly because Lack (1947) has suggested that decreased daylength in the tropics significantly decreases the time available for feeding young and thereby affects clutch size. Skutch on the other hand suggested that tropical birds do not rear as many young as they are capable of nourishing. According to his theory of adjusted reproduction, natural selection and density dependent regulating factors favor reduced clutch size in order to keep birth rate in balance with average annual mortality, which presumably is lower in the tropics. The birds do not produce as many young as they can, and natality is adjusted to balance mortality. One line of evidence that Skutch supplied to support this idea is an analysis of the time budget of tropical birds in relation to daylength. He showed that tropical parents should be able to feed at least one additional young. However his analysis contains an important fallacy in the assumption that all hours of daylight are equally usable by the birds. The present paper reviews Skutchs analysis to clarify the nature and usability of free time available.

PLUMAGE DEVELOPMENT AND MOLT IN LONG-TAILED MANAKINS (CHIROXIPHIA LINEARIS): VARIATION ACCORDING TO SEX AND AGE

Abstract Lek-mating Long-tailed Manakins (Chiroxiphia linearis) exhibit an unusual pattern of delayed plumage maturation. Each year, males progress through a series of predefinitive plumages before attaining definitive plumage in their fifth calendar year. Females also exhibit variation in plumage coloration, with some females displaying male-like plumage characteristics. Using data from mist-net captures in northwest Costa Rica (n = 1,315) and museum specimens from throughout the range of Long-tailed Manakins (n = 585), we documented the plumage sequence progression of males, explored variation in female plumage, and described the timing of molt in this species. Males progressed through a series of age-specific predefinitive plumages, which enabled the accurate aging of predefinitive-plumaged males in the field; this predefinitive plumage sequence is the basis for age-related status-signaling in these males. Females tended to acquire red coloration in the crown as they aged. However, colorful plumage in females may be a byproduct of selection on bright male plumage. Females exhibited an early peak of molt activity from February to April, little molt from May through July, and a second, more pronounced peak of molt activity in October. By contrast, males in older predefinitive-plumage stages and males in definitive plumage exhibited comparable unimodal distributions in molt activity beginning in June and peaking between July and October. Our data are consistent with selective pressure to avoid the costs of molt-breeding overlap in females and older males. Our findings have important implications for social organization and signaling in Long- tailed Manakins, and for the evolution of delayed plumage maturation in birds. Desarrollo del Plumaje y Muda en Chiroxiphia linearis: Variación de Acuerdo al Sexo y la Edad

Molt Cycles of the Orange-Crowned Warbler

Studies of the timing and extent of molt in congeneric species of migratory birds have revealed striking interspecific differences that reflect molt adaptation at the specific level (for example, see Johnson, 1963). However, with the exception of Millers (1928) work on the Loggerhead Shrike (Lanius ludovicianus), little attempt has been made to explain intraspecific variation in the timing and extent of molt in a migratory species in terms of adaptations at the subspecific level. The Orangecrowned Warbler (Vermivora celata) is a suitable species for such a study because it includes four well-marked geographic races which vary greatly in the extent and location of the breeding range, in the distance and period of migration, and in the timing of breeding. V. c. celata occupies a wide longitudinal range from central Alaska and the Alaskan peninsula eastward across Canada to western and central Ontario. V. c. orestera and V. c. lutescens, in contrast, occupy extensive latitudinal ranges. However, lutescens, which is found from southeastern Alaska to southern California, breeds along the Pacific coast and in the coast ranges, while orestera, an inland race, breeds in the Rocky Mountains and mountains of the Great Basin. V. c. sordida is restricted to the Channel Islands off the coast of southwestern California and to limited coastal areas on the adjacent mainland (ranges in all subspecies, A.O.U. Check-list, 1957). Furthermore, with the exception of an investigation of the Yellowthroat (Geothlypis trichas brachidactylus) by Stewart (1952), there has been, to my knowledge, no detailed study of molt in a parulid. Therefore, a study of the extent, sequence, and timing of the molt in the Orange-crowned Warbler was undertaken to determine the type and degree of possible interracial differences and to see if these differences represented adaptations at the subspecific level.