F. Gary Stiles

Ecology, Flowering Phenology, and Hummingbird Pollination of Some Costa Rican Heliconia Species

Nine hummingbird—pollinated species of Heliconia occur together at Finca La Selva, in the wet Caribbean lowlands of Costa Rica. In forest habitats, Heliconia clumps (clones) are typically small; in more open areas, many clumps attain large size. This probably reflects differences in light intensity and degree of vegetative competition in these habitats. Nine species of hummingbirds regularly visit Heliconia flowers at La Selva. The four hermits are nonterritorial, traplining foragers with long, curved bills. Non—hermits frequently hold territories at Heliconia clumps, and have short, straight bills. Pollination by hermits tends to produce more cross—pollination; territorial hummingbirds increase self—pollination. Different Heliconia species appear to be specialized for pollination by either hermits on non—hermits, largely through components of the caloric phenotype: amount and timing of nectar production, rate of inflorescene and flower production, and morphological paramerters that affect the energetic efficiency of nectar—harvesting hummingbirds. Habitat may influence pollination systems through its effects on clump size and thus on the number of flowers a clump can have at any one time. Ultimately, specialization for hermits or non—hermits may depend on the degree of self—compatibility of the different Heliconia species. Hermit—pollinated Heliconia mostly show sequential and nonoverlapping flowering peaks, probably resulting from competition for pollinators and/or selection against hybridization. Two hermit—pollinated species bloom simultanesoulsy, thereby inducing the birds to utilize an other—wise little—used microhabitat. Heliconia species pollinated by non—hermits bloom in the early to middle rainy season, and are mostly separated by habitat. Isolating mechanisms among sympatric Heliconia species involve both spatial and temporal patterns of partioning available pollinators. Floral parameters include mechanical (different site of pollen deposition on the bird) and ethological (caloric and visual factors affecting flower choice) mechanisms. Selection for pollinator specificity may result in convergence of blooming peaks, provided that other isolating mechanisms are present. Human activity has broken down some habitat barriers by producing large areas of second growth.

Taste Preferences, Color Preferences, and Flower Choice in Hummingbirds

Taste and color preferences have been experimentally demonstrated for a variety of birds, and the literature on food habits of wild birds is enormous, but as yet there is little to tie the two together. Laboratory experiments have usually involved taste stimuli that are probably irrelevant to birds in the wild; and the taste stimuli presented by insects, seeds, fruit, etc. are complex and difficult to duplicate for experimental purposes. The same is true to a considerable extent of color: the mostly granivorous species studied so far probably never experience bright, monochromatic colors in connection with feeding. Moreover, caloric parameters of birds’ foods are often difficult to measure or estimate but will almost certainly affect food selection, making taste and color preferences harder to assess. Hummingbirds are ideal subjects in studies of taste and color preferences. Their main foods are nectar and insects; flowers are visited almost exclusively for nectar, insects being captured elsewhere. Flower nectar is essentially an aqueous solution of 3 common sugars (Percival 1961, see also beyond). It can be easily sampled and its concentration, caloric value, and composition measured. The taste stimuli can be readily duplicated and manipulated experimentally. Assimilation of flower nectar is essentially 100% (Hainsworth 1974) making caloric parameters easy to estimate. Flowers visited by hummingbirds are usually brightly colored and fairly unpatterned, also favoring experimentation. The long controversy over the existence of color preferences in hummingbirds was reviewed by Grant and Grant ( 1968). The roles of color and taste factors have been greatly clarified by the feeding station experiments of Collias and Collias (1968); similar experiments by Miller and Miller (1971) further elucidated the role of position. These studies suggest a hierarchy of factors influencing feeder choice: sugar concentration and/or taste over position over color. As yet this hierarchy has not been subjected to experimental verification, or applied in detail to flower choice in the field. This paper presents laboratory experiments on taste and color preferences of the Anna Hummingbird (Calypte anna) and several other species. I also present the results of extensive field observations on flower choice by hummingbirds, and consider the taste and color stimuli presented by those flowers visited by hummingbirds. Hopefully, comparing the results of field and laboratory studies will permit a more realistic evaluation of the role of taste and color preferences in food choice by hummingbirds, and a better understanding of the coevolution of hummingbirds and the flowers they pollinate.

Patterns in floral nectar characteristics of some bird-visited plant species from Costa Rica

Nectar samples from a wide range of flowers (120 samples, 112 species, 22 families) visited by hummingbirds (subfamilies Phaethorninae and Trochilinae) were collected over a wide range of elevations and environments in Costa Rica. Some species visited are believed to be pollinated principally by perching birds, lepidopterans, and bees, however. We measured sugar composition (% fructose, glucose, and sucrose), daily secretion rates, and sugar concentration. In general, sugar compositions of all hummingbird nectars were found to be highly clustered toward the high-sucrose end of the composition spectrum and not significantly different from sugar compositions of other hummingbird nectar assemblages from northwestern Mexico and the southwestern United States. Significant correlations were detected between elevation and the percentage of fructose and of sucrose in the nectar, with the fructose percent positive and the sucrose negative. These correlations were found to be due to both elevation and hummingbirdgroup effects. Daily secretion rate and sugar concentration were both negatively correlated with elevation. Discriminant analysis indicated that nectar sugar concentration and daily secretion rates together could usually predict whether a species was pollinated by hermit (Phaethorninae) or nonhermit (Trochilinae) hummingbirds and could often predict which eco-morphological group of nonhermits.

Temporal Organization of Flowering Among the Hummingbird Foodplants of a Tropical Wet Forest

This paper seeks to elucidate causal factors in the temporal organization of blooming patterns, both within and between years, of a coadapted group of plants sharing a group of pollinators: the hummingbird flowers of Finca La Selva, Costa Rica. Quanititative data on flowering of 42 species over a four-year period are presented, with supplementary data on an additional 17 species. Peaks of flower availability for hummingbirds occur in the dry and early wet seasons; a severe and predictable flower shortage occurs in November and December, the wettest months of the year. This pattern is repeated in several other tropical areas, among hummingbird foodplants and the plant communities in general. A severe drought in March-April 1973 severely affected flowering of many species for up to four months thereafter; some species were noticeably affected as much as a year later. Hummingbird foodplants of large light gaps and second growth have longer peak blooming periods than do those of forest understory; vines bloom for shorter periods than do other growth forms. Mean year-to-year variation in various temporal parameters of flowering was about two months among La Selva hummingbird flowers, and about one month among those of a California locality. If, as in other groups of organisms, the action of photoperiod is to define the time span within which plants may be stimulated to flower by other factors like rainfall, then La Selva plants respond to a daylength change about half as great as do California plants. Two mechanisms may help maintain a regular sequence of blooming peaks from year to year in the face of annual variations in rainfall: displacement of the entire sequence, and opposite and compensating responses of individual plant species. In California and probably in tropical dry forest, the first mechanism predominates; at La Selva, both mechanisms may be important.

Energetics of Foraging: Rate and Efficiency of Nectar Extraction by Hummingbirds

The efficiency with which several species of hummingbirds extract nectar was estimated by converting the rate of extraction of nectar volumes to the energy expended and the energy obtained from the nectar. The extraction rates depend on corolla and bill morphologies, and the extraction efficiencies depend, in addition, on the bird weight and nectar concentration. Differential extraction efficiencies can be exploited by plants to increase pollinator specificity.

Aggressive and Courtship Displays of the Male Anna's Hummingbird

-The aggressive and courtship displays and vocalizations of the male Anna’s Hummingbird (Culypte anna) are described in detail, and various types of evidence and observations are used to reconstruct the typical courtship sequence. Initial contact is made by the female flying to the male’s territory and attempting to feed; she may have previously visited several other territories in order to evaluate territory quality. The well-known dive display is an aggressive maneuver by the male, although it may play a role very early in the courtship sequence. Following a lengthy chase towards the female’s nesting area, she alights low in dense vegetation. The male then gives the displays most critical for courtship: a back-and-forth “shuttle” display and high-intensity song. These hitherto undescribed displays occur immediately preceding copulation, and are probably the most important isolating mechanisms for the species. Many of the elements of courtship in C. an~la are widespread in hummingbirds. Practice and probably learning play a major role in the maturation of song and dive displays in the individual. The courtship sequence in this hummingbird probably represents the resultant of various selective pressures, some acting mainly on males and others on females. Hummingbirds are becoming among the beststudied of North American birds, particularly with respect to energetic aspects of nesting (Horvath 1967, Calder 197 1, 1974, Calder and Booser 1973, Smith et al. 1974) territoriality and foraging (Stiles 197 la, Ewald and Carpenter 1978, Kodric-Brown and Brown 1979, Pyke 1978, Gass 1979) and relationships with the flowers they visit and pollinate (Stiles 1973, Carpenter 1978, Waser 1978, KodricBrown and Brown 1979, Price and Waser 1979). Recent field studies in California have disclosed a striking amount of hybridization among several (newly?) sympatric species (Lynch and Ames 1970, Wells et al. 1978, Wells and Baptista 1979). Study of hybrids and parental types in nature offers a unique opportunity to document the ecological and behavioral effects of such hybridization. Analyses of these phenomena, however, have been hindered by the lack of reliable descriptions of courtship and aggressive displays of the species concerned. The conspicuous dive displays have been described with varying degrees of completeness and accuracy by Bent (1940) Banks and Johnson (196 l), Wells et al, (1978) and Wells and Baptista (1979), among others, but except for the perceptive discussions of Pitelka (1942, 19 5 1 a, b) their role in courtship has been misunderstood. Moreover, a whole family of close-range displays in these hummingbirds, far more crucial in courtship per se, has been overlooked. Finally, I know of no good descriptions of display sequences nor, indeed, of copulation itself in these birds. Behavioral isolating mechanisms in North American hummingbirds cannot be properly appraised without a clear understanding of what actually constitutes their courtship. Through several years of close study of the ecology and behavior of the Anna’s Hummingbird (Calypte annu) and other California species (Stiles 1973) I was able to observe courtship and aggressive displays ofthese birds in detail. My purpose here is to describe and interpret the vocalizations and displays of the Anna’s Hummingbird, to serve as a basis for comparisons with other species.

EVOLUTION OF PAIR COOPERATION IN A TROPICAL HUMMINGBIRD

In contrast with the many species of birds that form pairs during the breeding season, hummingbirds, in general, have a promiscuous mating system. The male mates with the female, and the female carries out the remainder of the reproductive effort, including nest-building, incubating the eggs and caring for the young, unaided by the male. The only reports to the contrary are for Colibri coruscans, the Sparkling Violet-ear of the Andes of South America, in which the male may aid in incubating the eggs and caring for the young (Moore, 1947; Schafer, 1954). However, Ruschi (1965) reported that only females attended the six nests of this species he studied, one in the wild and five in captivity. A pair bond in most hummingbirds, then, is limited to the short time during which a male and a female must cooperate to insure successful copulation. During the breeding season, the typical situation in hummingbirds is for males to hold mating and feeding territories at flowering plants, from which they attempt to exclude all other hummingbirds. In many cases, the female is treated aggressively by the male, both before and after mating, in a manner similar to the treatment accorded other conspecific intruders into his territory. The present report discusses pair relations of Panterpe insignis, the Fierythroated Hummingbird, during the breeding season. In this species a longer-term pair bond is apparently formed. The male still does not participate in the actual nesting effort, but indirectly does aid the female in

Consumption of fruits of the Melastomataceae by birds: how diffuse is coevolution?

The family Melastomataceae shows two major modes of dispersal for its small seeds: wind for capsular fruits, and birds for berry-type fruits. Distribution patterns of these two dispersal types differ. We focus on evolutionary diversification of bird-dispersed melastomes in the New World, in relation to that of their avian dispersers. We first examine in detail patterns of melastome fruit availability and consumption by understory birds in a Costa Rican rain forest. Although a relatively small portion of total fruit available in most months, melastomes comprised a major component of the diet of all bird groups studied, most notably manakins and most tanagers, which in turn were the major consumers of melastome fruits. Examination of data from several other neotropical areas confirms the importance of the manakins and tanagers as the avian frugivores most closely associated with the melastomes. These two groups differ in their manner of fruit consumption, being ‘gulpers’ and ‘mashers’, respectively. The patterns of species richness of small mashers in several Neotropical areas conform very closely to the distribution patterns of bird-dispersed melastomes; both are most speciose in wet forests of middle elevations. Gulpers, especially manakins, are most diverse at low elevations. Thus, the small-seeded melastome berry appears to have evolved primarily in association with masher-type frugivores such as tanagers; with the later addition of manakins to their disperser spectrum, the melastomes may have increased greatly in local abundance, but not (yet?) in diversity, in the wet lowland neotropics.

Untangling the influence of ecological and evolutionary factors on trait variation across hummingbird assemblages

Phylogenetic community ecology combines phylogenetic hypotheses with local species composition and functional-trait information to evaluate historical and contemporary mechanisms influencing local assemblage structure. Most studies assume that, if functional traits are conserved, then patterns of trait variation should match patterns of phylogenetic structure within local assemblages. Here we evaluated if we could predict trait structure by assuming that environmental filtering or biotic interactions work primarily on phylogenetically conserved functional traits. We investigated patterns of phylogenetic assemblage structure and functional-trait variation in bill length, wing length, and body mass in 236 hummingbird assemblages (126 species) across two major gradients in northern South America: elevation and precipitation. While mean trait values for assemblages vary predictably based on empirical knowledge of hummingbird biology, the distribution of trait values within assemblages do not correspond to tho...

Of Hummingbirds and Helicopters: Hovering Costs, Competitive Ability, and Foraging Strategies

Wing morphology and flight kinematics profoundly influence foraging costs and the overall behavioral ecology of hummingbirds. By analogy with helicopters, previous energetic studies have applied the momentum theory of aircraft propellers to estimate hovering costs from wing disc loading (WDL), a parameter incorporating wingspan (or length) and body mass. Variation in WDL has been used to elucidate differences either among hummingbird species in nectar‐foraging strategies (e.g., territoriality, traplining) and dominance relations or among gender‐age categories within species. We first demonstrate that WDL, as typically calculated, is an unreliable predictor of hovering (induced power) costs; predictive power is increased when calculations use wing length instead of wingspan and when actual wing stroke amplitudes are incorporated. We next evaluate the hypotheses that foraging strategy and competitive ability are functions of WDL, using our data in combination with those of published sources. Variation in hummingbird behavior cannot be easily classified using WDL and instead is correlated with a diversity of morphological and physiological traits. Evaluating selection pressures on hummingbird wings will require moving beyond wing and body mass measurements to include the assessment of the aerodynamic forces, power requirements, and power reserves of hovering, forward flight, and maneuvering. However, the WDL–helicopter dynamics model has been instrumental in calling attention to the importance of comparative wing morphology and related aerodynamics for understanding the behavioral ecology of hummingbirds.