Archilochus Colubris Research Articles

Transient hovering performance of hummingbirds under conditions of maximal loading.

Maximal load-lifting capacities of six ruby-throated hummingbirds (Archilochus colubris) were determined under conditions of burst performance. Mechanical power output under maximal loading was then compared with maximal hovering performance in hypodense gas mixtures of normodense air and heliox. The maximal load lifted was similar at air temperatures of 5 and 25 degrees C, and averaged 80% of body mass. The duration of load-lifting was brief, of the order of 1 s, and was probably sustained via phosphagen substrates. Under maximal loading, estimates of muscle mass-specific mechanical power output assuming perfect elastic energy storage averaged 206 W kg-1, compared with 94 W kg-1 during free hovering without loading. Under conditions of limiting performance in hypodense mixtures, maximal mechanical power output was much lower (131 W kg-1, five birds) but was sustained for longer (4 s), demonstrating an inverse relationship between the magnitude and duration of maximum power output. In free hovering flight, stroke amplitude and wingbeat frequency varied in inverse proportion between 5 and 25 degrees C, suggesting thermoregulatory contributions by the flight muscles. Stroke amplitude under conditions of maximal loading reached a geometrical limit at slightly greater than 180 degrees. Previous studies of maximum performance in flying animals have estimated mechanical power output using a simplified actuator disk model without a detailed knowledge of wingbeat frequency and stroke amplitude. The present load-lifting results, together with actuator disc estimates of induced power derived from hypodense heliox experiments, are congruent with previous load-lifting studies of maximum flight performance. For ruby-throated hummingbirds, the inclusion of wingbeat frequency and stroke amplitude in a more detailed aerodynamic model of hovering yields values of mechanical power output 34% higher than previous estimates. More generally, the study of performance limits in flying animals necessitates careful specification of behavioral context as well as quantitative determination of wing and body kinematics.

Hummingbird hovering performance in hyperoxic heliox: effects of body mass and sex.

Owing to their small size and hovering locomotion, hummingbirds are the most aerobically active vertebrate endotherms. Can hyperoxia enhance the flight performance of this highly oxygen-dependent group? Hovering performance of ruby-throated hummingbirds (Archilochus colubris) was manipulated non-invasively using hyperoxic but hypodense gas mixtures of sea-level air combined with heliox containing 35% O2. This manipulation sheds light on the interplay among metabolic power input, mechanical power output and aerodynamic force production in limiting flight performance. No significant differences in flight mechanics and oxygen consumption were identified between hyperoxic and normoxic conditions. Thus, at least in the present experimental context, hyperoxia did not change the major metabolic and mechanical parameters; O2 diffusive capacities of the respiratory system were probably not limiting to a significant extent. Compared with hummingbirds in our previous studies, the present experimental birds were heavier, had resultant shorter hover-feeding durations and experienced aerodynamic failure at higher air densities. Because hummingbirds have relatively stable wingbeat frequencies, modulation of power output was attained primarily through variation in stroke amplitude up to near 180 degrees. This result indicates that maximum hovering performance was constrained geometrically and that heavier birds with greater fat loads had less margin for enhancement of power production. Sexual dimorphism in flight adaptation also played a role, with males showing more limited hovering capacities, presumably as a trade-off for increased maneuverability.

Limits to Flight Energetics of Hummingbirds Hovering in Hypodense and Hypoxic Gas Mixtures

Hovering hummingbirds offer a model locomotor system for which analyses of both metabolism and flight mechanics are experimentally tractable. Because hummingbirds exhibit the highest mass-specific metabolic rates among vertebrates, maximum performance of hovering flight represents the upper limit of aerobic locomotion in vertebrates. This study evaluates the potential constraints of flight mechanics and oxygen availability on maximum flight performance. Hummingbird flight performance was manipulated non-invasively using air and gas mixtures which influenced metabolism via variable oxygen partial pressure and/or altered flight mechanics via variable air densities. Limits to the locomotor capacity of hovering ruby-throated hummingbirds (Archilochus colubris) were unequivocally indicated by aerodynamic failure in either air/helium or air/heliox mixtures. Air/helium mixtures are hypodense and hypoxic; failure to sustain hovering flight occurred at 63% of the density of sea-level air and at an oxygen concentration of 12%. Air/heliox mixtures are hypodense but normoxic; failure in hovering occurred at 47% of sea-level air density. Thus, hummingbirds demonstrated considerable power reserves in hovering flight as well as hypoxic tolerance. In air/helium mixtures, hovering was limited by oxygen supply and not by flight mechanics. Birds hovering in air/helium mixtures increased their mechanical power output but not their rate of oxygen consumption. By contrast, birds hovering in air/heliox mixtures increased both mechanical performance and metabolic expenditure. Under hypoxia, hovering hummingbirds demonstrated non-negligible, but still limited, capacities for anaerobic metabolism and/or oxygen storage. Depending on the physical context, hummingbird flight performance can therefore be limited by oxygen availability or by flight aerodynamics.

Effect of floral orifice width and shape on hummingbird-flower interactions.

Nectar guides are common among insect-pollinated plants, yet are thought to be rare or absent among hummingbird-pollinated plants. We hypothesize that the lower lips and trumpet-shaped orifices of many hummingbird flowers act as nectar guides to direct hummingbirds to the flowers' nectar and orient the birds for pollination. To test this hypothesis we conducted laboratory experiments using flowers of Monarda didyma (bee balm) and M. fistulosa (wild bergamot), which have orifice widths of about 4 mm and 2 mm, respectively, and latex flowers with orifice widths of 4 mm and 2 mm and three orifice shapes (trumpet, lipped, and lipless). Rubythroated hummingbirds (Archilochus colubris) made fewer errors during bill insertion and spent a smaller proportion of their feeding visit in error at M. didyma flowers than at M. fistulosa flowers, and at unaltered flowers of both species than at flowers with lower lips removed. Handling times were longer at both lipped and lipless flowers of M. didyma than at those of M. fistulosa, and at lipped than at lipless flowers of M. didyma. The average duration of contact between a hummingbird and a flower's anthers and stigma was longer at M. didyma than at M. fistulosa for both lipped and lipless flowers, and at lipped than at lipless M. didyma flowers. Hummingbirds missed the openings of latex flowers with their bills more frequently and spent a greater percentage of their total feeding visit in error at (i) 2-mm than at 4-mm flowers of all three shapes, (ii) lipless flowers than at trumpet or lipped flowers, and (iii) lipped flowers than at trumpet flowers of both widths. The duration of hummingbird/anther contact was longer at (i) 2-mm than at 4-mm flowers of all shapes, (ii) lipped than at trumpet or lipless flowers, and (iii) lipless than at trumpet flowers for both widths. No significant differences in handling times of hummingbirds were observed among any of the latex flower shapes or widths. Our results demonstrate that orifice shapes can act as guides by reducing the frequency of feeding errors by visiting hummingbirds, and that effects of orifice shape on pollination must be considered in conjunction with flower widths and locations of anthers and stigmas.

A new dimension to hummingbird-flower relationships.

The close correspondence between the bills of hummingbirds and the lengths of the flowers they feed from has been interpreted as an example of coadaptation. Observations of birds feeding at flowers longer and shorter than their bills, however, and the lack of experimental evidence for any feeding advantage to short bills, seem to contradict this interpretation. I address this problem by considering a little-studied dimension of floral morphology: corolla diameter. In laboratory experiments on female ruby-throated hummingbirds (Archilochus colubris), probing abilities (maximum extraction depths) increased with increasing corolla diameter. Handling times increased with decreasing corolla diameter, resulting in "handling time equivalents", i.e., flowers having the same handling times but different lengths and diameters. Longer-billed birds had greater maximum extraction depths and shorter handling times than shorter-billed birds at all corolla diameters greater than the width of the bill. In contrast, shorter-billed birds made fewer errors inserting their bills into narrow flowers. Hence, differences in bill lengths apparently are associated with trade-offs in foraging abilities, whereby longer-billed birds are able to feed at long flowers and may do so more quickly, whereas shorter-billed birds are able to feed more successfully at narrow flowers.

Mobility of Impatiens capensis flowers: effect on pollen deposition and hummingbird foraging.

Flexible pedicels are characteristic of birdpollinated plants, yet have received little attention in studies of hummingbird-flower interactions. A major implication of flexible pedicels is that flowers may move during pollination. We examined whether such motion affected interactions between ruby-throated hummingbirds (Archilochus colubris) and jewelweed (Impatiens capensis) by increasing pollen deposition and by altering the effectiveness of nectar removal. For I. capensis, flower mobility enhanced pollen deposition: birds had significantly longer contact with anthers and more pollen deposited on their bills and crowns when foraging at mobile flowers than at flowers that had been experimentally immobilized. In contrast, flower mobility imposed a cost on hummingbirds by significantly increasing their handling times and reducing their extraction rates relative to their interactions with immobile flowers. Field observations indicated that the motion observed during hummingbird visits did not occur when bees (Bombus spp., Apis mellifera) visited I. capensis flowers, which suggests that the mobility of I. capensis flowers is an adaptation for hummingbird pollination.

Limits to vertebrate locomotor energetics suggested by hummingbirds hovering in heliox

OXYGEN consumption1 and muscle power output2 of hovering hummingbirds are among the highest recorded for vertebrates. Maximum performance of hummingbirds thus approaches the upper limits of vertebrate aerobic locomotion3. Because air density is a major determinant of aerodynamic power requirements4, hovering flight performances5 can be manipulated non-invasively using nor-moxic gas mixtures of variable density6. Here we show that limits to the locomotor capacity of hovering ruby-throated hummingbirds are unequivocally indicated by aerodynamic failure at low densities less than half that of sea-level air. Hummingbirds demonstrate considerable power reserves, with muscle mass-specific power (assuming perfect elastic energy storage) averaging from 98 W kg-1 in normal air to a maximum value of 133 W kg-1 before aerodynamic failure. In contrast to such variable power expenditure, however, muscle efficiency remains approximately constant at 10%. Modulation of power output is attained primarily through variation in wing-stroke amplitude, with aerodynamic failure occurring near 180°.

Annual Molt in Ruby-Throated and Black-Chinned Hummingbirds

Details of the annual molt in Ruby-throated Hummingbirds (Archilochus colubris) and Black-chinned Hummingbirds (Archilochus alexandri) are presented. Young of both species hatch essentially naked but within three weeks acquire a full set of feathers. After the birds migrate to winter quarters, these feathers are replaced completely before the first year of age and annually thereafter. The postnuptial molt in adult males and females is similar within species and proceeds at about the same rate. The same is true for young birds in juvenal plumage, though molting is generally initiated and completed later than in adults. The period required for the molt of remiges and rectrices in an individual is estimated at around four months, while on a population basis it is underway for 7-8 months. Comparisons of the annual molt in Ruby-throated and Black-chinned Hummingbirds to other hummingbirds show that there are numerous similarities, differences being primarily attributable to migrant versus resident status

Factors Limiting Fecundity and Germination in Small Populations of Silene regia (Caryophyllaceae), a Rare Hummingbird-pollinated Prairie Forb

-The prairie forb Silene regia is restricted to small isolated populations in the eastern part of its range. I examined factors that limit fecundity and germination in these small populations. Although S. regia flowers rarely self due to dichogamy, geitonogamous crosses demonstrated self-compatibility. Substantial fruit production required pollination by ruby-throated hummingbirds (Archilochus colubris). Exclusion of hummingbirds (but not most insects) sharply reduced fruit production and seed numbers. Resource competition within individual inflorescences limited percent fruit set in some populations in certain years. Clipping of neighboring vegetation and watering S. regia did not increase reproductive output significantly. Silene regia seedings emerged at low rates (<5%) in field experiments, but field emergence increased twoto three-fold with soil disturbance and burning. Natural recruitment of seedlings occurred only in burned sites. Of all factors examined, burning had the greatest positive effects on reproductive output. In addition, controlling for the effects of burning, reproductive output was greater for plants in larger than smaller populations. By increasing fecundity and recruitment, prescribed burning can be used to expand population sizes of Silene regia.

Unusual Ruby-Throated Hummingbird Encounters and a Late Fall Record

Unusual Ruby-Throated Hummingbird Encounters and a Late Fall Record

AN ORNAMENTAL PASSION VINE FOR TEMPERATE ZONE CONDITIONS

Efforts to obtain edible-fruited passion vines through hybridization of Passiflora edulis and P. incarnata produced a population of tetraploid plants that survive freezing winters in central Georgia belowground, and grow aboveground in warm weather. One selection from this population blooms from late June or early July through October, yielding quantities of flowers from 8.5 to 9.6 cm diameter that have deep blue-colored sepals and petals surmounted by a disc of heavily-crimped filaments that are white at the outer margin. The nectar has proven a good food resource for the ruby-throated hummingbird, which breeds in much of the southeastern U.S. This clone is highly self-incompatible and sets no fruit when grown apart from cross-compatible clones. Its vines are vigorous, growing to 5 meters or more, and have dark green, markedly denticulate trilobed leaves 13 to 24.5 cm long by 15 to 25 cm wide. These afford a nursery habitat for caterpillars of 3 native butterflies, the zebra and the Gulf and variegated fritillaries. Because of its ease of culture and wide adaptation, this vine is recommended to plant in the continental U.S. for environmental enhancement.

A Possible Relationship between Reversed Sexual Size Dimorphism and Reduced Male Survivorship in the Ruby-Throated Hummingbird

An examination of 28 years of banding data from Powdermill Nature Reserve in southwestern Pennsylvania showed the following trends in the sex ratio (female: male) of Ruby-throated Hummingbirds (Archilochus colubris): 1.1:1 for immature hummingbirds caught in late summer and fall; 1.4:1 for adults in spring (April-May); 3.1:1 for adults in summer (June-7 August); and 4.1:1 for adults in fall (8 August-October). Hypotheses to explain these increasingly female-biased sex ratios include differential migration, capture bias and differential mortality. Banding data from eight other stations in eastern North America provided no evidence of different fall migration routes for males and females. We detected no capture bias at Powdermill, with respect to mesh size of mist nets, net wariness or net avoidance ability, but there was seasonal variation in the number of overlapping captures of males and females at individual mist nets: spring (81.5%); summer (27.6%); fall (44.7%). A capture bias resulting from differences between the sexes in the use of habitats sampled by our mist nets has likely contributed to observed sex ratio variation. Differential mortality, with males suffering greater losses than females, probably also contributed to this variation. Estimated annual survivorship calculated from returns of banded hummingbirds to Powdermill was lower for males (0.294) than females (0.446), but this difference was not significant. The mean minimum known age for returning male hummingbirds at Powdermill was significantly less than that for females; furthermore, the oldest known age for a returning male (three years) was half that observed for a female. The lower inferred survivorship for males may be related to reversed sexual size dimorphism in this species. The body mass of female Ruby-throated Hummingbirds was significantly greater than that of males and did not vary between May and August, whereas males weighed significantly less in June and July than they did in May and August. Low mid-summer mass in males, coupled with increased metabolic demands during the breeding season, may lead to a fatal "energy crisis" in this sex during nocturnal fasting or periods of inclement weather.

Hybridization and Introgression in Buckeyes (Aesculus: Hippocastanaceae): A Review of the Evidence and a Hypothesis to Explain Long-Distance Gene Flow

Evidence from morphology, distribution patterns, allozyme variation, and meiotic irregularities associated with decreased pollen germinability confirms the existence of a broad hybrid zone involving three parapatric species of Aesculus in the southeastern United States. The overall hybrid zone involving the three species is at least 200 km in width and probably represents the overlap of two hybrid zones: one between A. pavia and A. sylvatica and the other between A. flava and A. sylvatica. Both zones are highly asymmetrical, with hybrid populations occurring primarily in the Piedmont, where A. sylvatica is native. Detailed analyses of the hybrid zone involving A. pavia and A. sylvatica showed that hybrid populations consistently lack one or both of the putative parental species. Morphology and allozyme variation provide similar estimates of the position of the hybrid zone, but allozymes allow the detection of a larger zone than apparent on the basis of morphology. All available evidence is consistent with the hypothesis that extensive introgression has occurred among these species. Nevertheless, allozymic differentiation between these species is insufficient to reject hypotheses other than introgression that could generate the genetic structure observed in hybrid populations. Observations of pollinator activity in populations of A. pavia, A. sylvatica, A. flava, and their hybrids showed that these species share a number of important polli- nators, including several species of bumblebees (Bombus) and the ruby-throated hummingbird (Ar- chilochus colubris). Spring migration patterns of the ruby-throated hummingbird coincide closely with the flowering phenology of each of the Aesculus species, suggesting that hummingbirds could be vectors of long-distance pollen dispersal. The lifespan of pollen under field conditions is sufficient to permit transport of pollen in this fashion over distances of tens or hundreds of kilometers. Directional migration and arrival of hummingbirds after peak flowering would enforce a directional pattern of gene flow and could generate an asymmetrical hybrid zone of unusually great width.

Sexual Variations Among Plants of a Perfect-Flowered Species

Numerous environmental factors can influence the female and male reproductive success of hermaphroditic plants. These factors include, among others, the sexual status of neighboring plants, temporal changes in the number of seeds set in the population, and pollinator foraging. This study explores the interaction of such factors with the phenotypic gender of individual plants in a natural population of cardinal flower, Lobelia cardinalis. By measuring fecundity directly and estimating fecundation by assuming random mating, we show that the reproductive success of an individual is likely to be influenced by its phenotypic gender relative to others in the population, pollinator foraging, the timing of anthesis, temporal seed-set patterns in the population, and individual variation in seed and pollen production. Because the protandrous flowers are borne on an acropetally maturing raceme, the phenotypic gender of any individual plant begins as entirely male and changes to entirely female by the end of flowering. During the transition period from entirely male to entirely female, each plant exhibits considerable fluctuations in phenotypic gender. Moreover, the same trend holds for the entire population in which the percentage of open flowers in the staminate phase changes from 100% early in the season to 0% late in the flowering season but with unpredictable variations during mid-season. Mean seed set per day per pistillate-phase flower is also not constant during the flowering season. Much of the variation in daily seed set is attributable to the foraging behavior of the pollinator, the ruby-throated hummingbird. Supplementary hand-pollinations did not increase seed set relative to naturally pollinated plants, suggesting that seed production in this population is not limited by pollen reception. Considering this result and the environmental effects on male reproductive success, there appears to be greater certainty of success through the female than through the male reproductive functions. Although we have demonstrated that numerous environmental factors could partially obscure the relationship between a plant's phenotypic gender and its reproductive success through the female and male reproductive functions, our results indicate that the environment probably could not completely uncouple this relationship. This correlation between phenotypic and functional gender is likely because critical determinants of both gender estimates are differences among plants in seed and pollen production. These results are important because most theoretical studies of plant sexuality assume a strong correspondence between morphological and functional measures of sexuality. Our results, however, cannot be considered conclusive even for L. cardinalis plants; consequently, additional studies are needed.

Ruby-Throated Hummingbird Associating With a Foraging Hairy Woodpecker

Ruby-Throated Hummingbird Associating With a Foraging Hairy Woodpecker

Competition between hummingbirds and bumble bees for nectar in flowers of Impatiens biflora.

Using removal experiments and concurrent measurement of resource levels, evidence was obtained for exploitation competition between Ruby-throated hummingbirds and two bumble bee species (Bombus fervidus and B. vagans) foraging for nectar on Impatiens biflora.When all three species were active, flower visitors showed a complex pattern of resource partitioning involving both diel and spatial changes. Hummingbirds foraged almost exclusively from the outermost exposed flowers on plants from which they drained nectar levels beyond the reach of bees over most of the day. In contrast the longtongued bee species (B. fervidus), and the shorter-tongued B. vagans, displayed a preference for the innermost flowers on plants which were protected from hummingbird visitation by surrounding vegetation. The two Bombus spp. began foraging at different times during the day: B. vagans were most active in early morning but were replaced by B. fervidus later in the day.When hummingbirds were rare, only B. fervidus showed evidence of competitive release: an increase in the number of foragers and a broadening of flower choice to include more outer flowers. Workers of B. vagans showed a similar response to temporary removal of B. fervidus and also extended their foraging over the entire day. These responses were consistent with changes in the availability of nectar to different species.Removal experiments demonstrated that individuals of one species can be largely excluded from access to nectar resources as a direct result of exploitation of nectar by foragers of other species with longer tongues. Thus in this system interspecific exploitation is an important mechanism involved in resource partitioning.

Cues for orientation in hummingbird foraging: color and position

In this study we examined the relative influence of the color and position of food sources on the feeding patterns of hummingbirds. We studied ruby-throated hummingbirds (Archilochus colubns) in the field and rufous hummingbirds (Selasphorous rufus) in the laboratory. The field experiments demonstrated that the use of equally profitable feeders in arrays is more strongly influenced by position than by color. This suggests that hummingbirds learn and remember the locations of profitable food sources and return to them preferentially if the reward continues to be adequate. The laboratory experiments demonstrated that although hummingbirds learn quickly to return to a single profitable feeder in a linear array of unprofitable ones if it is marked by a consistent color cue, their learning is strongly influenced by position in the absence of color cues. This suggests that hummingbirds' spatial memory may be limited in resolution and that they may use visible landmarks to improve their performance. These results are consistent with the hypothesis that landmarks facilitate foragers' limited ability to learn and remember profitable locations in their environments.

Nectar extraction by hummingbirds: response to different floral characters.

Handling times of hummingbirds (Amazilia rutila and Cynanthus latirostris) visiting artificial flowers were a positive function of corolla length, nectar volume and nectar concentration. Corolla angle had no consistent effects on handling times. A multiple regression model explained 83% of the variation in handling times for these two species. The model also closely fit independent data from another hummingbird, Archilochus colubris, suggesting that it is general enough to apply to other medium-sized, short-billed hummingbird species. When examined across the range of variation normally encountered by hummingbirds in nature, corolla length and nectar volume had the largest effect on nectar extraction rates. At corolla lengths longer than a hummingbird's bill handling time increases markedly. Hummingbirds maximize their net rate of energy intake by selecting flowers with the shortest corolla, the highest nectar concentrations and the highest nectar volume. Since there is a positive relation between bill length and nectar extraction rate, it is surprising that most hummingbirds have relatively short bills.

What do foraging hummingbirds maximize?

Hainsworth and Wolf (1976) reported that under certain conditions hummingbirds made food choices which did not maximize their net rate of energy intake while foraging. They concluded that the birds were not foraging optimally. We show here that their birds probably maximized a different utility function, the net energy per unit volume consumed (NEVC), which appears to be an optimal choice on a time scale longer than that of a foraging bout. Our own experiments with Archilochus colubris support the conclusion that hummingbirds make foraging decisions that maximize NEVC. A simulation model shows that, in nature, NEVC maximization would require fewer foraging trips and visits to fewer flowers per day to balance daily energy budgets. For territorial birds this can lead to smaller territory sizes and reduced costs of territorial defense. Plants that evolutionarily increase corolla length to enhance pollinator specificity need only increase nectar concentration slightly to maintain the same net energy per unit volume consumed (NEVC) by a given hummingbird pollinator.

Hummingbird–sapsucker associations in northern climates

The ruby-throated hummingbird has the most extensive breeding distribution of any hummingbird species in North America. In its most northern distribution in Canada, its spring arrival often occurs before herbaceous flowers are in bloom, and in many parts of its range it breeds in habitats where floral nectar is scarce or unavailable. Evidence is presented to support the hypothesis that the dates of spring arrival and northern breeding distribution of the ruby throat, and probably the rufous hummingbird, are determined by a commensal relationship with the yellow-bellied sapsucker, in which the principal energy source for the hummingbirds is sucrose in sap from trees drilled by the sapsuckers.