Wintering hummingbirds in Alabama and Florida: species diversity, sex and age ratios, and site fidelity

ABSTRACT Over the past several decades, there have been numerous reports of hummingbirds wintering in the southeastern United States. However, little is known about the species present and their relative abundance. From November 1998 to March 2008, we examined the species diversity, sex and age ratios, and site fidelity of hummingbirds wintering in southern Alabama and northern Florida. We captured and banded 1598 individuals representing 10 species, and the most frequently captured species were Rufous Hummingbirds (Selasphorus rufus; 51.6%), Ruby‐throated Hummingbirds (Archilochus colubris; 23.5%), and Black‐chinned Hummingbirds (Archilochus alexandri; 16.9%). Other species captured included Buff‐bellied Hummingbirds (Amazilia yucatanensis), Calliope Hummingbirds (Stellula calliope), Allen's Hummingbirds (Selasphorus sasin), Broad‐tailed Hummingbirds (Selasphorus platycercus), Broad‐billed Hummingbirds (Cynanthus latirostris), Anna's Hummingbirds (Calypte anna), and Costa's Hummingbirds (Calypte costae). Most hummingbirds (71.8%) were captured in December and January. For most species, sex ratios were male‐biased for juveniles and female‐biased for adults, indicating possible differential mortality. Of 1598 hummingbirds captured, 144 representing five species returned to the same wintering location at least once. Female Rufous Hummingbirds (20.4% of individuals captured) exhibited the greatest site fidelity. Recaptures of banded Rufous Hummingbirds in autumn and early winter revealed that some individuals moved south into Alabama or Florida from Tennessee, northern Georgia, and northern Louisiana. Same‐season recaptures of banded Rufous Hummingbirds suggest that their spring migration route is west along the Gulf Coast. Our results suggest that Alabama and Florida are viable overwintering areas for several species of hummingbirds, with numbers of species and individuals higher than previously recognized. However, more study is needed to confirm migration routes and to determine if Ruby‐throated Hummingbirds wintering in our study area are year‐round residents or migrants.     

Structure of the vortex wake in hovering Anna's hummingbirds (Calypte anna)

Hummingbirds are specialized hoverers for which the vortex wake has been described as a series of single vortex rings shed primarily during the downstroke. Recent findings in bats and birds, as well as in a recent study on Anna''s hummingbirds, suggest that each wing may shed a discrete vortex ring, yielding a bilaterally paired wake. Here, we describe the presence of two discrete rings in the wake of hovering Anna''s hummingbirds, and also infer force production through a wingbeat with contributions to weight support. Using flow visualization, we found separate vortices at the tip and root of each wing, with 15% stronger circulation at the wingtip than at the root during the downstroke. The upstroke wake is more complex, with near-continuous shedding of vorticity, and circulation of approximately equal magnitude at tip and root. Force estimates suggest that the downstroke contributes 66% of required weight support, whereas the upstroke generates 35%. We also identified a secondary vortex structure yielding 8–26% of weight support. Lift production in Anna''s hummingbirds is more evenly distributed between the stroke phases than previously estimated for Rufous hummingbirds, in accordance with the generally symmetric down- and upstrokes that characterize hovering in these birds.     

Resource use,energetic profitability,and behavioral decisions in migrant rufous hummingbirds

Summary Migrant Rufous Hummingbirds (Selasphorus rufus) defend nectar resources at stopover sites while replenishing fat reserves needed for migratory flights. During late summer in the Sandia Mountains, central New Mexico, they defend the wasp- and bee-pollinated Scrophularia montana from other hummingbirds. Both hummingbirds and hymenopterans exploit Scrophularia nectar during the early part of its flowering period. As summer colony growth increases the densities of the eusocial hymenopterans by 100–150%, their exploitation of Scrophularia nectar lowers its mean standing crop in flowers by 200–300%. Sometime during the summer, Rufous Hummingbirds abandon and do not further use this resource for the remaining 3–4 weeks of its flowering period. The abandonment always occurs when the mean standing crop of nectar is approximately 0.2–0.3 L/ flower. This paper describes a model of Rufous Hummingbird energetics, that shows abandonment occurred 1–3 days after they passed the threshold at which the resource could have provided their minimum daily energy requirements. I suggest that constraints imposed by a highly competitive social environment severely reduced the options available to the hummingbirds, and caused them to continue to defend a resource that could no longer meet their energetic requirements.     

Hummingbird avoidance of nectar-robbed plants: spatial location or visual cues

Broad-tailed and rufous hummingbirds avoid plants and flowers that have recently been visited by nectar-robbing bees. However, the cues the hummingbirds use to make such choices are not known. To determine the proximate cues hummingbirds use to avoid visiting nectar-robbed plants, I conducted multiple field experiments and one aviary study using the nectar-robbed, hummingbird-pollinated plant Ipomopsis aggregata . In the first field experiment, free-flying hummingbirds were presented with plants in which I manipulated nectar volume and the presence of nectar-robber holes. Hummingbirds visited significantly more plants with nectar and probed more available flowers on those plants, regardless of the presence of nectar-robber holes. Thus, I hypothesized that hummingbirds may avoid robbed plants based on their spatial memory of unrewarding plants and/or visual cues that nectar absence provides. In an aviary study, I removed spatial cues by re-randomizing the position of plants after each hummingbird-foraging bout, but hummingbirds still selected plants with nectar. Nectar may provide a visual cue in I. aggregata flowers because corollas are translucent, and nectar is visible through the side of the corolla. To determine if hummingbirds use this visual cue to avoid plants with no nectar, I masked corolla translucence in a field study by painting flowers with acrylic paint. Hummingbirds still visited significantly more plants with nectar and probed more flowers on those plants, whether or not the corollas were painted. These results suggest that hummingbirds use nectar as a proximate cue to locate and avoid non-rewarding, nectar-robbed plants, even in the absence of spatial cues and simple visual cues.     

Hummingbirds fuel hovering flight with newly ingested sugar

We sought to characterize the ability of hummingbirds to fuel their energetically expensive hovering flight using dietary sugar by a combination of respirometry and stable carbon isotope techniques. Broadtailed hummingbirds (Selasphorus platycercus) were maintained on a diet containing beet sugar with an isotopic composition characteristic of C3 plants. Hummingbirds were fasted and then offered a solution containing cane sugar with an isotopic composition characteristic of C4 plants. By monitoring the rates of CO2 production and O2 consumption, as well as the stable carbon isotope composition of expired CO2, we were able to estimate the relative contributions of carbohydrate and fat, as well as the absolute rate at which dietary sucrose was oxidized during hovering. The combination of respirometry and carbon isotope analysis revealed that hummingbirds initially oxidized endogenous fat following a fast and then progressively oxidized proportionately more carbohydrates. The contribution from dietary sources increased with each feeding bout, and by 20 min after the first meal, dietary sugar supported approximately 74% of hovering metabolism. The ability of hummingbirds to satisfy the energetic requirements of hovering flight mainly with recently ingested sugar is unique among vertebrates. Our finding provides an example of evolutionary convergence in physiological and biochemical traits among unrelated nectar-feeding animals.     

Energetics of small body size and high latitude the Rufous Hummingbird in coastal Alaska

The Rufous Hummingbird is the smallest bird breeding at latitudes of 58° to 60°. Nesting chronology and food sequence was observed. The microclimate of the nest-site and its potential for heat loss during incubation were similar to those of hummingbirds in the Rocky Mountains at 39°N. Hummingbirds of lower latitudes increase their active-daylength in direct proportion to solar daylength (active day = 1.03 × solar day), whereas the Rufous Hummingbird tended to utilize less of the long daylength of coastal Alaska (active day = 197 min + 0.83 × solar day).     

Respiratory evaporative water loss during hovering and forward flight in hummingbirds

Hummingbirds represent an end point for small body size and water flux in vertebrates. We explored the role evaporative water loss (EWL) plays in management of their large water pool and its use in dissipating metabolic heat. We measured respiratory evaporative water loss (REWL) in hovering hummingbirds in the field (6 species) and over a range of speeds in a wind tunnel (1 species) using an open-circuit mask respirometry system. Hovering REWL during the active period was positively correlated with operative temperature (T_e) likely due to some combination of an increase in the vapor-pressure deficit, increase in lung ventilation rate, and reduced importance of dry heat transfer at higher T_e. In rufous hummingbirds (Selasphorus rufus; 3.3 g) REWL during forward flight at 6 and 10 m/s was less than half the value for hovering. The proportion of total dissipated heat (TDH) accounted for by REWL during hovering at T_e > 40 °C was < 40% in most species. During forward flight in S. rufus the proportion of TDH accounted for by REWL was ~ 35% less than for hovering. REWL in hummingbirds is a relatively small component of the water budget compared with other bird species (< 20%) so cutaneous evaporative water loss and dry heat transfer must contribute significantly to thermal balance in hummingbirds.     

Macroecological patterns of resource use in resident and migratory hummingbirds

Hummingbirds (Family Trochilidae) are key pollinators in several biodiversity hotspots, including the California Floristic Province in North America. Relatively little is known about how hummingbird diets change throughout the year, especially with regard to how migratory hummingbirds affect resident hummingbirds at stopover sites. In this study, we examine how hummingbird species, migratory status, sex, geographic region and local plant diversity influence floral resource use before, during, and after an influx of migratory hummingbirds (primarily Rufous hummingbirds, Selasphorus rufus) across California. We expected distinct floral resource use based upon species’ migratory status (resident vs. migrant), sex, sampling period, and geographic region. We employed DNA metabarcoding to detect plant DNA in hummingbird fecal samples to analyze diet diversity, composition, overlap, and interaction networks. We found significant effects of sex, sampling period, and migratory status on the alpha and beta diversity of plant taxa present in fecal samples. Analyses of Anna's hummingbirds (Calypte anna) alone revealed that female fecal samples contained higher plant species richness. In addition to hummingbird-pollinated plants, fecal samples also contained non-ornithophilous plants and species of agricultural importance. Diet overlap and plant-pollinator network analyses revealed high overlap in plant taxa used between hummingbird species, and networks were more connected, less nested, and less specialized than null models. DNA metabarcoding is minimally invasive and provides a detailed view of hummingbird diet, permitting large-scale studies. Insights into hummingbird diets are especially valuable given the logistical difficulties of directly observing floral visitation and foraging across broad temporal and spatial scales.     

A female Lucifer Hummingbird (Calothorax lucifer) with iridescent chin feathers

ABSTRACT.   We report an observation of a female Lucifer Hummingbird ( Calothorax lucifer ) with iridescent feathers on the chin, resembling the plumage of the juvenile male. The female and nest were found in a xeric shrubland in Barranca de Metztitlán Biosphere Reserve, Hidalgo State, Mexico. This is the first definitive report of a breeding female with such plumage, supporting a previous observation in which sex was not confirmed by behavior. Although this condition appears be rare in female Lucifer Hummingbirds, females in other species of hummingbirds exhibit much variation in the amount of iridescent plumage on the chin and in some, such as Costa's Hummingbirds ( Calypte costae ), females commonly exhibit colored feathers on the chin.     

Maximal horizontal flight performance of hummingbirds: effects of body mass and molt

Hovering and fast forward flapping represent two strenuous types of flight that differ in aerodynamic power requirement. Maximal capabilities of ruby-throated hummingbirds (Archilochus colubris) in hovering and forward flight were compared under varying body mass and wing area. The capability to hover in low-density gas mixtures was adversely affected by body mass, whereas the capability to fly in a wind tunnel did not show any adverse mass effect. Molting birds that lost primary flight feathers and reduced wing area also displayed mass loss and loss of aerodynamic power and flight speed. This suggests that maximal flight speed is insensitive to short-term perturbations of body mass but that molting is stressful and reduces the birds' speed and capacity for chase and escape. Hummingbirds' flight behavior in confined space was also investigated. Birds reduced their speeds flying through a narrow tube to approximately one-fifth of that in the wind tunnel and did not display differences under varying body mass and wing area. Hence, performance in the flight tube was submaximal and did not correlate with performance variation in the wind tunnel. For ruby-throated hummingbirds, both maximal mass-specific aerodynamic power derived from hovering performance in low-density air media and maximal flight velocity measured in the wind tunnel were invariant with body mass.     

Comparative energetics of the giant hummingbird (Patagona gigas)

Hummingbirds (family Trochilidae) represent an extreme outcome in vertebrate physiological design and are the only birds capable of sustained hovering. The giant hummingbird (Patagona gigas) is the largest trochilid, with a mass of ~20 g, and is found over an altitudinal range from 0 to 4,500 m above sea level. We report here measurements of daily, basal, and hovering rates of oxygen consumption in the giant hummingbird; compare these values with data from smaller hummingbirds; and assess overall metabolic and allometric limits to trochilid body size. The sustained metabolic scope (i.e., the ratio of daily energy expenditure to basal metabolic rate) in the giant hummingbird is higher than that in smaller hummingbirds but lies below a proposed theoretical maximum value for endotherms. Scaling exponents in the allometric relationships for different modes of energetic expenditure were comparable, suggesting that the giant hummingbird, although a clear outlier in terms of body size, does not obviously deviate from metabolic relationships derived from other trochilid taxa.     

Distinguishing Energy Maximizers from Time Minimizers: A Comparative Study of Two Hummingbird Species

SYNOPSIS. The potential reproductive success of a food energymaximizer increases with foraging time, while that of a foragingtime minimizer increases with time spent in nonforaging activitiesgiven a set energy requirement has been met. How can these foraging"goals" be distinguished for nonbreeding animals in the field?If individuals of two species occupying the same habitat consumethe same foods, face similar foraging constraints, and havesimilar meal sizes (food intake per foraging bout), then relativeto a time minimizer, an energy maximizer should: (1) spend moretime foraging, with greater foragingbout frequency, but no differencein foraging-bout duration; (2) spend less time sitting, withlower sitting-bout duration yet greater sitting-bout frequency;(3) gain mass more rapidly, if net energy intake results inmass accumulation; and (4) exhibit no other differences in timebudgeting. These assumptions and predictions were verified bypopulation- and individual-level comparisons of immature malesof two species of nectar-feeding hummingbirds studied over threefield seasons. The results suggest that, relative to each other,migrant Rufous Hummingbirds are energy maximizers and nonmigrantCosta Hummingbirds are time minimizers. Despite significantdifferences in time budgeting, by far the most striking differencebetween the species was that the Rufous gained mass four toeight times as rapidly as the Costa. This was due to the Rufousentering torpor at night, resulting in relatively little overnightloss in body mass. These patterns underscore the importanceof measuring net energy intake as directly as possible (in thiscase by fat accumulation) in testing foraging theory. Indirectmeasures (such as time budgets) may not always provide the resolutionnecessary to detect important energetic differences betweendifferent foragers.     

The role of size and dominance in the feeding behaviour of coexisting hummingbirds

Interspecific competition can strongly influence community structure and limit the distribution and abundance of species. One of the main factors that determine hummingbird community structure is competition for food. The temporal and spatial distribution of nectar has a strong impact on hummingbird assemblages, shaping foraging niches according to hummingbird dominance and foraging strategy. We investigated whether body size and the degree of aggressive dominance influence feeding behaviour of hummingbirds in a temperate forest in northwestern Mexico (El Palmito, Mexico) when winter migrant hummingbirds are present in the community. First, we determined the dominance status of hummingbirds and evaluated the relationship between dominance and body mass, wing disc loading and migratory status. Secondly, we determined how hummingbird species used plant species differently. Thirdly, we examined whether the most dominant hummingbird species defended floral patches with more energy and/or with a larger number of flowers. At each flower patch, hummingbird species, number of hummingbird interactions, feeding time and number of flowers present were recorded. The total number of calories available within each floral patch was also determined. Our results demonstrate that the dominance hierarchy of 13 hummingbird species (migratory and resident) was correlated with body size but not wing disc loading, and that members of the hummingbird community showed a clear separation in resource use (by plant species). Hummingbirds at the top of the dominance hierarchy defended and fed on the best flower patches, defined by the quantity of calories available. Hence, the feeding behaviour of hummingbirds at El Palmito depends on the abundance of plant species used by hummingbirds and on the amount of energy available from each flower patch. Thus, hummingbird body size, aggressive dominance and defence of quality flower patches determines niche partitioning among species.     

Hovering and intermittent flight in birds

Two styles of bird locomotion, hovering and intermittent flight, have great potential to inform future development of autonomous flying vehicles. Hummingbirds are the smallest flying vertebrates, and they are the only birds that can sustain hovering. Their ability to hover is due to their small size, high wingbeat frequency, relatively large margin of mass-specific power available for flight and a suite of anatomical features that include proportionally massive major flight muscles (pectoralis and supracoracoideus) and wing anatomy that enables them to leave their wings extended yet turned over (supinated) during upstroke so that they can generate lift to support their weight. Hummingbirds generate three times more lift during downstroke compared with upstroke, with the disparity due to wing twist during upstroke. Much like insects, hummingbirds exploit unsteady mechanisms during hovering including delayed stall during wing translation that is manifest as a leading-edge vortex (LEV) on the wing and rotational circulation at the end of each half stroke. Intermittent flight is common in small- and medium-sized birds and consists of pauses during which the wings are flexed (bound) or extended (glide). Flap-bounding appears to be an energy-saving style when flying relatively fast, with the production of lift by the body and tail critical to this saving. Flap-gliding is thought to be less costly than continuous flapping during flight at most speeds. Some species are known to shift from flap-gliding at slow speeds to flap-bounding at fast speeds, but there is an upper size limit for the ability to bound (~0.3 kg) and small birds with rounded wings do not use intermittent glides.     

Temporal partitioning of a floral resource by territorial hummingbirds

Most studies of territoriality in hummingbirds have focused on intraspecific competition for resources and the consequences for the spatial distribution of individuals within a habitat. As a result, we know little of the effects of interspecific competition for resources and less still of temporal resource partitioning. Here I describe the interactions of four species of tropical hummingbird which defended the same territory at different stages in the flowering period and at different times of the day. The pattern of territory defence was greatly influenced by the dominance hierarchy between species and the costs and benefits of territory ownership. I used a simple economic model to calculate the predicted territory size based on four potential strategies. Hummingbirds appeared to be defending territories of the smallest economical size, agreeing with two hypotheses: (1) that hummingbirds minimize the cost of territory ownership and (2) that hummingbirds maximize the time spent sitting. The model predicted accurately the observed pattern of territory acquisition; hummingbirds initiated defence as soon as the territory contained sufficient resources and were either displaced by a larger species or replaced by a smaller one as the value of the territory changed.     

Evidence of linked selection on the Z chromosome of hybridizing hummingbirds*

Levels of genetic differentiation vary widely along the genomes of recently diverged species. What processes cause this variation? Here, I analyze geographic population structure and genome-wide patterns of variation in the Rufous, Allen's, and Calliope Hummingbirds (Selasphorus rufus/Selasphorus sasin/Selasphorus calliope) and assess evidence that linked selection on the Z chromosome drives patterns of genetic differentiation in a pair of hybridizing species. Demographic models, introgression tests, and genotype clustering analyses support a reticulate evolutionary history consistent with divergence during the late Pleistocene followed by gene flow across migrant Rufous and Allen's Hummingbirds during the Holocene. Relative genetic differentiation () is elevated, and within-population diversity (π) is depressed on the Z chromosome in all interspecific comparisons. The ratio of Z to autosomal within-population diversity is much lower than that expected from population size effects alone, and Tajima's D is depressed on the Z chromosome in S. rufus and S. calliope. These results suggest that conserved structural features of the genome play a prominent role in shaping genetic differentiation through the early stages of speciation in northern Selasphorus hummingbirds, and that the Z chromosome is a likely site of genes underlying behavioral and morphological variation in the group.     

Observational Learning in the White-Eared Hummingbird (Hylocharis leucotis): Experimental Evidence

The adoption of new food resources can be facilitated by the ability to learn through observation of other individuals who use them. This behavior, termed observational learning, applies to any problem solving in which a naive individual who has observed an experienced individual learns a behavior faster than another who has not. Hummingbirds consume nectar from flowers of a large number of plant species, which are very diverse in morphology and color. During their local or migratory movements, they can observe the use of floral resources by conspecifics and heterospecifics which may change their foraging preferences. Although there is evidence that hummingbirds can use observational learning to exploit new floral resources, it is necessary to generate additional information by studying different hummingbird species. In this work, the learning performance of White‐eared hummingbirds (Hylocharis leucotis) was studied in the presence or absence of a knowledgeable tutor. In a first experiment, naïve hummingbirds learned to feed on arrays of artificial flower of two colors: red (previously known resource) and yellow (novel resource), where only one color had nectar. Naive hummingbirds visited red flowers faster and more often than rewarded yellow flowers. Individuals with the best performance on each color were further trained to ensure that they only visited flowers of a specific color, and were then used as tutors in the next experiment, in which new naive hummingbirds, caged individually, were allowed to observe them foraging on the artificial arrays. These naïve individual were then exposed alone to the same array used by their tutor. Tutored hummingbirds learned to feed faster and more frequently from nectar‐containing flowers of the array than naive individuals. Likewise, all tutored individuals only visited flowers of the color that had been previously visited by their tutors. This study provides experimental evidence that hummingbirds taken directly from the field can use observational learning as an efficient strategy to access new floral resources.     

Nectar robbing in Ipomopsis aggregata : effects on pollinator behavior and plant fitness

Hummingbirds foraging in alpine meadows of central Colorado, United States, face a heterogeneous distribution of nectar rewards. This study investigated how variability in nectar resources caused by nectar-robbing bumblebees affected the foraging behavior of hummingbird pollinators and, subsequently, the reproductive success of a host plant (Ipomopsis aggregata). We presented hummingbirds with experimental arrays of I. aggregata and measured hummingbird foraging behavior as a function of known levels of nectar robbing. Hummingbirds visited significantly fewer plants with heavy nectar robbing (over 80% of available flowers robbed) and visited fewer flowers on those plants. These changes in hummingbird foraging behavior resulted in decreased percent fruit set as well as decreased total seed set in heavily robbed plants. These results indicate that hummingbird avoidance of nectar-robbed plants and flowers reduces plant fitness components. In addition, our results suggest that the mutualisms between pollinators and host plants may be affected by other species, such as nectar robbers. Received: 22 April 1998 / Accepted: 12 May 1998     

The foraging behaviour of Australian honeyeaters: a review and some comparisons with hummingbirds

The foraging behaviour of Australian honeyeaters is reviewed in terms of diet, foraging selectivity, foraging flight mode, quality and quantity of nectar encountered per flower, flower densities encountered and effect of predation. At the same time comparisons are made between honeyeaters and hummingbirds. These two groups of birds are superficially similar. Both feed on nectar and insects. Both tend to have long curved bills and tongues adapted for removal of nectar from flowers. Both tend to feed at long, red flowers. However, on close inspection, honeyeaters and hummingbirds are quite dissimilar. For example, many honeyeaters include fruit in their diets. Hummingbirds almost never eat fruit. Honeyeaters appear to be considerably less nectarivorous and more insectivorous than hummingbirds. Honeyeaters are, for the most part, larger than hummingbirds and they usually perch while feeding whereas hummingbirds usually hover. Honeyeaters but not hummingbirds often flock while feeding. Predation appears to be considerably more important for honeyeaters than for hummingbirds. Territorial defense of flowers seems common in hummingbirds but uncommon in honeyeaters. These differences are discussed in detail and explanations are offered for them wherever possible.     

Nectar secretion dynamic links pollinator behavior to consequences for plant reproductive success in the ornithophilous mistletoe Psittacanthus robustus

The mistletoe Psittacanthus robustus was studied as a model to link flower phenology and nectar secretion strategy to pollinator behaviour and the reproductive consequences for the plant. The bright‐coloured flowers presented diurnal anthesis, opened asynchronously throughout the rainy season and produced copious dilute nectar as the main reward for pollinators. Most nectar was secreted just after flower opening, with little sugar replenishment after experimental removals. During the second day of anthesis in bagged flowers, the flowers quickly reabsorbed the offered nectar. Low values of nectar standing crop recorded in open flowers can be linked with high visitation rates by bird pollinators. Eight hummingbirds and two passerines were observed as potential pollinators. The most frequent flower visitors were the hummingbirds Eupetomena macroura and Colibri serrirostris, which actively defended flowering mistletoes. The spatial separation between anthers, stigma and nectar chamber promotes pollen deposition on flapping wings of hovering hummingbirds that usually probe many flowers per visit. Seed set did not differ between hand‐, self‐ and cross‐pollinated flowers, but these treatments set significantly more seeds than flowers naturally exposed to flower visitors. We suggest that the limitation observed in the reproductive success of this plant is not related to pollinator scarcity, but probably to the extreme frequency of visitation by territorial hummingbirds. We conclude that the costs and benefits of plant reproduction depend on the interaction strength between flowers and pollinators, and the assessment of nectar secretion dynamics, pollinator behaviour and plant breeding system allows clarification of the complexity of such associations.