Female Hummingbirds Research Articles

Vocal dimorphism in Anna's hummingbirds

Sexual dimorphism in vocal signals is found across many bird species. By evaluating correlations among sex, behaviors, and vocalizations, the meaning and utility of sound production may be inferred. Anna's hummingbirds Calypte anna exhibit pronounced sexual dimorphism, with males being larger and having more vibrant coloration than females, but vocal dimorphism in the species is less studied. A common vocalization of Anna's hummingbirds is the chip note, which is produced by both sexes in a wide array of contexts. Here, we correlated temporal parameters of recorded chip notes with individual sex and behavioral contexts gathered from field observations. The production rate of chip notes differed significantly between male and female hummingbirds but did not vary much with behavioral context. Although Anna's hummingbirds produce chips across a broad spectrum of behaviors, dimorphic chip production may be especially important in territorial behavior.

Free-living Allen’s hummingbirds (Selasphorus sasin) rarely use torpor while nesting

For reproducing animals, maintaining energy balance despite thermoregulatory challenges is important for surviving and successfully raising offspring. This is especially apparent in small endotherms that exhibit high mass-specific metabolic rates and live in unpredictable environments. Many of these animals use torpor, substantially reducing their metabolic rate and often body temperature to cope with high energetic demands during non-foraging periods. In birds, when the incubating parent uses torpor, the lowered temperatures that thermally sensitive offspring experience could delay development or increase mortality risk. We used thermal imaging to noninvasively explore how nesting female hummingbirds sustain their own energy balance while effectively incubating their eggs and brooding their chicks. We located 67 active Allen’s hummingbird (Selasphorus sasin) nests in Los Angeles, California and recorded nightly time-lapse thermal images at 14 of these nests for 108 nights using thermal cameras. We found that nesting females usually avoided entering torpor, with one bird entering deep torpor on two nights (2% of nights), and two other birds possibly using shallow torpor on three nights (3% of nights). We also modeled nightly energetic requirements of a bird experiencing nest temperatures vs. ambient temperature and using torpor or remaining normothermic, using data from similarly-sized broad-billed hummingbirds. Overall, we suggest that the warm environment of the nest, and possibly shallow torpor, help brooding female hummingbirds reduce their own energy requirements while prioritizing the energetic demands of their offspring.

Sex-specific flower resource use in hummingbird communities.

Research Highlight: Maglianesi, M. A., Maruyama, P. K., Temeles, E. J., Schleuning, M., Zanata, T. B., Sazima, M., Gutiérrez-Zamora, A., Marín-Gómez, O. H., Rosero-Lasprilla, L., Ramírez-Burbano, M. B., Ruffini, A. E., Salamanca-Reyes, J. R., Sazima, I., Nuñez-Rosas, L. E., Arizmendi, M. C., Rahbek, C., & Dalsgaard, B. (2022). Behavioural and morphological traits influence sex-specific floral resource use by hummingbirds across the Americas. Journal of Animal Ecology, 00: 00-00. https://doi.org/10.1111/1365-2656.13746. In their paper on intersexual differences in niche breadth and niche overlap in floral resource use in hummingbird communities, Maglianesi et al. compiled data of plant-hummingbird interactions based on pollen loads, and territoriality and morphological traits for 31 hummingbird species, and investigated whether patterns of resource use by females and males were related to sexual dimorphism and foraging behaviour. While accounting for evolutionary relatedness among species, the authors found a high level of resource partitioning between sexes (broader and more dissimilar floral niche breadth in females) and the sex-specific resource use by hummingbird species was related to territoriality and morphological traits; niche overlap between sexes was greater for territorial than non-territorial species, and lower for species with greater sexual dimorphism in bill curvature. This paper addresses two very timely issues, on the one hand on resource partitioning by sex to reduce intersexual competition in hummingbirds, and on the other hand highlight the much needed information on foraging ecology of female hummingbirds for better understanding of intersexual variation in shaping coexistence and species diversity in hummingbird communities and the interactions between plants and their hummingbird pollinators.

Why some female hummingbirds mimic males: it’s all about nectar

Why some female hummingbirds mimic males: it’s all about nectar

Nest description, nest survival, and habitat use of three resident hummingbird species in western Mexico

Nesting is a particularly sensitive period during which birds are highly vulnerable to predation, parasitism, and other natural events resulting in high mortality rates. Some factors can increase nesting success, such as habitat selection. In general, only female hummingbirds build nests, incubate eggs, and provide nestling care until chicks can feed independently. The purpose of our study was to describe nests (shape, size, and construction materials), nest survival, and habitat use of 3 resident hummingbirds in western Mexico: Cinnamon Hummingbird (Amazilia rutila), Golden-crowned Emerald (Cynanthus auriceps; a Mexican endemic), and Broad-billed Hummingbird (C. latirostris). In all 3 species, the nest is an open cup built with plant material and bound together with spider web. All 3 species used the same materials (silk cotton, wood, and grass) for nest construction. However, Cinnamon Hummingbird was the only one that used lichens at its outer surface, whereas Broad-billed Hummingbird used only seeds. We did not find interspecific differences among the materials used and their quantities. A low nest survival (26%) can be explained by the high rate of predation but also by the occurrence of natural events such as hurricanes. All 3 species showed preference for nesting in semi-deciduous forests but chose different nest placement sites that differed in (1) nest height, (2) the height of trees or shrubs where nests were built, and (3) the diameter of the supporting branch within a shrub or tree. Other factors, such as distance to the nearest stream, and distance to nesting-branch tip or trunk, did not differ among species.

True grit: ingestion of small stone particles by hummingbirds in West Mexico

BackgroundGrit is used by birds mainly for grinding hard food items but can also serve a nutritional role as a source of minerals. Ingestion of grit by birds has been documented primarily in species that feed on seeds and invertebrates. Although feeding mainly on nectar and small arthropods, hummingbirds also ingest grit, but why they do so is unclear. We quantified the number of grit particles in the stomachs of six species of hummingbirds during an annual cycle in a seasonal ecosystem of West Mexico.MethodsWe compared the number of grit particles in the stomachs of different hummingbird species (Mexican Violetear Colibri thalassinus, Amethyst-throated Mountaingem Lampornis amethystinus, White-eared Hummingbird Basilinna leucotis, Rivoli’s Hummingbird Eugenes fulgens, Broad-tailed Hummingbird Selasphorus platycercus, and Rufous Hummingbird S. rufus), and between sex and age categories during the different seasons of a year. To determine if grit was used to grind ingested arthropods, we examined the relationships between the number of grit particles, the biomass of arthropods ingested, and their chitin content.ResultsAlthough species did not differ in the number of grit particles in their stomachs, we found that grit was mostly ingested by female individuals, with only one male of one species (Mexican Violetear) presenting grit in its stomach. We also found that female hummingbirds had grit in their stomachs during the rainy and the cold-dry season (June–February) but not during the warm-dry season (March–May). Our analyses revealed no relationship between the number of grit particles and the amount of ingested arthropods and arthropod chitin content. However, high grit consumption was related to wasp ingestion on Mexican Violetears.ConclusionsOur results indicate that grit is used mainly by female hummingbirds. The seasonal variation in the ingestion of grit by female individuals suggests that it can be used to meet mineral requirements related to breeding; however, this topic needs further exploration. Additionally, the use of grit was proportionally higher in juvenile individuals, suggesting it is used for grinding arthropods during a period of fast development.

Microbiome composition of Anna's hummingbirds differs among regions of the gastrointestinal tract

The gastrointestinal (GI) microbiome is an important mediator and indicator of host physiology, health and fitness in many vertebrate systems, but sources of variation in microbiome composition within species are poorly understood, particularly in free‐ranging birds. Hummingbirds have GI anatomy and physiology similar to other avian species, but they have a relatively rapid gastrointestinal transit rate, which could affect microbiome structure. To date, it has not been elucidated if microbiomes isolated from feces and samples from sections of the GI tract differ in an avian organism that has a rapid gastrointestinal time. Here, we test the hypothesis that the microbiome in free‐ranging and wildlife rehabilitation‐sourced Anna's hummingbirds Calypte anna differ between fecal and samples from three sections of the GI tract and between birds that differ in sex, age and poxvirus infection status. We characterized bacterial composition in fecal and GI tract samples from Anna's hummingbirds by amplicon sequencing of the 16S rRNA gene. We found strong evidence of differentiation in bacterial composition among GI tract regions and compared it to fecal samples. Actinobacteria, primarily genus Corynebacterium, Firmicutes and Fusobacteria were abundant in bird samples from regions of the GI tract. In contrast, fecal bacterial communities were more diverse and variable compared to GI tract samples. Bacterial community composition differed between male and female hummingbirds and with bird age (hatch year vs after‐hatch year). Finally, birds with symptoms of avian poxvirus infection had a higher relative abundance of Staphylococcus spp. than birds with no symptoms of pox. Our results suggest that Anna's hummingbirds host differentiated microbiome among GI tract regions that is consistent among individual birds. The GI bacterial community also contained taxa not represented in fecal samples. This provides evidence for the possibility of a residential gut microbiome in Anna's hummingbirds, although functional significance of the bacterial microbiome remains unknown.

Male-like ornamentation in female hummingbirds results from social harassment rather than sexual selection

Ornamentation is typically observed in sexually mature adults, is often dimorphic in expression, and is most apparent during breeding, supporting a role for sexual selection in its evolution.1-4 Yet, increasing evidence suggests that nonsexual social selection may also have a role in the evolution of ornamentation, especially in females.5-9 Distinguishing between these alternatives remains challenging because sexual and nonsexual factors may both play important and overlapping roles in trait evolution.7,10 Here, we show that female ornamentation in a dichromatic hummingbird, the white-necked jacobin (Florisuga mellivora), cannot be explained by sexual selection. Although all males are ornamented, nearly 30% of females have male-like plumage. Remarkably, all juveniles of both sexes express ornamented plumage similar to adult males (androchromatism), but 80% of females acquire non-ornamented plumage (heterochromatism) as they age. This unique ontogeny excludes competition for mates as an explanation for female ornamentation because non-reproductive juveniles are more likely to be ornamented than adults. Instead, avoidance of social harassment appears to underlie this female-limited polymorphism, as heterochrome taxidermy mounts received more aggressive and sexual attention than androchrome mounts from this and other hummingbird species. Monitoring electronically tagged birds at data-logging feeders showed that androchrome females accessed feeders more than heterochrome females, presumably because of reduced harassment. Our findings demonstrate that ornamentation can arise purely through nonsexual social selection, and this hypothesis must be considered in the evolution of not only female-limited polymorphism but also the spectacular ornamentation often assumed to result from sexual selection.

Pairing automated mark-recapture and social network models to explore the effects of landscape configuration on hummingbird foraging patterns.

Landscape changes can alter pollinator movement and foraging patterns which can in turn influence the demographic processes of plant populations. We leveraged social network models and four fixed arrays of five hummingbird feeders equipped with radio frequency identification (RFID) data loggers to study rufous hummingbird (Selasphorus rufus) foraging patterns in a heterogeneous landscape. Using a space-for-time approach, we asked whether forest encroachment on alpine meadows could restrict hummingbird foraging movements and impede resource discovery. We fit social network models to data on 2221 movements between feeders made by 29 hummingbirds. Movements were made primarily by females, likely due to male territoriality and early migration dates. Distance was the driving factor in determining the rate of movements among feeders. The posterior mean effects of forest landscape variables (local canopy cover and intervening forest cover) were negative, but with considerable uncertainty. Finally, we found strong reciprocity in hummingbird movements, indicative of frequent out and back movements between resources. Together, these findings suggest that reciprocal movements by female hummingbirds could help maintain bidirectional gene flow among nearby subpopulations of ornithophilous plants; however, if the distance among meadows increases with further forest encroachment, this may limit foraging among progressively isolated meadows.

Female song in Costa's Hummingbird (Calypte costae)

ABSTRACT Bird song has been extensively studied in oscine passerines, but far less is known about song in hummingbirds, which are distantly related to passerines. Song is generally thought of as a male trait. Here we document song in 2 wild female Costa's Hummingbirds (Calypte costae) of 10 examined. Their vocalizations closely resembled male song and were produced, undirected, on territories just prior to the breeding season. Both birds were confirmed as female and not young males, which resemble females: one had an enlarged ovary, and the other laid eggs. Our results imply that at least some female hummingbirds may learn song as well as males and have similar song capacity as males. The function, if any, of female song was not immediately clear. Female song seemed rarer in this population than in males and thus easy to overlook. Presence of female song could also be an overlooked phenomenon in other hummingbird species.

Evaluation of Proctophyllodes huitzilopochtlii on feathers from Anna's (Calypte anna) and Black-chinned (Archilochus alexandri) Hummingbirds: Prevalence assessment and imaging analysis using light and tabletop scanning electron microscopy.

Proctophyllodes huitzilopochtlii Atyeo & Braasch 1966 (Acariformes: Astigmata: Proctophyllodidae), a feather mite, was found on feathers collected from five hummingbird species in California. This mite has not been previously documented on feathers from Anna’s (Calypte anna [Lesson 1829]) or Black-chinned (Archilochus alexandri [Bourcier & Mulsant 1846]) Hummingbirds. A total of 753 hummingbirds were evaluated for the presence of mites by species (Allen’s n = 112; Anna’s n = 500; Black-chinned n = 122; Rufous n = 18; Calliope n = 1), sex (males n = 421; females n = 329; 3 unidentified), and age (juvenile n = 199; after-hatch-year n = 549; 5 unidentified). Of these 753 hummingbirds evaluated, mites were present on the rectrices of 40.9% of the birds. Significantly more Anna’s Hummingbirds were positive for rectricial mites (59.2%) compared with 8.2% of Black-chinned, 0.9% of Allen’s, 5.6% of Rufous Hummingbirds, and 0% for Calliope (p-value < 0.0001). Across all hummingbird species, male hummingbirds (44.9%) had a higher prevalence of rectricial mites compared to female hummingbirds (36.2%; p-value = 0.004), while juvenile hummingbirds (46.2%) had a non-significantly higher prevalence compared to after-hatch-year hummingbirds (39.0%; p-value = 0.089). On average, the percentage of the long axis of the rachis occupied by mites for the outer rectrices (R4 and R5) was 19%, compared to 11% for inner rectrices (R1 and R2), a significant difference (p-value = <0.0001). There was a marginal lack of significance for symmetrical distribution of tail mites with the mean left side percentage of long axis of the rachis occupied by mites being 16% and very close to the mean right side score of 18% (p-value = 0.003). The identification of the feather mite species was based on light microscopic morphometry, and mite distribution on feathers was further evaluated using tabletop scanning electron microscopy (TSEM). The hummingbird–feather mite relationship is not well understood, but the specialized TSEM technique may be especially useful in examining natural positioning and developmental aspects of the mites since it allows in situ feather examination of live mites.

Resource partitioning by color in a tropical hummingbird

Many nectar-feeding animals use cues (floral signals) to locate, identify, and discriminate between floral resources. Whether competing pollinators can use cues to partition resources, however, has received little attention despite examples of pollinators differing in resource use. We examined whether sexes of the purple-throated carib hummingbird, Anthracothorax jugularis, partition artificial nectar sources (feeders) by color (red or yellow) or position (left or right), and whether such partitioning changes in response to changes in nectar reward. At equal nectar concentrations, sexes partitioned resources by color, but when one of the colors was reduced in quality, dominant males visited the more rewarding color, although they did not completely displace subordinate females. The position of the feeder had no effect on resource partitioning, and in the absence of color cues, male and female hummingbirds did not partition feeders. Our results differ from earlier studies where hummingbirds responded more to position than to color. We suggest that this difference may result from defense of patches as opposed to choice of individual food items as well as from differences between temperate and tropical hummingbird-plant communities in floral colors. Hummingbird-pollinated plants in many temperate communities converge on red flowers, which may eliminate color as a cue for resource partitioning, whereas the availability of plant species differing in floral color and pattern may provide tropical nectarivores with cues for resource subdivision. Our results have major implications for the evolution of species diversity. Because plants and pollinators may reciprocally affect each other’s evolution, partitioning of floral resources by color may not only affect traits of pollinators but also may maintain and drive the diversification of floral colors and other floral traits.

Female hummingbirds do not relocate rewards using colour cues

Males generally outperform females in spatial tasks. This difference in spatial performance may reflect differences in cue preference because males often use both spatial cues (distance and direction) and feature cues, whereas females prefer to use feature cues. However, studies in birds are few and results are conflicting. As wild male rufous hummingbirds, Selasphorus rufus, prefer to use spatial cues to relocate a rewarded flower, in the present study we tested free-flying wild female hummingbirds of three different species (rufous, white-eared, Hylocharis leucotis, and magnificent hummingbird, Eugenes fulgens) and males of one species (white-eared) for their cue preference in the same task in which rufous males have been tested previously. Birds were allowed to feed once from a four-flower array in which only one flower was rewarded. When the birds returned, the colour and the spatial cue designating the rewarded flower had been dissociated. Although we had expected females to visit the flower of the correct colour (feature cue) first, during the test phase most of the birds (males and females) went to the correct spatial location (spatial cue). It appears, then, that preference for spatial cues is not specific to males and it seems more likely to depend on the relevance or value of a cue to the solution of the task.

ANNUAL SURVIVAL AND RECRUITMENT IN A RUBY-THROATED HUMMINGBIRD POPULATION, EXCLUDING THE EFFECT OF TRANSIENT INDIVIDUALS

We estimated annual apparent survival, recruitment, and rate of population growth of breeding Ruby-throated Hummingbirds (Archilochus colubris), while controlling for transients, by using 18 years of capture-mark-recapture data collected during 1984–2001 at Hilton Pond Center for Piedmont Natural History near York, South Carolina. Resident males had lower apparent survival (0.30 ± 0.05 SE) than females (0.43 ± 0.04). Estimates of apparent survival did not differ by age. Point estimates suggested that newly banded males were less likely than females to be residents, but standard errors of these estimates overlapped (males: 0.60 ± 0.14 SE; females: 0.67 ± 0.09). Estimated female recruitment was 0.60 ± 0.06 SE, meaning that 60% of adult females present in any given year had entered the population during the previous year. Our estimate for rate of change indicated the population of female hummingbirds was stable during the study period (1.04 ± 0.04 SE). We suggest an annual goal of ≥64 adult females and ≥64 immature females released per banding area to enable rigorous future tests for effects of covariates on population dynamics. Development of a broader cooperating network of hummingbird banders in eastern North America could allow tests for regional or metapopulation dynamics in this species. Sobrevivencia Anual y Reclutamiento en una Población del Picaflor Archilochus colubris Excluyendo el Efecto de Individuos Ocasionales Resumen. Estimamos la sobrevivencia anual aparente, reclutamiento y tasa de crecimiento en una población reproductiva del picaflor Archilochus colubris, controlando por la presencia de individuos ocasionales. Utilizamos datos de 18 años de captura-marcaje-recaptura colectados entre 1984 y 2001 en Hilton Pond Center for Piedmont Natural History en cercanías de York, Carolina del Sur. Los machos residentes presentaron una sobrevivencia aparente menor (0.30 ± 0.05 EE) que las hembras (0.43 ± 0.04). Las estimaciones de la sobreviviencia aparente no difirieron entre edades. Estimaciones puntuales sugirieron que los machos marcados por primera vez tuvieron una menor probabilidad de ser residentes que las hembras, pero los errores estándar de estas estimaciones se sobrepusieron (machos: 0.60 ± 0.14 EE; hembras: 0.67 ± 0.09 EE). Para las hembras el reclutamiento estimado fue 0.60 ± 0.06 EE, lo que significa que el 60% de las hembras adultas presentes en un año determinado entraron a la población durante el año anterior. Nuestra estimación de la tasa de cambio indicó que la población de picaflores hembra fue estable durante el período de estudio (1.04 ± 0.02 EE). Para permitir futuras pruebas rigurosas sobre las covariables asociadas a las dinámicas poblacionales, sugerimos una meta anual de ≥64 hembras adultas y ≥64 hembras inmaduras liberadas por área de anillado. La creación de una amplia red cooperativa de anilladores de picaflores en el este de Norteamérica podría permitir estudiar y entender mejor las dinámicas regionales o metapoblacionales de esta especie.

Neural song control system of hummingbirds: Comparison to swifts, vocal learning (Songbirds) and nonlearning (Suboscines) passerines, and vocal learning (Budgerigars) and nonlearning (Dove, owl, gull, quail, chicken) nonpasserines

Males of certain hummingbird species such as Anna's hummingbirds (Calypte anna) learn their song during postnatal development. Here we report that male Anna's hummingbirds and male Amazilia hummingbirds (Amazilia amazilia), two singing hummingbird species, possess forebrain areas that are similar in morphological appearance, location, and connectivity to the song control areas RA (nucleus robustus archistriatalis), HVC (nucleus hyperstriatalis ventrale, pars caudale, or higher vocal center), and LMAN (lateral part of nucleus mangnocellularis anterioris) of oscine passerines (songbirds). The vocal control areas of songbirds are further defined by the expression of androgen receptors. Similarly, the singing hummingbird species express androgen receptors in the LMAN-like area and in the HVC-like area. The hummingbird RA projects to the medullary syringeal motonucleus nXIIts (nucleus hypoglossus pars tracheosyringealis) and the respiratory premotonucleus RAm (nucleus retroambigualis). The HVC-, RA-, and LMAN-like areas are rudimentary in adult male ruby-throated hummingbirds (Archilochus colubris) and Allen's hummingbirds (Selasphorus sasin) and not distinguishable in female hummingbirds, none of which sing. Vocal-area-like forebrain areas (delineated by the cytoarchitecture or androgen receptor expression) were not found in vocal nonlearning swifts and suboscines, the taxonomic sister groups of hummingbirds and songbirds, respectively. These areas were also missing in owls, ring doves, gulls, and gallinaceous species, nonpasserines that do not learn vocalizations. Budgerigars (vocal learners) are known to have forebrain vocal areas, but these areas do not express sex steroid receptors. These data suggest that hummingbirds and songbirds belong to two groups of birds that have common forebrain circuits. Parts of this circuit are organized as nuclear-like structures (LMAN, HVC, RA) in species that learn to sing.

Heart mass and the maximum cardiac output of birds and mammals: implications for estimating the maximum aerobic power input of flying animals

Empirical studies of cardiovascular variables suggest that relative heart muscle mass (relativeMh) is a good indicator of the degree of adaptive specialization for prolonged locomotor activities, for both birds and mammals. Reasonable predictions for the maximum oxygen consumption of birds during flight can be obtained by assuming that avian heart muscle has the same maximum physiological and biomechanical performance as that of terrestrial mammals. Thus, data onMhcan be used to provide quantitative estimates for the maximum aerobic power input (aerobicPi, max) available to animals during intense levels of locomotor activity. The maximum cardiac output of birds and mammals is calculated to scale with respect toMh(g) as 213Mh0.88±0.04(ml min−1), while aerobicPi,maxis estimated to scale approximately as 11Mh0.88±0.09(W). In general, estimated inter–species aerobicPi,max, based onMhfor all bird species (excluding hummingbirds), is calculated to scale with respect to body mass (Mbin kg) as 81Mb0.82±0.11(W). Comparison of family means forMhindicate that there is considerable diversity in aerobic capacity among birds and mammals, for example, among the medium to large species of birds the Tinamidae have the smallest relativeMh(0.25 %) while the Otidae have unusually large relativeMh(1.6 %). Hummingbirds have extremely large relativeMh(2.28 %), but exhibit significant sexual dimorphism in their scaling ofMhand flight muscle mass, so that when considering hummingbird flight performance it may be useful to control for sexual differences in morphology. The estimated scaling of aerobicPi,max(based onMhandMbin g) for male and female hummingbirds is 0.51Mb0.83 ±0.07and 0.44Mb0.85± 0.11(W), respectively. Locomotory muscles are dynamic structures and it might be anticipated that where additional energetic ‘costs’ occur seasonally (e.g. due to migratory fattening or the development of large secondary sexual characteristics) then the relevant cardiac and locomotor musculature might also be regulated seasonally. This is an important consideration, both due to the intrinsic interest of studying muscular adaptation to changes in energy demand, but also as a confounding variable in the practical use of heart rate to estimate the energetics of animals. Haemoglobin concentration (or haematocrit) may also be a confounding variable. Thus, it is concluded that data on the cardiovascular and flight muscle morphology of animals provides essential information regarding the behavioural, ecological and physiological significance of the flight performance of animals.

Behavioral, Ecological and Morphological Correlates of Foraging for Arthropods by the Hummingbirds of a Tropical Wet Forest

This paper seeks to clarify the ecological and evolutionary impacts of foraging for arthropods vs. nectar in hummingbirds, using data on morphology, sites, and tactics of arthropod foraging, and prey taken by 11 species of hummingbirds of La Selva, a Costa Rican lowland wet forest, as well as information on time budgets of male and female hummingbirds from two other sites. Hermit hummingbirds have long, curved bills and are nearly exclusively understory hover-gleaners that take mostly spiders. Most glean from webs but Threnetes ruckeri differs in foraging behavior and takes mostly jumping spiders. Non- hermits have shorter, straighter bills and employ a greater range of foraging tactics and sites, taking a wider variety of prey. In most species flycatching is frequent and flies and wasps are the predominant prey items, but some also take many spiders and ants; in particular, Heliothryx barroti is predominantly a hover-gleaner of the canopy and takes more spiders than flies. In Thalurania colombica, males and females differ in sites and tactics of arthropod foraging during the breeding season but not at other times. Breeding female hummingbirds spend much more time foraging for arthropods than do males in the same times and places, although flower visitation still constitutes the majority of foraging time. Among La Selva hummingbirds, bill curvature is strongly correlated with the proportion of gleaning vs. hawking, and with the proportion of prey taken from the substrate (spiders, ants) as opposed to volant prey (flies, wasps). Broad wings (low aspect ratio) and high wing disk loading are correlated with hovering, gleaning, and the proportion of spiders and ants in the diet; narrow wings and high wing disk loading, with flycatching. Except for the three species that engage in the most flycatching, females have broader wings than males and in virtually all species, females have higher wing disk loading. However, no parameter of wing morphology shows a clear relationship to strategies of nectar exploitation, probably reflecting the facultative nature of the latter. Hermits and nonhermits differ strongly in their preferred foraging levels in the vegetation, both for nectar and for arthropods, with the former using the understory almost exclusively, the latter using the canopy to a much greater extent, in both forest and second growth. Nevertheless, the two groups do not differ in the proportion of foraging attempts for nectar vs. arthropods, nor in the distribution of foraging through the day, in either habitat. In nearly all species, bills of females are longer than those of males, and this could increase their ability to forage for arthropods at least as much as for nectar. Much of the controversy regarding frequency and energetic importance of arthropod foraging vs. flower visitation reflects the biases inherent in observing each type of foraging or in interpreting data from stomach contents or emetic samples, or from failure to consider the specific tactics of foraging for arthropods. Reports that hummingbirds can exist for extended periods or breed without access to nectar or alternative sugar sources require confirmation, as does a report that availability of arthropods rather than nectar determines hummingbird breeding seasonality in dry areas. The major impact of arthropods in hum- mingbirds' diets appears to be at the daily, rather than seasonal, level. Data on arthropod prey do not support the notion that hummingbirds visit flowers for insects rather than nectar, and there is no evidence to suggest that their bills are specialized for extracting insects from

Widespread polychromatism in female sunangel hummingbirds (Heliangelus: Trochilidae)

In hummingbirds, a showy gorget of iridescent throat feathers is a characteristic male ornament in many sexually dichromatic species. Given that polygynous breeding systems are the only ones observed in hummingbirds, and that polygyny is often associated with dramatic male ornamentation and sexual dimorphism, it is not surprising that gorgets are usually interpreted as sexual display characters. This proposition is challenged by exceptional ‘polychromatic’ species in which adult females diner in their development of a male-like gorget. This analysis focuses on determining the prevalence of polychromatic variation in the Andean genus Heliangelus. In eight of nine species, females' gorgets range from a dull plumage characteristic of most female hummingbirds (female-like), to a bright iridescent plumage that matches that of the males (male-like). Analysis of museum study-skins reveals that individual variation in gorget colour is limited to females, and that it is independent of month and year of collection. Only H. exortis and H. amethysticollis exhibit significant geographic variation in the frequencies of different colour forms, though most male-like female H. amethysticollis occur in southern Ecuador-northern Peru. Some species-specific features of the male-like females' gorget appear to be linked to qualities of the male gorget (presence/absence, colour and maximum size). However, non-adaptive hypotheses, including genetic correlation between the sexes, hybridization and neutral variation, cannot account for the plumage variation among females. The absence of any association between female gorget colour and breeding condition suggests that gorget colour has a non-sexual function. I propose that gorgets can evolve solely as a non-sexual signal associated with different foraging behaviours.

"Prostitution" Behavior in a Tropical Hummingbird

For an animal to survive, it must acquire sufficient energy to meet metabolic requirements. In many vertebrates a territory helps insure adequate energy for the resident (Brown 1964, Fisler 1969). The size and quality of the territory will depend on the spatial and temporal characteristics of the food supply and competition for the food (Holmes 1970, Wolf and Stiles 1970). Most hummingbirds feed principally on nectar although they also eat small insects (Wagner 1946, Stiles 1971, Wolf and Hainsworth 1971). Male hummingbirds of many species defend groups of flowers from other hummingbirds, from other nectar-eating birds, and sometimes from insects. The ability of a male to maintain a territory depends on the relative dispersion of flowers and his dominance relationships with other species and individuals in the area (Brown 1964, Stiles and Wolf 1970, Wolf and Hainsworth 1971). In most hummingbirds that have been studied to date, males generally dominate females; there are a few exceptions (Wolf 1969). Male dominance over females may restrict the availability of energy to the female. Thus, when food is scarce or when the optimum resources can be controlled easily by the males, the subordinate females usually must exploit poorer nectar sources than the males. Female hummingbirds of most species apparently are solely responsible for the nesting effort and there is no long-term pair-bond (Wagner 1954, Wolf 1964, Lack 1968), although males of some species may provide indirect aid (Wolf and Stiles 1970, Snow and Snow 1973). This means that even during the breeding season, females of most species are not able to cohabit or regularly to use an area defended by the male. In most species nesting females do not and probably cannot hold territories around flowers (Wolf and Wolf 1971, Stiles 1973) and they are forced to forage at energetically poor, undefended sites. Females whose behavior enables them to exploit food defended by males would seem to have a selective advantage in both the breeding and nonbreeding season. This report describes use of aspects of mating behavior in the nonbreeding season by female Purplethroated Carib Hummingbirds (Eulampis jugularis) to gain ready access to nectar supplies on male territories. Since sexual behavior in these cases is being used for an energetic benefit for the female, I term this prostitution behavior.

Calculating the Nocturnal Energy Expenditure of an Incubating Anna's Hummingbird

Recently, attention has been focused on the bioenergetics of incubating female hummingbirds (Howell and Dawson 1954; Calder 1971, 1973). Hummingbirds are homeothermic with large surface area to volume ratios and have high rates of heat dissipation. During the night when cooler air temperatures prevail and feeding is prohibited, several species become torpid, lowering body temperatures and oxygen consumption (Bartholomew et al. 1957; French 1959; Pearson 1960; Lasiewski 1963; Calder 1971). This physiological state may be carefully regulated, as for Eulampis jugularis (Hainsworth and Wolf 1970). For incubating female hummingbirds, high nest temperatures maintained throughout the night indicate a nontorpid condition (Howell and Dawson 1954; Calder 1971). Pearson (1954) estimated that without torpor male Anna's Hummingbirds (Calypte anna) would require approximately 30% more metabolic energy during a 24-hr period and over five times more at night. Calder (1971) reviewed investigations on the nesting behavior of female hummingbirds and discounted increased feeding before roosting, decreased activity due to roosting, and lower metabolic rates for females as possible energymaintaining mechanisms. He concluded that for the Calliope Hummingbird (Stellula calliope) nest construction and the large overhanging limb which shields the nest from cold night skies must be important factors in nighttime energy conservation. The purpose of this investigation was to quantify the energy conservation properties of an Anna's Hummingbird nest and nest site and provide data on feeding behavior during incubation for comparisons with similar investigations. The analysis was accomplished by simultaneously measuring nesting behavior, nest temperatures, and microclimate for a nesting C. anna. A model was formulated to predict the nocturnal heat transfer of the bird and nest combination and used to compute the relative thermal advantages of nest properties and nest site.