Hummingbird Species Research Articles

A higher-level taxonomy for hummingbirds

In the context of a recently published phylo- genetic estimate for 151 hummingbird species, we provide an expanded informal taxonomy, as well as a formal phylogenetic taxonomy for Trochilidae that follows the precepts of the PhyloCode, but remains consistent with the hierarchical nomenclature of the Linnaean system. We compare the recently published phylogenetic hypothesis with those of prior higher-level and more taxonomically circumscribed phylogenetic studies. We recommend the recognition of nine new clade names under the PhyloCode, eight of which are consistent with tribes and one with a subfamily under the Linnaean system.

NECTAR INTAKE AND FORAGING EFFICIENCY: RESPONSES OF HONEYEATERS AND HUMMINGBIRDS TO VARIATIONS IN FLORAL ENVIRONMENTS

Costa Rican hummingbirds and Australian honeyeaters respond to an increase in the volume of nectar made available in tubular flowers by increasing the duration of foraging bouts, the rate at which nectar is ingested, and the energetic efficiency of that process. Enhanced intake is the result of increased nectar capture by the tongue during each lick rather than of any significant change in licking rate. All species react to an increase in floral length by spending more time foraging and reducing nectar intake, short-billed species being most affected. Decreased nectar capture per lick, rather than a change in licking rate, is responsible for this response. Honeyeater foraging times increase and nectar-intake rates decrease when the curvature of flowers is increased, though all but one of the short-billed hummingbird species were relatively insensitive to this change. Hummingbirds harvest nectar with equal proficiency whether foraging at erect or at pendulous flowers, whereas the bouts of honeyeaters are longer, and their nectar-uptake rates lower, when they visit pendulous flowers. Overall nectar-extraction rates of hummingbirds, as measured in the laboratory, were greater than those of honeyeaters; values for both groups were generally higher than rates recorded in the field. Hummingbird and honeyeater tongues are equally adept at extracting nectar from tubular flowers, though my results suggest that honeyeater tongues would be more effective in situations where nectar is thinly and widely dispersed.

Pollination ecology of Disterigma stereophyllum (Ericaceae) in south‐western Colombia

Several authors have recently expressed doubts that the 'pollination syndromes' as usually expressed are an adequate description of correlated suites of floral characters, or that they adequately describe evolutionary or ecological associations of plants with pollinators. Disterigma stereophyllum is a neotropical Ericaceae with floral characteristics intermediate between the 'entomophilous' syndrome and the 'ornithophilous' syndrome: the corolla is short, white and urceolate, but flowers produce large amounts of dilute nectar. We studied the pollination ecology of this species in south-western Colombia, and found it to be pollinated almost exclusively by hummingbirds at our study site. Two hummingbird species were responsible for about 75 of visits. Despite the fact that nectar standing crop remained more or less constant throughout the day, visit frequencies were highest in the morning and declined throughout the day. Pollinator efficiency, measured as the number of pollen grains deposited on a virgin stigma by each visitor after one visit, did not differ among the species of hummingbirds, but was lower for a nectar-robbing bird, Diglossa albilatera. This species does not contact the surface of the stigma during nectar robbing, but can produce some self-pollination indirectly because it shakes branches vigorously while piercing the flower. These findings indicate a need for further studies of neotropical Ericaceae in order to elucidate whether floral visitors of species like D. stereophyllum fluctuate through time or space, and whether floral characteristics reflect a compromise between such different visitors, or a transitional stage between pollination syndromes, or some other possibility.

Pollination and breeding system of Melochia tomentosa L. (Malvaceae), a keystone floral resource in the Brazilian Caatinga

The main goals of the present paper were to investigate the floral biology and the breeding system of Melochia tomentosa in a semi-arid region in Brazil, comparing the role of Apis mellifera with other native pollinators, and to discuss the importance of this plant species as a floral resource for the local fauna in maintaining different guilds of specialized pollinators in the Caatinga. M. tomentosa is very common in Caatinga areas and blooms year-round with two flowering peaks, one in the wet and another in the dry period. The pink, bright-colored flowers are distylous and both morphs are homogamous. The trichomatic nectary is located on the inner surface of the connate sepals, and the nectar (ca. 7 μl) is accumulated in the space between the corolla and the calyx. Nectar sugar concentration reaches an average of 28%. The results of controlled pollination experiments show that M. tomentosa is self-incompatible. Pollen viability varies from 94% to 98%. In spite of being visited by several pollen vectors, flower attributes of M. tomentosa point to melittophily, and A. mellifera was the most frequent visitor and the principal pollinator. Although honeybees are exotic, severely competing with native pollinators, they are important together with other native bees, like Centris and Xylocopa species, for the fruit set of M. tomentosa. This species represents a keystone floral resource favoring the maintenance of many species of bees, butterflies and hummingbirds through the year in this deciduous tropical dry forest.

Pollination networks and functional specialization: a test using Lesser Antillean plant–hummingbird assemblages

Network analysis has in recent years improved our understanding of pollination systems. However, there is very little information about how functionally specialized plants and pollinators interact directly and indirectly in pollination networks. We have developed a parameter, Functional specialization index, to quantify functional specialization in pollination networks. Using this parameter, we examined whether different sized hummingbirds visit a distinct set of flowers in five hummingbird‐pollinated plant assemblages from the Lesser Antilles, obtaining a simple relationship between hummingbird body size, network parameter and ecological function. In the Lesser Antilles, functionally specialized hummingbird pollination is distinct for plant species pollinated by the largest hummingbird species, whereas the pollination niche gradually integrates with the insect pollinator community as hummingbird body size decreases. The network approach applied in this study can be used to validate functional specialization and community‐level interdependence between plants and pollinators, and it is therefore useful for evaluating and predicting plant resilience to pollinator loss, presently a global concern.

Phylogenetic Systematics and Biogeography of Hummingbirds: Bayesian and Maximum Likelihood Analyses of Partitioned Data and Selection of an Appropriate Partitioning Strategy

Hummingbirds are an important model system in avian biology, but to date the group has been the subject of remarkably few phylogenetic investigations. Here we present partitioned Bayesian and maximum likelihood phylogenetic analyses for 151 of approximately 330 species of hummingbirds and 12 outgroup taxa based on two protein-coding mitochondrial genes (ND2 and ND4), flanking tRNAs, and two nuclear introns (AK1 and BFib). We analyzed these data under several partitioning strategies ranging between unpartitioned and a maximum of nine partitions. In order to select a statistically justified partitioning strategy following partitioned Bayesian analysis, we considered four alternative criteria including Bayes factors, modified versions of the Akaike information criterion for small sample sizes (AIC(c)), Bayesian information criterion (BIC), and a decision-theoretic methodology (DT). Following partitioned maximum likelihood analyses, we selected a best-fitting strategy using hierarchical likelihood ratio tests (hLRTS), the conventional AICc, BIC, and DT, concluding that the most stringent criterion, the performance-based DT, was the most appropriate methodology for selecting amongst partitioning strategies. In the context of our well-resolved and well-supported phylogenetic estimate, we consider the historical biogeography of hummingbirds using ancestral state reconstructions of (1) primary geographic region of occurrence (i.e., South America, Central America, North America, Greater Antilles, Lesser Antilles), (2) Andean or non-Andean geographic distribution, and (3) minimum elevational occurrence. These analyses indicate that the basal hummingbird assemblages originated in the lowlands of South America, that most of the principle clades of hummingbirds (all but Mountain Gems and possibly Bees) originated on this continent, and that there have been many (at least 30) independent invasions of other primary landmasses, especially Central America.

Fenologia reprodutiva, biologia floral e polinizadores de duas espécies simpátricas de Marantaceae em um fragmento de Floresta Atlântica, Nordeste do Brasil

Foi analisada a floração, a frutificação e a biologia da polinização em duas espécies de Marantaceae: Ischnosiphon gracilis (Rudge) Koern. e Stromanthe porteana A. Gris. As observações foram realizadas em populações naturais no Parque Estadual Dois Irmãos (8º7'30" S e 34º52'30" W), um remanescente de Floresta Atlântica em Pernambuco. Nas duas espécies foi verificado padrão fenológico contínuo, com diferentes picos de floração e frutificação. As inflorescências em I. gracilis produziram 14,4 ± 3,4 flores e 1,3 ± 0,6 frutos, enquanto em S. porteana, produziram 125,4 ± 14,8 flores e 7,4 ± 4,9 frutos. Foram verificadas baixa razão pólen/óvulo e reduzida produção natural de frutos nas duas espécies. Em I. gracilis, a concentração de açúcares no néctar foi alta (26%-32%), característica de flores visitadas por abelhas e em S. porteana o néctar foi menos concentrado (20%), situação comum em flores visitadas por beija-flores. Ischnosiphon gracilis é polinizada por três espécies de abelhas Euglossini (Euglossa sp., Eulaema bombiformis e E. cingulata), enquanto S. porteana é polinizada por uma espécie de abelha Euglossini (Eufriesea surinamensis) e por duas espécies de beija-flores (Phaethornis ruber e Amazilia versicolor). As diferenças observadas entre as flores das duas espécies evitam partilha e competição por polinizadores, o que pode garantir a manutenção delas em seu habitat. Entretanto, a longo prazo, a baixa produção de frutos pode afetar a estrutura da população, diminuindo o sucesso reprodutivo das duas espécies.

First Record of the White-tipped Sicklebill (Eutoxeres aquila aquila: Trochilidae) for Venezuela

A female White-tipped Sicklebill (Eutoxeres aquila aquila) was caught on 3 February 2006 at 1,050 m elevation in Agua Blanca, Municipio Andrés Bello, Mérida, Venezuela, 2 km southeast of La Azulita. This account represents the first record of this hummingbird species in Venezuela, extending the taxon's known distribution at least 500 km northeastward.

Geographical variation in floral traits of the tree tobacco in relation to its hummingbird pollinator fauna

Nicotiana glauca , a hummingbird pollinated plant, exhibits geographical variation in several floral traits. We examined whether geographical differentiation occurred for different flower characters and if this differentiation could be explained, at least in part, by the existence or abundance of different hummingbird species in the respective pollinator assemblages. The comparison between five populations showed significant variation in six floral traits and two female fitness measures. The traits that better discriminated between populations were corolla length and corolla width. There were metric correlations between corolla length and style length in all populations studied and, in four of the five populations, both corolla length and width were also correlated. Among plants in each population, seed weight was correlated positively and significantly with style exertion, suggesting that fruit quality is dependent on the degree of cross pollination. Assemblages of hummingbirds differed between populations in species composition, visitation frequencies, and bill length. Linear regression involving bill length of the more frequent hummingbird pollinators and corolla length yielded positive and significant relationships. Thus, there appears to be an adjustment between pollinators and flowers traits that have high incidence in the among population variation. © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 90 , 657‐667.

Neural specialization for hovering in hummingbirds: Hypertrophy of the pretectal nucleus lentiformis mesencephali

Hummingbirds possess an array of morphological and physiological specializations that allow them hover such that they maintain a stable position in space for extended periods. Among birds, this sustained hovering is unique to hummingbirds, but possible neural specializations underlying this behavior have not been investigated. The optokinetic response (OKR) is one of several behaviors that facilitates stabilization. In birds, the OKR is generated by the nucleus of the basal optic root (nBOR) and pretectal nucleus lentiformis mesencephali (LM). Because stabilization during hovering is dependent on the OKR, we predicted that nBOR and LM would be significantly enlarged in hummingbirds. We examined the relative size of nBOR, LM, and other visual nuclei of 37 species of birds from 13 orders, including nine hummingbird species. Also included were three species that hover for short periods of time (transient hoverers; a kingfisher, a kestrel, and a nectarivorous songbird). Our results demonstrate that, relative to brain volume, LM is significantly hypertrophied in hummingbirds compared with other birds. In the transient hoverers, there is a moderate enlargement of the LM, but not to the extent found in the hummingbirds. The same degree of hypertrophy is not, however, present in nBOR or the other visual nuclei measured: nucleus geniculatus lateralis, pars ventralis, and optic tectum. This selective hypertrophy of LM and not other visual nuclei suggests that the direction-selective optokinetic neurons in LM are critical for sustained hovering flight because of their prominent role in the OKR and gaze stabilization.

Do Pollination Syndromes Partition the Pollinator Community? A Test Using Four Sympatric Morning Glory Species

We adopted a comparative approach to evaluate whether pollination (floral) syndromes serve to partition the pollinator fauna among four coflowering species of morning glories (Ipomoea, Convolvulaceae). Two of the species (I. hederacea and I. trichocarpa) fit the “bee” syndrome: they produce blue and purple flowers that have large corolla openings, and they secrete smaller volumes of concentrated nectar. The other two (I. quamoclit and I. hederifolia) exhibit the “bird” syndrome: they have reddish flowers and narrow, tubular corollas, and they secrete larger volumes of dilute nectar. The pollinator fauna of 11 species of lepidoptera, bees, and hummingbird did not visit Ipomoea species in proportion to their relative abundance. The bumblebees foraged exclusively on the blue‐flowered I. hederacea and I. trichocarpa, and the hummingbirds visited only the red‐flowered I. hederifolia and I. quamoclit, while the lepidopterans exhibited less preference for a specific syndrome type. As a group, the Ipomoea species tended to be quite specialized in their use of pollinators. Ca. 75% of the visits to the blue‐flowered I. hederacea and I. trichocarpa were by bumblebees, while ca. 80% of visits to I. quamoclit and I. hederifolia were by sulfur butterflies. Our results support the notion that pollination syndromes do aid in partitioning the pollinator fauna.

Seasonal resource tracking by Amazonian hummingbirds

Many ecological theories predict a close match between food resources and the number and diversity of consumers but comparatively few empirical studies have simultaneously quantified seasonal variation in an entire assemblage and its resource base. Hummingbirds provide an excellent model system to examine this relationship because their primary food resource is conspicuous and easily quantified. Over a 2‐year period I used mist‐netting and direct observation of birds feeding at flowering plants to record the abundance of 16 species of hummingbird in lowland Amazonian rainforest. Although there was very little annual variation in temperature and daylength, both rainfall and river level showed significant seasonal variation. At the study site this resulted in an annual flood lasting for over 5 months, inundating much of the area to a depth of 2 m. I recorded marked seasonal patterns in flowering phenology and arthropod abundance, which correlated with river level and rainfall, respectively. In turn, these seasonal fluctuations in resource abundance correlated very closely with variation in hummingbird species richness, hummingbird abundance and the total biomass of hummingbirds in the assemblage. This study is the first to demonstrate tracking of seasonal resource variation in equatorial rainforest and provides evidence that hummingbirds are sensitive to resource availability at spatial and temporal scales much greater than previously documented.

Island–mainland difference in Nicotiana glauca (Solanaceae) corolla length: a product of pollinator-mediated selection?

A match between floral and pollinator traits, such as that between unique island plants and pollinators, is often thought to be the product of pollinator-mediated selection. I examined whether the floral morphology of an introduced hummingbird-pollinated plant, Nicotiana glauca (tree tobacco, Solanaceae), is under selection by pollinators on the California Channel Islands where it is a recent colonist. I first determined differences in floral morphology and pollinator composition between island and mainland populations of N. glauca. I found that island plants have detectably longer corollas (on average 1 mm) and are visited by hummingbird species with on average 1–2 mm longer bills than common mainland visitors. Corolla length differences were not found to be associated with site abiotic differences. Flower size does not vary consistently with season and corolla width is very consistent across sites. I tested whether island–mainland corolla length differences are the product of pollinator-mediated selection by measuring phenotypic selection and per visit effectiveness. Contrary to expectations, a longer corolla was not consistently associated with higher pollen transfer or seed count on the islands. Per visit effectiveness of longer and shorter-billed hummingbirds did differ; however, effectiveness did not depend on corolla length. Although I failed to detect expected patterns of selection for longer corollas on islands, I cannot rule out weak or past pollinator-mediated selection. It is also possible that despite the apparent match between pollinator and floral traits, island–mainland differences in corolla length are instead due to other environmental effects, selection unrelated to pollinators, or stochastic processes such as drift.

Polinização por beija-flores em uma área de caatinga no Município de Floresta, Pernambuco, Nordeste do Brasil

Ornithophilous plants were studied in a caatinga area (8o36'00 S and 38o34'5 W) in the State of Pernambuco, Northeast of Brazil, from July 2002 to June 2003. Eight ornithophilous species, belonging to five families and seven genera, were registered in the studied community. Cactaceae was the most representative family, with three species pollinated by hummingbirds, followed by Bromeliaceae with two species. Excepting for Bromelia laciniosa Mart. ex Schult. f. and Neoglaziovia variegata (Arruda) Mez (Bromeliaceae), the remaining species had flowering peak in the dry season. Fifty percent of the species have herbaceous habit, followed by shrubs (37.5%). Most species have red flowers (62.5%) and all of them have tubular flowers with corolla length of 20.2 ± 5.6 mm. Sugar concentration in the nectar varied from 18% to 33% and the volume from 22 to 41 µL. There were species flowering throughout the whole year. Five hummingbird species visited the flowers in the and only one was considered resident. Chlorostilbon aureoventris (d'Orbigny & Lafresnaye, 1838), due to its frequency and visiting behaviour, was considered as the dominant species. The number of ornithophilous species is lower than those found in other ecosystems as, for example, in the Atlantic forest remnants and some other Neotropical areas. Furthermore, a Trochilinae hummingbird, and not a Phaethornithinae, acted as the community organizer species.

Reprodução sexuada de Dyckia tuberosa (Vell.) Beer (Bromeliaceae, Pitcairnioideae) e interação planta-animal

Bromeliaceae is an important component in several Biomas due to its remarkable ecological diversity as well as biotic interactions. It presents a combination of two reproductive mechanisms: sexual and clonal propagation. We describe in this study the various attributes related to the sexual reproduction of Dyckia tuberosa as well as plant-animal interactions that take place in its reproductive structures. In the population studied, 55% of individuals blossomed and set fruit over an 11-month period. The flowers developed from the basis to the top and had morphometrical differences with regard to its position in inflorescence that cause also differences in number of seeds set. Sugar concentration in the nectar was 20% and total production of nectar was about 24 µL flower-1 day-1. Nectar production was bigger in the morning period. Dyckia tuberosa is self-incompatible, an unusual feature for Bromeliaceae, and the index of self-incompatibility (ISI) was 0.08. Only hummingbirds were responsible for the legitimate visits of flowers, contacting anthers and stigma, and 3.9 flowers were visited per hour. Chlorostilbon aureoventris, Colibri serrirostris and Phaethornis eurynome were the hummingbird species registered. Sexual reproduction on D. tuberosa depends on hummingbirds'visitation. Inflorescences of D. tuberosa were also visited by the ant species Camponotus rufipes, Camponotus cf. mus and Cephalotes sp. As sexual reproduction is absent in many individuals and this species is dominant on the granite outcrop of Pedra Grande, we consider clonal propagation an important strategy for the spread of this species in this area. Given the presence of food resources for pollinating and non-pollinating animals on reproductive structures of D. tuberosa, this system seems suitable to assess how variation on resources availablity change the results of these interactions as well as the reproductive output of D. tuberosa.

Phenology and floral visitors of two sympatric Heliconia species in the Brazilian Atlantic forest

The main goal of the present study was to test if two sympatric species of Heliconia, Heliconia spathocircinata and Heliconia laneana var. flava, differ in their reproductive seasons and guilds of flower visitors. Twenty-four sampling sessions were conducted monthly between February 2001 and January 2003. Individuals of H. spathocircinata and of H. laneana var. flava were tagged within a transect of 4000×4 m. Reproductive phenophases (flowering, unripe, and ripe fruits) were recorded, and circular statistics were used to test for the regularity and frequency of phenophases. Flower visitors were observed during two flowering periods for each species. Both species had well-defined and synchronous reproductive periods, with a small overlap, suggesting a sequential flowering model. Biotic and abiotic factors seem to affect phenophases of both species. However, herbarium data suggest that the flowering period in H. spathocircinata varies less than in H. laneana, despite its wider geographic distribution. The hummingbird Phaethornis idaliae was the main floral visitor of both Heliconia species studied. H. laneana was also legitimately visited by another hummingbird species, Glaucis hirsuta, and by Lepidoptera. Some Lepidoptera, particularly the bee Trigona spinipes, were illegitimate visitors (‘nectar robbers’). We suggest that the flower of H. spathocircinata allowed a wider array of visitors, because it is bigger.

Reproductive Biology and Pollination of the Parasitic Plant Psittacanthus Calyculatus (loranthaceae) in Central México1

Psittacanthus is the only genus in the family Loranthaceae that is considered a significant pest of woodlands and farmlands in Mexico and Central America, but relatively little attention has been given to the reproductive biology and pollination of natural populations. We present quantitative information on the floral morphology, breeding system, flowering phenology, nectar production, floral visitors, and pollinators of the mistletoe Psittacanthus calyculatus in Tlaxcala, México. Its flowering phenology extends from April to October, with a major peak in July. Flowers last 5 days and produce abundant nectar (2.34 ± 0.175 µl per flower) with a constant sugar concentration (ca. 22%) throughout flower life span. Hand self-pollinations showed that this species is partially self-compatible (44% fruit set), whereas emasculated flowers produced no seeds. Hand out-crossed flowers produced a similar number of fruits (96%) compared to unmanipulated control flowers visited by birds and insects (84%), showing no evidence for pollen limitation. Four hummingbird species were frequent visitors of mistletoe flowers, but they varied in visit duration and number of probes per flower. Artificial pollination of flowers with stuffed hummingbirds of the four species suggested that Hylocharis leucotis was the most effective pollinator.

Botanical composition and structure of hummingbird nests in different habitats from northwestern Patagonia (Argentina)

There are few detailed analyses of the building materials used in bird nests. This study was part of a project on interrelations between northern Patagonian plants and birds. Nests of two hummingbird species distributed in the Andean portion of southern South America: the Green‐backed Firecrown Sephanoides sephaniodes and the White‐sided Hillstar Oreotrochilus leucopleurus, were collected along a precipitation gradient in northwestern Patagonia (Argentina). Nest morphology, structure, and composition (mostly botanical in origin) were analysed. Plants used as building materials are presented for each nest, arranged by location. The main structural materials were mosses (especially those with falcate leaves) entangled with spider web. Nests were concealed in their environments by the presence of mimetic material on the outer layer (spider cocoons, leprose lichens, feathers, and hairs). Soft material was the main component of the lining (pappi, feathers, spider web, and manufactured cotton). The variety of nesting materials decreased along the precipitation gradient, while the main structural components are generally fixed.

HIGH-FLYING HUMMINGBIRDS BURST TO BEAT COMPETITORS

Locomotory behaviour and performance has long been assumed to be the key factor in determining the outcome of competition between animals for resources. However, empirical data to support this hypothesis has proved challenging to collect. In a recent publication in The American Naturalist,Douglas Altshuler sought to experimentally test the hypothesis that flight performance influences both territoriality, and success when competing for niches, by comparing the flight behaviour of several hummingbird species over a range of altitudes.Although simple measurements of wing disc loading in flying birds had previously been used to approximate the bird's flight performance, Altshuler instead used more complex aerodynamic theory to calculate more realistically the costs of flight. Altshuler also filmed the birds at different altitudes in the wild, and under low-pressure conditions in the lab, to identify their flight kinematics. Finally, Altshuler calculated the power requirements for flight, particularly in relation to maximal or `burst' power, as well as recording features of territorial behaviour during field observations.Travelling to the Andes and Colorado, Altshuler studied hummingbird flight in three experiments. First, he compared territorial behaviour of hummingbird species in the mountains from 400-3860 m and found that wing disc loading had no effect on territoriality, but burst power did. Territorial hummingbird species had higher burst aerodynamic power outputs than not-territorial species. Second, he studied two species of Selasphorus hummingbirds competing for access to nectar at 1875 and 2900 m. At the lower altitude, S. rufus males appeared to have a competitive advantage over S. platycerus males, but the roles were reversed as altitude increased. S. rufus had relatively lower burst power output and Altshuler found important difference in hovering kinematics between the two species of hummingbirds. Altshuler's measurements showed that S. rufus males used higher stroke angles when hovering and this constrained their ability to modulate aerodynamic force production when compared with S. platycerus. Tempting the hummingbirds with high-quality feeders (sucrose solutions) revealed that not only did altitude affect the way in which the hummingbirds selected feeding sites, but that the density of competitors for feeders also effected the bird's choice. The final component of the study involved laboratory studies of flight performance under high and low pressure conditions. Both species increased power output to compensate for lower air density.Altshuler's study has provided several intriguing insights into both the mechanical requirements for flight of territorial birds and the influence of environment on competitive interactions. The ability to produce bursts of power appears to be a pre-requisite for successful competitive outcomes at higher altitudes; however, at lower altitudes other flight parameters (such as manoeuvrability) can improve a hummingbird's advantage. Clearly, physiological and aerodynamic limitations become more important as oxygen availability and air density decline with increasing altitude. This study has shown how a bird's flight performance, and a bird's ability to modulate it, can affect its ability to compete with other species for resources.

Biologia reprodutiva de Melocactus glaucescens Buining & Brederoo e M. paucispinus G. Heimen & R. Paul (Cactaceae), na Chapada Diamantina, Nordeste do Brasil

Foi estudada a biologia reprodutiva de Melocactus glaucescens e M. paucispinus (Cactaceae) no Municipio de Morro do Chapeu, Chapada Diamantina, Bahia, sendo abordados aspectos da fenologia, biologia floral, polinizacao e sistema reprodutivo. Foram registrados os periodos de floracao e frutificacao, visitantes florais, frequencia e tipo de visitas, alem de estrategia e comportamento dos visitantes as flores. Foram realizadas polinizacoes experimentais para verificar o sistema reprodutivo das especies. As duas especies de Melocactus estudadas apresentaram sobreposicao de floracao ao longo do periodo de estudo. Os atributos florais de ambas as especies sao tipicos da sindrome da ornitofilia: cores atrativas, estrutura tubulosa e producao de nectar com baixa concentracao de solutos, entre 20% e 30%. O beija-flor Chlorostilbon aureoventris Boucier & Mulsant (1948) foi o visitante mais frequente, com 82% e 89% do total de visitas para M. paucispinus e M. glaucescens, respectivamente. Outras especies de beija-flores e borboletas tambem visitaram as flores das especies. A sobreposicao no periodo de floracao e a similaridade na composicao da guilda de polinizadores destas e demais especies simpatricas de Melocactus favorece a hibridacao no genero, como foi observado na regiao. Melocactus glaucescens apresentou auto-incompatibilidade e alogamia, enquanto M. paucispinus e auto-compativel e autogâmica, porem com menor frutificacao em autopolinizacao do que em polinizacao cruzada, possivelmente devido a ocorrencia de depressao endogâmica. Polinizacoes interespecificas indicam a ocorrencia de inter-compatibilidade entre M. paucispinus e M. concinnus Buining & Brederoo, sustentando hipoteses correntes de hibridacao entre estas especies.