Sugar preferences and digestive efficiency in an opportunistic avian nectarivore, the Dark-capped Bulbul Pycnonotus tricolor

It has recently been recognized that flowers pollinated by generalist opportunistic nectarivores tend to have different nectar properties to those pollinated by specialist nectarivores (including both hummingbirds and specialist passerines). While renewed interest in specialist avian nectarivore sugar preferences and digestive physiology has helped explain the concentrated sucrose-dominated nectar of plants they feed on, there has been little progress in understanding why generalist or occasional nectar-feeding birds tend to be associated with flowers that have dilute hexose-dominated nectar. We examined sugar preferences and assimilation efficiencies over a range of concentrations, and concentration preferences, in Dark-capped Bulbuls Pycnonotus tricolor, one of the more common occasional avian nectarivores in southern Africa. Dark-capped Bulbuls showed significant preference for hexose sugar solutions, irrespective of concentration, when given a choice between hexose and sucrose solutions in equicaloric pair-wise choice tests conducted at five different concentrations (5–25%). This contrasts with results from specialist nectarivore groups which generally show a significant concentration-dependant switch in preference from hexose at low concentrations to sucrose at high concentrations for equicaloric solutions. In addition, Dark-capped Bulbuls showed an unusual lack of preference for solutions of higher sugar concentration when simultaneously offered four solutions varying in concentration from 10 to 25%. Dark-capped bulbuls also showed a unique effect of concentration on sugar assimilation efficiency, assimilating relatively more energy on 5% diets than on 25% diets. Although able to assimilate sucrose effectively, assimilation rates of hexose sugars were marginally higher. These results shed new light on pollination systems involving occasional nectarivores and, in particular, help to explain the prevalence of low concentration hexose-dominated nectars in flowers pollinated by these birds.     

Nectar as food for birds: the physiological consequences of drinking dilute sugar solutions

 Nectarivory has evolved many times in birds: although best known in hummingbirds, sunbirds and honeyeaters, it also occurs on an opportunistic basis in a varied assortment of birds. We present a phylogenetic analysis of the distribution of nectarivory in birds. Specialised avian nectarivores are generally small, with an energetic lifestyle and high metabolic rates. Their high degree of dependence on nectar as a food source has led to convergence in morphological, physiological and behavioural adaptations. We examine the constituents of nectar which are most important to bird consumers, and how the birds deal with them in terms of physiology and behaviour. There are still unanswered questions: for example, the dichotomy between sucrose-rich nectars in hummingbird-pollinated plants and predominantly hexose-rich nectars in sunbird-pollinated plants appears to have little to do with bird physiologies and may rather reflect patterns of nectar secretion. Received November 28, 2002; accepted January 26, 2003 Published online: June 2, 2003     

Consequences of differences in body mass,wing length and leg morphology for nectar-feeding birds

Nectar-feeding birds are prominent in many parts of the world, and vary with respect to body size. Despite the availability of considerable morphometric data, few concerted efforts have been made to assess the influence of attributes such as mass, wing length and leg morphology upon the speed, acceleration, mode and energetic cost of movement by birds between flowers when foraging for nectar. This review attempts to consolidate and interpret available data and highlight areas where further investigations appear warranted. Australian honeyeaters are generally larger, and American hummingbirds smaller, than Hawaiian honeycreepers and sunbirds of Africa or Asia. Sunbirds, honeyeaters and honeycreepers generally perch while extracting nectar from flowers. Hummingbirds usually hover, apparently because suitable perches close to flowers are lacking, and not because hovering increases the speed at which flowers can be visited. Honeyeaters move from one flower to another at speeds that are at least as great as those for hummingbirds. Most passerine nectarivores need to ingest more nectar per day than hummingbirds in order to maintain energy balance, some species devoting more than 60% of the day to foraging. The major consequence of reduced foraging activity by hummingbirds, which spend only 5–30% of the day in this manner, appears to be male emancipation from nest construction and care of offspring. Large nectarivores have a greater capacity to store surplus food and to fast than smaller birds, and so can take advantage of short-lived peaks in nectar abundance. Nectarivores such as honeyeaters should therefore be favoured by the rapid diurnal changes in nectar availability which are characteristic of many Australian and African habitats. Body mass also determines the likely access to rich sources of nectar through size-related interspecific dominance hierarchies. In all families, larger species tend to monopolize the most rewarding nectar supplies, forcing smaller subordinate species to use poorer, more scattered sources. Within particular species, males usually have longer wings and greater masses than females. These variations imply that the two sexes differ with regard to their foraging ecology, although few supporting data are currently available.     

Response of avian nectarivores to the flowering of Aloe marlothii : a nectar oasis during dry South African winters

In southern Africa, Aloe marlothii flowers during the dry winter season and offers copious dilute nectar to a variety of birds. Avian abundance and community composition were monitored at an A. marlothii forest at Suikerbosrand Nature Reserve, South Africa. Sampling occurred during two summer months (February–March) when no flowers were present, and six months (May–October) that spanned the winter flowering. We hypothesized that an influx of occasional nectarivores to the A. marlothii forest during flowering would lead to significant changes in the avian community. Overall bird abundance increased 2–3 fold at the peak of nectar availability (August). We recorded 38 bird species, of 83 species detected during transects, feeding on A. marlothii nectar; this diverse assemblage of birds belonged to 19 families, including Lybiidae, Coliidae, Pycnonotidae, Sylviidae, Cisticolidae, Muscicapidae, Sturnidae, Ploceidae and Fringillidae. Surprisingly, only two species of sunbird (Nectariniidae) were observed feeding on A. marlothii nectar, and both occurred in low abundance. We predicted that competition for nectar resources would be high, but few aggressive inter- and intra-specific interactions occurred between birds while feeding on inflorescences. During peak flowering, insect feeders (insectivores, omnivores, nectarivores) fed on nectar during the cold morning when insect activity was low, whilst non-insect feeders (frugivores and granivores) fed on nectar in the middle of the day. Our study highlights the importance of A. marlothii nectar as a seasonal food and water source for a diverse assemblage of occasional nectarivores.     

瘦素在禽类中的研究进展

瘦素(leptin)是一种主要作用于下丘脑的重要激素,起到调控摄食和能量消耗的作用。另外,国内外越来越多的研究表明,leptin与动物的代谢、发育、繁殖和免疫调节等均有密切的联系。但是上述研究大多在哺乳动物中进行,在禽类中的研究还在起步阶段。现有的研究表明,禽类的leptin及leptin受体的作用与哺乳动物相比都有其特殊性。本文首先分析了禽类leptin及leptin受体的特点,在此基础上,从摄食、生长发育和繁殖三个方面综述了leptin对禽类的作用及可能机制。     

Competition Between Distantly Related Taxa In the Coevolution of Plants and Pollinators

Competition among distantly related plants for pollinators andamong distantly related animals for pollen and nectar playsa potentially important role in the organization of ecologicalcommunities and the coevolution of plant-pollinator relationships.Plants which rely on animals to disperse their pollen potentiallycompete for pollinators by processes similar to interferenceand exploitative competition. Coexisting plant species may evolveto avoid or reduce such competition by character displacementin floral morphology and/or phenology. One important differencebetween competition for pollinators and most other kinds ofcompetition is that pollinator resources are not used up andmade absolutely unavailable to competitors. Consequently, plantspecies can potentially overlap completely in their utilizationof pollinators. The disadvantages of competing apparently aresometimes outweighed by the advantages of sharing pollinators,because distantly related plant species frequently show evolutionaryconvergence in floral morphology, blooming time and nectar rewardsto utilize the same pollinators. Distantly related animal taxa may compete for floral nectarand pollen by both interference and exploitation. The mechanismsof such competition depend primarily on the energetic costsand benefits of foraging and aggression. Exploitative competitionis very important because nectar feeders of small body sizeand low energy requirements can forage economically and reducenectar availability to levels that will not support larger animals.Thus small nectarivores often can exclude larger competitorsfrom flowers to which both taxa have equal access. Plants mayevolve to influence the outcotre of competition among animalvisitors and favor species that provide the best pollinationservices. Thus flowers specialized for pollination by largeanimals often show morphological or phenological specializationswhich make rewards unava'lable to smaller animals. Interferenceis adaptive only when the benefits of exclusive use of a resourceoutweig.i the costs of defending it. Because distantly relatedkinds of flower visitors often differ in body size and energeticrequirements, interference competition among them is probablyrare although it is often important among closely related nectarivores. The community level consequences of competition in the ecologyand evolution of plant-pollinator associations are still poorlyunderstoood. Competition among distantly related pollinatorsfor plant floral rewards appears to play a major role, but competitionamong plants for pollinator services may be only a weak force.Although the basic interaction between plant and pollinatorusually is a mutualistic one, certain species of both plantsand animals parasitize this interaction and compete with themutualists for limited resources. Thus some animals rob nectarand pollen and compete with legitimate pollinators without providingpollination services. Similarly, some plants offer no floralrewards but obtain pollinator services by mimicing rewardingflowers of other species. The effects of these kinds of interactionson the organization of communities of plants and pollinatorsprovide a fertile area for future research.     

Asymmetric competition for nectar between a large nectar thief and a small pollinator: an energetic point of view

There are two alternative hypotheses related to body size and competition for restricted food sources. The first one supposes that larger animals are superior competitors because of their increased feeding abilities, whereas the second one assumes superiority of smaller animals because of their lower food requirements. We examined the relationship between two unrelated species of different size, drinking technique, energy requirements and roles in plant pollination system, to reveal the features of their competitive interaction and mechanisms enabling their co-existence while utilising the same nectar source. We observed diurnal feeding behaviour of the main pollinator, the carpenter bee Xylocopa caffra and a nectar thief, the northern double-collared sunbird Cinnyris reichenowi on 19 clumps of Hypoestes aristata (Acanthaceae) in Bamenda Highlands, Cameroon. For comparative purpose, we established a simplistic model of daily energy expenditure and daily energy intake by both visitor species assuming that they spend all available daytime feeding on H. aristata. We revealed the energetic gain–expenditure balance of the studied visitor species in relation to diurnal changes in nectar quality and quantity. In general, smaller energy requirements and related ability to utilise smaller resources made the main pollinator X. caffra competitively superior to the larger nectar thief C. reichenowi. Nevertheless, sunbirds are endowed with several mechanisms to reduce asymmetry in exploitative competition, such as the use of nectar resources in times of the day when rivals are inactive, aggressive attacks on carpenter bees while defending the nectar plants, and higher speed of nectar consumption.     

Interspecific competition in Australian honeyeaters—depletion of common resources

Many species of honeyeaters and other nectar-feeding birds occur in most habitats in South Australia. They frequently feed on nectar of the same species of plants. A succession of species of plants provide nectar for birds throughout the year. Nectar is most abundant in winter and early spring and least abundant in summer and autumn. There is more nectar per flower and more flowers in winter and spring. Nectar is often depleted by honeyeaters, and sometimes other visitors (silvereyes, lorikeets and insects) between December and May. It is at times reduced to a level at which it is uneconomical for some species to exploit. There are seasonal movements of honeyeaters into areas of abundant nectar and out of these areas when nectar becomes scarce. Breeding coincides with peak abundance of nectar. Diversity of honeyeaters is probably maintained by an interaction of two types of competition, exploitation and interference. The larger species use the richest sources of nectar and aggressively exclude the smaller species (interference) whereas the smaller species can use poorer sources of nectar because their energy requirements are less (exploitation).     

Proximate Costs of Competition for Nectar

Food competition among coexisting nectarivorous birds is conspicuousand often intense, affecting patterns of flower choice, dailybehavior budgets, and timing of successful reproduction. Exploitativecompetition involves loss of accumulated nectar to other individualsthat visited a flower first. Preliminary data support the useof Poisson models of the frequencies of point-source visitationand overlap for determining the probabilities of actual competitiveevents. Nectar losses from monitored flowers can be estimatedin terms of time intervals between visits weighted by flower-specificnectar production and bird-specific nectar removal capabilities.Foraging time budgets then provide a meaningful common denominatorfor assessing the impacts of competitive nectar losses, becausecompensatory increases in foraging time are required to maintaina balanced energy budget. Flexibility in foraging time budgetsmade possible by high efficiency foraging and predictably lowcompetitive losses may be an important determinant of reproductivetiming and success in nectar feeding birds. Aggressive displacement of competitors and territorial defenseof flowers are common forms of interference competition in nectar-feedingbirds. Aggression has definable caloric costs that ultimatelymust relate to caloric gains. Defense of flowers increases theaggressor's exclusive use of nectar, increases the predictabilityof a nectar supply, and increases the average amount of nectarobtained per flower. Simple cost-benefit models of territorialitydefine conditions when net benefits of territoriality are greaterthan those of alternatives.     

The nutritional significance of a winter‐flowering succulent for opportunistic avian nectarivores

The winter‐flowering succulent Aloe marlothii provides nectar for many opportunistic avian nectarivores in southern African savannas. We assessed the importance of A. marlothii nectar sugar for opportunistic nectarivores by analysing temporal changes in stable carbon isotope ratios (δ¹³C) in the tissues of birds in Suikerbosrand Nature Reserve, South Africa. The blood of the 11 most common non‐granivorous opportunistic nectarivores at our site was enriched in ¹³C by 3.4 ± 1.5‰ during the flowering period of A. marlothii, reflecting the enriched crassulacean acid metabolism (CAM) isotopic signature of nectar (?12.6 ± 0.5‰). This relatively small contribution of A. marlothii nectar to assimilated carbon in whole blood contrasted with that of exhaled CO₂ in African Red‐eyed Bulbuls Pycnonotus nigricans and Cape White‐eyes Zosterops capensis. In both these species, the δ¹³C of breath samples was significantly enriched compared with blood and feathers, and closely resembled that of the nectar, revealing combustion of ingested nectar rather than assimilation. Although our analysis was complicated by the presence of C₄ grasses, whose δ¹³C values are similar to those of CAM photosynthesizers, when considered with previously published feeding observations our data reveal that opportunistic nectarivores feeding on A. marlothii nectar obtain a relatively small fraction of their assimilated carbon, but most of their metabolized carbon, from this seasonally available carbohydrate food resource. Because the δ¹³C values of insects associated with C₃ plants also became enriched during the flowering season, some insect‐eating opportunistic nectarivores may have assimilated A. marlothii carbon indirectly from insects. This study highlights the importance of understanding isotopic routing when assessing the nutritional significance of specific dietary items to consumer communities.     

A review of the energetics of pollination biology

Pollination biology is often associated with mutualistic interactions between plants and their animal pollen vectors, with energy rewards as the foundation for co-evolution. Energy is supplied as food (often nectar from flowers) or as heat (in sun-tracking or thermogenic plants). The requirements of pollinators for these resources depend on many factors, including the costs of living, locomotion, thermoregulation and behaviour, all of which are influenced by body size. These requirements are modified by the availability of energy offered by plants and environmental conditions. Endothermic insects, birds and bats are very effective, because they move faster and are more independent of environmental temperatures, than are ectothermic insects, but they are energetically costly for the plant. The body size of endothermic pollinators appears to be influenced by opposing requirements of the animals and plants. Large body size is advantageous for endotherms to retain heat. However, plants select for small body size of endotherms, as energy costs of larger size are not matched by increases in flight speed. If high energy costs of endothermy cannot be met, birds and mammals employ daily torpor, and large insects reduce the frequency of facultative endothermy. Energy uptake can be limited by the time required to absorb the energy or eliminate the excess water that comes with it. It can also be influenced by variations in climate that determine temperature and flowering season.     

Altitude-related shift of relative abundance from insect to sunbird pollination in Elaeagnus umbellata (Elaeagnaceae)

The evolution of floral traits has been thought to be influenced by local, effective pollinators. However, little attention has been paid to the possibility that altitudinal variation in floral traits could be mediated by local pollinator functional groups, particularly a shift from bees to birds. Plant size, floral traits, pollinators and their pollination roles were investigated in the spring-flowering shrub Elaeagnus umbellata (Elaeagnaceae) at three altitudes (1160, 1676, and 2050 m) in Minshan, Sichuan Province, on the northern rim of the Hengduan Mountains, southwest China. Compared to lower altitudes, higher-altitude plants were smaller but the floral tubes were longer, with a larger volume of nectar of lower sugar concentration but with a greater proportion of sucrose. The visitation frequency of bees decreased with altitude, whereas the sunbirds did the opposite. Birds and bees foraged for nectar but not pollen, and birds deposited more pollen grains per visit relative to bees and least were syrphid flies. Excluding birds decreased seed set at high but not at mid- or low altitude. Our study of E. umbellata revealed an association between altitudinal variation in floral traits and a change in the relative abundance of the major pollinators with altitude from majority bees to majority sunbirds. Although abiotic factors also tend to vary with altitude and can affect floral traits, nectar properties of “pro-bird” pollination were observed at high altitude.     

Covariation of flower traits and bird pollinator assemblages among populations of Kniphofia linearifolia (Asphodelaceae)

Globally, bird-pollinated plants can be separated into two groups, one consisting of species pollinated by specialist nectarivores, and the other of plants pollinated by occasional nectarivores. There are marked differences in nectar properties among the two groups, implying that there has been pollinator-mediated selection on these traits. This raises the possibility that variation in bird assemblages among populations of a plant species could lead to the evolution of intraspecific variation in floral traits. We examined this hypothesis in Kniphofia linearifolia, a common and widespread plant in southern Africa. Although bees are common visitors to flowers of this species, exclusion of birds from inflorescences led to significant reductions in seed set, indicating that the species is primarily bird-pollinated. We showed that bird pollinator assemblages differ markedly between five different populations of K. linearifolia, and that variation in flower morphology and nectar properties between these populations are associated with the dominant guild of bird visitors at each population. We identified two distinct morphotypes, based on corolla length, nectar volume and nectar concentration, which reflect the bird assemblages found in each type. Further work is needed to establish if a natural geographic mosaic of bird assemblages are the ultimate cause of differentiation in floral traits in this species.     

Intake Responses in Nectar Feeding Birds: Digestive and Metabolic Causes, Osmoregulatory Consequences, and Coevolutionary Effects

Nectar-feeding vertebrates respond to variation in nectar sugarcontent by modulating volumetric intake. In some nectar feedinganimals, the intake response to sugar concentration can be accuratelypredicted from simple mathematical models that rely on knowledgeof gut morphology, in vitro rates of sugar digestion, and dailyenergy expenditures. Because most of the floral nectars consumedby vertebrates are dilute, these animals ingest large amountsof water while feeding. The water turnover rates of hummingbirdsfeeding on dilute nectar are more similar to those of amphibiousand aquatic organisms than to those of terrestrial vertebrates.Dilute nectars can pose osmoregulatory challenges for nectarivores.Nectarivorous birds exhibit renal traits that are well suitedto dispose of large water loads and that appear inadequate toproduce concentrated urine. Nectar-feeding birds prefer concentratedover dilute sugar solutions. However, the concentration differencethat they can discriminate is smaller at low than at high concentration.We hypothesize that this pattern is a consequence of the functionalform of intake responses that often results in deceleratingsugar intakes with increasing sugar concentration. The diminishingreturns in floral attractivity that may result from increasednectar concentration may be one of the reasons why the nectarsof hummingbird pollinated flowers are dilute in spite of thepreference of birds for higher concentrations. The intake responsesof nectar-feeding birds capture the integration of a behavioralresponse with the physiological processes that shape it. Becausethe behavior of nectar-feeding birds can have consequences forthe plants that they visit, the intake response may also havecoevolutionary effects.     

Bird pollinators differ in their tolerance of a nectar alkaloid

Although the function of nectar is to attract and reward pollinators, secondary metabolites produced by plants as anti‐herbivore defences are frequently present in floral nectars. Greater understanding is needed of the effects of secondary metabolites in nectar on the foraging behaviour and performance of pollinators, and on plant–pollinator interactions. We investigated how nectar‐feeding birds, both specialist (white‐bellied sunbirds Cinnyris talatala) and generalist (dark‐capped bulbuls Pycnonotus tricolor and Cape white‐eyes Zosterops virens), respond to artificial nectar containing the alkaloid nicotine, present in nectar of Nicotiana species. Preference tests were carried out with a range of nicotine concentrations (0.1–300 μM) in two sucrose concentrations (0.25 and 1 M), and for bulbuls also in two sugars (sucrose and hexose). In addition, we measured short‐term feeding patterns in white‐bellied sunbirds that were offered nicotine (0–50 μM) in 0.63 M sucrose. Both nicotine and sugar concentrations influenced the response of bird pollinators to nicotine. The birds showed dose‐dependent responses to nicotine; and their tolerance of high nicotine concentrations was reduced on the dilute 0.25 M sucrose diet, on which they increased consumption to maintain energy intake. White‐bellied sunbirds decreased both feeding frequency and feeding duration as the nicotine concentration in artificial nectar increased. Of the three species, bulbuls showed the highest tolerance for nicotine, and sugar type (sucrose or hexose) had no effect. The indifference of bulbuls to nicotine may be related to their primarily frugivorous diet. However, the response of white‐eyes to nicotine in the dilute sucrose solution was very similar to that of sunbirds, even though white‐eyes are generalist nectar‐feeders. Additional testing of other avian nectarivores and different secondary metabolites is required to further elucidate whether generalist bird pollinators, which utilise dilute nectars in which secondary metabolites have stronger deterrent effects, are more tolerant of ‘toxic’ nectar.     

Interspecific competition changes reproductive output but does not increase reproductive costs in a grassland perennial

In plants, it is hypothesized that allocation trade-offs may appear only when expenditures like seed production are high or external resources are scarce. In this study, we tested whether reproductive costs are more pronounced under enhanced interspecific competition.In a common garden, we investigated phenotypic correlations between sexual reproduction, clonal growth and storage structures in the grassland perennial, Succisa pratensis. During the past 50 years, habitats of this species have faced an expansion of clonal grasses that increase competition intensity. We simulated this process by growing five populations of Succisa from high- and low-production habitats with its clipped and non-clipped competitor, Agrostis capillaris. In addition, we experimentally removed flower heads of Succisa plants from one population grown with and without A. capillaris.We demonstrated costs of sexual reproduction by flower-head removal (resulting in increased plant size and relative allocation to belowground structures) but not by phenotypic correlations. We found no evidence that reproductive costs increase in a competitive environment and the opposite pattern was shown in both approaches used. However, high competition intensity reduced relative investment to flower-head production. In plants from low-production habitats, competition also reduced the absolute number of flower heads and belowground biomass as a result of smaller plant size. We assume that populations from low-production habitats are more prone to extinction as they have a reduced likelihood of local persistence and of escape to more suitable habitats during advancing succession.     

The sweet side of life: nectar sugar type and concentration preference in Wahlberg's epauletted fruit bat

Whether nectarivores or frugivores place selective pressure on the plants they feed on, in terms of nectar or fruit traits, is much debated. Globally sugar preferences, concentration preference and digestive ability of avian nectarivores have been extensively researched. In contrast, relatively little is known about mammalian nectarivores or frugivores in terms of these, particularly Old World species. Consequently effect of sugar type and concentration on food preference in Wahlberg's epauletted fruit bat Epomophorus wahlbergi was investigated. Pair-wise choice tests were conducted using equicaloric hexose and sucrose solutions at five different concentrations (5%-25%). It was expected that they would prefer hexose sugars as these are dominant in available indigenous fruits. However, bats preferred hexoses only when offered dilute (5%) concentrations. From 10% to 25% they showed a decrease in volume intake. Their body mass was generally higher and similar after feeding during the night with the exception of 5% concentration where the mean body mass decreased. When E. wahlbergi were offered a range of sucrose or hexose solutions (10%-25%) respectively, they showed no concentration preference in terms of total volume consumed, nor energy intake. These findings suggest that these fruit bats do not appear to act as a selective pressure on sugar composition in Old World fruit. In fruit bats with high energy requirements, dietary flexibility may be an advantage when faced with seasonal and unpredictable fruit availability.     

Diurnal foraging‐mode shifts and food availability in nectarivore assemblages during winter

Abstract Temporal fluctuations of food resources and foraging activities have been studied extensively, especially at longer timescales (monthly, seasonally, among years). However, short‐term variation (e.g. within days) is less well understood. Here we systematically quantified diurnal patterns of foraging by nectarivorous birds (meliphagid honeyeaters) that numerically dominated stands of a winter‐flowering eucalypt, the red ironbark, Eucalyptus tricarpa, in central Victoria, Australia. Diurnal variation in food resources also was measured. Data were collected in winter. Anecdotal observations that honeyeaters change from almost exclusive nectarivory early in the day to a higher fraction of insectivory – especially aerial hawking – later in the day were confirmed, although in areas of high flowering intensity, nectar‐feeding remained the dominant foraging activity throughout the day. Local climatic factors (ambient temperature, windiness and cloud cover) all varied systematically through the day. Together, results were consistent with a change in foraging emphasis to greater insectivory as a function of elevated activities of insects (especially aerial ones), which was probably fostered by higher ambient temperatures. Contrary to energetic expectations, the nectarivores were very active early in the morning when ambient temperatures averaged approximately 3°C, well below thermoneutral temperatures. We deduced that the potential benefits of gathering as much energy‐rich nectar as possible before it was depleted outweighed the high costs of activities at low temperatures.     

Intra-sexual competition alters the relationship between testosterone and ornament expression in a wild territorial bird

In a reliable signalling system, individual quality is expected to mediate the costs associated with ornamental displays, with relatively lower costs being paid by individuals of higher quality. These relative costs should depend not only on individual quality, but also on levels of intra-sexual competition. We explored the current and delayed effects that testosterone implants have on bird ornamentation in populations with contrasted population densities, as a proxy for intra-sexual competition. In a replicated experiment, we manipulated testosterone in 196 yearling male red grouse Lagopus lagopus scoticus in autumn in populations of high and low levels of intra-sexual competition. Males were assigned to one of three exogenous testosterone (T) treatments: empty implants (T0), small T implants (T1) or larger T implants (T2). We monitored subsequent changes in testosterone levels, ornament size and carotenoid-based colouration, carotenoid levels and body condition from autumn to spring. Testosterone implants increased testosterone levels, comb redness and comb size, and decreased body condition but these effects depended on levels of intra-sexual competition. Specifically, T2-implanted birds increased testosterone levels and comb size more, and reduced body condition more, in populations where intra-sexual competition was low. In the following spring, testosterone levels of T2-treated birds kept increasing in populations where intra-sexual competition was high but not in populations where intra-sexual competition was low. Our results highlight that levels of intra-sexual competition alter the relationship between testosterone levels and ornament expression, influencing their condition-dependence; they also indicate that the outcome of standard hormone manipulation conducted in free-living animals vary depending on the population context.     

Hummingbirds pay a high cost for a warm drink

Endotherms must warm ingested food to body temperature. Food warming costs may be especially high for nectar-feeding birds, which can ingest prodigious volumes. We formulated a mathematical model to predict the cost of warming nectar as a function of nectar temperature and sugar concentration. This model predicts that the cost of warming nectar should: (1) decrease as a power function of nectar concentration, and (2) increase linearly with the difference between body temperature and nectar temperature. We tested our model on rufous hummingbirds (Selasphorus rufus). A typical experiment consisted of feeding birds nectar of a given concentration at 39°C (equivalent to body temperature) and then at 4°C, and vice versa. We used the percentage change in metabolic rate between the two food temperatures to estimate the cost of warming nectar. The model's predictions were accurately met. When birds had to hover rather than perch during feeding bouts, estimated food-warming costs were only slightly lower. The cost of warming nectar to body temperature appears to be an important yet overlooked aspect of the energy budgets of nectar-feeding birds. Hummingbirds feeding on 5% sucrose solutions at 4^oC have to increase their metabolic rate by an amount equivalent to that elicited by a 15°C drop in ambient temperature.Abbreviations AE assimilation efficiency - C nectar concentration - H' cost of warming food to body temperature - SDA specific dynamic action - T_a ambient temperature - T_b body temperature - T_n nectar temperatureCommunicated by: G. Heldmaier