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.     

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.     

Sugar preferences of nectar feeding birds – a comparison of experimental techniques

Experiments to determine sugar preferences of nectarivorous animals have been conducted using a wide variety of experimental procedures, all of which aim at ensuring that the solutions offered in choices are “equivalent”. Each method used historically has controlled for a particular variable, such as number of molecules in solution, weight of sugar in solution, or amount of energy in solution, depending on what question the researchers have tried to answer. Biologists interpreting these results in terms of bird sugar preference have seldom taken these differences into account. The consequences of using different experimental procedures for sugar preferences exhibited by a nectarivorous bird, the malachite sunbird Nectarinia famosa, were examined using paired sucrose and hexose sugar solutions made up to be either equimolar, equiweight or equicaloric. We found the effect of methodology on bird sugar preference to be quite distinct, especially at low concentrations, where malachite sunbirds showed either sucrose preference, no preference, or hexose preference, depending on the method used. This study highlights the need for researchers to consider methodology when interpreting, or comparing among, results from previous studies.     

Sugar preferences of some southern African nectarivorous birds

Three southern African nectarivorous passerine birds, Gurney's Sugarbird Promerops gur-neyi , the Malachite Sunbird Nectarinia famosa and the Black Sunbird Nectarinia amethystina , were tested to determine their hexose and sucrose preferences. All three species preferred sucrose when offered a choice of 0.25 M solutions of glucose, fructose and sucrose. However, when the concentrations were increased to 0.73 M, the three species showed no preference for any of the three sugars. The choice made at low concentrations (equivalent to the lower limit of the range of nectar concentrations of preferred nectar-producing plants) may reflect preference for the sugar with the highest energy reward. We also examined the proposition that birds offered a choice of different concentrations of one sugar would show ranked preferences and maximize their rate of energy return by selectivity. In contrast to expectations, Gurney's Sugarbird and the Malachite Sunbird showed no preference for the highest concentrations. We suggest that dietary choices in these species indicate the birds had either reached a limit where they had sufficient energy intake or were affected by post-ingestion constraints.     

Added salt helps sunbirds and honeyeaters maintain energy balance on extremely dilute nectar diets

Nectar-feeding birds ingest excess water and risk loss of solutes when they excrete it. Previous work has shown that white-bellied sunbirds (Cinnyris talatala) are unable to maintain energy balance on extremely dilute sucrose diets without salts (e.g. <0.25 mol l⁻¹), and that they lose more electrolytes (i.e. Na⁺ and K⁺) via cloacal fluid on these diets than on more concentrated diets. Using white-bellied sunbirds and New Holland honeyeaters (Phylidonyris novaehollandiae) we tested the effect of adding electrolytes to a 0.1 mol l⁻¹ sucrose diet, by including equimolar NaCl and KCl at concentrations from 5 to 40 mmol l⁻¹ and the individual salts at 20 mmol l⁻¹. Addition of salts enabled both species to drink significantly more of the 0.1 mol l⁻¹ sucrose diet than in the absence of salts, and mass loss during the experiment was reduced when salt was included. The larger honeyeaters may be more susceptible to electrolyte depletion than the smaller sunbirds. On 20 mmol l⁻¹ combined salts, both sunbirds and honeyeaters consumed eight times their body mass in fluid daily. KCl alone had no effect. Birds are thus limited in their consumption of extremely dilute diets by increasing losses of Na⁺. This was confirmed by measuring plasma Na⁺ levels, which decreased in both species in the absence of dietary Na⁺. In addition, sucrose assimilation efficiencies were slightly, but significantly lower when sunbirds were fed salt-free diet, while glucose levels in ureteral urine remained extremely low. It is concluded that Na⁺ depletion on very dilute salt-free diets does not affect Na⁺-glucose transport activity in the kidney, but interferes with sugar digestion and/or assimilation in the intestine.     

Sugar Preferences in Nectar‐ and Fruit‐Eating Birds: Behavioral Patterns and Physiological Causes1

Sucrose, glucose, and fructose are the three sugars that commonly occur in floral nectar and fruit pulp. The relative proportions of these three sugars in nectar and fruit in relation to the sugar preferences of pollinators and seed dispersers have received considerable attention. Based on the research of Herbert and Irene Baker and their collaborators, a dichotomy between sucrose‐dominant hummingbird‐pollinated flowers and hexose‐dominant passerine flowers and fruits was proposed. Data on sugar preferences of several hummingbird species (which prefer sucrose) vs. a smaller sample of passerines (which prefer hexoses) neatly fitted this apparent dichotomy. This hummingbird–passerine dichotomy was strongly emphasized until the discovery of South African plants with sucrose‐dominant nectars, which are pollinated by passerines that are able to digest, and prefer sucrose. Now we know that, with the exception of two clades, most passerines are able to assimilate sucrose. Most sugar preference studies have been conducted using a single, relatively high, sugar concentration in the nectar (ca 20%). Thus, we lack information about the role that sugar concentration might play in sugar selection. Because many digestive traits are strongly affected not only by sugar composition, but also by sugar concentration, we suggest that preferences for different sugar compositions are concentration‐dependent. Indeed, recent studies on several unrelated nectar‐feeding birds have found a distinct switch from hexose preference at low concentrations to sucrose preference at higher concentrations. Finally, we present some hypotheses about the role that birds could have played in molding the sugar composition of plant rewards.     

Dietary intake effects on arginine vasotocin and aldosterone in cloacal fluid of whitebellied sunbirds (Nectarinia talatala)

The main objective of this study was to determine whether or not the renal outputs of the osmoregulatory hormones arginine vasotocin (AVT) and aldosterone (ALDO) reflect the osmotic status of whitebellied sunbirds (Nectarinia talatala). The birds were fed a range of sucrose concentrations (from 0.07 to 2.5 mol/l, with osmolalities of 70 to approximately 5,800 mosM/kg), and adjusted their intakes so that they drank large volumes of dilute diets and small volumes of concentrated diets. Renal fluid outputs were appropriately regulated so that large volumes of cloacal fluid (CF) were voided on the dilute diets and small volumes on the concentrated diets. Accordingly, plasma AVT concentrations increased with increasing sugar concentration; however, AVT outputs in CF did not change in a similar manner, rather they decreased as dietary concentration increased. It was not possible to measure plasma ALDO concentrations in the small sunbirds because of insufficient blood samples available; however, ALDO outputs in CF did vary with the sucrose diets and renal function, being highest on the most concentrated diet. In addition ALDO output in CF fell markedly when sodium was added to the 0.5 mol/l sucrose diet. We conclude that in sunbirds fed increasingly concentrated sucrose solutions, changes in CF outputs of ALDO, but not AVT, appear to reflect the water flux and hydration state of these birds.     

Whitebellied sunbirds (<Emphasis Type="Italic">Nectarinia talatala</Emphasis>, Nectariniidae) do not prefer artificial nectar containing amino acids

Amino acids are the most abundant class of compounds in nectar after sugars. Like its sugar concentration, the amino acid concentration of nectar has been linked to pollinator type, and it has been suggested that amino acid concentrations are high in the floral nectars of plant species pollinated by passerine birds compared to those pollinated by hummingbirds. We investigated the feeding response of whitebellied sunbirds (Nectarinia talatala) to the inclusion of amino acids in artificial nectar (0.63 M sucrose solution). The response to asparagine, glutamine, phenylalanine, proline, serine and valine, amino acids commonly found in floral nectars, was tested individually and using a mixture of all six amino acids, at two different concentrations (2 and 15 mM). Sunbirds showed no significant preference for amino acids in nectar, or avoided them, especially at the higher concentration. We discuss these findings in the light of the nitrogen requirements of nectarivorous birds and data on amino acids in floral nectars.     

Regulation of nutrient intake in nectar-feeding birds: insights from the geometric framework

A nectar diet is simple in nutritional composition and easily digested, but may vary greatly in its proportions of sugar and water. Here, we apply the geometric framework, a modelling approach for investigating how animals balance nutrient needs in multidimensional and dynamic nutritional environments, to captive whitebellied sunbirds (Cinnyris talatala). We address the question of how these small birds (~8?g) prioritise sugar and water intake, and how dietary salt content interacts with sugar and water intake. Sunbirds kept at 20°C and provided with moderate to high sucrose concentrations (≥1?M), together with supplementary water, converge on an intake target of 2.79?g?day(-1) of sucrose and 7.72?g?day(-1) of water: equivalent to 0.85?M sucrose. When the birds are given more dilute sucrose concentrations, they defend their sugar intake by over-ingesting water, up to a ceiling of 47?g?day(-1). Sugar intake thus gets priority over water intake, but the birds have a finite capacity to over-ingest water to gain the target level of sugar. Regulation appears to be less precise when birds are given a choice between two sucrose solutions than when they choose between a sugar solution and supplementary water. Intake targets vary in response to internal and external factors, and sunbirds increase their sugar intake in response to increased activity and cold, irrespective of nectar concentration. They also compensate for interruptions in foraging activity, whether overnight or during the day. Interactive effects become evident when sodium is included as a third nutrient: on very dilute nectar (≤0.1?M), where sunbirds lose body mass, the addition of sodium to the diet helps to achieve the carbohydrate intake target, while raising the ceiling on water intake. This analysis provides a new perspective on nectarivory, while adding to the comparative database on nutrient regulation and emphasising water as a nutrient.     

Bills and tongues of nectar-feeding birds: A review of morphology,function and performance,with intercontinental comparisons

The bills and tongues of nectar-feeding birds differ from continent to continent. The major differences are that: (i) the tongues of A Australian honeyeaters are broader any more fimbricated at the tip than the bifurcated tongues of sunbirds and hummingbirds; (ii) the bills of hummingbirds are more uniformly narrow and taper less markedly towards their tips than those of sun-birds and honeyeaters; and (iii) bill curvatures are generally greater for sunbirds and honey-creepers than for hummingbirds. A variety of hummingbirds has straight or even slightly upturned bills, while bills for all sunbirds, honeycreepers and honeyeaters are decurved to some extent. Despite differences in tongue morphology, hummingbirds, sunbirds and honeyeaters extract nectar at a similar range of rates, averaging approximately 40 γL s^?1 from ad libitum feeders, and 1–15 γL^?1 from flowers. All tongues collect nectar by capillarity, with licking rates of 6–17 s^?1. Licking behaviour has been little studied, although speeds of licking respond to changes in sugar concentration and corolla length. The tongues of honeyeaters are broad, and may need to be brush-tipped in order to allow capillary collection of nectar. Brush-tipped tongues can cover large surface areas on each lick, and may allow honeyeaters to exploit nectar and honeydew that is thinly spread over large surface areas. Bill lengths of nectarivorous birds are similar in all regions, though species of hummingbird have the shortest and longest bills. Bill lengths largely determine the range of floral lengths that can be legitimately probed. Maximum floral lengths exceed bill lengths, since hummingbirds, sunbirds and honeyeaters protrude their tongues beyond the tips of their bills. Rates of nectar extraction, however, decline rapidly once the floral length exceeds bill length. Decurved bills may have evolved in honeyeaters and sunbirds to enable perching birds to reach flowers at the ends of branches more easily. Consistent differences in bill length between the sexes suggest that males and females may exploit different floral resources or different proportions of the same resources. For honeyeaters and sunbirds, males have longer bills than females, but the reverse is true for many hummingbirds.     

Sugar gustatory thresholds and sugar selection in two species of Neotropical nectar-eating bats

Nectar-feeding bats play an important role in natural communities acting as pollinators; however, the characteristics that affect their food selection are unclear. Here we explore the role that sugar gustatory thresholds and sugar concentration play on sugar selection of Glossophaga soricina and Leptonycteris yerbabuenae. We offered bats paired feeders containing sugar solutions of sucrose (S), glucose (G) or fructose (F) vs. pure water, and sucrose vs. 1:1 equicaloric solutions of glucose–fructose at 5, 15 and 35% (wt./vol.). To see the effect of sweetness on sugar selection, we habituated the bats with a diet containing either sucrose or hexoses and subsequently evaluated sugar preferences. Sugar thresholds were S < G,F for G. soricina and G < S < F for L. yerbabuenae. These thresholds did not match with sugar preferences when the bats fed on dilute nectars. L. yerbabuenae changed its sugar preferences with concentration while G. soricina did not. Finally, the bats consistently preferred the sugar they were habituated to. Our results suggest that bats become accustomed to the sugar found in the most abundant plants they use, and thus prefer the most common sugars included in their diet. This could confer an advantage by allowing them shifting sugar preferences on the most common food present in their environment.     

Sucrose hydrolysis does not limit food intake by Pallas's long-tongued bats

Nectarivorous bats include very dilute nectar in their natural diet, and recent work with Pallas's long-tongued bat Glossophaga soricina showed that sugar (energy) intake rate decreased at dilute sucrose solutions. However, chiropterophillous nectar is composed mainly of the hexoses glucose and fructose. Because bats fed hexose nectar would save the delay of hydrolyzing sucrose, we hypothesized that sugar intake rate should be higher on this diet than on sucrose nectar. We compared intake response in Pallas's long-tongued bats offered 1 : 1 glucose-fructose (hexose) and sucrose diets at 5%, 10%, 20%, 30%, and 40% (mass/volume) sugar solutions. We also tested the hypothesis that sucrose hydrolysis limits food intake in bats. Intake response was the same in bats fed both types of diet: sugar intake rate was lower in dilute solutions and then increased with sugar concentration. Similar intake responses in both diets indicate that sucrose hydrolysis alone does not limit food intake and support the idea that the burden of processing excess water in dilute solutions plays a major role.     

Changes in nectar concentration: how quickly do whitebellied sunbirds (Cinnyris talatala) adjust feeding patterns and food intake?

Nectarivorous birds encounter varying nectar concentrations while foraging on different food plants and must adjust their consumption to maintain constant energy intake. We determined how rapidly captive whitebellied sunbirds (Cinnyris talatala) adjust their volumetric intake and feeding patterns after changes in diet concentration. On four consecutive days, birds were fed sucrose diets alternating between a standard diet of 16% w/w and test diets of 2.5, 8.5, 16 or 30% w/w, respectively, for 1.5 h periods. Feeding events were recorded with an infrared photo-detection system and food intake and body mass were monitored continuously by electronic balances interfaced to a computer. Generally, birds demonstrated a measurable increase in feeding frequency and food intake within 10 min after a decrease in sucrose concentration. However, individuals responded differently to the most dilute diet (2.5%): while most increased their food intake, others stopped feeding for a short while, appearing to dislike this diet. Furthermore, the number and duration of feeding events increased in the first 5 min after the switch from 2.5% back to 16%, as the birds attempted to compensate for previous reduced sugar intake. Daily sugar intake was lower when birds alternated between 2.5 and 16% diets than on other test days, but birds were able to maintain body mass, presumably through behavioural adjustments.     

Pollination by passerine birds: why are the nectars so dilute?

Bird-pollinated flowers are known to secrete relatively dilute nectars (with concentrations averaging 20–25% w/w). Many southern African plants that are pollinated by passerine birds produce nectars with little or no sucrose. Moreover, these hexose nectars are extremely dilute (10–15%). This suggests a link between sugar composition and nectar concentration. Nectar originates from sucrose-rich phloem sap, and the proportion of monosaccharides depends on the presence and activity of invertase in the nectary. Hydrolysis of sucrose increases nectar osmolality and the resulting water influx can potentially convert a 30% sucrose nectar into a 20% hexose nectar, with a 1.56 times increase in volume. Hydrolysis may also increase the gradient for sucrose transport and thus the rate of sugar secretion. When sucrose content and refractometer data were compared, some significant correlations were seen, but the occurrence of sucrose-rich or hexose-rich nectars can also be explained on phylogenetic grounds (e.g. Erythrina and Protea). Hexose nectars may be abundant enough to drip from open flowers, but evaporation leads to much variability in nectar concentration and increases the choices available to pollinators.     

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     

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.     

Preferences for nectar sugars in the peacock butterfly, Inachis io

1. Peacock butterflies, Inachis io , were tested experimentally for their preferences for nectar sugars.
2. In tests with different plain sugar solutions (25%, weight to total weight) the butterflies strongly preferred sucrose and fructose over glucose. They also preferred sucrose over fructose.
3. In tests with mixed sugar solutions the butterflies clearly preferred both sucrose-dominant (sucrose : hexoses = 5 : 1) and balanced sugar solutions (sucrose : glucose : fructose = 1 : 1 : 1) over hexose-dominant sugar solutions (sucrose : hexoses = 1 : 5).
4. Females consumed significantly more of the balanced sugar solution than did males.
5. These results are discussed with respect to previous experiments on nectar preferences of butterflies, nectar sugar composition of butterfly-pollinated flowers, and flower preferences, physiological and reproductive aspects of butterflies.     

Energy and water balance in the lesser double-collared sunbird (Nectarinia chalybea) feeding on different nectar concentrations

The water balance of nectarivores is tightly linked to their energy balance. When nectar is dilute, consumption of a large water excess is inevitable. We investigated energy and water balance in lesser double-collared sunbirds, Nectarinia chalybea (8 g), kept at 20 °C and fed different nectar concentrations (0.4, 0.8 M sucrose or 1.2 M sucrose). The mass of sucrose consumed, body mass, day-time mass gain and night-time mass loss were the same irrespective of diet, the birds compensating energetically for changes in sucrose concentration by drinking greater volumes of the more dilute solutions. Sunbirds consumed between 0.5 times and 1.8 times their body mass in preformed water per day, depending on sucrose concentration, and excreted around 75% of the water. The difference between water gain (preformed and metabolic water) and excreted water is assumed to equal evaporative water loss, and was similar on 1.2 M and 0.8 M sucrose, but was higher on a diet of 0.4 M sucrose. The osmolalities and K⁺ and Na⁺ concentrations of the excreted fluid were extremely low, so that sunbird urine resembled that of hummingbirds and freshwater vertebrates rather than that of typical terrestrial vertebrates. N. chalybea is able to maintain energy and water balance over a range of nectar concentrations by adjusting the volume of solution consumed and by excreting copious, dilute fluid. Accepted: 2 January 1999     

Nectar sugar composition of European Caryophylloideae (Caryophyllaceae) in relation to flower length,pollination biology and phylogeny

Floral nectar composition has been explained as an adaptation to factors that are either directly or indirectly related to pollinator attraction. However, it is often unclear whether the sugar composition is a direct adaptation to pollinator preferences. Firstly, the lower osmolality of sucrose solutions means that they evaporate more rapidly than hexose solutions, which might be one reason why sucrose‐rich nectar is typically found in flowers with long tubes (adapted to long‐tongued pollinators), where it is better protected from evaporation than in open or short‐tubed flowers. Secondly, it can be assumed that temperature‐dependent evaporation is generally lower during the night than during the day so that selection pressure to secrete nectar with high osmolality (i.e. hexose‐rich solutions) is relaxed for night‐active flowers pollinated at night. Thirdly, the breeding system may affect selection pressure on nectar traits; that is, for pollinator‐independent, self‐pollinated plants, a lower selective pressure on nectar traits can be assumed, leading to a higher variability of nectar sugar composition independent of pollinator preferences, nectar accessibility and nectar protection. To analyse the relations between flower tube length, day vs. night pollination and self‐pollination, the nectar sugar composition was investigated in 78 European Caryophylloideae (Caryophyllaceae) with different pollination modes (diurnal, nocturnal, self‐pollination) using high‐performance liquid chromatography (HPLC). All Caryophylleae species (Dianthus and relatives) were found to have nectar with more than 50% sucrose, whereas the sugar composition of Sileneae species (Silene and relatives) ranged from 0% to 98.2%. In the genus Silene, a clear dichotomous distribution of sucrose‐ and hexose‐dominant nectars is evident. We found a positive correlation between the flower tube length and sucrose content in Caryophylloideae, particularly in day‐flowering species, using both conventional analyses and phylogenetically independent contrasts.     

Do hummingbirds have a sweet-tooth? Gustatory sugar thresholds and sugar selection in the broad-billed hummingbird Cynanthus latirostris

Nectar is a solution of mainly three sugars: sucrose, glucose and fructose. Studies have demonstrated that pollinators have preferences according to the sugar composition presented in their diet, and these preferences may be caused by sugar assimilation capacities. However, sugar flavor could also play an important role for sugar preferences of nectar-feeding animals. We evaluated the sugar gustatory thresholds of the broad-billed hummingbird Cynanthus latirostris for sucrose, glucose, fructose and a 1:1 mixture of glucose-fructose. We presented eight C. latirostris to paired feeders containing either a sugar solution or pure water. Additionally, we conducted sugar preference tests at three different concentrations (146, 730 and 1022 mmol L^− 1), to relate sugar preferences with sugar gustatory thresholds. C. latirostris had different gustatory thresholds for the three different sugars tested. At low sugar concentrations (146 mmol L^− 1), sugar selection followed the gustatory thresholds. Hummingbird sugar preference patterns can be affected by different mechanisms, both pre- and post-ingestive. At low concentrations gustatory thresholds may play an important role to determine sugar selection. However, at intermediate and high concentrations, sugar assimilation rates, and velocity of food processing generated by osmotic constraints, can be the mechanisms that explain the sugar selection of these animals.