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.     

Specialised protagonists in a plant–pollinator interaction: the pollination of Blumenbachia insignis (Loasaceae)

• Analyses of resource presentation, floral morphology and pollinator behaviour are essential for understanding specialised plant‐pollinator systems. We investigated whether foraging by individual bee pollinators fits the floral morphology and functioning of Blumenbachia insignis, whose flowers are characterised by a nectar scale‐staminode complex and pollen release by thigmonastic stamen movements.
• We described pollen and nectar presentation, analysed the breeding system and the foraging strategy of bee pollinators. We determined the nectar production pattern and documented variations in the longevity of floral phases and stigmatic pollen loads of pollinator‐visited and unvisited flowers.
• Bicolletes indigoticus (Colletidae) was the sole pollinator with females revisiting flowers in staminate and pistillate phases at short intervals, guaranteeing cross‐pollen flow. Nectar stored in the nectar scale‐staminode complex had a high sugar concentration and was produced continuously in minute amounts (~0.09 μl·h^?1). Pushing the scales outward, bees took up nectar, triggering stamen movements and accelerating pollen presentation. Experimental simulation of this nectar uptake increased the number of moved stamens per hour by a factor of four. Flowers visited by pollinators received six‐fold more pollen on the stigma than unvisited flowers, had shortened staminate and pistillate phases and increased fruit and seed set.
• Flower handling and foraging by Bicolletes indigoticus were consonant with the complex flower morphology and functioning of Blumenbachia insignis. Continuous nectar production in minute quantities but at high sugar concentration influences the pollen foraging of the bees. Partitioning of resources lead to absolute flower fidelity and stereotyped foraging behaviour by the sole effective oligolectic bee pollinator.

     

Bird Pollination of Explosive Flowers While Foraging for Nectar and Caterpillars1

Mucuna (Fabaceae) has explosive flowers that open only if a pressure is applied on their wings and keel. The cacique Cacicus haemorrhous inserts its bill into a flower and spreading its mandibles apart it opens the flower to take nectar. This icterine bird also preys upon caterpillars of the butterfly Astraptes talus that pupates within the flowers. Foraging with use of bill movements to take nectar or insects within a flower is an adequate mechanism to open and pollinate explosive flowers. We suggest that a plausible behavioral scenario for the pollination relationship between icterines and Mucuna‐like flowers might start with the birds' searching for insects within flowers.     

The movement patterns of carpenter beesXylocopa micans and bumblebeesBombus pennsylvanicus onPontederia cordata inflorescences

The movement patterns of carpenter bees (Xylocopa micans) and bumblebees (Bombus pennsylvanicus) foraging for nectar on vertical inflorescences ofPontederia cordata were studied near Miami, Florida. The floral biology ofP. cordata is unique in several ways: (a) many short-lived flowers per inflorescence, (b) constant nectar production throughout the life span of each flower, and (c) abscence of vertical patterning of nectar and age of flowers. Inflorescences ranged between 3.5 and 15.8 cm long and had between 9 and 55 open flowers. Both carpenter bees and bumblebees arrived mostly on the bottom third of the inflorescence and left after visiting flowers on the top third of the inflorescence. The departure position from the inflorescence was higher up than observed in studies of other insect pollinators foraging on other speces of plants. This pattern of departure probably occurs in the absence of a vertical gradient of nectar or floral morphology.     

Choice of flowers by foraging honey bees (Apis mellifera): possible morphological cues

Abstract.

• 1 Honey bees foraging for nectar on lavender (Lavandula stoechas) chose inflorescences with more of their flowers open. The number of open flowers predicted whether an inflorescence was visited by bees, inspected but rejected, or ignored. Inflorescences chosen arbitrarily by observers had numbers of open flowers intermediate between those of visited and ignored inflorescences.
• 2 Differences in morphological characters between types of inflorescence correlated with nectar volume and sugar weight per flower so that visited inflorescences had a disproportionately greater volume of nectar and weight of sugar per flower and greater variance in nectar volume.
• 3 Although there were significant associations between nectar content and the morphological characters of inflorescences, discriminant function analysis revealed discrimination on the basis of morphology rather than nectar content.
• 4 Visited inflorescences tended to have smaller than average flowers but bees tended to probe the largest flowers on visited inflorescences.
• 5 Choice of flowers within inflorescences is explicable in terms of the relationship between flower size and nectar content.

     

Can nectar properties explain sex‐specific flower preferences in the Adonis Blue butterfly Lysandra bellargus?

1. Field observations in the Swiss Jura mountains showed that males and females of the bivoltine Adonis Blue butterfly Lysandra bellargus Rott. differed significantly in their flower visitation patterns. 2. In both generations, females visited a broader range of available nectar plants than did males. The specific flower visitation patterns of males and females were not affected by the general availability and abundance of potential nectar plant species during both flight periods, indicating high selectivity for nectar plants by both males and females. 3. In addition, the sexes differed in their nectar foraging behaviours: distances between successively visited flowers were significantly longer in males than in females, indicating that male and female butterflies have different foraging strategies. 4. Investigations of nectar characteristics showed that the sexes preferred flowers with different nectar compositions. Males of both generations preferred flowers with high proportions of sucrose and high amounts of total sugar, whereas females preferred flowers with high portions of glucose in their nectar, and, in the spring generation, flowers rich in amino acids. 5. Flowers visited exclusively by males or females in spring differed significantly in their amino acid composition. 6. This clear‐cut pattern did not hold for the autumn generation, most probably due to the limited availability of flowers. 7. The observed nectar foraging patterns underline the importance of adult feeding for longevity and reproduction in butterflies. The findings are particularly relevant for conservation, because L. bellargus is an increasingly threatened species in many European countries.     

Determinants of foraging profitability in two nectarivorous butterflies

ABSTRACT.

• 1 I studied flower selection and foraging energetics of Agraulis vanillae L. (Nymphalidae) and Phoebis sennae (Pieridae), two butterfly species common to north central Florida. I identified the major nectar resources exploited by several populations of these butterflies and, for each plant species, measured available nectar volumes and concentrations, corolla lengths, and density. I quantified foraging behaviour of each butterfly species at each nectar source (flower visitation rate and percentage of foraging time in flight), and used these data to estimate the net rate of energy intake of each butterfly species at each nectar source.
• 2 Estimated mean energy contents of individual flowers of the eleven exploited plant species spanned three orders of magnitude, ranging between 0.015 and 9.27 joules. Mean energy content of individual flowers was strongly correlated with mean foraging profit of both butterfly species.
• 3 Mean nectar volume strongly influenced energy content and varied widely within and among species, ranging from 0.0076 to 1.853 μ1. Nectar concentration varied between 17.1% and 40.4% sucrose-equivalents. Nectar volume was the best single predictor of foraging profitability (correlation coefficients of 0.994 and 0.984 for Phoebis and Agraulis respectively). Corolla length also strongly affected foraging profitability for both butterfly species; flower species with longer corollas were generally more profitable.
• 4 Flower density and nectar concentration showed weak or nonsignificant associations with foraging profitability.
• 5 The usefulness and limitations of these floral characteristics as bases for foraging selectivity, and the selective pressures foraging butterflies might place on the visited plants are discussed.

     

Butterfly nectaring flowers: butterfly morphology and flower form

The profitability of butterfly foraging depends in part on the corolla depth and clustering of flowers, and the tongue length, body mass and wing loading of butterflies. Interactions among these attributes of flowers and butterflies were investigated, using data from a field study in Cornwall and from Porter et al. (1992). The maximum corolla depth from which a butterfly can feed depends on tongue length, which correlates with the more easily measured attributes of body mass and wing loading. Small, short-tongued butterflies did not visit deep flowers. The quantity of nectar sugar per flower necessary for profitable foraging depends on foraging costs, which are expected to correlate with wing loading. Butterfly species with a high wing loading generally confined their visits to flowers that were clustered or very nectar-rich. Butterfly species with a low wing loading included solitary and less nectar-rich flowers in their diet. Body mass and wing loading affect a butterfly's load-carrying capacity (limiting the distance between fuelling stops) and cooling rate (limiting the distance between stops for basking or endothermic warming), and will therefore influence the capacity for floral selectivity and for migration and dispersal. Body mass, wing loading and tongue length characterised families or subfamilies of butterflies. For example vanessine nymphalids, with their long tongues and high wing loading, visited the deep, massed flowers of Buddleja davidii, but lycaenids, with their short tongues and low wing loading, did not. These often visited members of the Asteraceae. Eupatorium cannabinum, with massed flowers offering abundant and accessible nectar, was visited by butterflies of all tongue lengths and both high and low wing loading. These findings may help to inform habitat management for butterfly nectaring flowers.     

Competition between hummingbirds and bumble bees for nectar in flowers of Impatiens biflora

Summary Using removal experiments and concurrent measurement of resource levels, evidence was obtained for exploitation competition between Ruby-throated hummingbirds and two bumble bee species (Bombus fervidus and B. vagans) foraging for nectar on Impatiens biflora.When all three species were active, flower visitors showed a complex pattern of resource partitioning involving both diel and spatial changes. Hummingbirds foraged almost exclusively from the outermost exposed flowers on plants from which they drained nectar levels beyond the reach of bees over most of the day. In contrast the longtongued bee species (B. fervidus), and the shorter-tongued B. vagans, displayed a preference for the innermost flowers on plants which were protected from hummingbird visitation by surrounding vegetation. The two Bombus spp. began foraging at different times during the day: B. vagans were most active in early morning but were replaced by B. fervidus later in the day.When hummingbirds were rare, only B. fervidus showed evidence of competitive release: an increase in the number of foragers and a broadening of flower choice to include more outer flowers. Workers of B. vagans showed a similar response to temporary removal of B. fervidus and also extended their foraging over the entire day. These responses were consistent with changes in the availability of nectar to different species.Removal experiments demonstrated that individuals of one species can be largely excluded from access to nectar resources as a direct result of exploitation of nectar by foragers of other species with longer tongues. Thus in this system interspecific exploitation is an important mechanism involved in resource partitioning.     

Why do nectar-foraging bees and wasps work upwards on inflorescences?

Summary Wasps (Dolichovespula and Vespula spp.) worked predominantly upwards when foraging for nectar on inflorescences of the protogynous Scrophularia aquatica, in which the standing crop of nectar sugar per flower showed no clear pattern of vertical distribution up an inflorescence. Bumblebees taking nectar (Bombus hortorum visiting legally, and certain individuals of B. terrestris which positioned themselves head-upwards while taking nectar through holes bitten in the corolla) worked predominantly upwards on the racemose inflorescences of Linaria vulgaris, although the standing crop of nectar sugar per open flower increased up the raceme. Individuals of B. terrestris which robbed Linaria flowers in a head-down position worked predominantly downwards on inflorescences. The upward or downward directionality of intra-inflorescence movements by foraging insects may depend in part on the position these adopt during their flower visits.     

Effects of nectar theft by flower mites on hummingbird behavior and the reproductive success of their host plant, Moussonia deppeana (Gesneriaceae)

Hummingbird flower mites are transported in the nares of hummingbirds and may compete with them by "robbing" nectar secreted by the host plants. We have shown that Tropicoseius sp. flower mites consume almost half the nectar secreted by the long-lived, protandrous flowers of Moussonia deppeana (Gesneriaceae) pollinated by Lampornis amethystinus (Trochilidae). In this paper, we ask whether mimicking nectar consumption of flower mites alters some aspects of hummingbird foraging patterns, and, if so, how this affects host plant seed production. We observed hummingbirds foraging on (a) plants in which nectar was removed from the flowers and then filled with a sugar solution to half the volume of nectar simulating nectar consumption by flower mites, and (b) plants where nectar was removed and then filled with the sugar solution up to normal nectar volumes. Flower mites were excluded from both groups of plants to control for mite activity. Hummingbirds made fewer but longer visits to plants and revisited more the flowers with nectar removal than those without the treatment. We then conducted a pollination experiment on pistillate flowers using a stuffed L. amethystinus hummingbird to evaluate the effect of pollination intensity (number of bill insertions into one flower) on seed production. Flowers with more insertions produced significantly more seeds than those flowers that received fewer insertions. We conclude that the simulation of nectar consumption by hummingbird flower mites can influence the behavior of the pollinator, and this may positively affect seed production.     

Foraging strategy predicts foraging economy in a facultative secondary nectar robber

In mutualistic interactions, the decision whether to cooperate or cheat depends on the relative costs and benefits of each strategy. In pollination mutualisms, secondary nectar robbing is a facultative behavior employed by a diverse array of nectar‐feeding organisms, and is thought to be a form of cheating. Primary robbers create holes in floral tissue through which they feed on nectar, whereas secondary robbers, which often lack chewing mouthparts, feed on nectar through existing holes. Because primary robbers make nectar more readily available to secondary robbers, primary robbers facilitate the behaviors of secondary robbers. However, the net effect of facilitation on secondary robber fitness has not been empirically tested: it is unknown whether the benefit secondary robbers receive is strong enough to overcome the cost of competing with primary robbers for a shared resource. We conducted foraging experiments using the bumble bee Bombus bifarius, which can alternatively forage ‘legitimately’ (from the floral opening) or secondary‐rob. We measured the relative foraging efficiencies (handling time per flower, flowers visited per minute, proportion of foraging bout spent consuming nectar) of these alternative behaviors, and tested whether the frequency of primary robbing and nectar standing crop in primary‐robbed flowers of Linaria vulgaris (Plantaginaceae) affected foraging efficiency. Surprisingly, there was no effect of primary robbing frequency on the foraging efficiency of secondary‐robbing B. bifarius. Instead, foraging strategy was a major predictor of foraging efficiency, with legitimate foraging being significantly more efficient than secondary robbing. Legitimate foraging was the more common strategy used by B. bifarius in our study; however, it is rarely used by B. bifarius foraging on L. vulgaris in nature, despite indications that it is more efficient. Our results suggest the need for deeper investigations into why bees adopt secondary robbing as a foraging strategy, specifically, the environmental contexts that promote the behavior.     

Intra‐floral resource partitioning between endemic and invasive flower visitors: consequences for pollinator effectiveness

1. Sympatric flower visitor species often partition nectar and pollen and thus affect each other's foraging pattern. Consequently, their pollination service may also be influenced by the presence of other flower visiting species. Ants are solely interested in nectar and frequent flower visitors of some plant species but usually provide no pollination service. Obligate flower visitors such as bees depend on both nectar and pollen and are often more effective pollinators. 2. In Hawaii, we studied the complex interactions between flowers of the endemic tree Metrosideros polymorpha (Myrtaceae) and both, endemic and introduced flower‐visiting insects. The former main‐pollinators of M. polymorpha were birds, which, however, became rare. We evaluated the pollinator effectiveness of endemic and invasive bees and whether it is affected by the type of resource collected and the presence of ants on flowers. 3. Ants were dominant nectar‐consumers that mostly depleted the nectar of visited inflorescences. Accordingly, the visitation frequency, duration, and consequently the pollinator effectiveness of nectar‐foraging honeybees (Apis mellifera) strongly decreased on ant‐visited flowers, whereas pollen‐collecting bees remained largely unaffected by ants. Overall, endemic bees (Hylaeus spp.) were ineffective pollinators. 4. The average net effect of ants on pollination of M. polymorpha was neutral, corresponding to a similar fruit set of ant‐visited and ant‐free inflorescences. 5. Our results suggest that invasive social hymenopterans that often have negative impacts on the Hawaiian flora and fauna may occasionally provide neutral (ants) or even beneficial net effects (honeybees), especially in the absence of native birds.     

Dynamics of forager arrivals and nectar renewal in flowers of Anchusa strigosa

Summary Long-tongued Anthophora spp. bees collecting nectar from flowers of Anchusa strigosa (Boraginaceae) exhibit systematic foraging. Successive forager arrivals at individual flowers are not independent, and the time elapsed between successive arrivals at a particular flower is distributed more uniformly than be expected on the basis of a random arrival process. Distributions of inter-arrival time for individual flowers show standard deviation/mean ratios of 0.44–0.79, a range which is consistent with results obtained for two other plant-pollinator systems. The rate at which nectar is renewed between successive forager arrivals is independent of the amount of nectar in the flower, and the renewal process is strongly linear. Practical and theoretical implications of these results are discussed.     

Flowers morphology and nectar concentration determine the preferred food source of stingless bee,Heterotrigona itama

Heterotrigona itama is a stingless bee species from Meliponini tribe. The bee collects nectar, pollen and resin to produce honey, bee bread, and propolis. The bee is also known to visit and collect nectar from various types of flowers but there are limited studies on why this species of bee prefers to visit certain types of flowers. This study was conducted to identify the nectar concentration in selected flowers favoured by H. itama and the relationship between the bee and the morphology of the flowers. Nectar was obtained from different species of flowers and the concentrations were measured using a digital refractometer. The tube length of each flower species and the tongue length of the bees were also measured. The results revealed that flowers preferred by H. itama have high nectar concentrations. The tube lengths of the preferred flowers were between 2.0 and 4.0 mm, which is compatible with the tongue length of the bee. This study revealed that both nectar concentration and flower morphology are important factors for the bees in choosing their food sources. The results from this study will benefit the beekeepers in the identification of flowers that should be planted in their farms to improve stingless bee beekeeping activities. Understanding the relationship between the bees and their flower preferences could also help us to understand the importance of conserving both the bee colonies and the various species of flowering plants to ensure the sustainability of flora and fauna in the ecosystem.     

Flower choice by honey bees (Apis mellifera L.): sex-phase of flowers and preferences among nectar and pollen foragers

Bees foraging for nectar should choose different inflorescences from those foraging for both pollen and nectar, if inflorescences consist of differing proportions of male and female flowers, particularly if the sex phases of the flowers differ in nectar content as well as the occurrence of pollen. This study tested this prediction using worker honey bees (Apis mellifera L.) foraging on inflorescences of Lavandula stoechas. Female flowers contained about twice the volume of nectar of male flowers. As one would predict, bees foraging for nectar only chose inflorescences with disproportionately more female flowers: time spent on the inflorescence was correlated with the number of female flowers, but not with the number of male flowers. Inflorescence size was inversely correlated with the number of female flowers, and could be used as a morphological cue by these bees. Also as predicted, workers foraging for both pollen and nectar chose inflorescences with relatively greater numbers of both male and female flowers: time spent on these inflorescences was correlated with the number of male flowers, but not with the number of females flowers. A morphological cue inversely associated with such inflorescences is the size of the bract display. Choice of flowers within inflorescences was also influenced predictably, but preferences appeared to be based upon corolla size rather than directly on sex phase.     

Flower constancy and learning in foraging preferences of the green-veined white butterfly Pleris napi

Abstract.

• 1 Evolutionary pressure should select for efficient foraging strategies, within the constraints of other selective forces. We assess the mechanisms underlying flower choice in the butterfly, Pieris napi (L.), which as an adult forages for nectar. Experiments were carried out on a laboratory colony, using artificial flowers of two colours, and replicated on two successive generations.
• 2 When nectar was freely available from all flowers, equal numbers of butterflies visited each colour, but individual butterflies exhibited flower constancy, showing a strong preference for one colour or the other.
• 3 Following 3 day conditioning periods in which nectar was available from flowers of one colour only, butterflies responded by developing a preference for this colour, which persisted when both flower colours were refilled. This preference could subsequently be switched to the other flower colour following a further 3 days of conditioning. These are interpreted as adaptive (learned) responses, which would have obvious selective benefits in the field, enabling butterflies to avoid flower species which experience has shown are poor sources of nectar, and to adapt to temporal and spatial changes in nectar availability.

     

Effect of floral orifice width and shape on hummingbird-flower interactions

Nectar guides are common among insect-pollinated plants, yet are thought to be rare or absent among hummingbird-pollinated plants. We hypothesize that the lower lips and trumpet-shaped orifices of many hummingbird flowers act as nectar guides to direct hummingbirds to the flowers' nectar and orient the birds for pollination. To test this hypothesis we conducted laboratory experiments using flowers of Monarda didyma (bee balm) and M. fistulosa (wild bergamot), which have orifice widths of about 4 mm and 2 mm, respectively, and latex flowers with orifice widths of 4 mm and 2 mm and three orifice shapes (trumpet, lipped, and lipless). Rubythroated hummingbirds (Archilochus colubris) made fewer errors during bill insertion and spent a smaller proportion of their feeding visit in error at M. didyma flowers than at M. fistulosa flowers, and at unaltered flowers of both species than at flowers with lower lips removed. Handling times were longer at both lipped and lipless flowers of M. didyma than at those of M. fistulosa, and at lipped than at lipless flowers of M. didyma. The average duration of contact between a hummingbird and a flower's anthers and stigma was longer at M. didyma than at M. fistulosa for both lipped and lipless flowers, and at lipped than at lipless M. didyma flowers. Hummingbirds missed the openings of latex flowers with their bills more frequently and spent a greater percentage of their total feeding visit in error at (i) 2-mm than at 4-mm flowers of all three shapes, (ii) lipless flowers than at trumpet or lipped flowers, and (iii) lipped flowers than at trumpet flowers of both widths. The duration of hummingbird/anther contact was longer at (i) 2-mm than at 4-mm flowers of all shapes, (ii) lipped than at trumpet or lipless flowers, and (iii) lipless than at trumpet flowers for both widths. No significant differences in handling times of hummingbirds were observed among any of the latex flower shapes or widths. Our results demonstrate that orifice shapes can act as guides by reducing the frequency of feeding errors by visiting hummingbirds, and that effects of orifice shape on pollination must be considered in conjunction with flower widths and locations of anthers and stigmas.     

Nectar secretion pattern of the dish-shaped flower,Cayratia japonica (Vitaceae), and nectar utilization patterns by insect visitors

We studied the relationship between the diurnal nectar secretion pattern of flowers of Cayratia japonica and insect visiting patterns to these flowers. Flower morphology of C. japonica changed greatly for about 12 hours after flower-opening and the maximum duration of nectar secretion was 2 days. The nectar volume peaked at 11∶00 and 15∶00, and declined at night and at 13∶00 regardless of time elapsed after flower-opening. The nectar volume at the two peaks was, on average, 0.25 μl on bagged inflorescences and 0.1μl on unbagged inflorescences (both, sugar concentration=60%). The flower secreted nectar compensatory when the nectar was removed. This means that insects consume more nectar than the difference of nectar volume between bagged and unbagged flowers. Apis cerana is a primary visitor of this flower, and was the only species for which we confirmed pollen on the body, among many species of flower visiting insects to this flower. Apis cerana visited intensively at the two peaks of nectar secretion. Visits of the other insects were rather constant or intensive only when there was no nectar secretion. Thus flowers of C. japonica with morphologically unprotected nectaries may increase likelihood that their nectar is used by certain pollinators, by controlling the nectar secretion time in day. In this study the pattern of nectar secretion allowed A. cerana maximum harvest of nectar.     

Departure rules used by bees foraging for nectar: A field test

Summary Departure rules used by solitary long-tongued bees (Anthophora spp. andEucera spp.) collecting nectar from flowers ofAnchusa strigosa (Boraginaceae) were studied. The amount of nectar a bee receives from an individual flower was estimated by measuring the time elapsed since the previous bee visit to that flower. Measurements of nectar accumulation in experimentally emptied flowers indicated that this time interval is an accurate predictor of nectar volumes in flowers. We found that nectar rewards influence the probability of departure from individual plants, as well as distances of movements within plants. The probability of departure from individual plants was negatively related to the amount of reward received at the two lastvisited flowers. This result indicates that the bees used a probabllistic departure rule, rather than a simple threshold departure rule, and that rewards from both the current and the previously visited flower were important in determining departure points. Distances of inter-flower movements within plants were negatively related to the amount of reward received at the current flower. The overall results suggest that the pollinators ofA. strigosa make two types of departure decisions-departures from the whole plant and departures from the neighbourhood of individual flowers-and that they use different departure rules for each scale. Factors influencing the decision-making processes of the observed foraging behaviour are discussed.