New Holland Honeyeaters Research Articles

New Holland honeyeater (Phylidonyris novaehollandiae) microsatellites: isolation and characterization of 15 novel markers using an enrichment method

The New Holland honeyeater (Phylidonyris novaehollandiae) plays a significant role in the pollination and sustainability of Australia's endemic flora, and hence fauna. Despite this key ecosystem function, knowledge of P. novaehollandiae life-history traits and population dynamics remain poorly understood. We describe the development of primers amplifying 22 P. novaehollandiae microsatellite loci. Fifteen of the loci were found to be polymorphic, with observed heterozygosity between 0.500 and 1.000, and from four to 17 alleles per locus in adult birds from the study site.

Recognition of other species' aerial alarm calls: speaking the same language or learning another?

Alarm calls given by other species potentially provide a network of information about danger, but little is known about the role of acoustic similarity compared with learning in recognition of heterospecific calls. In particular, the aerial 'hawk' alarm calls of passerines provide a textbook example of signal design because many species have converged on a design that thwarts eavesdropping by hawks, and call similarity might therefore allow recognition. We measured the response of fairy-wrens (Malurus cyaneus) to playback of acoustically similar scrubwren (Sericornis frontalis) aerial alarm calls. First, if call similarity prompts escape independent of learning, then fairy-wrens should flee to playback of scrubwren calls outside their geographical range. However, fairy-wrens fled only in sympatry. Second, if call similarity is necessary for learning heterospecific calls, then fairy-wrens should not respond to sympatric species with different calls. We found, on the contrary, that fairy-wrens fled to the very different aerial alarm calls of a honeyeater (Phylidonyris novaehollandiae). Furthermore, response to the honeyeater depended on the specific structure of the call, not acoustic similarity. Overall, call similarity was neither sufficient nor necessary for interspecific recognition, implying learning is essential in the complex task of sifting the acoustic world for cues about danger.

The sweet life: diet sugar concentration influences paracellular glucose absorption

Small birds and bats face strong selection pressure to digest food rapidly in order to reduce digesta mass carried during flight. One mechanism is rapid absorption of a high proportion of glucose via the paracellular pathway (transfer between epithelial cells, not mediated by transporter proteins). Intestinal paracellular permeability to glucose was assessed for two nectarivorous passerines, the Australian New Holland honeyeater (Phylidonyris novaehollandiae) and African white-bellied sunbird (Cinnyris talatala) by measuring the bioavailability of radiolabelled, passively absorbed L-glucose. Bioavailability was high in both species and increased with diet sugar concentration (honeyeaters, 37 and 81% and sunbirds, 53 and 71% for 250 and 1,000 mmoll-1 sucrose diets, respectively). We conclude that the relative contribution of paracellular to total glucose absorption increases with greater digesta retention time in the intestine, and paracellular absorption may also be modulated by factors such as intestinal lumen osmolality and interaction with mediated glucose uptake. The dynamic state of paracellular absorption should be taken into account in future studies.

Ticks (Ixodes sp.) and blood parasites (Haemoproteus spp.) in New Holland Honeyeaters (Phylidonyris novaehollandiae): evidence for site specificity and fitness costs

New Holland Honeyeaters (Phylidonyris novaehollandiae, Meliphagidae) were examined for tick and blood parasite prevalence and intensity during a single breeding season (June-November 2004) at four locations in South Australia. We examined 126 birds for ticks and 110 of these birds for blood parasites. Twenty-two birds (17.5%) were infested with one to two ticks from the genus Ixodes. Ten birds (9.1%) were infected with blood parasites from the genus Haemoproteus (intensity was low, on average 0.016% infected blood cells). There was no relation between blood parasite and tick prevalence within birds and across sites. We found no relationship between blood parasites and site, or age, sex, or body condition of hosts. Tick prevalence was significantly related to host age (being higher in juveniles than adults), site (ticks were only found in coastal areas), and reduced body condition. This study provides preliminary evidence for a geographical pattern of tick (Ixodes sp.) distribution: we found ticks on Kangaroo Island and in a coastal area of the Fleurieu Peninsula, but not at sites more inland within the Mount Lofty Ranges.

How specialized is the plant–pollinator association between Eucalyptus globulus ssp. globulus and the swift parrot Lathamus discolor?

Abstract The swift parrot Lathamus discolor (Shaw) (Psittacidae) evolved from granivorous ancestors to become a specialized flower‐feeder in a monotypic genus. Its reproduction is dependent largely on flowers of Eucalyptus globulus Labill. ssp. globulus (Myrtaceae), the birds migrating to breed within the natural distribution of this tree. This paper investigates the extent to which this dependence of L. discolor on E. globulus is mirrored by dependence of the tree on the bird. It was found that L. discolor carried significantly more eucalypt pollen within 22 mm of its bill tip than did the New Holland honeyeater, Phylidonyris novaehollandiae (Latham) (Meliphagidae), and that pollen was concentrated on the regions of the head of L. discolor that consistently contact stigmas. Larger pollen loads on L. discolor can be attributed to it consuming both pollen and nectar, while honeyeaters take nectar only. The short thick bill of L. discolor necessitates regular stigmatic contact while the long slender bills of honeyeaters are unlikely to contact stigmas as often in these bowl‐shaped flowers. These factors suggest that L. discolor has a greater capacity to deposit pollen on stigmas of E. globulus than do honeyeaters. However, the characteristics of L. discolor that make it such an effective pollinator of E. globulus are also exhibited by a lorikeet (Psittacidae) that feeds on flowers of E. globulus. The association between E. globulus and L. discolor is therefore only moderately specialized because the flowers are also adapted to the more recently associated lorikeet and are almost certainly also pollinated by honeyeaters.

Non-breeding territoriality in the New Holland Honeyeater, Phylidonyris novaehollandiae, in an unpredictable environment—short-term energy costs for possible long-term reproductive benefits

The short-term energetic and long-term reproductive costs and benefits of aggressive social behaviour of New Holland Honeyeaters were examined in an open forest where nectar availability for part of the year was highly unpredictable. Despite the variability in food supply, birds consistently maintained priority and, to varying degrees, exclusive access to certain areas. Feeding territories held between the autumn and spring breeding seasons did not always provide sufficient energy to cover the owner's daily costs of occupation. An indirect assessment of the net energy benefits revealed that owners may experience considerable periods of food shortage. Despite this, owners remained site faithful as the areas defended were all close to, or overlapped, sites used for nesting. These areas had a high density of both Banksia spp. (food plants) and Leptospermum spp. (nest plants). It appears that while non-breeding territories may be unprofitable in the short term, owners that maintain them benefit in the long term by ensuring access to nesting sites.

Breeding biology and nesting success of the Eastern Yellow Robin and the New Holland Honeyeater in a southern Victorian woodland

This study examines the nesting success and the role of nest predation in the breeding biology of birds in a woodland remnant of southern Victoria. In total, 225 nests of 21 species were found within the study area during the 1999–2000 breeding season, including 90 nests of the Eastern Yellow Robin (Eopsaltria australis), and 52 nests of the New Holland Honeyeater (Phylidonyris novaehollandiae). Overall, only 39% of nests were successful, and 48% of nests were preyed upon. For nests of the Eastern Yellow Robin and the New Holland Honeyeater, the predation rate was 50% and 42%, respectively. The mean clutch size was 2.3 (±0.06, s.e.) for the Eastern Yellow Robin and 2.1 (±0.05, s.e.) for the New Holland Honeyeater. Eastern Yellow Robin clutches of three eggs were initiated significantly later in the season than clutches of two eggs. The clutch initiation dates of successful New Holland Honeyeater nests were significantly earlier than New Holland Honeyeater nests that were preyed upon, but this was not the case for the Eastern Yellow Robin. Nests of the Eastern Yellow Robin with two eggs were no more successful than nests with three eggs. The incubation and nestling periods of the Eastern Yellow Robin were 15.9 (±0.1, s.e.) and 12.9 (±0.2, s.e.) days, respectively, and for the New Holland Honeyeater were 13.6 (±0.2, s.e.) and 12.2 (±0.3, s.e.) days, respectively. There was no significant difference in the nestling periods of Eastern Yellow Robin nests with a clutch size of two and three.

Regulation of water and sodium balance in the field by Australian honeyeaters (Aves: Meliphagidae).

We evaluated the use of water and sodium by free-living individuals of several species of Australian honeyeaters (Acanthorhynchos superciliosus, Phylidonyris novaehollandiae, Phylidonyris nigra, Manorina flavigula, and Anthochaera carunculata). Water and Na fluxes were highly variable between species, largely reflecting differences in diet. Water fluxes ranged from approximately 300% of total body water per day in 10-g, nectarivorous A. superciliosus to approximately 45% of total body water per day, typical of a desert species, in M. flavigula, a 50-g, insectivorous, arid-zone bird. Similarly, Na fluxes ranged from nearly 60% of Na pool per day in A. superciliosus to about 25% per day in M. flavigula. Despite these different fluxes, values of regulated osmoregulatory variables, including plasma osmolality, hematocrit, plasma concentrations of Na+ and K+, and exchangeable Na pool, were relatively invariant both between species and within species at different seasons. In contrast, values of variables reflecting the operation of regulatory systems did differ between species and seasons. Urine concentrations were highest in M. flavigula and, in A. carunculata, varied seasonally (higher in summer than winter). Plasma concentrations of aldosterone were lowest in A. carunculata (5-25 pg/mL), highest in P. novaehollandiae (70-200 pg/mL), and in the latter species were higher in winter than summer. Concentrations of arginine vasotocin ranged from 5 pg/mL in A. carunculata to greater than 30 pg/mL in M. flavigula. Our data demonstrate that within the family Meliphagidae, there exists substantial variation in the fluxes of water and Na and that these relate in part to body size variation but more importantly to diet. The different fluxes between species are reflected in the values of numerous osmoregulatory variables.

Reproductive biology of the rare and endangered Banksia brownii Baxter ex R. Br. (Proteaceae)

Abstract Banksia brownii is an endangered species, now limited to ∼ 15 disjunct populations in southwestern Western Australia. Data on flowering phenology, plant size, fruit set, pollination and the mating system were gathered for two of these populations between March and October 1993. Flowering for both populations followed a similar pattern, with open flowers first evident in April, and the number of inflorescences with open flowers peaking in June. At both locations, trees differed considerably with respect to their size, the total number of inflorescences produced and the length of their flowering season. Fruiting success was typically low, with approximately half of all inflorescences failing to develop into infructescences. Only 1. 8% of the flowers originally present on inflorescences developed into follicles. The distribution of follicles along each infructescence was non‐random, with most forming in the middle third of the infructescence for reasons relating to nutrient supply and pollinator behaviour. More flowers opened during the day than at night, although pollen was lost from individual flowers during both periods. Honeyeaters such as Phylidonyris novaehollandiae were common at the two study sites, and often carried large loads of B. brownii pollen. Though less frequently caught, the nocturnal mammals Rattus fuscipes and Tarsipes rostratus also bore substantial amounts of pollen. Most inflorescences from which these mammals and birds were excluded remained barren. Fruiting success was further reduced when invertebrates such as Apis mellifera were also prevented from visiting inflorescences. The ability of B. brownii to set at least some fruit in the absence of biotic poli‐nators indicates that the species is partially self‐compatible. Honeyeaters foraged preferentially at inflorescences with one to two thirds of their flowers open, probing mainly along the ‘advancing front’ of open flowers. These animals moved more frequently between inflorescences on the same plant than between those on different plants, and were often recaptured in the same locations. Mammals also appeared to be sedentary. Both B. brownii populations had mixed mating systems, with genetically determined outcrossing rates of ∼0.7. The unusually high level of selfing in each population is presumably a reflection of the species’ self‐compatibility and the foraging behaviour of its pollinators.

Aggression And Nectar Use in Territorial Non-Breeding New Holland Honeyeaters Phylidonyris novaehollandiae in Eastern Australia

Territorial non-breeding New Holland Honeyeaters Phylidonyris novaehollandiae were studied in an open forest in eastern Australia. During winter, birds maintained priority of access to nectar from Banksia inflorescences but the extent to which this use was exclusive was variable. I examined the aggressive responses of these birds to intruders of different sizes (interference behaviour), and their use of defended nectar (exploitation behaviour). Owners displayed interspecific discrimination with all honeyeater species being excluded except the much larger wattlebird which was harassed while feeding. Owners expelled a greater proportion of intruding conspecifics compared to other species and used more aggressive behaviour (chases) against them. Energy inexpensive aggression (displacements) was used against smaller intruder species who were also more likely to be attacked if they attempted to feed. The rate at which intruding honeyeaters were expelled (birds/h/m²) increased with both nectar abundance and intruder pressure but actual territory exclusiveness (proportion of intruders expelled) was highest only at moderate nectar levels (lower when nectar was poor or very rich). Owners visited inflorescences in their territories more often than intruding honeyeaters, and fed at the most productive inflorescence stages. Nectar within territories was depleted at a faster rate then undefended nectar. Consequently, the territories appeared to be operated as nectar 'sinks', rather than as the 'sources' typical of other nectarivorous birds.

Territoriality in Honeyeaters: Reviewing the Concept and Evaluating Available Information

We define territorial behaviour as aggressive behaviour that occurs repeatedly in about the same location with associated submissive behaviour on the part of the individuals or groups to which the aggression is directed. Of a worldwide total of about 170 honeyeater species (Meliphagidae), 36 have been described as being territorial and we consider that 28 of these have been shown to satisfy our definition of territoriality. We discuss the consequences of territorial behaviour and the determination of the boundaries and areas of territories. We also review the available information regarding territorial behaviour exhibited by the New Holland honeyeater (Phylidonyris novaehollandiae), the most studied of all honeyeater species. We recommend that future descriptions of territorial behaviour attempt to determine the intensity of the behaviour, sharpness of the territory boundary, degree of exclusive use of the relevant resource, and the extent to which areas separate from the territory are used by the territory owner or owners. For territoriality to become a useful concept for honeyeaters, behaviour that leads to the label of 'territorial' must be fully documented.

Field Metabolic Rate and Water Flux of Nectarivorous Honeyeaters

Field metabolic rate (FMR) and water influx of New Holland honeyeaters (Phylidonyris novaehollandiae), eastern spinebills (Acanthorhynchus tenuirostris) and a crescent honeyeater (P. pyrrhoptera) were measured by the doubly labelled water technique. New Holland honeyeaters had just finished breeding and were beginning their summer moult. They ranged in mass from 15.4 to 21.0 g (mean = 17.3 g, n = 12) and had FMRs averaging 8.8 mt CO2 g(-1) h(-1) or 77.6 kJ day(-1), which was 2.8 times their measured basal metabolic rate (BMR). Their water influx rate averaged 10.7 mL day(-1). Eastern spinebills were still feeding young and had yet to begin moulting. They ranged in mass from 8.0 to 10.7 g (mean = 9.7 g, n = 6), had FMRs averaging 10.9 mL CO2 g(-1) h(-1) or 52.9 kJ day(-1) (2.5 times their measured BMR), and had an average water influx rate of 8.7 mL day(-1). FMR and water influx of a single 14.6-g crescent honeyeater, which was in late primary moult, were 75.9 kJ day(-1) (2.7 times measured BMR) and 12.5 mL day(-1). The FMR of New Holland honeyeaters varied inversely with mean standard operative temperature (T-es) calculated for values of T-es below 20 degrees C as follows: FMR (kJ day(-1)) = 134 - 5.47 T-es (n = 12, r(2) = 0.52). Honeyeater FMRs were much lower than would be predicted allometrically for hummingbirds of the same mass, reflecting the honeyeaters' low-cost foraging tactic of consuming nectar while perched.

Responses of Territorial New Holland Honeyeaters Phylidonyris novaehollandiae to Short-term Fluctuations in Nectar Productivity

New Holland Honeyeaters Phylidonyris novaehollandiae occupying feeding territories during the nonbreeding season were studied in an open forest in northeastern New South Wales. The energy content of these territories fluctuated from day-to-day both naturally and as a result of experimental manipulations. In response to artificial increases in nectar, territory owners did not alter territory area but did exclude a greater proportion of intruders, reduced the level of interspecific discrimination in aggression, and employed more energetically expensive aggression. However, at very high levels of nectar productivity the owners expelled fewer intruders. The results indicated a graded aggressive behaviour dependent on the prevailing nectar productivity and intruder rate. There was no increase in intruder pressure as productivity was increased during the experiments.

The Vocal Repertoires of Six Honeyeater (Meliphagidae) Species from Adelaide, South Australia

Honeyeaters (Meliphagidae) are prominent members of the avifauna in Australia, typically with four to five resident species in any area. We recorded vocalisations from six species resident around Adelaide in South Australia: the Red Wattlebird Anthochaera carunculata, Little Wattlebird Anthochaera chrysoptera, Noisy Miner Manorina melanocephala, White-plumed Honeyeater Lichenostomus penicillatus, New Holland Honeyeater Phylidonyris novaehollandiae and Crescent Honeyeater Phylidonyris pyrrhoptera. The larger honeyeaters (Red and Little Wattlebird) produced characteristically harsh vocalisations covering a wide range of frequencies. Crescent and White-plumed Honeyeaters had a varied and complex repertoire of calls and songs which were quite musical in nature. Noisy Miners and New Holland Honeyeaters produced vocalisations dominated by simple single note whistles. Each species had specific vocalisations emitted in different contexts: alarm, distress, begging and contact calls as well as song. Alarm and distress calls of the six species had common characteristics and typically induced responses from other honeyeater species.

An inexpensive photographic technique for identifying nest predators at active nests of birds.

Change in rate of nest predation due to environmental modification is considered a major cause of population decline of many bird species. Our ability to adequately understand and effectively manage this effect is limited by our ability to identify the relative roles of individual nest predators. This is because nest predation is seldom witnessed despite its high frequency. We describe and evaluate an inexpensive photographic technique for identifying nest predators at active nests. Each camera unit (A$220) was triggered by circuitry (A$30), using a magnetic reed switch attached to a supplementary egg. A total of 51 nests of New Holland honeyeaters (Phylidonyris novaehollandiae) was monitored with the equipment. Of these, 39 were preyed upon. Predation was never witnessed, but predators were captured on film for 72% of nests at which predation occurred.

Ecomorphological constraints imposed by the kidney component measurements in honeyeater birds inhabiting different environments

The histological renal anatomy of 10 species of honeyeaters was examined quantitatively, using stereology. The kidneys of five species of predominantly wet zone inhabiting birds: the western spinebill Acanthorhynchus superciliosus, White‐checked honeyeater Phylidonyris nigra. New Holland honeyeater Phylidonyris novaehollandiae, little wattlebird anthochaera chrysoptera and red wattlebird Anthochaera carunculata, were compared to five predominantly arid zone inhabiting birds: the grey‐fronted honeyeater Meliphaga plumula, white‐plumed honeyeater Meliphaga penicillata, white‐fronted honeyeater Phylidonyris albifrons, spiny‐checked honeyeater Acanthogenys rufogularis and yellow‐throated miner Manorina flarigula. The kidneys were asymmetrical, with the left kidney being larger than the right kidney. Kidney mass was directly proportional to body mass (coefficient of correlation, r =+0·95), as was kidney volume to kidney mass (r =+1·0). Wet zone honeyeaters generally and a higher percentage and absolute volume of renal cortex, whilst arid zone honeyeaters generally had a significantly higher percentage and absolute volume of renal medulla. There were few differences between species, in either the percentage or absolute volume or luminal surface area of nephron components within the cortex. Within the medulla, wet zone honeyeaters generally had a higher percentage and absolute luminal surface area of collecting ducts, whilst arid zone honeyeaters had a higher percentage and absolute surface area of capillaries. This may be due to factors such as variations in diet and climate between habitats resulting in differences in honeyeater renal morphology which enable arid zone birds to conserve water and wet zone birds to conserve ions more efficiently.

Use of heathland and adjoining forest by honeyeaters: Results of a radiotracking study

Abstract New Holland (Phylidonyris novaehollandiae) and White‐cheeked (Phylidonyris nigra) Honeyeaters that are resident in heathland during February and March, when there is negligible nectar production in that habitat, are expected to forage for nectar at that time in the adjacent open forest habitat. As nectar production in the heathland increases from then to June, while that in the forest decreases, the proportion of time spent by these birds in the forest should decrease over the same period. Radiotracking revealed that during February the residents did, as expected, spend time feeding on nectar in the forest and more time was spent in the forest in February than in April or June. However, the residents spent time away from the heathland throughout the year and spent more time away in June than in April. The resident honeyeaters spent about 80% of their time on our heathland grid overall. Radiotracking also indicated that there was no sharp distinction in behaviour between residents and other birds because birds that had been resident in the past but were not considered to be so at the time (absent residents), birds that would later be considered resident (future residents) and some birds that were never resident, all spent about half their time on the grid. The area that included 90% of heathland locations, estimated by radiotracking, for P. novaehollandiae and P. nigra averaged 0.48 ha per bird.

The avian population of an urban bushland reserve at Wollongong, New South Wales: implications for management

This study focuses on the avian ecology and management of a 43.5 ha remnant hind-dune and estuary ecosystem in suburban Wollongong (population 200 000). In total, 62 land-bird species and 24 water-bird species were present between July 1984 and February 1987 (32 months). During area searches ( n=12) in each calendar month, an average of 427 individuals of 31 species was recorded with an average species diversity of 70%. The avian population was characterised by a small number of species with high abundance and vice versa. Five species were migratory. Resident species ranged in abundance from the Eastern Whipbird Psophodes olivaceus (maximum population eight birds) to the New Holland Honeyeater Phylidonyris novaehollandiae (maximum population 200 birds). Numbers of the latter species increased markedly in autumn and winter when banksia and coral trees were rich with nectar. Most terrestrial species were habitat specialist, linked to the various microhabitats for food, nesting or protection. Recommendations to preserve the bird community are as follows: (1) the catchment should be managed to control siltation and contamination of the wetland, rather than dredging the estuary itself; (2) wetland diversity should be maintained by allowing water levels to vary naturally; (3) terrestrial microhabitats should be preserved by maintaining their structural and floristic diversity; (4) refuge areas should be established to minimise the impact of human-related disturbance; (5) a network of corridors should be provided to facilitate movement of birds into the reserve; (6) a buffer zone should be maintained around the reserve to shield the birdlife from suburban disturbance and domestic animals.

Correlation between nectar supply and aggression in territorial honeyeaters: causation or coincidence?

A fundamental prediction of food-based economic models of territoriality is that animals will not defend territories if food is so abundant that defense will not improve access to food. Several studies of nectar-feeding birds support this prediction, with territoriality being rare or absent in years when nectar was particularly abundant. However, these results could potentially be an artefact of changes in bird density with nectar availability, and in at least some cases the correlations between territory defense and nectar availability could be purely coincidental. This paper reports the first experimental test of whether cessation of territory defense in nectar-feeding birds results from a direct response to abundance of nectar. New holland honeyeaters Phylidonyris novaehollandiae and white-cheeked honeyeaters P. nigra show pronounced changes in their levels of territorial aggression over the 7–8 months that they breed. These changes are predictable from economic considerations in that the birds are least aggressive in the months when nectar is extremely abundant. I tested whether the birds were responding to changes in nectar availability by providing sugar-water feeders at “neutral” locations that were easily accessible to territory holders, but far enough away from territories that intrusion rates were unaffected. I tested for responses at two time scales feeders were put out for 48-h periods in 1987, and were left out continuously from January to October 1988. The only effect was that territory holders visited feeders instead of flowers when floral nectar was scarce. They continued to defend their territories aggressively at those times, showed seasonal changes in aggressiveness similar to birds on a site without feeders, and did not shift their territories toward feeders. I conclude that the observed changes in aggressiveness are not responses to changes in nectar availability, and suggest alternative explanations.

Effects of nectar volume and concentration on sugar intake rates of Australian honeyeaters (Meliphagidae)

Sugar intake rates of captive Australian honeyeaters (Meliphagidae) feeding at artificial flowers varied across species, and as a function of nectar volume and concentration within each species. Red Wattlebirds (Anthochaera carunculata, 110 g), achieved higher intake rates than New Holland Honeyeaters (Phylidonyris novaehollandiae, 20 g), and both achieved higher rates than Eastern Spinebills (Acanthorhynchus tenuirostris, 10 g). These results reflect differences in bill and tongue dimensions as well as in body mass. Sugar intake rates for all three species increased with volume (5-50 μl) at any given concentration (10-60% mass/mass sucrose). For a given volume, sugar intake rates peaked at intermediate concentrations: 40-50% for the two larger species, and 30-40% for the smallest species. Published studies for other nectarivores foraging at unlimited volume feeders also show optimal nectar concentrations of 30-50%. However, biophysical theory predicts optima at 20-26% for small volumes, and plants presumed to be adapted for bird-pollination often have dilute nectar (20-30%). To explore this discrepancy further, we presented New Holland Honeyeaters with a range of sucrose concentrations (10-50%) using two presentation schemes. In the first we varied concentration but kept volume constant, thus varying gross sugar reward available in each concentration. This gave maximum sugar intake rates at 50%. In the second we varied both volume and concentration so that gross sugar rewards were equal for all solutions, decoupling high concentrations and large sugar rewards. This gave optima at 20%. We argue that variation among plants in nature more closely resembles the latter, "equal sugar presentation" scheme, and therefore, that dilute nectars may indeed represent adaptations for bird pollination.