Why hummingbirds have such large crops

Summary Male Anna's Hummingbirds (Calypte anna) defend territories that contain a predictable food source, floral nectar. For such a hummingbird, the meal size that maximizes long-term net energy intake is less than the maximal crop volume. Smaller meals must be consumed more frequently, but larger meals increase body mass and therefore flight cost. Individuals without territories or with inadequate territories do not have easy access to nectar and intrude on territories owned by otherC. anna, where they may be chased at nay time. It was predicted that these intruders should minimize the number of potentially risky intrusions necessary for maintenance by ingesting as much nectar as possible whenever they manage to feed without being chased (usually when owners are temporarily absent). Therefore, relative to uninterrupted feeding by owners, uninterrupted intruders should feed longer and take larger meals. Field observations supported these predictions. Intruders apparently filled their crops in all seasons, whereas owners ingested smaller amounts (0.21–0.22 ml) and fed for lengths of time consistent with the prediction of an optimization model (0.21 ml). Thus, owners may energetically optimize meal size whereas intruders fill their crops whenever they are not chased. Under most conditions, hummingbirds only fill their crops one-tenth to one-third full, leading to the question why hummingbirds have such large crops. This study demonstrates that a large crop volume may be of survival value when an individual lacks a territory or has inadequate access to resources and must poach on others' territories.     

The foraging behaviour of Australian honeyeaters: a review and some comparisons with hummingbirds

The foraging behaviour of Australian honeyeaters is reviewed in terms of diet, foraging selectivity, foraging flight mode, quality and quantity of nectar encountered per flower, flower densities encountered and effect of predation. At the same time comparisons are made between honeyeaters and hummingbirds. These two groups of birds are superficially similar. Both feed on nectar and insects. Both tend to have long curved bills and tongues adapted for removal of nectar from flowers. Both tend to feed at long, red flowers. However, on close inspection, honeyeaters and hummingbirds are quite dissimilar. For example, many honeyeaters include fruit in their diets. Hummingbirds almost never eat fruit. Honeyeaters appear to be considerably less nectarivorous and more insectivorous than hummingbirds. Honeyeaters are, for the most part, larger than hummingbirds and they usually perch while feeding whereas hummingbirds usually hover. Honeyeaters but not hummingbirds often flock while feeding. Predation appears to be considerably more important for honeyeaters than for hummingbirds. Territorial defense of flowers seems common in hummingbirds but uncommon in honeyeaters. These differences are discussed in detail and explanations are offered for them wherever possible.     

The foraging of New Zealand honeyeaters

Abstract

New Zealand has three species of honeyeaters, all of which feed on nectar, fruit, and ‘insects’. There is disagreement between published data and those becoming available from long-term studies on the relative proportion of these items in the diet. The effect of factors such as body size, dominance status, degree of movement, and time of year on diet and foraging behaviour are outlined, and predictions of differences between species and between sexes are made. A brief comparison of foraging in relation to the flora is made between New Zealand and Australian species.     

Status and foraging in New Zealand honeyeaters

Abstract

Aggression is a common feature of sociality among nectar-feeding birds, and these have been used to consider many aspects of ecological theory, for example community structure, foraging patterns, social organisation and plant-pollinator systems.

Knowledge of aggression among and within New Zealand's honeyeater species is reviewed. Additional information on the importance of various asymmetric cues as learned predictors of status is considered. High-status individuals appear to gain access to nectar in all situations.

Honeyeaters are monogamous and the relationship between the sexes is examined. Even when mates are not formally associated, both appear to benefit from the presence of the other. Examples of the effect of status on pollination efficiency are also considered.

Workers are urged to consider status and individual differences if we are to increase our understanding of community structure, foraging and plant-pollinator interaction.     

A higher-level taxonomy for hummingbirds

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

Energy regulation by traplining hummingbirds

1. A published model of constant diurnal energy accumulation by territorial hummingbirds does not accurately reflect the temporal distribution of feeding behaviour of traplining hummingbirds, Phaethornis longirostris (Long-Tailed Hermit Hummingbirds).
2. In an enclosure study, gross nectar intake by P . longirostris decreased through the day, mirroring nectar production rates in its natural food-flowers and mimicking its natural foraging patterns.
3. Using a simulation model, the energetic consequences of constant and decreasing net energy intake rates for traplining hummingbirds are compared.
4. Given natural patterns of nectar production, model birds with decreasing diurnal net intake rates met their energetic needs with fewer flowers than those with constant net intake, and spent less time foraging.
5. It is concluded that P . longirostris do not satisfy the physiological assumptions of the published model, and that in this way they are different from the territorial species on which the model has previously been tested.     

Temporal patterning of feeding by hummingbirds

For five species of hummingbirds in the laboratory, time between meals was related to energy intake on the first meal and rate of energy expenditure between meals. Field observations gave similar results. Average meal sizes were similar at one intake rate independent of food caloric density; females averaged longer bouts than males. When rate of intake was approximately halved, meal duration approximately doubled and volume intake remained similar. We postulate that feeding is initiated when crop contents reach a lower threshold and that feeding is terminated after ingestion of an optimal volume determined by the added weight of the meal.     

Evidence of mutualistic synzoochory between cryptogams and hummingbirds

Endozoochory is usually involved in seed dispersal mutualisms, whereas ectozoochory is non‐rewarding, and therefore neutral (or even negative) for the animal vector. Synzoochory is an intermediate dispersal type between endo and ectozoochory in which propagules are deliberately transported (usually in the mouth) but with no ingestion or gut passage involved. We present empirical evidence of synzoochoric mutualism between the hummingbird Sephanoides sephaniodes and cryptogams (one fern and seven moss species). Two species (Lophosoria quadripinnata and Ancistrodes genuflexa) constituted the bulk of nest biomass, and another six moss species were present in lesser quantity. The hummingbird was selective when collecting nest material so that the nests contained a higher density of reproductive structures (that could be dispersed further) than natural patches of the cryptogam species. Even after one year, the nests maintained half of the original reproductive structures (sporangia, sporophytes) and biomass, constituting an important dispersal source. These results show a new type of mutualism in which mosses could be dispersed throughout longer distances (several km) by hummingbirds and to higher positions (particularly for ground‐living species, promoting dispersal potential). The hummingbird benefits from collecting cryptogam material for nest building, and cryptogams benefit from the concentration and relocation of diaspore sources into more effective recruiting sites. Similar mutualistic relationships could be a general phenomenon, of importance in many ecosystems.     

Speciation dynamics in the Australo-Papuan Meliphaga honeyeaters

The Australo-Papuan Meliphaga honeyeaters have diversified over a wide range of habitats and elevational zones and are one of the few regionally known cryptic avian radiations. Using a combined 1580 bp of mitochondrial and nuclear DNA we investigate the species limits, systematic affinities and biogeographic history of Meliphaga. We also investigate the role of spatial sorting mechanisms, including altitudinal replacement and niche partitioning, as mechanisms underlying the adaptive radiation of this group. Phylogenetic analysis indicates that the genus Meliphaga comprises at least 16 species, three more than recognized in current classifications. The genus divides into two clades; the species-poor lewinii group, and the larger analoga group that has diversified into a wider range of vertical, vegetational and elevational niches. The basal division of each clade into an Australian and New Guinean assemblage was likely induced by the formation of the Arafura Sea during the early Pliocene ( approximately 4 MYA) with a single reinvasion of Australia by the open forest species M. gracilis during the early Pleistocene (1.2-1.5 MYA) via intermittent land bridges or island hopping. Most recent sister species were found to replace each other geographically within the same ecological and elevational zone conforming to the classical allopatric mode of speciation. In contrast, M. orientalis (650-1950 m) and M. analoga (0-1100 m) were found to replace each other altitudinally across ecological zones providing empirical support for altitudinal speciation as a mechanism of diversification in a montane avifauna. We find no evidence of sympatric speciation (co-existing sister lineages) and suggest that spatial segregation within the habitat (niche partitioning) is primarily a mechanism enabling more divergent species to coexist.     

Cost of territory establishment in hummingbirds

Summary Time spent in territorial defense was measured during territory establishment for non-breeding Black-chinned (Archilochus alexandri) and Anna's (Calypte anna) hummingbirds. Newly established territory holders spent more time chasing intruders than neighboring established owners, which served as controls. This higher investment in defense by the new owners was due to 1) a longer time spent per each chase and 2) in some cases, a higher frequency of chases at the onset of territory ownership.     

Context-dependent foraging decisions in rufous hummingbirds

A core assumption implicit in economic models of animal choice is that subjects assign absolute utilities to options that are independent of the type and number of alternatives available. Humans sometimes appear to violate this assumption and employ relative, as opposed to absolute, currencies when making choices. Recent evidence suggests that animals too might sometimes employ relative choice mechanisms. We tested this idea by measuring the foraging preferences of rufous hummingbirds (Selasphorus rufus) faced with choices analogous to those in which human use of relative currencies is evident. The birds experienced three treatments: a binary choice between two artificial flower types designated concentration (20 microl, 40% sucrose solution) and volume (40 microl, 20%), and two trinary treatments in which a third decoy option (either concentration decoy: 10 microl, 30% or volume decoy: 30 microl, 10%) was added to the set. The birds' preferences differed significantly across the three treatments. In the trinary treatments, the effect of the decoy options was to increase the preference for the option that dominated the decoy. These results are similar to those reported in the human choice literature, and are compatible with the hummingbirds using a relative evaluation mechanism in decision making.     

Resource partitioning and competition in honeyeaters of the genus Meliphaga

Six species of Meliphaga studied in detail in South Australia all differ in some important aspect of their ecology consistent with the concept of interspecific competition. Four species are very similar in their feeding ecology but show distinct habitat preferences. The two remaining species are rather different in their feeding ecology and frequently overlap with their congeners in habitat. Two other species, not studied in detail, also appear to have distinct habitat preferences. One species has increased its range of habitat on Kangaroo Island, in the absence of potential competitors. The case for interspecific competition playing a part in the moulding of the niches of species in this genus is thus strengthened.     

High reproductive skew in tropical hover wasps.

A plethora of recent models examines how genetic and environmental factors might influence partitioning of reproduction ('skew') in animal societies, but empirical data are sparse. We used three microsatellite loci to estimate skew on 13 nests of the Malaysian hover wasp, Liostenogaster flavolineata. Groups are small in L. flavolineata (1-10 females) and all females are capable of mating and laying eggs. Despite considerable variation between nests in parameters expected to influence skew, skew was uniformly high. On 11 of the 13 nests, all female eggs had been laid by a single dominant female. A second female had laid one to two out of 5-10 eggs respectively on the two remaining nests. A likelihood analysis suggested that on average, 90% of the male eggs had also been laid by the dominant. The slightly lower skew among male eggs might reflect the lower average relatedness of subordinates to male versus female offspring of the dominant. We suggest that high skew in L. flavolineata may result from strong ecological constraints and a relatively high probability that a subordinate will eventually inherit the dominant, egg-laying position.     

Energy,predators and the behavior of feeding hummingbirds

Summary Current evidence suggests that many animals trade off energy gain against the risk of predation while feeding. In contrast, energetic considerations alone have proven successful in explaining and predicting the behaviour of feeding hummingbirds. This success may reflect the relative lack of natural predation on hummingbirds, but this study suggests that it may additionally reflect the lack of studies taking a predation perspective. In particular, when Anna's hummingbirds (Calypte anna) are faced with an obstructed view of their surroundings, they engage in behaviour suggestive of anti-predatory vigilance. In doing so, they voluntarily reduce their rate of energy intake. These birds also forgo better feeding opportunities that occur close to the ground, where observations suggest they are wary of opportunistic predators such as roadrunners (Geococcys californianus). While energy-based concepts will remain useful in the study of hummingbird feeding behaviour, the lack of predation on these birds should not be equated with an insensitivity to the risk of predation. This realization may lead to further insights into hummingbird-plant interactions, and hummingbird biology in general.     

Asymmetrical expression of transsexual phenotypes in hummingbirds

I present evidence for asymmetry in the expression of transsexual traits in adult hummingbirds. Among females, individuals with male-like plumage are common and define a continuous range of variation. Among males, individuals with female-like plumage are rare and define discontinuous plumage morphs. Quantitative characters also distinguish transsexuals from other members of their sex, but the characters involved differ for male-like females (bill length) and female-like males (bill, wing and tail length). Gonadal development is correlated with transsexuality only in males; female-like males have significantly smaller testes than male-like males. Both sexes demonstrate a significant negative association between plumage brightness and bill length. This association suggests an ecological basis for transsexuality because differences in plumage and bill morphology are associated with differences in foraging behaviour within and between hummingbird species. Morphological differences between transsexuals and non-transsexuals imply that plumage sexual dimorphism is more likely to evolve through changes in the frequency of female, rather than male, transsexual variants.     

Energy intake rates and nectar concentration preferences by hummingbirds

Summary In a series of daul choice tests with large volume feeders, rufous hummingbirds preferred sucrose concentrations near those that maximized their instantaneous rates of energy intake. As predicted on theoretical grounds, energy intake rates increased with increasing sucrose concentration to a maximum then decreased above this maximum. Earlier experimental studies suggested that hummingbirds always prefer the highest available concentration. Our results are consistent with the data of these studies, but by using a wider range of concentrations than previous workers, we found that the hummingbirds discriminated against very concentrated solutions.     

Growth responses of age 0 white perch and yellow perch from field-enclosure experiments

In this paper, we investigate the potential for competition between age 0 white perch Morone americana and yellow perch Perca flavescens in field-enclosure experiments. In 1986 and 1987, different densities of white perch and yellow perch were stocked into cages in the western basin of Lake Erie and their growth responses determined. Fish were weighed and measured pre- and post-stocking. In 1986 when fish were food-limited, individual growth of both species were negatively affected by increased total fish density; yellow perch to a greater extent. In 1987, experiments were carried out later in the year at lower temperatures. Yellow perch were shown to feed and continue to grow at lower water temperatures than white perch. We conclude that competition in summer may be mitigated in the fall because of differences in thermal optima between these two species.