Do past experience and competitive ability influence foraging strategies of parasitoids under interspecific competition?

Abstract 1. In solitary parasitoids, several species can exploit the same host patch and competition could potentially be a strong selective agent as only one individual can emerge from a host. In cereal crops, Aphidius rhopalosiphi and A. ervi share the grain aphid Sitobion avenae as host. 2. The present work studied foraging strategies of both species on patches already exploited by the other species. The study analysed larval competition in multi‐parasitised hosts and compared the foraging behaviour of females with and without previous experience. 3. It was found that A. ervi wins larval competition three times more often than A. rhopalosiphi. Both species spent less time on patches exploited by a heterospecific than on unexploited ones. When they foraged on heterospecifically exploited patches, experienced females induced less mortality in aphids than inexperienced ones. 4. Although A. rhopalosiphi is a specialist on cereal aphids and is the most abundant species due to its early appearance in the season, S. avenae is still a profitable host for A. ervi, because: (i) A. rhopalosiphi leaves patches partially exploited, (ii) A. ervi wins larval competition in three out of four multi‐parasitised hosts, and (iii) A. ervi is only slightly deterred by the cornicular secretions of the host and can thus easily parasitise hosts.     

Root foraging traits and competitive ability in heterogeneous soils

The responses of plant roots to nutrient patches in soil may be an important component of competitive ability. In particular, the scale, precision, and rate of foraging for patchy soil resources may influence competitive ability in heterogeneous soils. In a target–neighbor experiment in the field, per-individual and per-gram competitive effects were measured for six old-field species with known root foraging scale, precision, and rate. The presence and number of nutrient patches were also manipulated in a full factorial design. Number and presence of patches did not influence the outcome of competition. Competitive ability was not related to total plant size, growth rate, or root:shoot allocation, or to root foraging precision. Per-individual competitive effects were marginally correlated with root foraging scale (biomass of roots) and root foraging rate (time required to reach a patch). Therefore, competitive ability was more closely related to ability to quickly fill a soil volume with roots than to ability to preempt resource-rich patches.     

Some effects of energy costs on foraging strategies

We consider the effect of including energy costs on the optimal strategy for animals exploiting a depleting food resource. In the context of central place foraging this leads to the problem of what load size should be brought back to the central place. Two strategies are discussed: (i) maximize gross rate of energy delivery and (ii) maximize net rate of energy delivery. The optimal load size (or optimal patch time) for net maximizers is not always larger than for gross maximizers, as has been claimed. Instead, the difference in optimal load size has the same sign as the difference between metabolic rates of travelling and foraging. We point out that the influence of costs has not always been correctly incorporated in experimental tests of the theory.     

Respiratory evaporative water loss during hovering and forward flight in hummingbirds

Hummingbirds represent an end point for small body size and water flux in vertebrates. We explored the role evaporative water loss (EWL) plays in management of their large water pool and its use in dissipating metabolic heat. We measured respiratory evaporative water loss (REWL) in hovering hummingbirds in the field (6 species) and over a range of speeds in a wind tunnel (1 species) using an open-circuit mask respirometry system. Hovering REWL during the active period was positively correlated with operative temperature (T_e) likely due to some combination of an increase in the vapor-pressure deficit, increase in lung ventilation rate, and reduced importance of dry heat transfer at higher T_e. In rufous hummingbirds (Selasphorus rufus; 3.3 g) REWL during forward flight at 6 and 10 m/s was less than half the value for hovering. The proportion of total dissipated heat (TDH) accounted for by REWL during hovering at T_e > 40 °C was < 40% in most species. During forward flight in S. rufus the proportion of TDH accounted for by REWL was ~ 35% less than for hovering. REWL in hummingbirds is a relatively small component of the water budget compared with other bird species (< 20%) so cutaneous evaporative water loss and dry heat transfer must contribute significantly to thermal balance in hummingbirds.     

Flying in the rain: hovering performance of Anna's hummingbirds under varied precipitation

Flight in rain represents a greater challenge for smaller animals because the relative effects of water loading and drop impact are greater at reduced scales given the increased ratios of surface area to mass. Nevertheless, it is well known that small volant taxa such as hummingbirds can continue foraging even in extreme precipitation. Here, we evaluated the effect of four rain intensities (i.e. zero, light, moderate and heavy) on the hovering performance of Anna's hummingbirds (Calypte anna) under laboratory conditions. Light-to-moderate rain had only a marginal effect on flight kinematics; wingbeat frequency of individuals in moderate rain was reduced by 7 per cent relative to control conditions. By contrast, birds hovering in heavy rain adopted more horizontal body and tail positions, and also increased wingbeat frequency substantially, while reducing stroke amplitude when compared with control conditions. The ratio between peak forces produced by single drops on a wing and on a solid surface suggests that feathers can absorb associated impact forces by up to approximately 50 per cent. Remarkably, hummingbirds hovered well even under heavy precipitation (i.e. 270 mm h(-1)) with no apparent loss of control, although mechanical power output assuming perfect and zero storage of elastic energy was estimated to be about 9 and 57 per cent higher, respectively, compared with normal hovering.     

Mating strategies based on foraging ability: an experiment with grasshoppers

Female mate choice and the benefits of this behavior are criticalaspects of Darwinian sexual selection, but they are seldom documentedbecause it is difficult to identify the male trait(s) that femalesmay be seeking. We conducted experiments with grasshoppers (Melanoplussangutnipes: Orthoptera, Acrididae) to examine this behavior.Males that feed more intensively and select a diet mix thatpermits greater food intake (food intake per body mass per time)in laboratory trials were preferentially selected by females.These better foraging males on average provide greater paternalinvestment (greater spermatophore mass) to the female, whichincreases her reproductive rate (eggs produced per body massper time). However, paternal investment may not entirely explainfemale choice of better foraging males, because these maleswere still selected even if they had their food intake restrictedor had been allowed to recently mate, which reduces spermatophoreproduction. Furthermore, males change their mating strategyin response to female choice and the foraging abilities of surroundingmales. Poorer foraging males attempt forcible copulation ratherthan displaying and allowing female choice. A male will facultativelyswitch between these strategies depending on the foraging abilitiesof the surrounding males. While females attempt to reject forciblecopulation, forcible copulation reduces the frequency with whichfemales successfully copulate with better foraging males. Therefore,males that are less "attractive" to females adopt alternativemating strategies to counter female choice which would excludethem from mating.[Behav Ecol 7: 438–444 (1996)]     

Optimal foraging in hummingbirds: Rule of movement between inflorescences

The movements of hummingbirds between inflorescences of scarlet gilia (Ipomopsis aggregata) were studied. These movements exhibited the following patterns: (1) Although the hummingbirds appeared to avoid moving to the previous inflorescence, no significant correlation was found between the directions of successive inter-inflorescence movements. (2) The frequency distribution of inter-inflorescence flight distances was found to be leptokurtic. (3) The hummingbirds were more likely to move to an inflorescence the larger and/or closer it was. (4) The hummingbirds moved to inflorescences of greatest apparent size (i.e. ratio of number of flowers available to distance from present inflorescence) more often than they moved to the largest inflorescence, the closest infloresence, or the inflorescence estimated to yield the greatest rate of energy gain. (5) The frequency distribution of moves to the inflorescence having the ith greatest apparent size is well fitted by a geometric distribution. This is consistent with the hummingbrids choosing the inflorescence of greatest apparent size (excluding the previous inflorescence) from within some scanning sector. These movement patterns are consistent with the expectations of optimal foraging theory only if the hummingbirds cannot or do not determine the directions of possible inflorescences relative to the direction of arrival at the present inflorescence and if they cannot assess independently the sizes and distances of possible inflorescences.     

Flight costs of long,sexually selected tails in hummingbirds

The elongated tails adorning many male birds have traditionally been thought to degrade flight performance by increasing body drag. However, aerodynamic interactions between the body and tail can be substantial in some contexts, and a short tail may actually reduce rather than increase overall drag. To test how tail length affects flight performance, we manipulated the tails of Anna''s hummingbirds (Calypte anna) by increasing their length with the greatly elongated tail streamers of the red-billed streamertail (Trochilus polytmus) and reducing their length by removing first the rectrices and then the entire tail (i.e. all rectrices and tail covert feathers). Flight performance was measured in a wind tunnel by measuring (i) the maximum forward speed at which the birds could fly and (ii) the metabolic cost of flight while flying at airspeeds from 0 to 14 m s⁻¹. We found a significant interaction effect between tail treatment and airspeed: an elongated tail increased the metabolic cost of flight by up to 11 per cent, and this effect was strongest at higher flight speeds. Maximum flight speed was concomitantly reduced by 3.4 per cent. Also, removing the entire tail decreased maximum flight speed by 2 per cent, suggesting beneficial aerodynamic effects for tails of normal length. The effects of elongation are thus subtle and airspeed-specific, suggesting that diversity in avian tail morphology is associated with only modest flight costs.     

The twin threshold model: risk-intermediate foraging by rufous hummingbirds, Selasphorus rufus

I developed two versions of the twin threshold model (TTM) to assess risk-sensitive foraging decisions by rufous hummingbirds. The model incorporates energy thresholds for both starvation and reproduction and assesses how three reward distributions with a common mean but different levels of variance interact with these critical thresholds to determine fitness. Fitness, a combination of survival and reproduction, is influenced by both the amount of variance in the distributions and the relative position of the common mean between the thresholds. The model predicts that risk-intermediate foraging is often the optimal policy, and that risk aversion is favoured as the common mean of the distributions approaches the starvation threshold, whereas risk preference is favoured as the common mean approaches the reproduction threshold. Tests with free-living hummingbirds supported these predictions. Hummingbirds were presented with three distributions of nectar rewards that had a common mean but Nil, Moderate or High levels of variance. Birds preferred intermediate levels of variance (Moderate) when presented with all three rewards simultaneously, and became more risk-averse as the mean of the distributions was decreased but more risk-prone as the mean was increased. Birds preferred Nil when it was paired with Moderate or with High, but preferred Moderate in the presence of Nil and High together. This reversal of preference is a violation of regularity, conventionally interpreted as irrational choice behaviour. I provide an alternative version of the TTM demonstrating that violations of regularity can occur when relative instead of absolute evaluation mechanisms are used.     

Flight performance and competitive displacement of hummingbirds across elevational gradients

Hummingbirds, with their impressive flight ability and competitive aerial contests, make ideal candidates for applying a mechanistic approach to studying community structure. Because flight costs are influenced by abiotic factors that change systematically with altitude, elevational gradients provide natural experiments for hummingbird flight ecology. Prior attempts relied on wing disc loading (WDL) as a morphological surrogate for flight performance, but recent analyses indicate this variable does not influence either territorial behavior or competitive ability. Aerodynamic power, by contrast, can be derived from direct measurements of performance and, like WDL, declines across elevations. Here, I demonstrate for a diverse community of Andean hummingbirds that burst aerodynamic power is associated with territorial behavior. Along a second elevational gradient in Colorado, I tested for correlated changes in aerodynamic power and competitive ability in two territorial hummingbirds. This behavioral analysis revealed that short-winged Selasphorus rufus males are dominant over long-winged Selasphorus platycercus males at low elevations but that the roles are reversed at higher elevations. Several lines of evidence support the hypothesis that the burst rather than sustained aerodynamic performance mediates competitive ability at high elevation. A minimum value for burst power may be required for successful competition, but other maneuverability features gain importance when all competitors have sufficient muscle power, as occurs at low elevations.     

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.     

Experimental increase of flying costs in a pelagic seabird: effects on foraging strategies,nutritional state and chick condition

A central point in life history theory is that parental investment in current reproduction should be balanced by the costs in terms of residual reproductive value. Long-lived seabirds are considered fixed investors, that is, parents fix a specific level of investment in their current reproduction independent to the breeding requirements. We tested this hypothesis analysing the consequences of an experimental increase in flying costs on the foraging ecology, body condition and chick condition in Cory’s shearwaters Calonectris diomedea. We treated 28 pairs by reducing the wing surface in one partner and compared them with 14 control pairs. We monitored mass changes and incubation shifts and tracked 19 foraging trips per group using geolocators. Furthermore, we took blood samples at laying, hatching and chick-rearing to analyse the nutritional condition, haematology, muscle damage and stable isotopes. Eighty-day-old chicks were measured, blood sampled and challenged with PHA immune assay. In addition, we analysed the effects of handicap on the adults at the subsequent breeding season. During incubation, handicapped birds showed a greater foraging effort than control birds, as indicated by greater foraging distances and longer periods of foraging, covering larger areas. Eighty-day-old chicks reared by treated pairs were smaller and lighter and showed a lower immunity than those reared by control pairs. However, oxygen demands, nutritional condition and stable isotopes did not differ between control and handicapped birds. Although handicapped birds had to increase their foraging effort, they maintained physical condition by reducing parental investment and transferred the experimentally increased costs to their partners and the chick. This result supports the fixed investment hypothesis and is consistent with life history theory.     

Leg autotomy in a spider has minimal costs in competitive ability and development

A number of species have the ability to autotomize limbs voluntarily, but animals that have lost limbs often face substantial costs. We examined the frequency of leg loss and its effects on competitive ability and development in the spider Holocnemus pluchei (Araneae: Pholcidae), a family of spiders known for its readiness to autotomize legs. Leg loss was common in field populations, with 7.5% of all surveyed spiders missing at least one leg, most commonly one of the anterior pair. More spiders were missing multiple legs than expected by chance, suggesting that leg loss events are not independent. Large adult spiders were missing legs more frequently than were small spiders. The competitive ability of injured males was tested in three contexts. In the field, no effect of leg loss was found on the ability of spiders to remain in webs into which they were introduced. In the laboratory, no effect of leg loss was found on the ability to fight with a single opponent over a prey, except that injured spiders were more likely to lose high-intensity fights. There was no difference between intact and injured males in their ability to compete with three females for limited prey. Leg loss significantly affected development time. The moult interval during the instar in which the injury occurred increased by approximately 15%. However, the growth rate for injured spiders was slightly but not significantly faster in the instar following leg loss, and total development time of the two instars together did not differ significantly between treatments. No spider showed any signs of regeneration. We conclude that, although there were some statistically significant differences between intact and injured males, these are unlikely to have major impacts on fitness, in contrast to findings in other species. Copyright 1999 The Association for the Study of Animal Behaviour.     

Tactile discriminatory ability and foraging strategies in Kangaroo rats and pocket mice (Rodentia: Heteromyidae)

Summary A comparative study of seasonal food hoarding activity and tactile discriminatory ability in four species of heteromyid rodents (Dipodomys panamintinus, D. merriami, Perognathus longimembris, and P. formosus) was conducted in laboratory test arenas. Animals were tested individually to determine their treatment of seed (white millet) and seed mimics (glass beads and gravel) offered as food. In general, all animals showed low levels of millet hoarding activity during winter months with higher levels in fall and spring. Observations revealed that all species manipulated (with the forepaws) each potential food item prior to eating, pouching, or rejecting it. These tactile cues appear to surpass visual and olfactory cues as critical factors in distinguishing between food and food mimics. Pocket mice (Perognathus) showed high levels of tactile discriminatory ability which may serve as the mechanism by which they achieve high foraging efficiency in nature when filter-feeding for widely dispersed seed resources. Kangaroo rats (Dipodomys), on the other hand, are less adept at distinguishing between food and very similar non-food items. The fact that, in nature, kangaroo rats depend heavily on clumped food resources may obviate the need for highly efficient tactile discriminatory abilities.     

Towards a cognitive niche: divergent foraging strategies resulting from limited cognitive ability of foraging herbivores in a spatially complex environment.

A model was developed to explain one mechanism whereby differential optimal foraging strategies can occur between species as a result of selection for competition avoidance. This is the primary requirement for niche differentiation to evolve without a difference in the underlying foraging ability or morphology. The model used an individual-based patch choice mechanism, whereby herbivores move from patch to patch seeking food with the highest nutrient intake characteristics. The choice of patch was governed by a parameter, mu, which determined to what extent information in the landscape at different distances from the herbivore was used by it to make foraging decisions. A genetic algorithm was used to optimise the value, mu, in a complex landscape. The value of mu quickly converged to a single value with stabilising selection occurring when there was only a single species foraging. When there was a competing species with a fixed value of mu, the value of mu evolved to be above or below the mean for the single species mean depending on whether the value of mu for the competitor was below, or above the single-species mean, respectively. This was indicative of niche segregation. However mu tended to vary unstably over time when allowed to vary simultaneously in both species, although there was evidence for interaction between the two values. These results indicate that there can be a competitive advantage in choosing a cognitive strategy that is complementary to that used by other species.     

Wingstroke frequency of foraging and hovering bumblebees in relation to morphology and temperature

Abstract Wingstroke frequency, morphometries and thoracic temperatures of freely foraging bumblebees were examined in the field at ambient temperatures varying from 10 to 29^oC. Frequency was strongly correlated with morphometric parameters, particularly wing length, but was not correlated with either ambient or thoracic temperature. Magnitudes and scaling of frequency of foraging bees were comparable to values obtained for bees hovering in a closed chamber. These data indicate that frequency is primarily determined by morphometric characteristics which determine lift requirements and do not support the hypothesis that frequency is varied in response to environmental conditions as a means of in-flight thermoregulation.     

Effect of sexual dimorphism in bill length on foraging behavior: an experimental analysis of hummingbirds

We examined whether sexual differences in trophic morphology are associated with sexual differences in foraging behavior through two laboratory experiments on rufous hummingbirds (Selasphorus rufus) designed to compare probing abilities (maximum extraction depths) and handling times of sexes at flowers. Bills of female S. rufus are about 10.5% longer than bills of males, and this difference was associated with sexual differences in foraging abilities. Maximum extraction depths of female S. rufus were significantly greater than those of males, and no overlap between the sexes was observed. Moreover, handling times of females were shorter than handling times of males at flowers having longer corollas (15 mm). Thus, because of their longer bills, female S. rufus have the potential to feed from longer flowers than males, and can do so more quickly. We suggest that no single mechanism is responsible for the evolution of sexual dimorphism in bill lengths of hummingbirds, but rather that the dimorphism probably reflects the combined effects of reproductive role division and intersexual food competition, and possibly, sexual selection.