Food Supplementation Experiment Research Articles

Habitat‐specific effects of a food supplementation experiment on immunocompetence in Eurasian MagpiePica picanestlings

Parasite pressure and nutrition are two of the most important factors affecting the trade‐off between nestling growth and immune development. During development, energy and nutrients are often limited, and nestlings should only dedicate differentially more valuable resources to their immune system when the associated benefits are high (i.e. in situations of an increased risk of parasitism). In this study, we manipulated nutritional condition of Eurasian MagpiePica picanestlings by providing a food supplement. Additionally, the study area was subdivided into two categories of habitat (irrigated and arid) based on the presence of irrigation canals. Nestling diet composition was more varied in the irrigated compared with in the arid habitat. In addition, nestlings of the irrigated habitat showed a significantly higher infestation of both ectoparasites and blood parasites and a significantly higher cell‐mediated immunity, but lower tarsus length compared with nestlings of the arid habitat. Food supplementation to nestlings did not affect tarsus length, but increased nestling cell‐mediated immunity in the arid habitat only. Based on the recent demonstrated trade‐off between growth and immunocompetence in nestlings, we suggest that differences between habitats in nestling diet and parasite prevalence may have caused different priority rules in the allocation of resources between both fitness traits.

Food supplementation experiments revisited: verifying that supplemental food is used by its intended recipients

Field-based food supplementation experiments can determine whether populations are limited by natural food availability. However, they may yield spurious results if added food is hoarded by dominants, exploited by immigrants, or delivered ineffectively. Surprisingly, population-level approaches accounting for these potential sources of bias have not been established. We explored responses of free-ranging snowshoe hares ( Lepus americanus Erxleben, 1777) to food supplementation by contrasting per capita browsing rates on three study sites with added food with those on three control sites during two winters. Food augmentation reduced foraging pressure by hares; this reduction was significant when browse species of high dietary importance were considered. By implication, hares on manipulated sites switched from favored natural foods to supplemental food, meaning that the extra food found its mark and was not heavily exploited by immigrants or hoarded by dominants. Demographic responses to food addition were not detected on the manipulated sites, indicating that the study population was not food limited. Given the success with which we detected the signal of supplementation and eliminated potential confounding factors, we suggest that future studies using food addition in the context of herbivore population dynamics would benefit from a more mechanistic approach; this may involve measurement of browsing rates on manipulated and control sites.

Mass Loss by Breeding Female Songbirds: Food Supplementation Supports Energetic Stress Hypothesis in Black-Throated Blue Warblers

Abstract Reproduction is physiologically stressful for many animals. Female birds often lose body mass after their young hatch, which has been commonly attributed either to an energetic deficit incurred while breeding (the energetic stress hypothesis) or to an adaptive reduction in wing-loading to save energy during flight (the wing-loading hypothesis). We tested these two hypotheses for an open-cup nesting passerine, the Black-throated Blue Warbler (Dendroica caerulescens), using a food-supplementation experiment. We found that females provided with extra food lost less body mass between the incubation and nestling periods than did control females and that mass loss varied seasonally, with females in both treatments that nested earlier in the season losing more mass. We conclude that greater mass loss in control females supports the energetic stress hypothesis in Black-throated Blue Warblers.

MASS LOSS BY BREEDING FEMALE SONGBIRDS: FOOD SUPPLEMENTATION SUPPORTS ENERGETIC STRESS HYPOTHESIS IN BLACK-THROATED BLUE WARBLERS

Abstract Reproduction is physiologically stressful for many animals. Female birds often lose body mass after their young hatch, which has been commonly attributed either to an energetic deficit incurred while breeding (the energetic stress hypothesis) or to an adaptive reduction in wing-loading to save energy during flight (the wing-loading hypothesis). We tested these two hypotheses for an open-cup nesting passerine, the Black-throated Blue Warbler (Dendroica caerulescens), using a food-supplementation experiment. We found that females provided with extra food lost less body mass between the incubation and nestling periods than did control females and that mass loss varied seasonally, with females in both treatments that nested earlier in the season losing more mass. We conclude that greater mass loss in control females supports the energetic stress hypothesis in Black-throated Blue Warblers.

Sex‐specific mass loss in chick‐rearing South Polar Skuas Stercorarius maccormicki– stress induced or adaptive?

Mass loss of chick‐rearing birds can be the direct consequence of physiological stress (reproductive stress hypothesis) or an adaptive mass adjustment in response to the increased demands on flight efficiency during the flight‐intensive chick‐rearing period (adaptive mass loss hypothesis). To test which of these hypotheses best explains mass loss in South Polar Skuas Stercorarius maccormicki rearing chicks, a food supplementation experiment was carried out in the austral summer 2000/01 at King George Island, Antarctica. Half of the breeding pairs were fed about 20% of the chick's daily energy demand every second day and chick growth and adult nest attendance were recorded. Parents were caught at the start and the end of chick‐rearing to calculate adult mass loss. Male parents of food‐supplemented pairs attended their nest territories more than control males but females kept their attendance constant. Chick growth was only minimally affected and the treatment probably had no fitness consequences. Male Skuas in control pairs had a higher deviation from the body size–mass regression at the end of chick‐rearing compared with the start, supporting the stress hypothesis, whereas female deviation remained unchanged. Males of food‐supplemented pairs were heavier than unsupplemented males at the end of the breeding cycle but not significantly so. Food‐supplemented females were lighter at the end, supporting the adaptive mass loss hypothesis. Adult mass loss is thus best explained by the reproductive stress hypothesis in males but by the adaptive mass loss hypothesis in females. However, the two hypotheses are not mutually exclusive and the results do not exclude the possibility that mass loss in females is stress‐induced but the amount of mass lost is an adaptive adjustment to the reliability of the food supply. The finding that members of a breeding pair may follow different strategies of mass adjustment has implications for the use of mass loss as an index of parental effort. Without knowing which strategy each sex has adopted it is of little use to compare mass loss between parents.

Food availability affects diurnal nest predation and adult antipredator behaviour in song sparrows, Melospiza melodia

Food and predators are thought to have independent effects on avian reproductive success. Several studies have now shown that extra food can reduce nest predation rates. Most hypotheses suggest that this is due to changes in adult and/or nestling behaviour. A critical test as to whether behaviour is key, is to examine when predation events occur for birds with and without extra food. If behaviour is important, extra food should lead to fewer nest predation events during the day, when birds are active, but have no effect at night. We conducted a food supplementation experiment on song sparrows and tested for food effects on timing of nest predation events, adult antipredator behaviour and nestling begging behaviour. Food supplementation significantly reduced the relative frequency of daytime nest predation especially during incubation. During incubation, fed females had shorter foraging bouts and longer bouts on the nest, and these behaviours were associated with a reduced probability of being preyed upon. During brooding, we found trends (P < 0.1) towards lower daytime predation and greater adult nest attendance for fed birds than for unfed birds, but nestling begging did not differ with food treatment. Our results show that food availability does influence nest predation through its effect on antipredator behaviour. We highlight the importance of examining adult behaviour and vulnerability to predation at both stages of the nesting period because behavioural changes with extra food were extreme at the egg stage and were more effective at reducing nest predation than were behavioural changes at the nestling stage.

Food supplementation and abundance estimation in the white-footed mouse

Food availability often drives consumer population dynamics. However, food availability may also influence capture probability, which if not accounted for may create bias in estimating consumer abundance and confound the effects of food availability on consumer population dynamics. This study compared two commonly used abundance indices (minimum number alive (MNA) and number of animals captured per night per grid) with an abundance estimator based on robust design model as applied to the white-footed mouse ( Peromyscus leucopus (Rafinesque, 1818)) in food supplementation experiments. MNA consistently generated abundance estimates similar to the robust design model, regardless of food supplementation. The number of animals captured per night per grid, however, consistently generated lower abundance estimates compared with MNA and the robust design model. Nevertheless, the correlations between abundance estimates from MNA, number of animals captured, and robust design model were not influenced by food supplementation. This study demonstrated that food supplementation is not likely to create bias among these different measures of abundance. Therefore, there is a great potential for conducting meta-analysis of food supplementation effect on consumer population dynamics (particularly in small mammals) across studies using different abundance indices and estimators.

FOOD LIMITATION DURING BREEDING IN A HETEROGENEOUS LANDSCAPE

Abstract Breeding success in birds may be determined by the availability of food that parents can provide to growing nestlings. A standard method for testing the occurrence of food limitation is to provide supplemental food during different parts of the breeding period. If there is spatial variation in the strength of food limitation, the effect of such an experiment should also vary spatially. We investigated whether the strength of food limitation during nestling rearing in the European Starling (Sturnus vulgaris) was related to the management intensity of agricultural landscapes. We fed birds mealworms during the nestling period in landscapes with high or low local availability of pasture, the preferred foraging habitat. Both habitat and food supplementation affected growth and survival of nestlings; the effects of the food-supplementation experiment were generally stronger than those of habitat. Mortality mainly struck the last-hatched chick. Both habitat and food supplementation positively affected n...

Food Limitation During Breeding in a Heterogeneous Landscape

Abstract Breeding success in birds may be determined by the availability of food that parents can provide to growing nestlings. A standard method for testing the occurrence of food limitation is to provide supplemental food during different parts of the breeding period. If there is spatial variation in the strength of food limitation, the effect of such an experiment should also vary spatially. We investigated whether the strength of food limitation during nestling rearing in the European Starling (Sturnus vulgaris) was related to the management intensity of agricultural landscapes. We fed birds mealworms during the nestling period in landscapes with high or low local availability of pasture, the preferred foraging habitat. Both habitat and food supplementation affected growth and survival of nestlings; the effects of the food-supplementation experiment were generally stronger than those of habitat. Mortality mainly struck the last-hatched chick. Both habitat and food supplementation positively affected nestling growth, measured as nestling tarsus length. In addition, food supplementation positively affected feather growth and asymptotic mass. Contrary to expectation, no interactions existed between effects of habitat and food supplementation, which suggests that breeding success was limited by food availability in both landscapes. Potential reasons for this lack of effect are parental compensation and low statistical power. Also, breeding densities were higher in landscapes with more pastures, possibly equalizing the per-capita availability of food. Thus, our results demonstrate that reproductive success was limited by availability of food when local availability of preferred foraging habitat was either low or high, but fail to demonstrate spatial variation in the strength of food limitation. Escasez de Alimentos durante el Período Reproductivo en un Paisaje Heterogéneo

Behavioral Response of Resident Jamaican Birds to Dry Season Food Supplementation1

ABSTRACTWe used plot‐level manipulations and analyses to test the effects of food availability on the behavior and condition of resident dry‐forest birds in Jamaica. Two control plots were monitored in each of 2 yr. Food was supplemented on five plots over 2 yr with piles of cut oranges distributed around plots, which served as a direct source of carbohydrates and water, and an indirect source of ground arthropods due to increased above‐ground activity. We reduced ants on five plots over 2 yr; however, we found no difference in total ground arthropod biomass between control and reduction treatments, so we pooled these plots for analysis. We selected nine focal resident bird species for study of relative abundance, body condition, and breeding condition. Birds were sampled prior to, and 5 to 6 weeks after the initiation of treatments. Seven of nine species had higher relative abundance following food supplementation. Three species were recaptured more frequently in supplementation plots than in control plots. These abundance and persistence responses did not cause any changes in body condition. In one species, Bananaquit (Coereba flaveola), food supplementation resulted in higher concentrations of individuals in breeding condition. These results demonstrate a functional response to dry‐season food availability and suggest a limiting mechanism. This study helps explain mechanisms by which bird populations respond to resource availability, and is the first successful plot‐level food supplementation experiment for tropical forest birds.

PARADOXICAL AVOIDANCE OF ENRICHED HABITATS: HAVE WE FAILED TO APPRECIATE OMNIVORES?

Omnivores have not figured prominently in our understanding of food webs and prey dynamics even though they can have substantial direct and indirect effects on the structure of ecological communities and on the dynamics of interacting species. The important role of omnivores is implicated by the paradoxical results of a food-supplementation experiment. The experiment was designed to test theories that predict how habitat change affects the distribution of habitat-selecting species. According to theories of habitat selection, a quantitative change in habitat (as caused by supplemental food) should increase consumer population size and alter habitat selectivity. Related theories of patch use predict that consumers should increase their use of enriched patches. A two-year experiment on two species of small mammals in Canada's boreal forest failed to alter population densities of red-backed voles, but did cause a dramatic shift in vole habitat use. Rather than increasing their use of feeding stations as predicted by classical theory, voles avoided them. Deer mice did not respond to the experimental treatments. The paradoxical results occurred because omnivorous black bears altered prey behavior by increasing predation risk at feeding stations. Revised theory confirms the indirect omnivore effect, and demonstrates that the behavioral paradox is far more likely for omnivores than for other types of predators. The behavioral paradox of enrichment highlights not only important new, and potentially stabilizing, roles for omnivores, but also the pervasive influences of behavior and habitat selection on population dynamics and regulation.

FOOD LIMITS ANNUAL FECUNDITY OF A MIGRATORY SONGBIRD: AN EXPERIMENTAL STUDY

In short-lived species, fecundity strongly influences population size. For those species with multiple breeding attempts per breeding season, variance in fecundity is best explained by the number of breeding attempts. For birds, multiple brooding may be influenced by food availability. Here, we report results of a food supplementation experiment that tests the role of food as a mechanism driving variation among individuals in the frequency of multiple brooding in a Neotropical migrant songbird, the Black-throated Blue Warbler (Dendroica caerulescens). Supplementally fed females produced more second broods, spent less time foraging and more time loafing, and stayed closer to their nests than did control females. Fed and control females did not differ in the number or mass of young fledged from the first nesting attempt. Supplemental food increased the probability that females would initiate second broods in both a low and an average food year, suggesting that this population is food limited during the breeding season in most years. Our results thus demonstrate that food availability can strongly influence annual fecundity in migratory bird species breeding in temperate forests, which, in turn, affects annual recruitment rates and population size.

High levels of hatching failure in an insular population of the South Island robin: a consequence of food limitation?

Endangered birds in insular environments like New Zealand are often translocated to predator-free offshore islands for conservation purposes. Some translocated populations however exhibit reduced fecundity, and it has been suggested that either inbreeding (due to small number of founders) or resource shortages (due to high population density) may limit reproductive success. For example, the South Island robin ( Petroica a. australis) population on Motuara Island (59 ha) was founded by only five birds but has increased to ∼600 individuals, or a density >10 times that of the mainland. Despite this apparent success, the rate of hatching failure in this population is three times higher than in mainland populations, with more than a third of eggs failing to hatch. To test if elevated hatching failure is the result of food limitation, we carried out a food supplementation experiment by providing females with mealworms equivalent to ∼50% of their daily energy requirements. Food supplementation had no effect on hatching success, clutch size, incubation attentiveness or nest size. Egg volume increased with food supplementation in one of the two years of this study, but both egg volume and incubation attentiveness were unrelated to rate of hatching failure. As previous genetic analyses confirmed that the bottlenecked population of robins on Motuara Island have significantly less genetic variation than their source population, we conclude that inbreeding depression, and not food limitation, is the most likely explanation for their high rate of hatching failure. We suggest that the experimental translocation of genetically dissimilar individuals be considered as a possible remedy for low productivity in island populations that were initiated with few founders.

Effects of a food supplementation experiment on reproductive investment and a post‐mating sexually selected trait in magpies Pica pica

Food availability is an important factor affecting breeding success in birds. Food supplementation experiments in birds have in general focused on the effects on reproductive success in terms of female investment (laying date, clutch size, egg size), however, it is also known that the estimation of mate quality based on sexually selected signals influences female reproductive investment. In the particular case of magpies, females use nest size, a post‐mating sexually selected signal, to assess male's likelihood to invest in reproduction, and accordingly adjust reproductive investment (clutch size). Then, the possible effects of food supplementation on female reproductive investment could be mediated by other variables related to parental quality, such as nest size in magpies. In the present study, we explore if higher food availability in a magpie territory affected both male sexually selected traits (i.e. nest size) and female reproductive investment (laying date, egg size, clutch size). We performed a food supplementation experiment in which we experimentally increased food availability in several magpie territories, keeping others as controls. In food‐supplemented territories, males built significantly larger nests and females significantly increased egg size by 4.1% compared to control females. Results suggest that the continuous provisioning of protein rich food allowed magpie females to increase egg size. However, laying date and clutch size did not differ between control and food‐supplemented magpie pairs. Food availability also affected the relationship between female reproductive investment and nest size. In control territories, females decreased their egg size in response to a larger nest, whereas a tendency for the opposite relationship was revealed in food‐supplemented territories. We discuss the possibility that magpie females adopt different strategies for reproductive investment according to food availability.

Effect of Prey Biomass on Reproductive Success and Mating Strategy of Bicknell's Thrush (Catharus Bicknelli), a Polygynandrous Songbird

Abstract Resource availability and distribution are hypothesized to be the primary ecological mechanisms driving variation in avian mating systems. Although food supplementation experiments have been used to examine plasticity in mating strategies, variation in avian mating strategies across naturally occurring gradients of prey availability has rarely been examined. Here, we test the hypothesis that the polygynandrous mating system of Bicknell's Thrush (Catharus bicknelli) results from food limitation on female home ranges in the species' harsh montane breeding environment. In 2000, number of chicks fledged per nest was significantly positively correlated with prey biomass on a female's home range, and number of male feeders at a nest was significantly negatively correlated with prey biomass. Neither relationship was significant in 2001, a year with high nest-predation rates, which suggests that nest predation may ultimately limit the species' productivity. These results are consistent with the hypothesis that (1) female productivity is limited by prey biomass, and (2) female mating decisions may be influenced by their assessment of home-range food abundance prior to egg laying.

EFFECT OF PREY BIOMASS ON REPRODUCTIVE SUCCESS AND MATING STRATEGY OF BICKNELL'S THRUSH (CATHARUS BICKNELLI), A POLYGYNANDROUS SONGBIRD

Abstract Resource availability and distribution are hypothesized to be the primary ecological mechanisms driving variation in avian mating systems. Although food supplementation experiments have been used to examine plasticity in mating strategies, variation in avian mating strategies across naturally occurring gradients of prey availability has rarely been examined. Here, we test the hypothesis that the polygynandrous mating system of Bicknell's Thrush (Catharus bicknelli) results from food limitation on female home ranges in the species' harsh montane breeding environment. In 2000, number of chicks fledged per nest was significantly positively correlated with prey biomass on a female's home range, and number of male feeders at a nest was significantly negatively correlated with prey biomass. Neither relationship was significant in 2001, a year with high nest-predation rates, which suggests that nest predation may ultimately limit the species' productivity. These results are consistent with the hypothesi...

Automated feeders: new technology for food supplementation experiments with mammals

Experimental manipulations of food supply in wild populations are often confounded by a number of factors, such as induced immigration to local populations receiving food supplements and intraspecific competition for access to feed stations. Here, we describe newly developed automated feeding stations, designed to dispense weighed food supplements to specific experimental animals. The feeders incorporate a data logger that records the identification and weight of each animal and the time of the visit to the feeder.Using the common brushtail possum (Trichosurus vulpecula) as a model medium-sized mammal, we present results of an 18-month field trial. Each free-ranging experimental possum was fitted with a collar containing a transponder chip, allowing them access to a feeder. During the field trial, experimental possums were found to show a significant increase in body mass compared with control animals, which showed a slight decrease in mass. Body masses recorded by the feeders for experimental possums did not differ from mass data recorded during live-trapping sessions.The automated feeding stations represent an advance over previous methodology and the first time supplementary food has been delivered in a controlled automated fashion in a wild mammal population. The implications of the feeders to future studies of resource supplementation in mammals are discussed.

Why do Both Parents Incubate in the Kentish Plover?

AbstractIncubation by both parents is a common parental behaviour in many avian species. Biparental incubation is expected if the survival prospects of offspring are greatly raised by shared care, relative to the costs incurred by each parent. We investigated this proposition in the Kentish plover Charadrius alexandrinus, in which both parents incubate the clutch, but one parent (either the male or the female) usually deserts after hatching of the eggs. We carried out a mate‐removal and food supplementation experiment to reveal both the role of the sexes and food abundance in maintaining biparental incubation by removing either the male or the female from the nest for a short period of time. In some nests we provided supplementary food for the parent that remained at the nest to reduce the costs of incubation, whereas other nests were left unsupplemented. Although males spent more time on incubation after their mate had been removed, females’ incubation did not change. Notwithstanding the increased male incubation, total nest attentiveness was lower at uniparental nests than at biparental controls. However, incubation behaviour was not influenced by food supplementation. We conclude that offspring desertion during incubation is apparently costly in the Kentish plover, and this cost cannot be ameliorated with supplementary food.

Proximate and ultimate control of sex ratios in Myrmica brevispinosa colonies.

The literature on sex ratio evolution in ant colonies is dominated by inclusive fitness arguments. In general, genetic theory makes good predictions about sexual investment in ant populations, but understanding colony-level variance in sex investment ratios has proven more difficult. Recently, however, more studies have addressed ecological factors that influence colony-level sex investment ratios. Food availability, in particular, has been manipulated because larval nutrition influences female caste determination, thus implying that resource availability should be of critical importance for colony sex investment ratios. However, results from food supplementation experiments are equivocal, and it is clear that ant colony response to food supplementation is dependent on the ecological background of the population. We presented field colonies of the ant Myrmica brevispinosa with two food types (proteins and carbohydrates), and assessed their relative impact on colony-level sex investment ratios. We show that colonies receiving carbohydrate enhancement invested in more female sexuals and produced more female-biased sex allocation ratios than protein-fed or control colonies. Thus, our study is the first, to our knowledge, to demonstrate that sex ratios in social insect colonies might be sensitive to resource quality. Male investment was not influenced by food treatment, but was positively correlated with colony size. Therefore, the shift in sex ratios in our study must have been mediated through nutritional influences on female caste determination rather than male brood elimination. We also used our data to evaluate evidence for sex ratio compensation by queenright colonies in response to male production by workers from orphaned colonies.

Effects of experimental food supplementation on movements of juvenile northern goshawks (Accipiter gentilis atricapillus).

Food availability is thought to strongly influence the leaving phase of natal dispersal in animals with condition-dependent dispersal. We conducted a food supplementation experiment to determine the influence of extra food on the onset of dispersal and early dispersal movement patterns of 42 radio-tagged northern goshawks (Accipiter gentilis atricapillus) from 28 broods in north-central New Mexico during 1992 and 1993. We randomly assigned half of the broods each year as treatments and the other half as controls. Treatment broods were given supplemental food from hatching (late April) until mid-October. Control broods received the same visitation rate but no food. Birds were located approximately every 2 days from fledging until mid-October in 1992 and 1993. Timing of fledging and independence were not affected by the treatment. However, extra food significantly influenced post-fledging movements of juvenile goshawks. During the late fledgling-dependency period (>65 days of age until independence) control birds were located in the natal area (<2 km from nest tree) more frequently than supplemented birds. This pattern reversed after independence (approximately 82 days of age) when supplemented birds were located more frequently in the natal area than controls. After independence the control birds were never located in the natal area and by the end of September in both years the controls had all left the study area (study area boundaries were approximately 25 km from nest tree). Supplemented birds were never located outside of the study area for the duration of the experiment. We conclude that the control birds dispersed out of the study area and the supplemented birds remained. Since the experimentally fed juveniles remained near a known food source and the controls did not, our study demonstrates that food availability influences at least the first 4 months of post-fledging movement patterns in this population. These results also suggest individuals base dispersal decisions on knowledge of their environment at a local scale, which can influence juvenile recruitment.