Food Availability Fails to Explain Asynchronous Breeding of Two Syntopic Oriental Trogons

The hypothesis that migratory bird populations are limited during the non-breeding season is increasingly supported by empirical studies that also suggest consequences that carry-over into subsequent seasons. Although variation in food supply is the best supported explanation for non-breeding season limitation, the ecological mechanisms and physiological consequences are not well understood. We both supplemented and reduced Ovenbird (Seiurus aurocapilla) food availability on replicate plots in Jamaica in each of 3 years to determine the direct role of food in limiting the physical condition of Ovenbirds. Annual variation in rainfall and food supply created a natural experiment in parallel with manipulations. Sex and age-classes of Ovenbirds did not respond differently in terms of body condition to either food manipulation or natural variation in environmental conditions, suggesting that this population is not structured by strong dominance relationships. Ovenbird body mass, fat, and pectoralis muscle shape were positively and predictably related to manipulated food availability. Feather regrowth rate also responded positively to food supplementation and negatively to food reduction in the drier of 2 years. Prior to manipulation, annual variation in body mass corresponded to annual variation in food supply and rainfall, providing additional, correlational evidence of food limitation. Since multiple intercorrelated body condition indices of Ovenbirds responded directly to food supply, and since food supply influenced body condition independently of other habitat features, we argue that food is a primary driver of non-breeding season population limitation. Moreover, since these effects were observed during the late non-breeding period, when individuals are preparing to migrate, we infer that food availability likely initiates carry-over effects.

Food supply is widely considered to be a major factor in determining life history traits and reproductive performance of birds. However, large spatial and temporal variation in natural available food supply is not always paralleled by proportional changes in energy demand by breeding birds. This necessarily results in variation in the supply–demand ratio and the amount of food available per unit mass of nestling. Because reproductive performance should respond to increases in available supply as a saturation curve, reaching a plateau above a certain threshold of food supply, we predict that variation in supply should change the intensity of selection on reproductive traits. We first tested this prediction using long-term data on nestling growth and survival in Blue Tits (Parus caeruleus) breeding over a gradient of habitat richness in Corsica, France. This long-term data analysis evaluates the effect of variation in food supply available to breeding tits using three surrogate variables: interannual variation in peak caterpillar abundance (caterpillar frass fall), offset between breeding date and peak caterpillar abundance, and natural variation in clutch size. We also used an experimental brood size manipulation (±3 chicks) to test the effect of varying brood demand on nestling growth. Results of the long-term data analysis show that all three variables affect fledging mass and fledging success in poor habitats, while only fledging mass is affected by variation in offset in rich habitats. Moreover, mean annual fledging success and fledging mass is strongly affected by annual variation in peak caterpillar abundance at low levels of abundance, but these effects disappear when food becomes abundant (saturation threshold level of 373 and 560 mg frass·m−2·d−1 for fledging success and fledging mass, respectively). Brood size manipulations confirm these results. In rich habitats, breeding birds can raise three extra chicks without any apparent effects on chick growth, while in poor habitats, chick growth is significantly reduced with brood enlargement. Our study shows that although food limitation can play an important role, it may not always be the primary force shaping life history traits.

Food supply fluctuations constrain group sizes of kangaroos and in turn shape their vigilance and feeding strategies

Even though growth rate is an important fitness component, it is still controversial to what extent parent birds adjust the timing of offspring hatch to natural variations in food supply to maximize offspring growth. We studied the role of food availability in explaining inter- and intra-seasonal variation of growth rate in goslings of greater snow geese over 5 years. The peak of hatching generally coincided with the peak of food availability. However, early-hatched goslings usually grew faster than birds hatched at the peak, which in␣turn grew faster than late-hatched goslings, although this phenomenon was not observed in all years. There was considerable variation in growth rate among the five years, the smallest goslings produced in the best year (1991) being larger than the largest goslings of the poorest year (1994). We developed three indices of food availability, based on the cumulative availability of plant biomass and nitrogen content during the growth period, and showed that the cumulative exposure to nitrogen biomass explained up to 43% of variation (intra- and inter-annual) in body size just before fledging. In years with good feeding conditions, early-hatched goslings had access to more nitrogen during their growing period than those hatching on or after the peak and they grew faster. In years of lower food availability, early-hatched goslings had no detectable advantage over peak- or late-hatched birds for access to protein-rich food and no seasonal decline in growth rate was observed. These results confirm the critical role of food supply in the seasonal variation of growth rate in Arctic-nesting geese.

Antarctic fur seals breed synchronously in a highly seasonal environment. There is considerable inter-annual variation in food supply, which affects breeding performance. However, food supply may also influence future performance through effects on female ovulation, implantation and/or pregnancy. We used foraging trip duration, pup growth rate and weaning mass as indicators of the food available to females during the pup-rearing period (December-April) and examined relationships between these and pup production and timing of breeding in the following year. Productivity (pup production) was positively correlated with growth rate and weaning mass of both male and female pups in the previous season (P < 0.05) and negatively correlated with the variation around the mean birth mass of males (P < 0.05), suggesting that poor feeding conditions one season led to lower production the next. The timing of birth was positively correlated with foraging trip duration (P < 0.05) and negatively correlated with the birth and weaning masses of male and female pups, the duration of the perinatal period, growth of male pups, and the variation in the late-season growth (last two months) of male and female pups (P < 0.05 in all cases). This indicates that females give birth later in the season following a year when food resources were scarce.

Earlier reports suggested that seasonal variation in food-caching behavior (caching intensity and cache retrieval accuracy) might correlate with morphological changes in the hippocampal formation, a brain structure thought to play a role in remembering cache locations. We demonstrated that changes in cache retrieval accuracy can also be triggered by experimental variation in food supply: captive mountain chickadees (Poecile gambeli) maintained on limited and unpredictable food supply were more accurate at recovering their caches and performed better on spatial memory tests than birds maintained on ad libitum food. In this study, we investigated whether these two treatment groups also differed in the volume and neuron number of the hippocampal formation. If variation in memory for food caches correlates with hippocampal size, then our birds with enhanced cache recovery and spatial memory performance should have larger hippocampal volumes and total neuron numbers. Contrary to this prediction we found no significant differences in volume or total neuron number of the hippocampal formation between the two treatment groups. Our results therefore indicate that changes in food-caching behavior and spatial memory performance, as mediated by experimental variations in food supply, are not necessarily accompanied by morphological changes in volume or neuron number of the hippocampal formation in fully developed, experienced food-caching birds.

Bird species adapted to variable environments tend to have slow lean tissue growth rates and high fat deposition rates, allowing survival during food shortages. This emphasis on fat deposition may be a fixed physiological trait. Alternatively, tissue allocation may be adjusted facultatively according to the proximate food supply. We consider two models of facultative adjustment that could account for the emphasis on fat deposition: (1) the fat‐priority model, in which no lean growth occurs when food is scarce, and (2) the lean‐priority model, in which a minimal level of lean growth always occurs but nutrients are otherwise allocated to fat deposition. We tested these two models using Welcome Swallows Hirundo neoxena, a species we show to have a variable food supply that is influenced by weather. We reduced food supply to chicks experimentally, by enlarging broods or excluding parents from chicks, and tested for reduction in wing growth (an indicator of lean growth) and mass growth (an indicator of fat deposition). Mass growth was retarded by both manipulations, but not wing growth, corroborating the lean‐priority model. This growth strategy may function not to cope with violent variation in food supply, but to maintain development and symmetry of wings and feathers in the face of moderate variation in food supply. Our results contrast with those of a similar experiment on the Black Noddy Anous minutus, a species with more severe variation in food supply.

Food availability is likely to influence body condition and, in turn, fitness. The intensity of this response may vary between populations of the same species on a small spatial and temporal scale. We used 5 yr of data from 6 Eurasian red squirrel Sciurus vulgaris populations from the southern Alps to explore differences in body size and body mass among neighbouring populations, in relation to habitat type and variation in food supply. We also investigated sexual dimorphism in these traits and whether phenotypic variation affects local survival and female reproductive success. Mean hind foot length, a measure of body size, did not differ between sexes but differed between areas. Seasonal variation in body mass was small with no evidence for fattening in autumn. Females were slightly heavier than males, but this difference was largely explained by mass gain of females during reproduction. The size of conifer seed crops, the major food supply, varied strongly over years and between habitats, but this variation corresponded only weakly with autumn body mass. Differences in size and mass between populations were partially explained by habitat‐related differences in body size and variability of seed‐crops, suggesting differential selection for smaller squirrels in spruce‐larch forests against selection for larger and heavier animals in mixed broadleaves and conifer forests and in Scots pine forests with more stable seed production. The probability of reproduction by females increased with body mass, but varied strongly between habitats and years, with more females reproducing in years with rich seed‐crops. In both sexes, body mass positively affected probability of settlement and length of residency. Our results suggest that in temporally variable environments that differ in overall amount of food resources, individual variation in body mass is related to habitat type, and that having a relatively high body mass, within each population, positively affects male and female settlement success and local survival, and female reproductive success.

Food availability is likely to influence body condition and, in turn, fitness. The intensity of this response may vary between populations of the same species on a small spatial and temporal scale. We used 5 yr of data from 6 Eurasian red squirrel Sciurus vulgaris populations from the southern Alps to explore differences in body size and body mass among neighbouring populations, in relation to habitat type and variation in food supply. We also investigated sexual dimorphism in these traits and whether phenotypic variation affects local survival and female reproductive success. Mean hind foot length, a measure of body size, did not differ between sexes but differed between areas. Seasonal variation in body mass was small with no evidence for fattening in autumn. Females were slightly heavier than males, but this difference was largely explained by mass gain of females during reproduction. The size of conifer seed crops, the major food supply, varied strongly over years and between habitats, but this variation corresponded only weakly with autumn body mass. Differences in size and mass between populations were partially explained by habitat-related differences in body size and variability of seed-crops, suggesting differential selection for smaller squirrels in spruce-larch forests against selection for larger and heavier animals in mixed broadleaves and conifer forests and in Scots pine forests with more stable seed production. The probability of reproduction by females increased with body mass, but varied strongly between habitats and years, with more females reproducing in years with rich seed-crops. In both sexes, body mass positively affected probability of settlement and length of residency. Our results suggest that in temporally variable environments that differ in overall amount of food resources, individual variation in body mass is related to habitat type, and that having a relatively high body mass, within each population, positively affects male and female settlement success and local survival, and female reproductive success.

1. The immune system plays an important role in fitness, and interindividual variation in immunocompetence is due to several factors including food supply. 2. Seasonal variation in food resources may therefore explain why immunocompetence in bird nestlings usually declines throughout the breeding season, with chicks born early in the season receiving more food than chicks born later, and thereby possibly developing a more potent immune system. Although there are studies supporting this hypothesis, none has been experimental. 3. We performed an experiment in the kittiwake Rissa tridactyla by manipulating the food supply of pairs that were left to produce a first brood, and of pairs that were induced to produce a late replacement brood. 4. If food supply mediates, at least partially, seasonal variations in chick immunocompetence, non-food-supplemented chicks would show a stronger seasonal decline in immunocompetence than food-supplemented chicks. 5. Food supplementation improved humoral immunocompetence (the production of immunoglobulins Y), but not T-cell immunocompetence (phytohaemagglutinin, PHA response). T-cell immunocompetence of food-supplemented and non-food-supplemented chicks decreased through the season but to a similar extent, whereas the humoral immunocompetence of non-food-supplemented chicks decreased more strongly than that of food-supplemented chicks. 6. Our results suggest that the seasonal decline in humoral immunocompetence can be explained, at least partly, by variations in food supply throughout the breeding season.

Among species which feed their young, particularly those with large size dimorphism, parental investment trade-offs between growth and survival of male and female offspring, and parental fitness, may be significant (sensu Trivers & Willard 1973). In contrast, little is known about the effects of variation in food supply on sex-differential growth and survival in species with precocial young. In such cases, where parental investment is generally smaller, the trade-off is more proximate ; how should individual offspring allocate resources to maximize their fitness. We examined this question by assessing the effects of seasonal variation in feeding conditions on growth and survival of male and female offspring of an obligate avian herbivore with precocial young, the lesser snow goose (Anser caerulescens caerulescens L.), using long-term observational data from 1969 to present. Snow geese show limited sexual size dimorphism, with males being 2-6% larger at all ages post-hatching. Growth of snow goose goslings has been previously shown to be extremely sensitive to variation in food supply, and previous analysis of this species indicated even small differences in growth rates may significantly affect the probability of survival. We found a highly significant difference in the relative body mass, but not structural size, of male and female goslings at fledging in response to seasonal declines in food supply, with males showing a greater proportional fledging mass decrease than females. Despite growth differences there was no detectable seasonal variation in gosling survival between the sexes, and no difference between male and female goslings in survival to fledging overall. Since the analysis involved only goslings which had survived to fledging, it can be difficult to determine if the seasonal decrease in dimorphism reflects either (i) seasonal differences in relative growth of male and female goslings, or (ii) increasing mortality of larger male goslings later in the season. However, the failure to find any sex-specific differences in mortality generally favours the hypothesis of proportionately slower growth of males hatched later in the season. The lack of sexual differences in survival despite measurable differences in sex-specific growth may reflect some level of adaptation to maximize the probability of survival of each sex. However, the possibility that the proportionately greater seasonal declines exhibited by males may reflect a simple constraint, rather than an adaptation, cannot presently be ruled out.

Hydrodynamic Control of the Supply of Suspended Chlorophyllato Infaunal Estuarine Bivalves

Resident populations of the Coppery—rumped Hummingbird, Amazilia tobaci, occupy the West Indian Islands of Trinidad and Tobago. Trinidad has 15 other hummingbird species, Tobago only 4 others. We predicted that A. tobaci on Tobago would have a broader feeding niche and a denser population than would A. tobaci on Trinidad. We monitored populations of Amazilia tobaci for 13 mo at study sites in the Arima Valley of northern Trinidad and in central Tobago. The two sites had nearly identical climates, and similar assemblages of flowers visited by hummingbirds. At both sites, food available to nectar—feeding birds varied > 100 fold over the year. For 9 mo (December—August), neither A. tobaci population faced a severe food shortage. From September through November, nectar was scarce at both sites. During this period, Tobago A. tobaci faced only two other, uncommon species of nectar—feeding birds, which had very different feeding habits. The Trinidad population endured the food shortage along with six other hummingbird species. At this time Tobago A. tobaci utilized variously sized and shaped flowers of numerous plant species. On Trinidad, where many flowers were visited by the other hummingbird species, A. tobaci primarily used flowers that matched their bill size, which resulted in narrower feeding niche than on Tobago. These effects of "ecological release" on Tobago were apparent for only 3 mo, however, and when data for an entire year are compared there were few statistically significant contrasts between Tobago and Trinidad populations. At both sites, niche (diet) breadth responded in density—dependent fashion to changes in food density, broadening with high densities of A. tobaci relative to food, narrowing when food was abundant. Median values for niche breadth, population density, and population density per unit resource were not significantly greater on Tobago than on Trinidad. Ecological differences between the two populations during the food shortage may be linked to a phenotypic difference. We found no increase on Tobago in the intrapopulation variance of any morphological parameter important to feeding. However, Tobago, A. tobaci had longer wings than Trinidad conspecifics, resulting in lower wing disc loadings (ratio of body mass to area swept out by the wings). Among hummingbirds in general, low wing disc loading indicates a capacity to forage at a wide variety of flowers, which suggests that the Tobago population has undergone "within—phenotype nich expansion." Two experiments showed that individuals from the Tobago population of A. tobaci used a wide variety of artificial flowers, and a wide range of flower densities in field manipulations, than did conspecifics from Trinidad. The results suggest that occasional food shortages play a crucial role in the ecology and evolution of island populations, and that this might be undetected in short—term studies.

Within three decades, the barnacle goose population wintering on the European mainland has dramatically increased in numbers and extended its breeding range. The expansion has occurred both within the Arctic as well as by the colonization of temperate areas. Studies of performance of individuals in expanding populations provide information on how well species can adapt to novel environments and global warming. We, therefore, studied the availability of high quality food as well as timing of reproduction, wing moult, fledgling production and postfledging survival of individually marked geese in three recently established populations: one Arctic (Barents Sea) and two temperate (Baltic, North Sea). In the Barents Sea population, timing of hatching was synchronized with the peak in food availability and there was strong stabilizing selection. Although birds in the Baltic and North Sea populations bred 6–7 weeks earlier than Arctic birds, timing of hatching was late in relation to the peak in food availability, and there was moderate to strong directional selection for early breeding. In the Baltic, absolute timing of egg laying advanced considerably over the 20‐year study period, but advanced little relative to spring phenology, and directional selection on lay date increased over time. Wing moult of adults started only 2–4 weeks earlier in the temperate populations than in the Arctic. Synchronization between fledging of young and end of wing moult decreased in the temperate populations. Arctic‐breeding geese may gradually accumulate body stores from the food they encounter during spring migration, which allows them to breed relatively early and their young to use the peak of the Arctic food resources. By contrast, temperate‐breeding birds are not able to acquire adequate body stores from local resources early enough, that is before the quality of food for their young starts to decrease. When global temperatures continue to rise, Arctic‐breeding barnacle geese might encounter similar problems.

The biomass of the macrobenthic animals living in intertidal flats of the Wadden Sea varies annually and seasonally. However, the variation in prey biomass harvestable by wading birds such as knot Calidris canutus, which feed mainly on the middle range of their prey size classes, is even larger. The majority of first-year Cerastoderma edule, Mya arenaria or Mytilus edulis are too small to be profitable as prey for knot. Yet, by the end of the subsequent growing season, these same prey are too large to be ingested and/or live at a depth that puts them out of reach of the birds' bills. Macoma balthica is a major prey for knot, because (1) its annual spatfall is less erratic than in the other bivalve species, and (2) it grows more slowly, and each cohort is therefore available as food for knot for at least 3 yr. Knot feed in flocks which roam over the feeding area, but they are more often observed in food-rich than in food-poor areas, A similar relationship between bird and food densities was found in one locality, when the annual numbers of knot were compared with the yearly variation in food supply. Since the numbers of knot in the whole area were the same over many years, the birds were apparently able to find other feeding areas when the local food supply was low, i.e. the food supply harvestable by knot (prey not too small, not too large and not too deep) was less than about 4 g ash-free dry weight m-2. Knot arrive in the study area at the end of July and leave after only a few weeks en route to Africa. They depart before d serious decline in the harvestable prey biomass takes place, which results from a decrease in the body condition of individual prey and an increase in the fraction of the prey which burrows out of the reach of the knot's bill.