VARIATION IN BLUE TIT BREEDING PERFORMANCE ACROSS GRADIENTS IN HABITAT RICHNESS

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.

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.

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.

Carbon, nitrogen, and phosphorus (C, N, P) stoichiometry influences the growth of plants and nutrient cycling within ecosystems. Indeed, elemental ratios are used as an index for functional differences between plants and their responses to natural or anthropogenic variations in nutrient supply. We investigated the variation in growth and elemental content of the rootless terrestrial bromeliad Tillandsia landbeckii, which obtains its moisture, and likely its nutrients, from coastal fogs in the Atacama Desert. We assessed (1) how fog nutrient supply influences plant growth and stoichiometry and (2) the response of plant growth and stoichiometry to variations in nutrient supply by using reciprocal transplants. We hypothesized that T. landbeckii should exhibit physiological and biochemical plastic responses commensurate with nutrient supply from atmospheric deposition. In the case of the Atacama Desert, nutrient supply from fog is variable over space and time, which suggests a relatively high variation in the growth and elemental content of atmospheric bromeliads. We found that the nutrient content of T. landbeckii showed high spatio-temporal variability, driven partially by fog nutrient deposition but also by plant growth rates. Reciprocal transplant experiments showed that transplanted individuals converged to similar nutrient content, growth rates, and leaf production of resident plants at each site, reflecting local nutrient availability. Although plant nutrient content did not exactly match the relative supply of N and P, our results suggest that atmospheric nutrient supply is a dominant driver of plant growth and stoichiometry. In fact, our results indicate that N uptake by T. landbeckii plants depends more on N supplied by fog, whereas P uptake is mainly regulated by within-plant nutrient demand for growth. Overall, these findings indicate that variation in fog nutrient supply exerts a strong control over growth and nutrient dynamics of atmospheric plants, which are ubiquitous across fog-dominated ecosystems.

Hydrodynamic Control of the Supply of Suspended Chlorophyllato Infaunal Estuarine Bivalves

The finding of a previously undescribed cold-water coral reef (Banana Reef) in the Scottish Mingulay reef complex, with denser coverage of living Lophelia pertusa than the principal Mingulay 1 Reef, was the incentive for a comparative study of the food supply to the 2 reefs. Suspended particulate matter (SPM) samples from the surface and bottom water covering a tidal cycle were compared with respect to lipids, pigments, and delta C-13 and delta N-15. Lipid profiles and stable isotope signatures of SPM were compared with those of coral tissue samples. Concurrently, hydrographic measurements were conducted to track the movement of the water masses across both reefs. Between-reef differences in SPM lipid concentrations were small compared to those in coral tissue. Corals at Banana Reef had lower lipid concentrations, pointing to less favourable food conditions than at Mingulay 1. Stable isotopes signatures and lipid profiles showed that corals on both reefs feed primarily on surface algal matter, within the timeframe of our study. At Mingulay 1, fresh microalgae are supplied to the coral reef by local downwelling. This downwelling pulse is tidally advected to Banana Reef. Food conditions observed during this study at both reefs do not explain the between-reef difference in coral coverage. A speculative explanation for the denser coral coverage at the deeper Banana Reef encompasses the slightly lower temperature that exhibits lower metabolic stress on corals, in combination with a higher current speed and particle encounter rate.

We studied recruitment age (i.e. the age that a gull bred for the first time) and the probability that an adult reproduced during its life in three cohorts of Western Gulls Larus occidentalis breeding on southeast Farallon Island. We examined the effects of sex, hatching date, hatching order, brood size and annual variation in food supply during the recruitment period. Modal recruitment ages of males and females were 4 and 5 years, respectively, a difference that was significant. None of the factors examined, including hatching date, hatching order and brood size, had a significant effect on recruitment age. The probability of breeding in males who experienced low food supply in their fourth and fifth years was lower than that of males who experienced favourable food supply in their fourth and fifth years. Recruitment probability (the proportion of the sample group that recruited among those available to recruit during a given year) of 4‐ and 5‐year‐old males was lower in the food‐poor 1983 El Nino than in years of favourable food supply. The only variable affecting recruitment probability of females was age: recruitment probability was greater among 5‐year‐olds than 4‐year‐olds. Recruitment age in males was more sensitive to annual variation in food supply, probably because females have more difficulty recruiting due to an excess of adult females in the population and because of the different roles in reproduction in which males are primarily responsible for provisioning chicks and maintaining the territory.

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.

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

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.

Colonies of Common Puffins (Fratercula arctica), differing in numbers of breeding Herring Gulls (Larus argentatus) present, also differed in puffin numbers, burrow placement, and burrow activity rate. All differences indicated that puffins avoided gulls. Sites differing in gull numbers did not differ, however, in puffin fledging success or weights of fledged chicks. Chick weights and fledging success were low. The pattern of weight gain by chicks was irregular over the season, although consistent among sites for individual periods. These results imply that food usually was hard to find, although occasionally locally abundant. Pressures on puffin reproductive efforts due to variation in food supply and levels of kleptoparasitism do not appear to be additive; rather, low food availability decreases the effect of gull kleptoparasitism as well.

Among the range of determinants of post‐fledging survival in altricial birds, the energy supply to the growing juveniles is likely to play a central role. However, the exact mechanisms shaping post‐fledging survival are poorly understood. Using a food supplementation experiment, we determined the effect of variation in food supply on the survival of juvenile Little Owls Athene noctua from hatching to 2 months post‐fledging. Experimental broods were food‐supplemented for 36 days during the nestling and the early post‐fledging period. The fate of 307 juveniles (95 of them provided with extra food) was determined by nest monitoring and radiotelemetry. In unsupplemented birds, the rates of survival measured at 5‐day intervals were lowest during the nestling stage, remained low during the early post‐fledging stage and steadily increased after about 2 weeks post‐fledging. Food supplementation substantially increased nestling survival, but we detected no direct treatment effect on post‐fledging survival. Instead, we found a strong indirect effect of food supplementation, in that fledglings of good physical condition had markedly higher chances of surviving the post‐fledging period compared with those in poor condition. Experimental food supplementation increased survival over the first 3 months from 45% to 64.6%. This suggests that energy reserves built up during the nestling stage influence post‐fledging survival and ultimately parental reproductive output. The low nestling and post‐fledging survival shows that the early life‐history stages constitute a crucial bottleneck of reproductive ecology in Little Owls. The strong treatment effects on the number of independent offspring indicate that natural variation in food supply is an important determinant of spatio‐temporal patterns in Little Owl demography.

Managing variations in dairy cow nutrient supply under grazing

We investigate whether a mosaic of habitats of different quality functions as a source-sink system for the Blue Tit Parus caeruleus L. Breeding parameters, especially laying date, clutch size and breeding success have been studied in relation to the food supply in three habitats: two habitats, one rich and one poor, next to each other on the mainland (southern France) and one poor habitat on the island of Corsica. Food resources are more abundant and are available earlier in the season in the rich habitat than in both the mainland and the island poor habitats. The timing of breeding is nicely timed on the food peak of abundance in the rich mainland habitat and in the poor insular one but tits are mistimed in the poor mainland habitat because they start to breed too early in relation to food availability. Such patterns strongly suggest that the rich mainland habitat where birds produce many fledglings functions as a source from which birds emigrate in the poor habitat which functions as a sink. These birds which are genetically programmed to breed in the source habitat become mistimed in the sink. Tits on Corsica which are isolated from any mainland population have adjusted their breeding traits on the local patterns of food availability and abundance. This hypothesis is supported by the existence of a strong genetic component of laying date which has been experimentally proved.