Food supply controls the body condition of a migrant bird wintering in the tropics

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 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.

The reproductive patterns (birth seasonality, litter size, litters per year) of two sympatric species of galago (Galago zanzibaricus and G. garnettii) were studied in a coastal forest in Kenya for a two-year period. Trap-retrap and radio tracking methods were employed. G. zanzibaricus has one infant twice per year; G. garnettii has one infant once per year. Both species are seasonal breeders. These East African galagos are intermediate in reproductive patterns when compared with galagos from South African woodland (G. senegalensis moholi and G. crassicaudatus umbrosis) and West African rainforest (G. alleni and G. demidovii). Climatic patterns (total annual rainfall, seasonal variability of rainfall, variability in total annual rainfall, and annual temperature variability) are also compared for the three regions. Climatically, East Africa is intermediate between West and South Africa in total annual rainfall and in seasonality of rainfall, but not in year-to-year variability in rainfall. East Africa shows the highest variability in annual rainfall. South Africa has the coldest dry seasons and highest variability in temperatures. The results of this study suggest that "r-selection" and "K-selection" do not provide adequate explanations of galago reproductive patterns.

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.

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.

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.

Conservation tillage and sustainable intensification of agriculture: regional vs. global benefit analysis

Understanding rainfall characteristic is very important for aviation operational planning and for other activities. This study investigated annual, seasonal and monthly rainfall climatology and variability in Ethiopian Active airports from 1991-1994 using time series Climate Hazards Group Infrared Precipitation with Stations (CHIRPS) rainfall datasets. Long year mean, standard deviation and coefficient of variations were used to analyze Annual, Monthly and seasonal rainfall climatology and variability spatially and on active airports as a point. Western and southwestern airports such as Jimma, Asosa, Bahir Dar, and Gambella exhibit the highest annual mean rainfall, consistently exceeding 1,200mm, while eastern and northeastern Airports like Gode and Semera, experience very low annual totals (often below 300mm. For instance, Jimma airport records monthly averages exceeding 200mm in peak months, while Airports like Axum, Humera, and Gode receive less than 1mm in dry months. The Kiremt season contributes the bulk of annual rainfall in most parts of the country, for instance Nekemte Airport (1324.5mm), Bahir Dar airport (1147.4mm), and Jimma airport (881.0mm) leading as high climatological mean rainfall. Belg (February-May) and Bega (October-January) are secondary rainfall seasons and dry seasons respectively but are significant in southern and southwestern parts of the country such as Jinka and Arba Minch. More than 80% of airports fall under low annual rainfall variability (CV < 20%), indicating high reliability for long-term aviation planning. Only Gode airport shows high annual rainfall variability (CV = 41%), indicating operational unpredictability and risk exposure. CV values during dry months often exceed 200%, particularly in arid and semi-arid regions, indicating unreliable rainfall patterns. In contrast, wet months (July-August) in western airports exhibit low CVs (e.g., Bahir Dar: 11.8% in August), supporting predictable seasonal operations. Generally climatological and variability insights highlight the spatial and temporal heterogeneity of rainfall across Ethiopian airports, driven by topography, regional climate systems, and global drivers like ENSO. Understanding both average conditions (climatology) and year-to-year fluctuations (variability) is critical for aviation safety, infrastructure resilience, and operational efficiency.

The Darwin's finches on Isla Daphne Major, Galapagos, were studied between July 1975 and June 1978. Geospiza fortis and G. Scandens are residents, while G. fuliginosa and G. magnirostris are regular immigrants. The Daphne climate is unpredictably dry. The island has a simple plant community displaying marked annual and spatial variation in the foods which form finch diets. Breeding is stimulated by rain falling irregularly between January and April; G. scandens laid eggs with as little as 16 mm of rain, but G. fortis required 35 mm or more before laying. G. scandens consistently bred prior to the rains, associated with specialized exploitation of dry season Opuntia cactus flowers. The breeding system of both species was similar to that of other Geospiza species: monogamous matings on small, permanent, all—purpose territories. Reproductive output of both species varied. In 1976 single broods were produced by both species at high densities, with modes of three young. In 1977, only 24 mm of rain fell during the breeding season and G. scandens alone bred, with poor success. By 1978, G. scandens populations had declined by 66% and G. fortis by 85%. In 1978, both species laid an average of three clutches per pair, with a mode of four young per brood. During the 1977 drought, the sex ratio became skewed in favor of males in both species, and as a consequence some females bred successively with up to three different males in 1978. The skewed sex ratio retarded population recovery following the drought. The population decline was associated with a decline in seed abundance in the drought. As food disappeared, G. fortis diets widened to include a broad selection of food items, while G. scandens diets contracted to Opuntia seeds. Even in normal years, both species showed pronounced seasonal variation in feeding habits. Both fed heavily on Opuntia flowers in the late dry season, followed by a mixed diet of insects, fresh seed, and other green matter during the breeding season. After breeding, the birds fed primarily on seeds, the two species selecting different proportions of the available range of seed sizes. Overall, G. fortis displayed the feeding, breeding, and population dynamics characteristic of an ecologically generalized species, whereas G. scandens is a highly specialized species. Large variation in food supply, caused by large variation in rainfall, is responsible for the presence of only two species with breeding populations, for large fluctuations in their population sizes, and for their large clutch sizes and opportunistic breeding. Long—term studies are essential for an understanding of communities in such variable environments because some important events are rare.

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.

The adult sex ratio (ASR) is an important property of populations. Comparative phylogenetic analyses have shown that unequal sex ratios are associated with the frequency of changing mates, extrapair mating (EPM), mating system and parental care, sex-specific survival, and population dynamics. Comparative demographic analyses are needed to validate the inferences, and to identify the causes and consequences of sex ratio inequalities in changing environments. We tested expected consequences of biased sex ratios in two species of Darwin's finches in the Galápagos, where annual variation in rainfall, food supply, and survival is pronounced. Environmental perturbations cause sex ratios to become strongly male-biased, and when this happens, females have increased opportunities to choose high-quality males. The choice of a mate is influenced by early experience of parental morphology (sexual imprinting), and since morphological traits are highly heritable, mate choice is expressed as a positive correlation between mates. The expected assortative mating was demonstrated when the Geospiza scandens population was strongly male-biased, and not present in the contemporary Geospiza fortis population with an equal sex ratio. Initial effects of parental imprinting were subsequently overridden by other factors when females changed mates, some repeatedly. Females of both species were more frequently polyandrous in male-biased populations, and fledged more offspring by changing mates. The ASR ratio indirectly affected the frequency of EPM (and hybridization), but this did not lead to social mate choice. The study provides a strong demonstration of how mating patterns change when environmental fluctuations lead to altered sex ratios through differential mortality.

Simple SummaryThe present study examines how the variations in food supply and winter weather are reflected in the body condition of female and male Tawny Owls Strix aluco of different colour morphs in a population near the southern coast of Finland. Winter weather conditions before breeding seemed to have effects on the food availability of Tawny Owls: the depth of the snow cover showed a positive relationship, and the frequency of temperature fluctuations around the freezing point had a negative relationship. In females, intrinsic factors such as colour morph and age, as well as the body condition of the mate and the stage of the season, governed body condition. In males, only age and the stage of the season suggested associations with body condition. Probably due to the efficient use of alternative prey, the effects of fluctuations of vole populations on the body condition of Tawny Owls are only moderate.The body condition of boreal species of vole-eaters seems to vary largely according to fluctuations in vole populations and weather conditions of the preceding winter. I studied females and males of the Tawny Owl Strix aluco of temperate origin near the northern limit of the species’ range in southern Finland to reveal if they show similar patterns to the boreal species. Winter weather conditions before breeding seemed to have pronounced effects on the food availability of Tawny Owls. In females, intrinsic factors such as colour morph and age, as well as the body condition of the mate and the stage of the season (Julian date), governed body condition. In males, only age and Julian date showed pronounced relationships with body condition. The results suggest that deep snow cover protects vole populations through winter until spring better than a minor amount of snow and that frequent temperature fluctuations around the freezing point in early spring make voles more available for owls that are preparing for breeding. This was also reflected positively in the body condition of female owls. Probably due to the efficient use of alternative prey, the effects of fluctuating vole populations on the body condition of Tawny Owls are, in general, only moderate.

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.