Does the Jarman–Bell principle at intra-specific level explain sexual segregation in polygynous ungulates? Sex differences in forage digestibility in Soay sheep

Fat accumulation by blackcaps (Sylvia atricapilla) is a prerequisite for successful migratory flight in the autumn and has recently been determined to be constrained by availability of drinking water. Birds staging in a fruit-rich Pistacia atlantica plantation that had access to water increased their body mass and fat reserves both faster and to a greater extent than birds deprived of water. We conducted a series of laboratory experiments on birds captured during the autumn migration period in which we tested the hypotheses that drinking water increases food use by easing limitations on the birds’ dietary choices and, consequently, feeding and food processing rates, and that the availability of drinking water leads to improved digestion and, therefore, to higher apparent metabolizable energy. Blackcaps were trapped in autumn in the Northern Negev Desert, Israel and transferred to individual cages in the laboratory. Birds were provided with P. atlantica fruit and mealworms, and had either free access to water (controls) or were water-deprived. In experiment 1, in which mealworm availability was restricted, water-deprived birds had a fourfold lower fruit and energy intake rates and, consequently, gained less fat and total mass than control birds. Water availability did not affect food metabolizability. In experiment 2, in which mealworms were provided ad libitum, water availability influenced the birds’ diet: water-restricted birds ate more mealworms, while control birds consumed mainly P. atlantica fruit. Further, in experiment 2, fat and mass gain did not differ between the two treatment groups. We conclude that water availability may have important consequences for fat accumulation in migrating birds while they fatten at stopover sites, especially when water-rich food is scarce. Restricted water availability may also impede the blackcap’s dietary shift from insectivory to frugivory, a shift probably necessary for successful pre-migratory fattening.     

Daily body mass regulation in dominance-structured coal tit (Parus ater) flocks in response to variable food access: a laboratory study

In a dominance-structured flock, social status may determinepriority of access to food. Birds of low social status mayperceive present and future access to food as less predictable,and so have a higher risk of starvation, than birds of highsocial rank. Theoretical models predict that subordinate birdsshould carry larger fat reserves and incur higher mass-dependentcosts than dominants. However, empirical tests of the assumptionsof these models are still scarce and controversial. We investigatedthe effect of dominance rank on daily mass gain under conditionsof fluctuating food availability in a laboratory experimentusing four flocks of four coal tits (Parus ater) each. Thesame amount of food was delivered in two treatments, but inone treatment the food was offered at a constant rate betweendays (fixed treatment), while in the other treatment the dailyfood supply varied in an unpredictable sequence between days(variable treatment). All birds showed greater variance inbody mass in the variable treatment than in the fixed treatment.Body mass within birds showed the same variability at dawn thanat dusk in the fixed treatment, but less variability at dawnthan at dusk in the variable treatment. This may be a mechanismto reduce the immediate risk of starvation at the beginningof the day, when fat reserves are at their lowest and the aggressionbetween flock members when feeding highest. Subordinate birdswere excluded from the feeders by dominants more often in theearly morning than in the rest of the day, and they showedmore variability in daily mass gain and body mass at dawn thandominant birds. These results support the hypothesis that subordinatebirds have a reduced probability of surviving when food availabilitychanges unexpectedly compared to dominants.     

The effects of cache loss on choice of cache sites in black-capped chickadees

Food-storing birds lose a great deal of their stored food toother animals. We examined whether blackcapped chickadees (Parusairicapillus) modify their choice of cache sites using informationthat predicts cache loss. In experiment 1, birds learned toavoid caching at spatial locations where cache loss had previouslyoccurred, but they did not avoid caching near local color cuesthat predicted cache loss. Birds did not modify their generaluse of space in the aviary. Birds also learned to reduce searchingfor caches where spatial location predicted cache loss. Experiment2 confirmed the birds’ ability to discriminate among thespatial locations and the local color cues used in experiment1. In experiment 3, learning a food-rewarded approach to potentialcache sites occurred without any change in the choice of sitesfor caching. We discuss how chickadees selectively associatethe choice of cache site with its consequences, even over delaysof several hours between caching and cache recovery.     

Can conditions experienced during migration limit the population levels of birds?

Populations of migratory birds are usually considered to be limited by conditions in breeding or wintering areas, but some might be limited by conditions encountered on migration. This could occur at stopover sites where competition for restricted food supplies can reduce subsequent survival or breeding success, or during the flights themselves, when adverse weather can occasionally kill large numbers of individuals. Competition for food could act in a density-dependent manner and help to regulate populations, whereas weather effects are more likely to act in a density-independent manner. The evidence for these views is explored in this paper. When preparing for migration, birds must normally obtain more food per day than usual, in order to accumulate the body reserves that fuel their flights. Birds often concentrate in large numbers at particular stopover sites, where food can become scarce, thus affecting migratory performance. Rates of weight gain, departure weights, and stopover durations often correlate with food supplies at stopover sites, sometimes influencing the subsequent survival and reproductive success of individuals, which can in turn affect subsequent breeding numbers. Many studies have provided evidence for interference and depletion competition at stopover sites, relatively few for migration conditions influencing the subsequent breeding or survival of individuals, and even fewer for effects on subsequent breeding numbers. Migrants in flight occasionally suffer substantial mortality in storms, especially over water, sometimes involving many thousands of birds at a time. Other mass mortalities have resulted from atypical ‘winter-like’ weather, occurring soon after the arrival of summer migrants in their breeding areas or just before their departure in autumn. Again, many thousands of birds at a time have been killed in such incidents, causing reductions of 30–90% in local breeding densities. In some bird species, migration-related events can at times have substantial effects on the year-to-year changes in breeding population levels. Nonetheless, the difficulties involved in investigating migrating birds at different points on their migration routes have so far limited the number of studies on the influence of events during migration periods on population levels.     

Stopover duration of Palearctic passerine migrants in the western Sahara – independent of fat stores?

Birds on migration spend much more time on stopover sites to refuel for the next migration step than aloft, but empirical data on stopover duration are rare, especially for Palearctic trans-Sahara migrants whilst crossing the desert. Previous studies suggest that stopover duration of fat birds in oases is much shorter than that of lean birds. During 2003 and 2004 capture–recapture data of migrating passerines from two inland oases in spring and from one coastal site in autumn in Mauritania, West Africa, were analysed to test whether the probability of being a transient and the stopover duration depend on fuel stores at first capture. The application of capture–recapture models revealed that during autumn migration at the coast the proportion of transients (individuals that stop over only for 1 day) was relatively high (77–90%) in three out of four species investigated and stopover duration was short (1.9–4.6 days). In the inland oases in spring, transients were detected in only four out of 12 analyses. Stopover duration was longer than at the coast in autumn and surprisingly long in some species with durations of up to 30 days. Models taking into account the initial fat load of birds on the first capture occasion were, with one exception, never the most parsimonious ones. This indicates that the time spent after and before capture at the stopover site did not depend on the fat stores at first capture. Therefore, we cannot confirm the assumption that birds arriving at stopover sites in the desert with low fat loads stay longer than birds that arrive with high fat loads.     

Management of fat reserves and food caches in tufted titmice (Parus bicolor) in relation to unpredictable food supply

In the temperate zone, permanent-resident birds and mammalsthat do not hibernate must survive harsh winter conditions oflow ambient temperature, long nights, and reduced food levels.To understand the energy management strategy of food-hoardingbirds, it has been hypothesized that such birds respond to increasedstarvation risk by increasing the number of their hoards ratherthan by increasing their fat reserves and that they cache earlyin the day and retrieve their caches later to achieve fat reservesnecessary to survive the night We tested these hypotheses byobserving the responses in captivity of a caching bird, thetufted titmouse (Parus bicolor), to the combined influencesof reduced predictability of food and naturally occurring ambienttemperature and photoperiod. When the food supply was unpredictable,birds significantly increased both internal fat reserves atdusk and external food caches. Initially leaner birds tendedto increase their fat reserves to a greater extent and initiallyfatter birds tended to cache more food and to fly significantlyless. Half the birds also increased their dawn and mean dailybody mass. All birds tended to forage, gain body mass, and cachefood at significantly lower rates in the morning and at significantlyhigher rates in the evening. Cache retrieval showed the oppositetrend, with birds retrieving most of their caches in the morning.Our results do not support the hypothesis that caching birdsincrease caching rate but not body mass under an unpredictablefood regime. Instead fat reserves and food caches are both importantcomplementary sources of energy in food-hoarding birds. Energymanagement by wintering birds occurs in response to a numberof biotic and abiotic factors acting simultaneously; thus futuremodels must incorporate independent variables in addition tothe state of the food supply and time of day     

Effects of predictable and unpredictable food restriction on the stress response in molting and non-molting European starlings (Sturnus vulgaris)

This study tested whether an ethologically relevant stressor, a three-week period of food restriction where food was unavailable for four hours a day, caused chronic stress in molting and non-molting captive European starlings. Although all birds increased weight during the Food Restriction period, only non-molting birds increased food intake. Morning baseline heart rates increased during the Food Restriction period and all birds showed a decrease in heart rate when food was absent from the cage. In non-molting birds, there were no differences in either baseline or stress-induced corticosterone (CORT) concentrations, whereas molting birds showed attenuated baseline CORT, stress-induced CORT, and fecal glucocorticoid metabolite levels over the Food Restriction period. Although several parameters, such as increased morning heart rate, are consistent with chronic stress, the majority of these data suggest that restricting food availability is not chronically stressful. Furthermore, making the timing of food removal less predictable by randomizing when food was removed during the day did not enhance any of the above responses, but did alter the frequency of maintenance and feeding behaviors. In conclusion, starlings appear resistant to developing symptoms of chronic stress from repeated food restriction.     

Phenotypic flexibility in digestive system structure and function in migratory birds and its ecological significance

Birds during migration must satisfy the high energy and nutrient demands associated with repeated, intensive flight while often experiencing unpredictable variation in food supply and food quality. Solutions to such different challenges may often be physiologically incompatible. For example, increased food intake and gut size are primarily responsible for satisfying the high energy and nutrient demands associated with migration in birds. However, short-term fasting or food restriction during flight may cause partial atrophy of the gut that may limit utilization of ingested food energy and nutrients. We review the evidence available on the effects of long- and short-term changes in food quality and quantity on digestive performance in migratory birds, and the importance of digestive constraints in limiting the tempo of migration in birds. Another important physiological consequence of feeding in birds is the effect of diet on body composition dynamics during migration. Recent evidence suggests that birds utilize and replenish both protein and fat reserves during migration, and diet quality influences the rate of replenishment of both these reserves. We conclude that diet and phenotypic flexibility in both body composition and the digestive system of migratory birds are important in allowing birds to successfully overcome the often-conflicting physiological challenges of migration.     

Food consumption and retention time in captive whooping cranes (Grus americana)

Food consumption, digesta retention time, and food preference were measured for captive whooping cranes fed pelleted diets. The basal commercial diet was compared to four mixtures containing 70% basal and 30% of one of four important winter foods for the whooping crane: blue crab (Callinectes sapidus), wolfberry fruit (Lycium carolinianum), live oak acorn (Quercus virginiana), or common Rangia clam (Rangia cuneata). Because captive birds would not eat whole foods, we were prevented from direct food preference tests. Food passed through the gut rapidly, with almost complete elimination within 7 hr. There was some indication that retention time was shorter for the low fiber and high ash and calcium clam diet. Cranes ate less wolfberry feed (g/day) than the other feeds, and all birds ate less wolfberry feed on the day it was first fed, compared to basal diet the previous day. Birds ate more low energy feed than high energy feed. Due to combined effects of low energy content, lower metabolizable energy coefficients, and reduced feed consumption, less energy was assimilated for study diets than basal diet. Apparent shorter retention times for some diets containing whooping crane foods may partly explain lower digestibilities and metabolizable energy of winter whooping crane foods compared to commercial crane diet. Zoo Biol 16:519–531, 1997. © 1997 Wiley-Liss, Inc.     

Strategic Regulation of Body Mass and Singing Behavior in New Zealand Robins

The ‘small bird in winter’ paradigm states that body mass is a balance between the conflicting demands of carrying enough energy to survive nightly fasts while minimizing the risk of predation associated with carrying additional fat reserves. We conducted a short‐term food‐supplementation experiment during which New Zealand robins (Petroica australis) were provided with food on the second day of a 3‐d trial. This allowed us to test two predictions from models of strategic mass regulation in small birds: (1) individual birds reach the same end‐of‐day mass despite differences in their initial morning mass while, (2) using surplus energy for increased singing. As expected, robins gained mass at a higher rate early in the morning on the fed day than they did on either of the two control days, but there was no significant difference in their evening masses across the 3 d of the experiment despite birds on day 3 starting at higher initial masses than birds on day 1. Robins displayed a significantly higher rate of singing when receiving food supplements on day 2, supporting a link between energetic reserves and behavior. Our results suggest that potentially energetically costly behaviors, such as song production, are sensitive to short‐term changes in energy reserves, and that both state and behavioral predictions can be successfully integrated to provide tests of state‐based models of behavior.     

长距离迁徙鸟类对应于能量积累状态的取食行为调整

迁徙鸟类能够预计到迁徙过程中对能量需求的增加和迁徙途中获得能量的不确定性。最佳迁徙理论指出：迁徙停留期的一系列决策受到体内能量状态、取食机会和迁徙时间的影响。利用刚完成春季跨越墨西哥湾迁徙的鸫类,我们研究了取食行为、体内能量状态和能量积累速度的相互关系。我们用雾网捕获了停留的鸫,然后给每只鸟进行了环志和称重,并估测了表皮下积累的脂肪。为了检测体内能量积累大小对取食行为的影响,我们把在野外观察到的鸟分成肥、瘦两组。当一天里被捕获鸟的平均体重低于相关种的瘦体重时,这一天被观察到的取食鸟被归到瘦组,反之就属于肥组。我们同时对一部分鸟用有色环进行了标记,以便能在野外观察到它们时能准确地知道每一只鸟的初始能量积累状态。在春天完成跨越墨西哥湾迁徙以后的鸫大约有50％在到达停留地时就已经消耗了所有的表皮下脂肪。与肥组鸟相比,瘦组的个体在停留期扩展了取食方法和取食基底,增加了取食速度。与此相关的是,瘦鸟表现出体重积累更多、速度更快。我们的数据表明迁徙鸟在到达迁徙中途停留地时的能量状态会影响到它们停留期间的取食行为和继续迁徙时的生理机能,从而影响停留期的长短和在迁徙途中停留与否的决定[动物学报51(1)：12—23,2005]。     

Interaction of bodymass,fat, foraging and stopover period in trans-Sahara migrating passerine birds

Summary The effect of body mass and fat reserves on the choice of the stopover place and on the stopover period was investigated in 3 species of passerine birds migrating through the Sahara in Egypt. Birds grounded in an oasis with food and water were more than 10% lighter than those from a desert stopover place which offered shade only. Stopover period was in general one day except for the light portion of oasis birds which stopped for up to three weeks during which they regained fat. A calculation of the maximum range which can be covered with the available fat depots revealed that nearly all birds from the desert stopover place should be able to cross the desert whereas about 60% of the oasis birds should not. An intermittant migratory strategy for the desert crossing is proposed with flight during the night and stopover during the day, even without drinking and feeding.     

Intermittent feeding in a migratory omnivore: digestion and body composition of American black duck during autumn

Birds fast intermittently during weather disturbances and migration. We tested responses of black duck to lost feeding days during autumn mass gain. Nine adult males were fed a pelleted diet (1.5% fat, 15.8% protein, and 18.3% neutral detergent fiber) and caged indoors during September and October (12 h light; 17 degrees -24 degrees C) to measure balances over 14 d when fed ad lib. each day and fasted intermittently for 2 d wk(-1) (short fast) or 4 d wk(-1) (long fast). Body mass (1,081 g), body water content, and metabolizable intakes of energy and protein were maintained as daily intakes of dry matter increased to 1.65 (short fast) and 2.35 (long fast) times the unfasted level. Intermittent feeding reduced metabolizability of dry matter, energy, protein, and acid detergent fiber. Concentrations of Mn provided similar estimates of metabolizability to direct measures in unfasted birds but underestimated measures of birds on long fasts. Fasting regimes continued outdoors for 9 wk when temperatures declined to -9 degrees C. Birds on short fasts were heavier (1,373 vs. 1,241 g) and fatter (159 vs. 58 g) than those on long fasts, while body water (894 g) and protein (316 g) were similar between groups after 5 wk. Birds on long fasts subsequently gained mass when fed daily, but those on short fasts lost mass when fed each day. Omnivorous waterfowl combine ingestive and digestive flexibility with plasticity of body lipid to contend with uncertain food availability.     

Dominance, competition, and energetic reserves in the European starling, Sturnus vulgaris

We investigated the relationships between social dominance,competition for food, and strategies of body mass and fat regulationin the European starling (Sturnus vulgaris). In birds housedin groups of three, subdominant birds stored more fat than dominants.A removal experiment established a causal link between socialdominance and fat reserves; in groups that had the dominantindividual removed, the remaining birds reduced body mass andfat, relative to control groups that had the subordinate removed.In a second experiment, we investigated the influences of degreeof competition for food and dominance on body mass and fat reserves.Birds under high competition increased fat reserves and tendedto have higher body mass than birds under low competition. Theincrease in fat reserves was higher in the subdominants thanin the dominants. These results are consistent with hypothesesconcerning dominance-dependent access to food; subdominant birds,or birds under increased competition, may store more fat asan insurance against periods when food cannot be obtained. However,relations between dominance, body mass, and fat reserves mayalso arise through other proximate factors relating to dominance-dependentcosts and benefits of fat storage, such as predation risk andenergetic expenditure.     

Test of a digestion optimization model: effect of variable-reward feeding schedules on digestive performance of a migratory bird

Birds on migration often alternate between feeding and nonfeeding periods, in part because food resources may be patchily distributed and in part because birds on migration may adopt a risk-prone foraging strategy characterized by selection of variable rather than constant food rewards. Optimal digestion models predict that increases in intermeal interval like those encountered by some migratory birds should result in longer retention time of digesta and higher digestive efficiency if birds are maximizing their rate of energy intake. We tested these predictions by comparing residence time of digesta and extraction efficiency of lipid for captive yellow-rumped warblers (Dendroica coronata) feeding adlibitum and when we added intervals of time when the birds received no food. We increased the likelihood that the warblers were maximizing their rate of energy intake by increasing light levels during spring to induce hyperphagia (treatment birds (16L:8D light: dark cycle) ate 2.13 ± 0.14 g dry food day⁻¹ (n = 8) while control birds (10L:14D) ate 1.25 ± 0.03 g dry food day⁻¹ (n = 6)). Treatment birds offered food only every other 2–3 h ate 50% more during the 4-h test period than when they were always feeding adlibitum. Despite these differences in food intake, extraction efficiency of glycerol trioleate remained high and constant (93%), and mouth-to-anus total mean retention time (TMRT) did not change (overall mean: 54.8 ± 6.0 min). Residence time of lipid in the stomach increased whereas residence time of lipid in the intestine decreased when birds fed only every other 2–3 h compared to when birds always fed ad libitum. None of the results were consistent with the predictions of the optimal digestion model unless we assume that birds were minimizing their feeding time rather than maximizing their rate of energy gain. Furthermore, the ability of yellow-rumped warblers to maintain high extraction efficiency with no change in TMRT suggests some spare digestive capacity when food intake increases by as much as 50%. Received: 14 June 1997 / Accepted: 20 November 1997     

Night conditions affect morning incubation behaviour differently across a latitudinal gradient

Intermittently incubating birds alternate between sessions of egg warming and recesses for foraging during the day, but stay on the nest continuously at night. Hence, energy costs of nocturnal incubation (which increase during longer and colder nights) cannot be replenished until the next day. Night conditions might therefore be expected to affect morning incubation behaviour the day after. We tested this prediction by exploring latitudinal and seasonal trends in characteristics of the first recess in Eastern Bluebirds Sialia sialis over a 1400-km latitudinal gradient in the continental USA. The time from civil dawn to leaving the nest (latency) increased with latitude early in the breeding season but decreased with latitude late in the season. Birds breeding at higher latitudes also took longer first recesses throughout the season, which led to a larger drop in nest temperature. At the local scale, birds rose earlier after longer nights if the night was also cold, but night length did not predict latency following warm nights. The first recess was longer if the night was warmer, probably because birds could replenish reserves at lower risk of low egg temperature. Our study shows that characteristics of the night led to behavioural changes in features of early morning incubation that were evident at both continental and local scales. These responses also affected nest temperature. Hence, night conditions carry over to incubation behaviour the following morning, which in turn may impose thermal constraints on embryonic development.     

Morning flight behavior of nocturnally migrating birds along the western basin of Lake Erie

Many species of birds that normally migrate during the night have been observed engaging in so‐called morning flights during the early morning. The results of previous studies have supported the hypothesis that one function of morning flights is to compensate for wind drift that birds experienced during the night. Our objective was to further explore this hypothesis in a unique geographic context. We determined the orientation of morning flights along the southern shore of Lake Erie's western basin during the spring migrations of 2016 and 2017. This orientation was then compared to the observed orientation of nocturnal migration. Additionally, the orientation of the birds engaged in morning flights following nights with drifting winds was compared to that of birds following nights with non‐drifting winds. The morning flights of most birds at our observation site were oriented to the west‐northwest, following the southern coast of Lake Erie. Given that nocturnal migration was oriented generally east of north, the orientation of morning flight necessarily reflected compensation for accumulated, seasonal wind drift resulting from prevailingly westerly winds. However, the orientation of morning flights was similar following nights with drifting and non‐drifting winds, suggesting that birds on any given morning were not necessarily re‐orienting as an immediate response to drift that occurred the previous night. Given the topographical characteristics of our observation area, the west‐northwest movement of birds in our study is likely best explained as a more complex interaction that could include some combination of compensation for wind drift, a search for suitable stopover habitat, flying in a direction that minimizes any loss in progressing northward toward the migratory goal, and avoidance of a lake crossing.     

Field studies of avian nocturnal migratory orientation II. Experimental manipulation of orientation in white-throated sparrows (Zonotrichia albicollis) released aloft

Migrant white-throated sparrows (Zonotrichia albicollis) were released from boxes carried aloft by balloon and tracked with radar. All birds were released on clear nights when winds were light and opposed to the normal migration direction for the season. Birds were treated in one of two ways: Lens birds were fitted with frosted lenses prior to release; No Lens birds were released without lenses. No Lens birds that engaged in straight and level flight generally headed in the predicted migratory direction and as a group were not oriented with respect to wind direction. Lens birds did not head in the predicted migratory direction, but instead oriented downwind. This orientation behaviour is consistent with the relationship of orientation cues inferred from the field observations described in part I of this paper. The data show that flying birds deprived of all detailed form vision can determine wind direction.     

Optimal fat loads and long-distance flights by migrating Knots Calidris canutus, Sanderlings C. alba and Turnstones Arenaria interpres

Arctic waders often build up large fat loads and complete their migratory journeys by a few long-distance flights between traditional staging sites. Optimal fat loads and choices of staging sites differ depending on whether the birds are adapted to minimize energy or time spent on migration. In the latter case, we predict that the birds will depart for the next staging site when the instantaneous speed of migration expected after arrival at the next site, exceeds the corresponding speed at the departure site. The instantaneous migration speed is a function of the rate of fat deposition and the current fat load. As a consequence of this, overloading (birds deposit larger fat loads than needed merely for covering the flight distance to the next destination) and by-passing of possible, but low-quality staging sites, are expected under specific conditions in time-selected migration.
Estimates of fat deposition rates and departure fat loads were obtained by captures of Knots Calidris canutus , Sanderlings C. alba and Turnstones Arenaria interpres in W. Iceland during spring migration. Further fat deposition data referring to spring migration of these species were compiled from the literature. Fat deposition rates at different sites, as measured by the daily gain in mass relative to lean body-mass, range between 1.0 and 3.6%/day, and departure fuel loads (in % of lean body-mass) between 27 and 73%.
Comparison with flight range estimates suggests that overloading may be a regular phenomenon during spring migration of Knots, Sanderlings and Turnstones. Furthermore, fat deposition rates at different staging sites, and the general difference in migration patterns between spring and autumn, indicate that by-passing of possible staging sites may well occur. Hence, it cannot be excluded that the waders' migratory habits primarily serve to maximize the overall speed of migration.     

Regulation of cache stores and body mass in Carolina chickadees (Panes carolinensis)

Here I report the results of laboratory experiments on cachingbehavior of Carolina chickadees (Parus carolinensis) designedto test the following predictions from a recent dynamic optimizationmodel: under limited resources, small birds, when fat, shouldcache food instead of eating it, but they should eat insteadof caching when lean; when resources are abundant, birds shouldcache less when fat and more when lean. In addition, when resourcesare abundant the amount of time spent in foraging-related activityshould decrease with an increase in body mass. Chickadees weretested for 2 weeks in either a poor-quality environment (wherethey were given four 5-min periods per day of access to a feederfilled with sunflower seeds) or a richer environment (wherethey were given four 10-min access periods); they were thenswitched to the alternative environment for an additional 2weeks. The entire experiment lasted from October through June.Within-individual comparisons showed that birds in both thepoor and rich environments exhibited the predicted correlationsbetween the probability that a seed was cached and body massmeasured at dawn. The number of seeds already stored had a weakand variable effect on caching decisions. Time budgets changedas predicted; birds spent less time with food-related behaviorswhen their energy stores were high compared to when they werelow, and overall spent a higher fraction of their time foragingin the poor-quality environment compared to the rich environment.Two other variables also affected caching behavior: experienceand season. Individuals were more likely to cache in the poorenvironment when they had been tested in the rich environmentthe previous 2 weeks, as compared with birds started in thepoor environment. However, this was true only for tests donein the winter. In late spring, all birds stopped caching orreduced caching rates when faced with limited food availability,irrespective of previous experience. The birds harvested seedsfaster when they cached instead of eating seeds; as a result,long-term weight regulation in the poor environment was affectedby caching decisions. Birds tested in late spring lost weightin the poor environment, whereas those tested in the wintermaintained a stable weight when switched from the rich to thepoor environment. Thus, annual endogenous cues directly affectcaching decisions and indirectly influence the long-term regulationof body mass