The Energetic Value of Land-Based Foods in Western Hudson Bay and Their Potential to Alleviate Energy Deficits of Starving Adult Male Polar Bears

Climate change is predicted to expand the ice-free season in western Hudson Bay and when it grows to 180 days, 28–48% of adult male polar bears are projected to starve unless nutritional deficits can be offset by foods consumed on land. We updated a dynamic energy budget model developed by Molnar et al. to allow influx of additional energy from novel terrestrial foods (lesser snow geese, eggs, caribou) that polar bears currently consume as part of a mixed diet while on land. We calculated the units of each prey, alone and in combination, needed to alleviate these lethal energy deficits under conditions of resting or limited movement (2 km d^-1) prior to starvation. We further considered the total energy available from each sex and age class of each animal prey over the period they would overlap land-bound polar bears and calculated the maximum number of starving adult males that could be sustained on each food during the ice-free season. Our results suggest that the net energy from land-based food, after subtracting costs of limited movement to obtain it, could eliminate all projected nutritional deficits of starving adult male polar bears and likely other demographic groups as well. The hunting tactics employed, success rates as well as behavior and abundance of each prey will determine the realized energetic values for individual polar bears. Although climate change may cause a phenological mismatch between polar bears and their historical ice-based prey, it may simultaneously yield a new match with certain land-based foods. If polar bears can transition their foraging behavior to effectively exploit these resources, predictions for starvation-related mortality may be overestimated for western Hudson Bay. We also discuss potential complications with stable-carbon isotope studies to evaluate utilization of land-based foods by polar bears including metabolic effects of capture-related stress and consuming a mixed diet.     

What to eat now? Shifts in polar bear diet during the ice-free season in western Hudson Bay

Under current climate trends, spring ice breakup in Hudson Bay is advancing rapidly, leaving polar bears (Ursus maritimus) less time to hunt seals during the spring when they accumulate the majority of their annual fat reserves. For this reason, foods that polar bears consume during the ice‐free season may become increasingly important in alleviating nutritional stress from lost seal hunting opportunities. Defining how the terrestrial diet might have changed since the onset of rapid climate change is an important step in understanding how polar bears may be reacting to climate change. We characterized the current terrestrial diet of polar bears in western Hudson Bay by evaluating the contents of passively sampled scat and comparing it to a similar study conducted 40 years ago. While the two terrestrial diets broadly overlap, polar bears currently appear to be exploiting increasingly abundant resources such as caribou (Rangifer tarandus) and snow geese (Chen caerulescens caerulescens) and newly available resources such as eggs. This opportunistic shift is similar to the diet mixing strategy common among other Arctic predators and bear species. We discuss whether the observed diet shift is solely a response to a nutritional stress or is an expression of plastic foraging behavior.     

The early bear gets the goose: climate change,polar bears and lesser snow geese in western Hudson Bay

As climate change advances the date of spring breakup in Hudson Bay, polar bears are coming ashore earlier. Since they would have lost some of their opportunities to hunt ringed seals from a sea ice platform, they may be deficient in energy. Subadult polar bears appear to come ashore before more mature individuals and the earliest subadults are beginning to overlap the nesting period of the large colony of snow geese also occupying the Cape Churchill Peninsula. The eggs these bears are known to eat could make up some of their energy shortfall. The earlier these eggs are consumed during the snow goose nesting period, the greater would be the energy that is available. Recent studies have shown that the annual survival rate for subadult bears declined in contrast to that of prime aged individuals. If this reduction in survival is related to an increasing energy deficit, as suggested by some, the consumption of goose eggs may reverse the trend and help stabilize the population, at least for some period of time. The total number of polar bears that could benefit from this resource will depend on the increasing temporal overlap with the nesting period and on the foraging behaviors of individuals eating the eggs. It is likely that other food sources will also have to play a role if the polar bears are to persist.     

LOW VARIATION IN BLOOD δ13C AMONG HUDSON BAY POLAR BEARS: IMPLICATIONS FOR METABOLISM AND TRACING TERRESTRIAL FORAGING

We investigated the use of stable-carbon isotope analysis of serum and cellular fractions of blood to detect the extent of terrestrial feeding in polar bears on land during the ice-free period in western Hudson Bay. We compared blood in bears that were restricted entirely to coastal areas, who showed no evidence of terrestrial feeding, with blood in bears sampled at inland locations and who were known to have fed on berries of Vaccinium uliginosum and Empetrum nigrum . Despite a separation of approximately 9‰ between terrestrial and marine foods, we found no statistical difference in blood 6¹³C values between these two groups of bears. This suggests that (1) carbon pathways associated with feeding on berries result in minor incorporation of terrestrial-based carbon into bulk plasma or cellular fractions of blood, (2) bears feed insignificantly on berries despite observational evidence to the contrary, or (3) carbon mobilized from endogenous lipid reserves overwhelmed the terrestrial signal or could not be segregated isotopically from carbon derived from berry carbohydrates. We discuss evidence for each of these scenarios and suggest that a more effective approach to using stable-carbon isotope analysis to delineate the importance or use of terrestrial foods to polar bears on land in Hudson Bay during the ice-free period might be through the isotopic analysis of exhaled carbon dioxide rather than blood components.     

Projected Polar Bear Sea Ice Habitat in the Canadian Arctic Archipelago

Background

Sea ice across the Arctic is declining and altering physical characteristics of marine ecosystems. Polar bears (Ursus maritimus) have been identified as vulnerable to changes in sea ice conditions. We use sea ice projections for the Canadian Arctic Archipelago from 2006 – 2100 to gain insight into the conservation challenges for polar bears with respect to habitat loss using metrics developed from polar bear energetics modeling.

Principal Findings

Shifts away from multiyear ice to annual ice cover throughout the region, as well as lengthening ice-free periods, may become critical for polar bears before the end of the 21^st century with projected warming. Each polar bear population in the Archipelago may undergo 2–5 months of ice-free conditions, where no such conditions exist presently. We identify spatially and temporally explicit ice-free periods that extend beyond what polar bears require for nutritional and reproductive demands.

Conclusions/Significance

Under business-as-usual climate projections, polar bears may face starvation and reproductive failure across the entire Archipelago by the year 2100.     

Trophic matches and mismatches: can polar bears reduce the abundance of nesting snow geese in western Hudson Bay?

Climate change driven advances in the date of sea ice breakup will increasingly lead to a loss of spring polar bear foraging opportunities on ringed seal pups creating a phenological trophic ‘mismatch’. However, the same shift will lead to a new ‘match’ between polar bears and ground nesting birds. This new match will be especially prevalent along the Cape Churchill Peninsula of western Hudson Bay where both polar bears and nesting snow geese are abundant. Easily foraged goose eggs will provide at least some of the earlier arriving polar bears with compensation for the energy deficit accrued through lost seal hunting opportunities. We examine the potential impact of changes in the extent and pattern of polar bear egg predation on snow goose abundance using projection models that account not only for increases in the temporal overlap of the two species but also for autocorrelation and stochasticity in the processes underlying polar bear onshore arrival and snow goose incubation. Egg predation will reduce reproductive output of the nesting lesser snow geese and, under all but trivial rates, will lead to a reduction in the size of their nesting population on the Cape Churchill Peninsula. Stochasticity associated with the asymmetrical advances in polar bear onshore arrival and the snow goose incubation period will lead to periodic mismatches in their overlap. These, in turn, will allow snow goose abundance to increase periodically. Climate driven changes in trophic matches and mismatches may reduce snow goose numbers but will not eliminate this over‐abundant species that poses a threat to Arctic landscapes.     

Correlates of seasonal change in the body condition of an Arctic top predator

Climate‐driven sea ice loss has led to changes in the timing of key biological events in the Arctic, however, the consequences and rate of these changes remain largely unknown. Polar bears (Ursus maritimus) undergo seasonal changes in energy stores in relation to foraging opportunities and habitat conditions. Declining sea ice has been linked to reduced body condition in some subpopulations, however, the specific timing and duration of the feeding period when bears acquire most of their energy stores and its relationship to the timing of ice break‐up is poorly understood. We used community‐based sampling to investigate seasonality in body condition (energy stores) of polar bears in Nunavut, Canada, and examined the influence of sea ice variables. We used adipose tissue lipid content as an index of body condition for 1,206 polar bears harvested from 2010–2017 across five subpopulations with varying seasonal ice conditions: Baffin Bay (October–August), Davis Strait and Foxe Basin (year‐round), Gulf of Boothia and Lancaster Sound (August–May). Similar seasonal patterns were found in body condition across subpopulations with bears at their nadir of condition in the spring, followed by fat accumulation past break‐up date and subsequent peak body condition in autumn, indicating that bears are actively foraging in late spring and early summer. Late season feeding implies that even minor advances in the timing of break‐up may have detrimental effects on foraging opportunities, body condition, and subsequent reproduction and survival. The magnitude of seasonal changes in body condition varied across the study area, presumably driven by local environmental conditions. Our results demonstrate how community‐based monitoring of polar bears can reveal population‐level responses to climate warming in advance of detectable demographic change. Our data on the seasonal timing of polar bear foraging and energy storage should inform predictive models of the effects of climate‐mediated sea ice loss.     

贵州草海越冬斑头雁日间行为模式及环境因素对行为的影响

2011年12月至2012年2月,在贵州省威宁县草海国家级自然保护区,采用瞬时扫描取样法对越冬斑头雁(Anser indicus)种群进行了日间行为时间分配和活动节律的研究.结果表明,斑头雁越冬期间的主要行为是取食和静息,分别占50.48％和43.79％,并呈现出早上和傍晚的双取食高峰,中午的静息高峰.各时间段的取食、静息和饮水行为存在显著差异.对不同生境中斑头雁行为的统计结果表明,在陆地生境中的主要行为是取食,在浅水沼泽生境中的主要行为是静息,推测食物丰富度和干扰水平是影响不同栖息地行为模式的主要因素.利用偏相关分析环境温度和湿度对斑头雁行为的影响表明,气温是影响斑头雁日间行为模式的主要因素.气温升高时,斑头雁增加静息行为减少取食行为;气温降低时则减少静息行为增加取食行为.     

Habitat‐mediated timing of migration in polar bears: an individual perspective

Migration phenology is largely determined by how animals respond to seasonal changes in environmental conditions. Our perception of the relationship between migratory behavior and environmental cues can vary depending on the spatial scale at which these interactions are measured. Understanding the behavioral mechanisms behind population‐scale movements requires knowledge of how individuals respond to local cues. We show how time‐to‐event models can be used to predict what factors are associated with the timing of an individual's migratory behavior using data from GPS collared polar bears (Ursus maritimus) that move seasonally between sea ice and terrestrial habitats. We found the concentration of sea ice that bears experience at a local level, along with the duration of exposure to these conditions, was most associated with individual migration timing. Our results corroborate studies that assume thresholds of >50% sea ice concentration are necessary for suitable polar bear habitat; however, continued periods (e.g., days to weeks) of exposure to suboptimal ice concentrations during seasonal melting were required before the proportion of bears migrating to land increased substantially. Time‐to‐event models are advantageous for examining individual movement patterns because they account for the idea that animals make decisions based on an accumulation of knowledge from the landscapes they move through and not simply the environment they are exposed to at the time of a decision. Understanding the migration behavior of polar bears moving between terrestrial and marine habitat, at multiple spatiotemporal scales, will be a major aspect of quantifying observed and potential demographic responses to climate‐induced environmental changes.     

Has early ice clearance increased predation on breeding birds by polar bears?

Past studies suggest that polar bears (Ursus maritimus) consume terrestrial food only opportunistically and derive little nutritional benefit from it. Here, we present observations of at least 6 bears consuming large numbers of snow goose (Chen caerulescens) eggs at two locations in the eastern low Arctic in 2004 and 2006. We also report two records of a polar bear eating the eggs and chicks of cliff-nesting thick-billed murres (Uria lomvia) in 2000 and 2003. Climatic warming has resulted in progressively earlier ice break-up in Hudson Bay, forcing bears ashore much earlier than historical records indicate. Advancement in the nesting dates of birds has been more modest, and this mismatch in timing could lead to an increasing overlap between the nesting period of birds and the period during which bears are on land. At these sites in these years, bears were on land prior to the hatch of nests, and the predation that ensued was catastrophic for the birds at a local scale. Although anecdotal, our observations highlight the complexity of trophic interactions that may occur in a changing Arctic.     

Polar Bears from Space: Assessing Satellite Imagery as a Tool to Track Arctic Wildlife

Development of efficient techniques for monitoring wildlife is a priority in the Arctic, where the impacts of climate change are acute and remoteness and logistical constraints hinder access. We evaluated high resolution satellite imagery as a tool to track the distribution and abundance of polar bears. We examined satellite images of a small island in Foxe Basin, Canada, occupied by a high density of bears during the summer ice-free season. Bears were distinguished from other light-colored spots by comparing images collected on different dates. A sample of ground-truthed points demonstrated that we accurately classified bears. Independent observers reviewed images and a population estimate was obtained using mark–recapture models. This estimate (: 94; 95% Confidence Interval: 92–105) was remarkably similar to an abundance estimate derived from a line transect aerial survey conducted a few days earlier (: 102; 95% CI: 69–152). Our findings suggest that satellite imagery is a promising tool for monitoring polar bears on land, with implications for use with other Arctic wildlife. Large scale applications may require development of automated detection processes to expedite review and analysis. Future research should assess the utility of multi-spectral imagery and examine sites with different environmental characteristics.     

Could brown bears (Ursus arctos) have survived in Ireland during the Last Glacial Maximum?

Brown bears are recorded from Ireland during both the Late Pleistocene and early–mid Holocene. Although most of the Irish landmass was covered by an ice sheet during the Last Glacial Maximum (LGM), Irish brown bears are known to have hybridized with polar bears during the Late Pleistocene, and it is suggested that the Irish brown bear population did not become extinct but instead persisted in situ through the LGM in a southwestern ice-free refugium. We use historical population modelling to demonstrate that brown bears are highly unlikely to have survived through the LGM in Ireland under any combination of life-history parameters shown by living bear populations, but instead would have rapidly become extinct following advance of the British–Irish ice sheet, and probably recolonized Ireland during the end-Pleistocene Woodgrange Interstadial from a closely related nearby source population. The time available for brown bear–polar bear hybridization was therefore restricted to narrow periods at the beginning or end of the LGM. Brown bears would have been extremely vulnerable to extinction in Quaternary habitat refugia and required areas substantially larger than southwestern Ireland to survive adverse glacial conditions.     

Increasing nest predation will be insufficient to maintain polar bear body condition in the face of sea ice loss

Climate change can influence interspecific interactions by differentially affecting species‐specific phenology. In seasonal ice environments, there is evidence that polar bear predation of Arctic bird eggs is increasing because of earlier sea ice breakup, which forces polar bears into nearshore terrestrial environments where Arctic birds are nesting. Because polar bears can consume a large number of nests before becoming satiated, and because they can swim between island colonies, they could have dramatic influences on seabird and sea duck reproductive success. However, it is unclear whether nest foraging can provide an energetic benefit to polar bear populations, especially given the capacity of bird populations to redistribute in response to increasing predation pressure. In this study, we develop a spatially explicit agent‐based model of the predator–prey relationship between polar bears and common eiders, a common and culturally important bird species for northern peoples. Our model is composed of two types of agents (polar bear agents and common eider hen agents) whose movements and decision heuristics are based on species‐specific bioenergetic and behavioral ecological principles, and are influenced by historical and extrapolated sea ice conditions. Our model reproduces empirical findings that polar bear predation of bird nests is increasing and predicts an accelerating relationship between advancing ice breakup dates and the number of nests depredated. Despite increases in nest predation, our model predicts that polar bear body condition during the ice‐free period will continue to decline. Finally, our model predicts that common eider nests will become more dispersed and will move closer to the mainland in response to increasing predation, possibly increasing their exposure to land‐based predators and influencing the livelihood of local people that collect eider eggs and down. These results show that predator–prey interactions can have nonlinear responses to changes in climate and provides important predictions of ecological change in Arctic ecosystems.     

Determinants of Home Range Size for Polar Bears (Ursus maritimus)

The mean home range size of female polar bears ( Ursus maritimus ; 125 100 km² ± 11 800; n  = 93) is substantially larger than the predicted value (514 km²) for a terrestrial carnivore of similar weight. To understand this difference, we correlated home range size and sea ice characteristics. Home range size was related to (i) the ratio of land vs. sea within a given home range (42% of explained variance), and (ii) seasonal variation in ice cover (24%). Thus, bears using land during the ice-free season had larger home ranges and bears living in areas of great seasonal variation in ice cover also had larger home ranges. In another analysis we investigated how variation in a bear's environment in space and time affects its choice of home range. We found that polar bears adjusted the size of their home range according to the amount of annual and seasonal variation within the centre of their home range. For example, polar bears experiencing unpredictable seasonal and annual ice tended to increase their home range size if increasing home range size resulted in reducing variation in seasonal and annual ice. Polar bears make trade-offs between alternate space-use strategies. Large home ranges occur when variable ice cover is associated with more seals but also a more unpredictable distribution of those seals.     

Polar bear stress hormone cortisol fluctuates with the North Atlantic Oscillation climate index

Polar bears are heavily dependent on sea ice for hunting sufficient prey to meet their energetic needs. When the bears are left fasting, it may cause a rise in the levels of the stress hormone cortisol. Cortisol is the major corticosteroid hormone in most mammals, including polar bears. Production and regulation of this stress hormone are vital for the body as it is part of a myriad of processes, including in relation to metabolism, growth, development, reproduction, and immune function. In the present study, we examined the correlation between East Greenland polar bear hair cortisol concentration (HCC), a matrix that reflects longer-term hormone levels, and the fluctuations of the North Atlantic Oscillation (NAO) index, a large-scale climate phenomenon applied as a proxy for sea ice extent in the Greenland Sea along the coast of East Greenland. In doing so, a significant positive correlation (r = 0.88; p = 0.0004) was found between polar bear hair cortisol and the NAO, explaining 77 % of the variation in HCC observed between years over the period 1989–2009. This result indicates that interannual fluctuations in climate and ice cover have a substantial influence on longer-term cortisol levels in East Greenland polar bears. Further research into the implications and consequences inherent in this correlation are recommended, preferably across multiple polar bear populations.     

Genomic Evidence for Island Population Conversion Resolves Conflicting Theories of Polar Bear Evolution

Despite extensive genetic analysis, the evolutionary relationship between polar bears (Ursus maritimus) and brown bears (U. arctos) remains unclear. The two most recent comprehensive reports indicate a recent divergence with little subsequent admixture or a much more ancient divergence followed by extensive admixture. At the center of this controversy are the Alaskan ABC Islands brown bears that show evidence of shared ancestry with polar bears. We present an analysis of genome-wide sequence data for seven polar bears, one ABC Islands brown bear, one mainland Alaskan brown bear, and a black bear (U. americanus), plus recently published datasets from other bears. Surprisingly, we find clear evidence for gene flow from polar bears into ABC Islands brown bears but no evidence of gene flow from brown bears into polar bears. Importantly, while polar bears contributed <1% of the autosomal genome of the ABC Islands brown bear, they contributed 6.5% of the X chromosome. The magnitude of sex-biased polar bear ancestry and the clear direction of gene flow suggest a model wherein the enigmatic ABC Island brown bears are the descendants of a polar bear population that was gradually converted into brown bears via male-dominated brown bear admixture. We present a model that reconciles heretofore conflicting genetic observations. We posit that the enigmatic ABC Islands brown bears derive from a population of polar bears likely stranded by the receding ice at the end of the last glacial period. Since then, male brown bear migration onto the island has gradually converted these bears into an admixed population whose phenotype and genotype are principally brown bear, except at mtDNA and X-linked loci. This process of genome erosion and conversion may be a common outcome when climate change or other forces cause a population to become isolated and then overrun by species with which it can hybridize.     

Spatial context influences patch residence time in foraging hierarchies

Understanding responses of organisms to spatial heterogeneity in resources has emerged as a fundamentally important challenge in contemporary ecology. We examined responses of foraging herbivores to multi-scale heterogeneity in plants. We asked the question, “Is the behavior observed at coarse scales in a patch hierarchy the collective outcome of fine scale behaviors or, alternatively, does the spatial context at coarse scales entrain fine scale behavior?” To address this question we created a nested, two-level patch hierarchy. We examined the effects of the spatial context surrounding a patch on the amount of time herbivores resided in the patch. We developed a set of competing models predicting residence time as a function of the mass of plants contained in a patch and the distance between patches and examined the strength of evidence in our observations for these models. Models that included patch mass and inter-patch distance as independent variables successfully predicted observed residence times (bears: r ²=0.67–0.76 and mule deer: r ²=0.33–0.55). Residence times of grizzly bears (Ursus arctos) and mule deer (Odocoileus hemionus) responded to the spatial context surrounding a patch. Evidence ratios of Akaike weights demonstrated that models containing effects of higher levels in the hierarchy on residence time at lower levels received up to 34 times more support in the data than models that failed to consider the higher level context for grizzly bears and up to 48 times more support for mule deer. We conclude that foraging by large herbivores is influenced by more than one level of heterogeneity in patch hierarchies and that simple empirical models offer a viable alternative to optimal foraging models for the prediction of patch residence times.     

Individual patterns of prey selection and dietary specialization in an Arctic marine carnivore

The cumulative effect of individual‐level foraging patterns may have important consequences for ecosystem functioning, population dynamics and conservation. Dietary specialization, whereby an individual exploits a subset of resources available to the rest of the population, can develop in response to environmental or intrinsic population factors. However, accurate assessment of individual diets may be difficult because analyses of recent food intake may misrepresent foraging variability within a heterogeneous environment. We used quantitative fatty acid signature analysis (QFASA) and a novel index of longitudinal dietary change to examine the individual foraging patterns of 64 polar bears Ursus maritimus successively sampled in Western and Southern Hudson Bay between 1994–2003. Estimated diets varied between and within age and sex classes, with adult male polar bears consuming significantly more bearded seal Erignathus barbatus than adult female or subadult bears, whose diets were dominated by ringed seal Pusa hispida. Among individual adult males, consumption of bearded seal accounted for 0–98% of the diet and bearded seal consumption was positively correlated with individual dietary specialization, as measured by proportional similarity (PS_i) to the rest of the population. Most individual diets were consistent from year‐to‐year and were therefore not a product of short‐term heterogeneity in prey distribution. However, a novel dietary change index indicated that adult male polar bears had the most temporally variable diets with 23% of adult males switching their diet from predominantly ringed seal to predominantly bearded seal or vice versa. We conclude that QFASA is well‐suited to analyses of individual‐level foraging because it reflects an animal's diet over the preceding weeks to months. The subpopulations of bears in this study were near the southern limit of their species range and have experienced negative individual‐ and population‐level impacts related to sea ice loss and climate warming. The tightly constrained diets of some individuals, particularly adult females and subadults, may make them especially sensitive to future climate change.     

Home ranges in moving habitats: polar bears and sea ice

Home range size estimates are often used to assess the amount of space required for animals to perform the activities essential for their survival and reproduction. However, in moving environments, traditional home range estimates may be ill suited to this task. In particular, traditional home range estimates are inaccurate representations of the space required by polar bears Ursus maritimus. The sea ice is the prime foraging platform of polar bears, and estimating the amount of ice encountered by bears may provide a better approximation of space use. We develop a technique to make these estimates. Our results confirm that polar bears use more space than terrestrial carnivores to find the resources and conditions they require. We also show that the traditional geographic home range can underestimate both the movement of bears and the amount of space encountered. Moreover, area of ice encountered increased with ice drift, indicating that bears living on highly mobile ice might be exposed to higher energetic costs, and potentially larger energetic gains, than bears inhabiting more stable ice. The methods and concepts presented here can serve as a foundation for new approaches to study the space use of the many species living in moving environments.     

Modeling optimal responses and fitness consequences in a changing Arctic

Animals must balance a series of costs and benefits while trying to maximize their fitness. For example, an individual may need to choose how much energy to allocate to reproduction versus growth, or how much time to spend on vigilance versus foraging. Their decisions depend on complex interactions between environmental conditions, behavioral plasticity, reproductive biology, and energetic demands. As animals respond to novel environmental conditions caused by climate change, the optimal decisions may shift. Stochastic dynamic programming provides a flexible modeling framework with which to explore these trade‐offs, but this method has not yet been used to study possible changes in optimal trade‐offs caused by climate change. We created a stochastic dynamic programming model capturing trade‐off decisions required by an individual adult female polar bear (Ursus maritimus) as well as the fitness consequences of her decisions. We predicted optimal foraging decisions throughout her lifetime as well as the energetic thresholds below which it is optimal for her to abandon a reproductive attempt. To explore the effects of climate change, we shortened the spring feeding period by up to 3 weeks, which led to predictions of riskier foraging behavior and higher reproductive thresholds. The resulting changes in fitness may be interpreted as a best‐case scenario, where bears adapt instantaneously and optimally to new environmental conditions. If the spring feeding period was reduced by 1 week, her expected fitness declined by 15%, and if reduced by 3 weeks, expected fitness declined by 68%. This demonstrates an effective way to explore a species' optimal response to a changing landscape of costs and benefits and highlights the fact that small annual effects can result in large cumulative changes in expected lifetime fitness.