Kyle H. Elliott

Individual specialization in diet by a generalist marine predator reflects specialization in foraging behaviour

1. We studied chick diet in a known-age, sexed population of a long-lived seabird, the Brünnichs guillemot (Uria lomvia), over 15 years (N = 136; 1993-2007) and attached time-depth-temperature recorders to examine foraging behaviour in multiple years (N = 36; 2004-07). 2. Adults showed specialization in prey fed to offspring, described by multiple indices calculated over 15 years: 27% of diet diversity was attributable to among-individual variation (within-individual component of total niche width = 0.73); average similarity of an individuals diet to the overall diet was 65% (mean proportional similarity between individuals and population = 0.65); diet was significantly more specialized than expected for 70% of individuals (mean likelihood = 0.53). These indices suggest higher specialization than the average for an across-taxa comparison of 49 taxa. 3. Foraging behaviour varied along three axes: flight time, dive depth and dive shape. Individuals showed specialized individual foraging behaviour along each axis. These foraging strategies were reflected in the prey type delivered to their offspring and were maintained over scales of hours to years. 4. Specialization in foraging behaviour and diet was greater over short time spans (hours, days) than over long time spans (years). Regardless of sex or age, the main component of variation in foraging behaviour and chick diet was between individuals. 5. Plasma stable isotope values were similar across years, within a given individual, and variance was low relative to that expected from prey isotope values, suggesting adult diet specialized across years. Stable isotope values were similar among individuals that fed their nestlings similar prey items and there was no difference in trophic level between adults and chicks. We suggest that guillemots specialize on a single foraging strategy regardless of whether chick-provisioning and self-feeding. With little individual difference in body mass and physiology, specialization likely represents learning and memorizing optimal feeding locations and behaviours. 6. There was no difference in survival or reproductive success between specialists and generalists, suggesting these are largely equivalent strategies in terms of evolutionary fitness, presumably because different strategies were advantageous at different levels of prey abundance or predictability. The development of individual specialization may be an important precursor to diversification among seabirds.

Central-place Foraging in an Arctic Seabird Provides Evidence for Storer-Ashmole's Halo

ABSTRACT. Breeding seabirds are extreme central-place foragers, commuting long distances between colonies and feeding areas. Central-place foraging theory predicts that prey items close to the colony will be preferred over prey items distant from the colony, which can lead to prey depletion near the colony (“Storer-Ashmoles halo”). To investigate the relevance of these ideas to a single-prey loader, we equipped chick-rearing Thick-billed Murres (Uria lomvia) with time—depth recorders (1999–2007) and monitored prey deliveries (1993–2008). Because feeding rates were constant for chicks 3–15 days old, we restricted analyses to that age group. Between-date relationships were examined within individuals to avoid confounding effects of specialization and parental quality. The mass of prey items increased with foraging distance0,5, which suggests that large prey items were depleted by foraging pressure. Foraging distance for pelagic species increased through the season in years without spawning. After accounting for bathymetry, foraging effort decreased with distance from the colony for benthic fish and, in years without spawning, for pelagic fish. Within each season, Thick-billed Murres “fished down the food web”; they began by feeding on large fish, progressed to medium-sized prey (small fish), and finished feeding on small prey (invertebrates). We concluded that pelagic species responded to seabird foraging pressure by moving away from the colony, creating a three-dimensional halo. Benthic species, meanwhile, were depleted from a nearby shelf but remained abundant at a more distant offshore bench. We suggest that populations of central-place foragers are partially regulated by prey depletion.

High flight costs, but low dive costs, in auks support the biomechanical hypothesis for flightlessness in penguins

Flight is a key adaptive trait. Despite its advantages, flight has been lost in several groups of birds, notably among seabirds, where flightlessness has evolved independently in at least five lineages. One hypothesis for the loss of flight among seabirds is that animals moving between different media face tradeoffs between maximizing function in one medium relative to the other. In particular, biomechanical models of energy costs during flying and diving suggest that a wing designed for optimal diving performance should lead to enormous energy costs when flying in air. Costs of flying and diving have been measured in free-living animals that use their wings to fly or to propel their dives, but not both. Animals that both fly and dive might approach the functional boundary between flight and nonflight. We show that flight costs for thick-billed murres (Uria lomvia), which are wing-propelled divers, and pelagic cormorants (Phalacrocorax pelagicus) (foot-propelled divers), are the highest recorded for vertebrates. Dive costs are high for cormorants and low for murres, but the latter are still higher than for flightless wing-propelled diving birds (penguins). For murres, flight costs were higher than predicted from biomechanical modeling, and the oxygen consumption rate during dives decreased with depth at a faster rate than estimated biomechanical costs. These results strongly support the hypothesis that function constrains form in diving birds, and that optimizing wing shape and form for wing-propelled diving leads to such high flight costs that flying ceases to be an option in larger wing-propelled diving seabirds, including penguins.

Tracking Marine Pollution

Seabird monitoring studies are providing a global picture of an increasing range of marine pollutants. Visit a beach almost anywhere and you will see plastic waste floating in the water and heaped above the tide lines. That debris is both a source and an overt signal of the even more pervasive contamination of marine biota by persistent chemicals. Present at ultra-trace levels but often highly toxic, chemical pollutants can be challenging to measure and understand. As the most problematic compounds biomagnify in food chains, sampling of marine top predators yields a global picture of ocean pollution.

Inbreeding effects on immune response in free-living song sparrows (Melospiza melodia)

The consequences of inbreeding for host immunity to parasitic infection have broad implications for the evolutionary and dynamical impacts of parasites on populations where inbreeding occurs. To rigorously assess the magnitude and the prevalence of inbreeding effects on immunity, multiple components of host immune response should be related to inbreeding coefficient (f) in free-living individuals. We used a pedigreed, free-living population of song sparrows (Melospiza melodia) to test whether individual responses to widely used experimental immune challenges varied consistently with f. The patagial swelling response to phytohaemagglutinin declined markedly with f in both females and males in both 2002 and 2003, although overall inbreeding depression was greater in males. The primary antibody response to tetanus toxoid declined with f in females but not in males in both 2004 and 2005. Primary antibody responses to diphtheria toxoid were low but tended to decline with f in 2004. Overall inbreeding depression did not solely reflect particularly strong immune responses in outbred offspring of immigrant–native pairings or weak responses in highly inbred individuals. These data indicate substantial and apparently sex-specific inbreeding effects on immune response, implying that inbred hosts may be relatively susceptible to parasitic infection to differing degrees in males and females.

PCBs and DDE, but not PBDEs, increase with trophic level and marine input in nestling bald eagles

Concentrations of persistent contaminants often vary widely among individuals within a population. We hypothesized that such variation was caused mainly by differences in diet (biomagnification) and in coastal systems by the tendency of marine systems to act as contaminant sinks. We examined the relationship between contaminant concentrations and stable isotope ratios in nestling plasma from an apex predator with a particularly broad diet. Our study included freshwater, estuarine, inshore and pelagic breeding sites. Bald eagles (Haliaeetus leucocephalus) at the pelagic marine sites showed high trophic level and marine input, eagles at the freshwater sites showed low trophic level and marine input, and eagles at the estuarine and inshore marine sites had intermediate values. The relationship between trophic level and marine input may reflect longer food chains in pelagic compared to terrestrial ecosystems. summation operator PCBs and DDE concentrations generally increased with trophic level and marine input, with the exception of the freshwater sites, while summation operator PBDEs, hydroxylated-PBDEs and hydroxylated-PCBs increased with marine input, but were independent of trophic level. The relationships for summation operator PCBs and DDE were often slightly stronger with marine input than trophic level, suggesting that oceanographic processes may be more important than trophic level. At freshwater locations, spatial variation may be more important than trophic level due to the heterogeneity of contaminant profiles between feeding locations (lakes, rivers, agricultural fields). Adults had similar isotopic composition to their chicks but higher contamination. Based on nests where prey composition was determined independently, isotopic enrichment values for nestling plasma were 1.6+/-0.1 (delta(15)N) and -0.4+/-0.2 (delta(13)C). We conclude that trophic level and marine influence are significant factors influencing PCB and DDE concentrations in eagles. However, trophic level in particular did not influence PBDEs, possibly due to their being metabolized by eagles.

Accelerometry predicts daily energy expenditure in a bird with high activity levels

Animal ecology is shaped by energy costs, yet it is difficult to measure fine-scale energy expenditure in the wild. Because metabolism is often closely correlated with mechanical work, accelerometers have the potential to provide detailed information on energy expenditure of wild animals over fine temporal scales. Nonetheless, accelerometry needs to be validated on wild animals, especially across different locomotory modes. We merged data collected on 20 thick-billed murres (Uria lomvia) from miniature accelerometers with measurements of daily energy expenditure over 24 h using doubly labelled water. Across three different locomotory modes (swimming, flying and movement on land), dynamic body acceleration was a good predictor of daily energy expenditure as measured independently by doubly labelled water (R2 = 0.73). The most parsimonious model suggested that different equations were needed to predict energy expenditure from accelerometry for flying than for surface swimming or activity on land (R2 = 0.81). Our results demonstrate that accelerometers can provide an accurate integrated measure of energy expenditure in wild animals using many different locomotory modes.

Time allocation by a deep-diving bird reflects prey type and energy gain

Central-place foragers must decide how to trade off travel time with foraging time when energy costs and gains differ between available prey types. Oxygen consumption places a fundamental constraint on the available options for deep-diving birds. Dives exceeding the aerobic dive limit (ADL) are considered costly because they extend the interdive surface time. Nevertheless, dives exceeding ADL, combined with short surface pauses, may be an efficient strategy if (1) prey density or (2) the probability of losing contact with ephemeral prey is high. We examined surface pause duration, dive duration and dive depth during dive bouts of Brunnichs guillemots, Uria lomvia , immediately before prey delivery. Surface pauses were more strongly related to dive depth than to duration and were both ‘anticipatory’ (of short dives) and ‘reactive’ (to long dives). Surface pauses decreased weakly, but significantly, with prey mass and were not shorter for ephemeral than for benthic prey once prey mass was accounted for. Dive duration was a decelerating function of dive depth and was unaffected by prey type (benthic versus pelagic). Dive duration (as a function of depth), bottom time (as a function of depth and duration) and an index for prey acquisition rate based on these parameters all increased with prey mass. Thus, surface pause duration was not reduced during dive bouts for ephemeral prey and only slightly reduced during dive bouts for larger prey, suggesting that guillemots alter other components of the dive cycle (bottom time, transit time) to accommodate differences in prey type and energy gain.

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.

Age-related variation in energy expenditure in a long-lived bird within the envelope of an energy ceiling

Energy expenditure in wild animals can be limited (i) intrinsically by physiological processes that constrain an animals capacity to use energy, (ii) extrinsically by energy availability in the environment and/or (iii) strategically based on trade-offs between elevated metabolism and survival. Although these factors apply to all individuals within a population, some individuals expend more or less energy than other individuals. To examine the role of an energy ceiling in a species with a high and individually repeatable metabolic rate, we compared energy expenditure of thick-billed murres (Uria lomvia) with and without handicaps during a period of peak energy demand (chick-rearing, N = 16). We also compared energy expenditure of unencumbered birds (N = 260) across 8 years exhibiting contrasting environmental conditions and correlated energy expenditure with fitness (reproductive success and survival). Murres experienced an energy ceiling mediated through behavioural adjustments. Handicapped birds decreased time spent flying/diving and chick-provisioning rates such that overall daily energy expenditure remained unchanged across the two treatments. The energy ceiling did not reflect energy availability or trade-offs with fitness, as energy expenditure was similar across contrasting foraging conditions and was not associated with reduced survival or increased reproductive success. We found partial support for the trade-off hypothesis as older murres, where prospects for future reproduction would be relatively limited, did overcome an energy ceiling to invest more in offspring following handicapping by reducing their own energy reserves. The ceiling therefore appeared to operate at the level of intake (i.e. digestion) rather than expenditure (i.e. thermal constraint, oxidative stress). A meta-analysis comparing responses of breeding animals to handicapping suggests that our results are typical: animals either reduced investment in themselves or in their offspring to remain below an energy ceiling. Across species, whether a handicapped individual invested in its own energy stores or its offsprings growth was not explained by life history (future vs. current reproductive potential). Many breeding animals apparently experience an intrinsic energy ceiling, and increased energy costs lead to a decline in self-maintenance and/or offspring provisioning.