David B. Irons

FORAGING AREA FIDELITY OF INDIVIDUAL SEABIRDS IN RELATION TO TIDAL CYCLES AND FLOCK FEEDING

I studied the foraging behavior of Black-legged Kittiwakes (Rissa tridactyla) using radio transmitters, remote receiving stations, and a boat to track individuals. I tested the hypothesis that individual birds exhibit foraging area fidelity. I also investigated evidence for information transfer among birds and the role of tidal cycles in foraging behavior. Flight speed, feeding methods, chick growth rates, and fledging success were similar for radio-tagged birds and birds without radio-tags, suggesting that radio-tags had no significant effect. Birds made an average of two foraging trips per day for 13 d, during a spring and a neap tide series. Twenty-four of 26 birds returned repeatedly (X = 19.2 trips) to particular areas to feed; these birds were considered faithful to a foraging area. During spring and neap tides, 17 and 14 birds, respectively, timed their foraging trips to coincide with daily tidal cycles. Foraging area fidelity indicates that kittiwakes learn and remember when and where to forage an...

Prevalence of Influenza A viruses in wild migratory birds in Alaska: Patterns of variation in detection at a crossroads of intercontinental flyways

BackgroundThe global spread of the highly pathogenic avian influenza H5N1 virus has stimulated interest in a better understanding of the mechanisms of H5N1 dispersal, including the potential role of migratory birds as carriers. Although wild birds have been found dead during H5N1 outbreaks, evidence suggests that others have survived natural infections, and recent studies have shown several species of ducks capable of surviving experimental inoculations of H5N1 and shedding virus. To investigate the possibility of migratory birds as a means of H5N1 dispersal into North America, we monitored for the virus in a surveillance program based on the risk that wild birds may carry the virus from Asia.ResultsOf 16,797 birds sampled in Alaska between May 2006 and March 2007, low pathogenic avian influenza viruses were detected in 1.7% by rRT-PCR but no highly pathogenic viruses were found. Our data suggest that prevalence varied among sampling locations, species (highest in waterfowl, lowest in passerines), ages (juveniles higher than adults), sexes (males higher than females), date (highest in autumn), and analytical technique (rRT-PCR prevalence = 1.7%; virus isolation prevalence = 1.5%).ConclusionThe prevalence of low pathogenic avian influenza viruses isolated from wild birds depends on biological, temporal, and geographical factors, as well as testing methods. Future studies should control for, or sample across, these sources of variation to allow direct comparison of prevalence rates.

ADULT PREY CHOICE AFFECTS CHICK GROWTH AND REPRODUCTIVE SUCCESS IN PIGEON GUILLEMOTS

Abstract Pigeon Guillemots (Cepphus columba) are diving seabirds that forage near shore and feed their chicks demersal and schooling fishes. During nine years between 1979 and 1997, we studied chick diet, chick growth rate, and reproductive success of Pigeon Guillemots at Prince William Sound, Alaska, to determine factors limiting breeding populations. We found evidence for prey specialization among breeding pairs and detected differences in reproductive success between specialists and generalists. Pairs that specialized on particular prey types when foraging for their chicks fledged more chicks than those that generalized, apparently because they delivered larger individual prey items. Reproductive performance also varied among guillemot pairs as a function of the proportion of high-lipid schooling fishes fed to the chicks. Pairs that delivered primarily high-lipid fishes (Pacific sand lance [Ammodytes hexapterus] and Pacific herring [Clupea pallasii]) had higher overall reproductive success than pairs that delivered primarily low-lipid demersal fishes (e.g. sculpins, blennies, stichaeids, and pholidids) and gadids. The proportion of high-lipid fishes in the diet was positively related to chick growth, suggesting that piscivorous seabird chicks benefit from eating species with high-energy densities during development. The diet of Pigeon Guillemot chicks showed high annual variation from 1979 to 1997, presumably because of fluctuations in abundance of Pacific sand lance, a high-lipid schooling fish. Regression analyses suggest that the percent occurrence of high-lipid fishes in the diet affected chick growth rate at the population level. We conclude that Pigeon Guillemots benefit by specializing when selecting prey for their chicks, and that high-lipid schooling fishes enhance chick growth and reproductive success.

PREY RESOURCES, COMPETITION, AND GEOGRAPHIC STRUCTURE OF KITTIWAKE COLONIES IN PRINCE WILLIAM SOUND

Mostly indirect evidence has shown that the size of bird colonies is often related positively to the size of foraging areas, in turn an indirect measure of prey availability. We report here results of an unusual opportunity to directly investigate the relationships among the size and distribution of colonies of Black-legged Kittiwakes (Rissa tridactyla), the degree of overlap of colony-specific foraging areas, and the availability of kittiwake prey, principally Pacific herring (Clupea pallasi) and sand lance (Ammodytes hexapterus), in Prince William Sound, Alaska, 1995–1999. Aerial surveys to assess prevalence of prey schools visible at the surface (the kittiwake mode of prey search) allowed quantification of prey availability in foraging areas. Foraging areas and foraging ranges were determined by radio telemetry. Large colonies occurred as far apart as physically possible and not within foraging range (∼40 km) of one another; only small colonies occurred within the respective ranges of large colonies. F...

Foraging Strategies of Glaucous-Winged Gulls in a Rocky Intertidal Community

Foraging strategies of Glaucous-winged Gulls (Larus glaucescens) were studied in rocky intertidal habitats of the western Aleutian Islands, Alaska. Daily foraging activity was most intense at maximum low tide, and was concentrated in the lowest intertidal zones available to the birds. Barnacles (Balanus glandula) and mussels (Mytilus edulis) comprised most of the gulls diet during neap low tides, but these species were almost entirely abandoned during spring low tides in favor of sea urchins (Strongylocentrotus polyacanthus), chitons (Katharina tunicata), and limpets (Collisella pelta and Notoacmaea scutum). Sea urchins, chitons, and limpets, which had positive prey selection indices, were most abundant in the lower intertidal zones; barnacles and mussels, which had negative prey selection indices, were most abundant in the upper zones. Gulls also generally selected the larger individuals from each prey species, although sea urchins larger than the commissural bill width were avoided and limpets were selected in proportion to availability. Variation in prey availability also occurred among study areas with varying densities of sea otters (Enhydra lutris). With increasing depression of invertebrate prey by sea otter predation, gulls fed on a more diverse prey resource, and they switched to neritic fishes under intense sea otter predation. Preference experiments were conducted in the field, in which the common species and sizes of prey were made equally available to foraging gulls, thus eliminating search and capture times. In comparison with natural food choice, where sea urchins were most preferred, chitons became most preferred. We suggest that chitons are infrequent in natural diets because they adhere more strongly to the substratum than do sea urchins. Benefits of selective foraging were determined by comparing the net rate of energy gain of simulated random foragers with energy gained by selection of intertidal zones, prey species, and prey sizes. Observed selection patterns provided increased energy as gulls became more selective, and averaged 155% more than that obtained by the simulated random foragers. Results of the study support the two main predictions of optimal foraging theory in that (1) foraging patches (intertidal zones) and diets were selected such that net rates of energy gain were maximized, and (2) gulls became more selective foragers when energetically more profitable prey were more available.

Determinants of reproductive costs in the long-lived Black-legged Kittiwake: A multiyear experiment

We studied reproductive costs of Black-legged Kittiwakes (Rissa tridactyla) in Prince William Sound, Alaska (USA) by removing entire clutches from randomly selected nests over four successive years, and then contrasting survival and fecundity of adults from manipulated and unmanipulated nests in each subsequent year. To elucidate mechanisms that lead to the expression of reproductive costs, we simultaneously characterized several behavioral and physiological parameters among adults in the two treatment groups. We also examined naturally nonbreeding adults that previously bred to determine their survival and future nonbreeding probabilities. Food availability varied during the study, being generally poor at the onset, and improving in later years. Adult nest attendance and body condition (assessed late in the chick- rearing period) varied accordingly among years, and between adults raising chicks and adults that had their eggs removed. Adults from unmanipulated nests incurred significant survival costs in all years, although fecundity costs were strongly expressed in only one of four years. Neither survival nor fecundity costs were strongly influenced by body condition or food availability, and no difference in reproductive costs was detected between the sexes. Although unmanipulated breeders survived at lower rates than manipulated breeders due to costs of reproduction, their survival rates were elevated compared to those of natural nonbreeders, presumably due to differences in individual ability. These findings indicate that models of adult survival must consider not only an organisms reproductive state, but also the factors that lead to that state. Although body condition appeared to be weakly related to survival, it was insufficient to explain the full magnitude of survival costs observed. We suggest that other parameters that were found to differ between treatment groups (e.g., rates of energy turnover, baseline levels of stress, and patterns of allocating body reserves) may be important mechanistic determinants of reproductive costs in kittiwakes, and potentially other long-lived species. Future efforts should move beyond simple assessments of body condition and toward more integrated measures of physiological condition when attempting to identify factors that influence how long-lived species balance the costs and benefits of reproduction.

Prey Patch Patterns Predict Habitat Use by Top Marine Predators with Diverse Foraging Strategies

Spatial coherence between predators and prey has rarely been observed in pelagic marine ecosystems. We used measures of the environment, prey abundance, prey quality, and prey distribution to explain the observed distributions of three co-occurring predator species breeding on islands in the southeastern Bering Sea: black-legged kittiwakes (Rissa tridactyla), thick-billed murres (Uria lomvia), and northern fur seals (Callorhinus ursinus). Predictions of statistical models were tested using movement patterns obtained from satellite-tracked individual animals. With the most commonly used measures to quantify prey distributions - areal biomass, density, and numerical abundance - we were unable to find a spatial relationship between predators and their prey. We instead found that habitat use by all three predators was predicted most strongly by prey patch characteristics such as depth and local density within spatial aggregations. Additional prey patch characteristics and physical habitat also contributed significantly to characterizing predator patterns. Our results indicate that the small-scale prey patch characteristics are critical to how predators perceive the quality of their food supply and the mechanisms they use to exploit it, regardless of time of day, sampling year, or source colony. The three focal predator species had different constraints and employed different foraging strategies – a shallow diver that makes trips of moderate distance (kittiwakes), a deep diver that makes trip of short distances (murres), and a deep diver that makes extensive trips (fur seals). However, all three were similarly linked by patchiness of prey rather than by the distribution of overall biomass. This supports the hypothesis that patchiness may be critical for understanding predator-prey relationships in pelagic marine systems more generally.

PREY SWITCHING AND VARIABLE FORAGING STRATEGIES OF BLACK-LEGGED KITTIWAKES AND THE EFFECT ON REPRODUCTIVE SUCCESS

Abstract We studied the diets, foraging strategies, and reproduction of Black-legged Kittiwakes (Rissa tridactyla) during five years at two colonies within Prince William Sound, Alaska. Years with reduced occurrence of 1-year-old Pacific herring (Clupea pallasi) in kittiwake diets were associated with increased foraging trip duration, distance, and travel time at both colonies. Foraging range was consistently greater at the large, fjord colony with an annual mean trip duration of 4 hr and mean distance to the farthest feeding location of 40 km in years when 1-year-old herring dominated diets; these numbers increased to a maximum mean of 6 hr and 60 km during a year when kittiwakes consumed primarily young-of-year (YOY) herring and Pacific sand lance (Ammodytes hexapterus). Foraging trips of kittiwakes at the small, island colony averaged 2 hr and 5 km during years when 1-year-old herring dominated diets and increased when capelin (Mallotus villosus) and YOY herring and sand lance were consumed. Consequences of reduced herring availability were greatest at the large colony where alternative prey was limited, resulting in reduced reproductive success. In contrast, kittiwakes from the small colony were able to compensate for reduced herring availability by obtaining sand lance and capelin in relatively close proximity, and maintained above average reproductive success. Time spent traveling and trip distance increased with greater trip duration. However, search and prey capture times were only weakly related to trip duration and may reflect foraging strategies that vary with different species, age classes, or availability of prey consumed.

Raising young reduces body condition and fat stores in black-legged kittiwakes

Abstract We conducted a manipulative experiment to investigate how raising chicks affects the body condition (body mass scaled by body size) and body composition (percent fat vs. lean mass) of black-legged kittiwakes (Rissa tridactyla). For 4 consecutive years (1991–1994) we removed eggs from randomly selected nests and then compared adults raising chicks with adults that had their eggs removed. At the end of the chick-rearing period, adults raising chicks were significantly lighter for their size than adults that had their eggs removed. Adults raising chicks also had a significantly lower percent body fat (by 28%) than adults from manipulated nests. The difference in percent body fat between the two groups was apparent at all levels of condition, suggesting that adults that are raising chicks apportion their reserves differently than adults that are working only to meet their own metabolic needs. End-of-season body condition of adults from manipulated and unmanipulated nests varied significantly among 5 years of study, and appeared to reflect differences in local foraging conditions. In all years, females were in worse condition than males at the end of the breeding season. This sex-specific condition difference did not, however, appear to indicate a greater short-term reproductive cost among females. Females were lighter for their size than males in both the manipulated and unmanipulated groups. Our results suggest that adult kittiwakes compromise their body condition and body composition during chick rearing to increase the likelihood of successfully fledging young, even though such adjustments may decrease their own post-reproductive survival probabilities. Prior to estimating the body composition of the experimental birds, we evaluated the usefulness of several noninvasive techniques for predicting fat mass in kittiwakes. We used cross-validation techniques to compare multiple regression models that included total body electrical conductivity (TOBEC), total body water (TBW), and morphometric measurements as independent variables. The most parsimonious model for predicting fat mass was based on TOBEC and mass measurements. TBW and morphometrics were of little utility in predicting fat mass in kittiwakes. Previous studies that have evaluated the usefulness of TOBEC as a predictor of fat mass have shown mixed results. We suggest that the size of the experimental subject relative to the size of the TOBEC measurement chamber may affect the accuracy of this technique.

NINE YEARS AFTER THE EXXON VALDEZ OIL SPILL: EFFECTS ON MARINE BIRD POPULATIONS IN PRINCE WILLIAM SOUND, ALASKA

Abstract We compared post Exxon Valdez oil-spill densities of marine birds in Prince William Sound from 1989–1991, 1993, 1996, and 1998 to pre-spill densities from 1984–1985. Post-spill densities of several species of marine birds were lower than expected in the oiled area of Prince William Sound when compared to densities in the unoiled area. These negative effects continued through 1998 for five taxa: cormorants, goldeneyes, mergansers, Pigeon Guillemot (Cepphus columba), and murres. Black Oystercatchers (Haematopus bachmani) and Harlequin Ducks (Histrionicus histrionicus) exhibited negative effects in 1990 and 1991. Loons showed a weak negative effect in 1993. Black-legged Kittiwakes (Rissa tridactyla) showed relative decreases in 1989, 1996, and 1998 which may have been caused by shifts in foraging distribution rather than declines in populations. Glaucous-winged Gulls (Larus glaucescens) showed positive effects in most post-spill years. Murrelets and terns showed relative increases in 1993, 1996, and 1998. Generally, taxa that dive for their food were negatively affected, whereas taxa that feed at the surface were not. Effects for some taxa were dependent upon the spatial scale at which they were analyzed. Movements of birds and the mosaic pattern of oiling reduced our ability to detect oil-spill effects, therefore our results may be conservative. Several marine bird species were negatively affected at the population level and have not recovered to pre-spill levels nine years after the oil spill. The reason for lack of recovery may be related to persistent oil remaining in the environment and reduced forage fish abundance.