Age-dependent vigilance in winter aggregations of cooperatively breeding white-winged choughs (Corcorax melanorhamphos)

Summary Winter aggregation of cooperatively breeding white-winged choughs (Corcorax melanorhamphos) was examined to elucidate the advantages for different group members. Adults have higher vigilance burdens than juveniles when foraging in their regular group. Aggregation of groups in winter enables the older members to reduce vigilance and increase the time spent foraging. Vigilance was lowest and foraging time highest in aggregations of 20 individuals but the size of cooperatively breeding groups did not exceed fifteen. Despite increased foraging time, choughs did not consume more food items when aggregating.     

Sulfur isotopes link overwinter habitat use and breeding condition in Double-crested Cormorants.

North American Double-crested Cormorant (Phalacrocorax auritus) populations have increased greatly. Both breeding and overwintering ground factors have likely contributed to these increases. However, demonstrating how overwintering conditions may affect breeding birds has not been possible because of the difficulty in linking breeding birds to their wintering grounds. Here, we demonstrate the utility of stable sulfur isotopes to elucidate overwintering habitat use by cormorants breeding on Lake Erie. Sulfur isotopes in feathers grown on overwintering grounds provided insights into the degree to which birds used freshwater vs. marine environments. The proportion of birds utilizing freshwater habitats increased through time. This change may have reflected increases in freshwater aquaculture (i.e., catfish) in the U.S. south. Examination of body condition in birds returning to breed on Lake Erie indicated that those individuals that solely used marine habitats for at least a portion of the overwintering period were in poorer condition than birds using freshwater. Enhanced foraging opportunities at aquaculture facilities may improve the fitness of individuals that have returned to breed after overwintering at such locations. This study is the first to demonstrate a linkage between overwinter habitat use and breeding ground parameters in Double-crested Cormorants. These results underscore that factors throughout the Mississippi flyway are likely acting together to regulate cormorant populations.     

Fattening strategies in dominance-structured greenfinch (Carduelis chloris) flocks in winter

Relationships between social status and levels of body reserves stored by members of greenfinch (Carduelis chloris) flocks in winter were investigated. In addition, the adjustment of reserves by birds of different rank to experimental changes in food predictability and to changed weather conditions were examined. Birds with low social status carried overall larger body reserves than high-ranked birds. The results of the experiments suggest that this was mainly because subordinates, due to their low priority to food access, perceived future foraging success as less predictable than dominants. In response to severe weather, which probably increased the risk of starvation for birds, dominants temporarily increased their reserves more than subordinates. This response also indicated that birds with low social status carried larger reserves than high-ranked birds, and demonstrated that dominants could increase food intake when there was a risk of energetic shortfall. The results suggest that fattening strategies in greenfinches depend on social status. In winter, subordinates may be forced to carry larger reserves than dominants to safeguard against social constraints in access to food during critical times. As body reserves may be costly to carry and acquire, this should reduce the probability of surviving the winter for subordinates compared to dominants. Received: 2 March 1995/Accepted after revision: 13 April 1996     

Persistent bimodal activity patterns in wild and captive black-tailed godwit <Emphasis Type="Italic">Limosa limosa</Emphasis> under different environmental conditions

There is a large literature dealing with daily foraging routines of wild birds during the non-breeding season. While different laboratory studies have showed that some bird activity patterns are a persistent property of the circadian system, most of field studies preclude the potential role of an endogenous circadian rhythm in controlling bird’s foraging routines. In this study we compared the patterns of diurnal foraging activity and intake rates of migrating black-tailed godwits, Limosa limosa (radio-tagged and non-tagged individuals) at two stopover sites (habitats) with different environmental characteristics, aiming at identifying proximate factors of bird activity routines. To gain insights into the role of food availability in control of such foraging routines, we also estimated foraging activity patterns in captive godwits subjected to constant food availability. Captive and wild black-tailed godwits showed a persistent bimodal activity pattern through daylight period. Food availability had a significant effect on the intake rates, but had a subtler effect on foraging and intake rate rhythms. Temperature and wind speed (combined in a weather index) showed non-significant effects on both rhythms. Although we could not discard a role for natural diurnal changes in light intensity, an important timing cue, our findings support the idea that an endogenous circadian rhythm could be an important proximate factor regulating foraging activity and food items taken per unit time of wild black-tailed godwits during migration.     

Food or sex—males and females in a sex role reversed pipefish have different interests

In a sex role reversed pipefish, Syngnathus typhle, we found that basic life history allocations were directly influenced by sexual selection. We investigated time allocation to foraging and mating, respectively, in a choice experiment, giving males and females, of small or large body size, a choice between food and a potential partner. We found that males were more interested in foraging than mating, i.e., were more frequently observed in front of the food than in front of the partner, whereas females were more interested in the potential partner. This reflects sexual selection operating differently on the two sexes, as males and females are relatively similar in other life history traits, such as growth, mortality, age of maturity, dispersal, and parental expenditure. Moreover, large individuals allocated more time to mating activities, small to feeding. Individuals more interested in mating compared to food were subsequently more critical when given a choice between a large (high-quality) and a small (low-quality) partner, whereas individuals more interested in food were not selective. These findings are consistent with our predictions: sex-role reversed males can be relatively sure of achieving one or more matings, and should allocate more time to feeding and, hence, to parental investment, growth and/or future reproduction. Females, on the other hand, have more uncertain mating prospects and should allocate time to imminent reproductive activities, thereby foregoing other life history traits such as growth and future egg production. By this, they also sacrifice future fecundity and attractiveness.     

Energy budgets reveal equal benefits of varied migration strategies in northern gannets

We investigated migration and wintering of adult northern gannets (Morus bassanus) breeding in east Scotland, North Sea, by deploying geolocation loggers over three winters. The wintering ranges of these birds varied from the North Sea to the Atlantic off West Africa. Flight time was taken as a proxy for migration and foraging effort. Gannets wintering off Africa had higher total flight times during migration than birds wintering further north. Total flight times in different wintering regions were generally low. Birds off West Africa consistently spent < 25 % of daylight hours in flight, but birds further north showed more variable values that may reflect more variable weather or food availability. Winter sea surface temperatures ranged from 9 °C (North Sea) to 16 °C (West Africa). Thermostatic costs in winter as estimated by measuring thermal conductance in carcasses in still air and water were 28 % higher in North Sea than off West Africa. This effect is aggravated by higher thermostatic costs caused by stronger wind chills in the North Sea compared to the conditions off West Africa. Birds wintering close to the UK arrived at the colony on average 12 days earlier than birds wintering off West Africa. We conclude that the net cost-benefit analysis may be similar for all wintering areas investigated.     

The amphipod Corophium multisetosum (Corophiidae) in Ria de Aveiro (NW Portugal). I. Life history and aspects of reproductive biology

The population of Corophium multisetosum Stock, 1952 in Areão displayed a semiannual, iteroparous life history. Mean longevity was ～6?mo, with the estimated life span longer for overwintering individuals born in autumn than for individuals born in spring. Length-frequency data indicated that the length increment per moult is probably higher in males than females; however females moulted more frequently and achieved a larger body size. Preliminary growth rates were 100?μm?d^?1 for juveniles and 19 to 29?μm?d^?1 for mature females, with the lower values occurring during the winter. It was estimated that under favourable conditions females may attain reproductive size and mature within 1?mo. Although incubating females were present all year round, recruitment occurred in spring, almost ceased during the summer, peaked in autumn, and decreased again during the winter. Extreme temperatures and very low salinities during winter and summer may have deterred breeding, while moderate temperatures (15 to 20?°C) and salinities > 1?psu in spring and autumn were apparently favourable for reproduction. The unfavourable summer conditions constrained breeding and synchronised the timing of reproduction. In late-autumn and during the winter, as temperature decreased and brooding time increased, synchrony was progressively lost. Brood size varied as a function of embryonic developmental stage, size of incubating females, and season. The life-history pattern and reproductive features of C. multisetosum in Areão are closely related to temperature and salinity; other environmental conditions such as oxygen content of the water and food availability may also be relevant.     

Specific targeting of host individuals by a kleptoparasitic bird

Kleptoparasitism is a tactic used to acquire food opportunistically and has been shown to provide several benefits, including greater food intake rate and the acquisition of items not normally available during self-foraging. Host individuals may differ in their ability to defend themselves against kleptoparasitic attacks and therefore identifying those host individuals that are particularly vulnerable to attack could both provide energetic benefits and increase the efficiency of kleptoparasitism as a foraging strategy. Here, we show that the kleptoparasitic fork-tailed drongo (Dicrurus adsimilis) specifically targets juveniles when following groups of cooperatively breeding pied babblers (Turdoides bicolor). Drongos give alarm calls upon sighting a predator, thus providing extra predator vigilance to foraging pied babblers. However, drongos also use alarm calls to steal food items. During kleptoparasitic attacks, drongos give false alarm calls and then swoop down to steal food items dropped by alarmed babblers. Juvenile pied babblers are particularly vulnerable to attack because they (a) spend a longer period handling prey items prior to consumption and (b) respond to alarm calls primarily by immediately moving to cover, in contrast to adults who respond by looking up and visually scanning the surrounding area. Drongos attack juvenile babblers significantly more often than adults, with attacks on juveniles more likely to result in the successful procurement of a food item. This patterns of attack suggests that drongos are able to differentiate between individuals of different age when targeting pied babblers, thus increasing the efficiency of kleptoparasitism as a foraging strategy.     

Reproductive bioenergetics within the summer and winter generations of Euchaeta norvegica (Copepoda)

The predatory copepod Euchaeta norvegica from Korsfjorden, western Norway, has a half-year life cycle. Egg sacs produced by the winter generation are heavier on average than those produced by the summer generation. This is probably determined by the supply of food during the breeding period rather than by the average weight of mature females at the beginning of the breeding period. Measurements on individuals from a Swedish fjord show that the respiratory rate is relatively low and stable over the year. Bioenergetic budgets for the period prior to and during breeding reveal pronounced differences between individuals from the summer and winter generation, the latter being the more efficient at converting energy from a lower trophic level. It is suggested that the most important environmental factor governing this is the abundance of prey organisms.     

Critical threshold size for overwintering sandeels (<Emphasis Type="Italic">Ammodytes marinus</Emphasis>)

Several ecologically and commercially important fish species spend the winter in a state of minimum feeding activity and at lower risk of predation. To enable this overwintering behaviour, energetic reserves are generated prior to winter to support winter metabolism. Maintenance metabolism in fish scales with body size and increases with temperature, and the two factors together determine a critical threshold size for passive overwintering below which the organism is unlikely to survive without feeding. This is because the energetic cost of metabolism exceeds maximum energy reserves. In the present study, we estimated the energetic cost of overwintering from a bioenergetic model. The model was parameterised using respirometry-based measurements of standard metabolic rate in buried A. tobianus (a close relative to A. marinus) at temperatures from 5.3 to 18.3°C and validated with two independent long-term overwintering experiments. Maximum attainable energy reserves were estimated from published data on A. marinus in the North Sea. The critical threshold size in terms of length (L _th) for A. marinus in the North Sea was estimated to be 9.5 cm. We then investigated two general predictions: (1) Fish smaller than L _th display winter feeding activity, and (2) size at maturation of iteroparous species is larger than L _th to ensure sufficient energy reserves to accommodate both the metabolic cost of passive overwintering and reproductive investments. Both predictions were found to be consistent with data on size at maturation and total body energy in December and February.     

Foraging activity and resource use by three parrotfish species at Fernando de Noronha Archipelago, tropical West Atlantic

Parrotfishes are important components of the herbivore and detritivore guilds of tropical and subtropical reefs. Most of parrotfish species are protogynous hermaphrodites that change colour and sex, from initial phase females or males (IP) to terminal phase males (TP). We studied the foraging behaviour of Sparisoma amplum, S. axillare and S. frondosum, three syntopic scarids on the rocky reefs of Fernando de Noronha Archipelago, Tropical West Atlantic. The three parrotfish species differed in food selection and preference, but IP and TP individuals of the same species preferred the same food types, except for S. amplum. Feeding rates of IP individuals were higher than those of TP individuals, but the distribution of feeding frequencies throughout the day of IP and TP individuals of the same species was similar. IP individuals had higher feeding rates than TP ones, which seems related to the fact that TP individuals spend a large amount of time patrolling their territories and chasing away conspecific individuals at the study site. The general foraging pattern we found for S. amplum, S. axillare and S. frondosum is similar to patterns found for other parrotfish species in the Western Atlantic.     

A patch use model to separate effects of foraging costs on giving-up densities: an experiment with white-tailed deer (<Emphasis Type="Italic">Odocoileus virginianus</Emphasis>)

The giving-up density of food (GUD), the amount of food remaining in a patch when a forager ceases foraging there, can be used to compare the costs of foraging in different food patches. But, to draw inferences from GUDs, specific effects of foraging costs (predation risk, metabolic and missed opportunities costs) on GUDs have to be identified. As high predation risk, high metabolic costs and abundant food all should produce high GUDs, this does not allow us to infer directly the quality of a habitat. In order to separate the effect of each foraging cost, we developed an optimal foraging model based on food supplementation. We illustrate the use of our model in a study where we assessed the impact of a power line right-of-way in a white-tailed deer (Odocoileus virginianus) winter yard by determining whether the negative effects of cover loss outweigh the positive effects of browse regeneration.     

Energy expenditure and food consumption of foraging Imperial cormorants in Patagonia, Argentina

Energy management during the breeding season is crucial for central place foragers since parents need to feed themselves and their offspring while being spatially and temporally constrained. In this work, we used overall dynamic body acceleration as a measure of activity and also to allude to the foraging energy expenditure of breeding Imperial cormorants Phalacrocorax atriceps. We also analyzed how changes in the time or energy allocated to different activities affected the foraging trip energy expenditure and estimated the daily food requirements of the species. Birds spent 42 % of the total energy flying to and from the feeding areas and 16 % floating at sea. The level of activity underwater was almost 1.5 times higher for females than for males. The most expensive diving phase in terms of rate of energy expenditure was descending though the water column. The total foraging trip energy expenditure was particularly sensitive to variation in the amount of time spent flying. During the breeding season, adult cormorants breeding along the Patagonian coast would consume approximately 10,000 tons of food.     

Foraging behaviour and habitat use by brown skuas Stercorarius lonnbergi breeding at South Georgia

Top predators are critical to ecosystem function, exerting a stabilising effect on the food web. Brown skuas are opportunistic predators and scavengers. Although skuas are often the dominant land-based predator at seabird colonies, this is the first detailed study of their movements and activity during breeding. The study was carried out at Bird Island, South Georgia (54°00′S, 38°03′W), in the austral summer of 2011/2012 and included GPS data from 33 breeding adults tracked during the late incubation and early chick-rearing periods. Brown skuas spent on average more than 80 % of time in the territory, and it was extremely rare for both partners to leave the territory simultaneously. Much more time was spent foraging at the coast than in penguin colonies and, based on saltwater immersion data, adults never foraged at sea. None of the tracked birds appeared to specialise in catching small petrels at night. Fewer foraging trips were made per day, and hence, more time was spent in the territory, during incubation than chick-rearing. Despite the pronounced sexual size dimorphism, there were no effects of sex on territorial attendance, foraging time or habitat use. Skuas at Bird Island show higher territorial attendance and are less likely to leave the territory unattended than those breeding elsewhere, suggesting closer proximity to more diverse or abundant food resources than at other colonies. The results tie in with previous diet studies, indicating that brown skuas at this site feed mostly on seal placentae and carrion and that birds may rely on a broader range of food resources as the season progresses.     

Movements of vervets (Cercopithecus aethiops) and patas monkeys (Erythrocebus patas) as estimators of food resource size, density, and distribution

The effect of food resources on behavior has been difficult to measure. Here we use animals themselves to describe “effective” food abundance and distribution by comparing, relative to where individuals stopped to eat, movements of (1) adult females living in a small group of vervet monkeys (Cercopithecus aethiops) with those living in a large group and (2) vervets and patas monkeys (Erythrocebus patas). Although females in the large vervet group travelled farther and stopped to eat more often than females in the small vervet group, these differences resulted from foraging in Acacia drepanolobium habitat. In A. xanthophloea habitat, females in the large group travelled less far, travelled shorter distances between foods, and stopped as often as females in the small group. Greater foraging costs of females in larger vervet groups may be offset by access to home ranges of better quality. Compared to patas, vervets travelled shorter distances, moved shorter distances between food sites, stopped less often, and had longer feeding bouts, suggesting that foods of vervets are denser and larger, overall, than foods of patas. When vervets foraged in A. drepanolobium habitat, also the habitat of patas, their foraging behavior became more like that of patas. Vervets travelled farther, stopped more often, and spent less time at food sites in A. drepanolobium habitat than in A. xanthophloea habitat, suggesting that foods are smaller and less usurpable in A. drepanolobium habitat. Distance between foods, a component of food distribution, did not increase, however. The critical variable underlying usurpability of foods may be food site depletion time, a temporal measure. Received: 14 March 1997 / Accepted after revision: 19 October 1997     

Vertical distribution and feeding of the copepod<Emphasis Type="Italic"> Chiridius armatus</Emphasis>

Adult females of the copepod Chiridius armatus inhabited the lower half of a 200-m water column during winter and spring. Their distribution became gradually shallower during summer and autumn. Female C. armatus carried out diel vertical migrations, with shallower population distribution at night compared to day, although normally with few individuals captured in the upper 50 m. Enumeration of fecal pellets produced by freshly collected individuals showed that C. armatus was foraging throughout the year. Production of fecal pellets was at a minimum in February and at a maximum in April. Individuals from all depths produced fecal pellets, but the numbers seemed to decrease by depth. Numbers of fecal pellets were greatest for individuals captured at night. The omnivorous C. armatus consistently produced many more fecal pellets than the concurrently studied, strictly carnivorous copepod Paraeuchaeta norvegica. We ascribe the relatively high number of fecal pellets to the higher content of indigestible components in plant material than in animal food. The assumption of phytoplankton as a prevailing food source got qualitative support from scanning electron microscope pictures, showing that fecal pellets mainly contained phytoplankton.Communicated by L. Hagerman, Helsingør     

An energy–distance trade-off in a central-place forager,the Antarctic fur seal (<Emphasis Type="Italic">Arctocephalus gazella</Emphasis>)

We tested the prediction that lactating fur seals (Arctocephalus gazella) at South Georgia will take prey of greater energy density with increasing distance of foraging from the colony. The study investigated the differences in diet of fur seals foraging within two regions, one near the breeding colony and the other at greater distance. Diet varied significantly in relation to foraging location. Dietary items of low quality were eaten in both regions but more food items with a high-energy content appeared in the diet of seals travelling to distant oceanic waters. We conclude that there is likely to be a trade-off between energy gain and distance travelled which enables female fur seals to maintain a relatively constant rate of energy delivery to their offspring irrespective of the distance travelled to find food.     

Polymorphic color vision in white-faced capuchins (Cebus capucinus): Is there foraging niche divergence among phenotypes?

Many species of platyrrhine primates are characterised by sex-linked color vision polymorphism. This presents an opportunity to study the biology and ecology of individuals with different phenotypes living in the same group. Several evolutionary processes could maintain polymorphic genes in populations. In this study, we evaluate the hypothesis that foraging niche divergence among monkeys explains the presence of multiple color vision phenotypes. Specifically, we test whether dichromats and trichromats differ in foraging time devoted to cryptic vs brightly colored resources. We did not find any differences in foraging time spent on different food types by dichromatic and trichromatic monkeys in two groups of white-faced capuchins (Cebus capucinus) living in a tropical dry forest. We conclude that in so far as these variables are concerned, niche divergence does not likely explain color vision polymorphism in our study population.     

Autotomy shapes the trade-off between seeking cover and foraging in larval damselflies

Animals commonly choose between microhabitats that differ in foraging return and mortality hazard. I studied the influence of autotomy, the amputation of a body part, on the way larvae of the damselfly Lestes sponsa deal with the trade-off between foraging or seeking cover. Survival of Lestes larvae when confronted with the odonate predator Aeshna cyanea was higher in a complex than in a simple microhabitat, indicating that this more complex microhabitat was safer. Within the simple microhabitat, larvae without lamellae had a higher risk for mortality by predation than larvae with lamellae, showing a long-term cost of autotomy. When varying the foraging value (food present or absent) and predation risk (encaged predator or no predator) in the simple microhabitat, larvae with and without lamellae responded differentially to the imposed trade-off. All larvae spent more time in the simple microhabitat when food was present than when food was absent. Larvae without lamellae, however, only sporadically left the safe microhabitat, irrespective of the presence of the predator. In contrast, larvae with lamellae shifted more frequently towards the risky microhabitat than those without lamellae, and more often in the absence than in the presence of the predator. These decisions affected the foraging rates of the animals. I show for the first time that refuge use is higher after autotomy and that this is associated with the cost of reduced foraging success. The different microhabitat preferences for larvae with and without lamellae are consistent with their different vulnerabilities to predation and demonstrate the importance of intrinsic factors in establishing trade-offs. Received: 4 June 1999 / Received in revised form: 18 August 1999/ Accepted: 18 August 1999     

Dominance in feeding territories relates to foraging success and offspring growth in brown skuas <Emphasis Type="Italic">Catharacta antarctica lonnbergi</Emphasis>

Territoriality should lead to strict dominance, as territory holders typically control access to resources and exclude others from their use. In feeding territories, dominance should be reflected in foraging success and ultimately in reproduction differences; however, these successive links have rarely been made explicit. Therefore, we investigated a population of brown skuas Catharacta antarctica lonnbergi, in which only part of the breeding population occupied feeding territories within penguin colonies. We identified the dominance hierarchy and determined the foraging success of the participants in fights for access to penguin carcasses within the territories. Furthermore, we monitored offspring growth from parents with and without feeding territories. Our results indicated a clear dominance hierarchy with territorial birds in their own territory dominating over territorial breeders from other territories, non-territorial breeders and non-breeding birds. However, territory owners could not completely exclude others from access to food. Foraging success was positively related to dominance scores: The dominant territory owners received 63% of a carcass, whereas non-territorial pairs could get less than 10%. The link between foraging success and offspring development was less clear: Although male chicks of non-territorial parents suffered from lower growth rates and, thus, delayed fledging, there were no such differences in female chicks. Territoriality in skuas did not imply a complete occupation of food, but guaranteed optimal growth conditions for offspring. Non-territorial individuals were forced to search for alternative resources, and the restricted access to the preferred food resulted in inferior conditions for offspring development, making this foraging strategy less rewarding.