Sex-specific provisioning behaviour in a monomorphic seabird with a bimodal foraging strategy

Sexual differences in foraging and provisioning behaviour have been observed in several size-dimorphic seabird species. These differences are usually thought to be driven by size-related mechanisms such as the ability to compete for food or defend the nest. However, recent studies on monomorphic species suggest that sexual differences in foraging may arise independently of size. Selective forces driving sex-specific patterns are poorly known but essential to understand parental strategies. In this study, we examine sex differences in the provisioning behaviour of a monomorphic species, the Little Auk Alle alle . Using automated recording systems during two consecutive seasons at two colonies, we found that both sexes used a bimodal foraging strategy in which they regularly alternated single foraging trips of long duration with a cycle of several short-trips. The duration of long-trips was substantially longer in females than in males, and the sexes differed in the number of short-trips they performed in between long-trips, resulting in male-biased provisioning rates in both years. In species with a bimodal foraging strategy, long-trips have been interpreted as self-feeding trips to replenish body reserves. Our results therefore suggest that female Little Auks allocate more time to self-maintenance at the cost of chick provisioning, possibly due to different energetic constraints of the sexes prior and/or subsequent to chick-rearing. Our findings contribute to accumulating evidence that sex-specific foraging patterns may be widespread in sexually size-monomorphic seabird species.     

Fledging success of little auks in the high Arctic: do provisioning rates and the quality of foraging grounds matter?

Long-lived birds often face a dilemma between self-maintenance and reproduction. In order to maximize fitness, some seabird parents alternate short trips to collect food for offspring with long trips for self-feeding (bimodal foraging strategy). In this study, we examined whether temporal and spatial variation in the quality of foraging grounds affect provisioning and fledging success of a long-lived, bimodal forager, the little auk (Alle alle), the most abundant seabird species in the Arctic ecosystem. We predicted that an increase in sea surface temperature (SST), with an associated decrease in the preferred Arctic zooplankton prey, would increase foraging trip durations, decrease chick provisioning rates and decrease chick fledging success. Chick provisioning and survival were observed during three consecutive years (2008–2010) at two colonies with variable foraging conditions in Spitsbergen: Isfjorden and Magdalenefjorden. We found that a change in SST (range 1.6–5.4 °C) did not influence trip durations or provisioning rates. SST was, however, negatively correlated with the number of prey items delivered to a chick. Furthermore, provisioning rates did not influence chick’s probability to fledge; instead, SST was also negatively correlated with fledging probability. This was likely related to the prey availability and quality in the little auk’s foraging grounds. Our findings suggest that predicted warmer climate in the Arctic will negatively influence the ability of parents to provide their chicks, and consequently, the fledging prospects of little auk chicks.     

Diving pattern and performance in relation to foraging ecology in the gentoo penguin, Pygoscelis papua

We present data on diving pattern and performance (dive depth, duration, frequency and organization during the foraging trip) in gentoo penguins Pygoscelis papua , obtained using time-depth recorders ( n = 9 birds, 99 foraging trips). These data are used to estimate various parameters of foraging activity, e.g. foraging range, prey capture rates, and are compared in relation to breeding chronology. Foraging trip duration was 6 h and 10 h, and trip frequency 1.0/day and 0.96/day, during the brooding and creche periods, respectively. Birds spent on average 52%of each foraging trip diving. Dive depth and duration were highly bimodal: shallow dives (< 21 m) averaged 4 m and 0.23 min, and deep dives (> 30 m) 80 m and 2.5 min, respectively. Birds spent on average 71%and 25%of total diving time in deep and shallow dives, respectively. For deep dives, dive duration exceeded the subsequent surface interval, but shallow dives were followed by surface intervals 2–3 times dive duration. We suggest that most shallow dives are searching/exploratory dives and most deep dives are feeding dives. Deep dives showed clear diel patterns averaging 40 m at dawn and dusk and 80–90 m at midday. Estimated foraging ranges were 2.3 km and 4.1 km during the brood and creche period, respectively. Foraging trip duration increased by 4 h between the brood and creche periods but total time spent in deep dives (i.e. time spent feeding) was the same (3 h). Of 99 foraging trips, 56%consisted of only one dive bout and 44%of 2–4 bouts delimited by extended surface intervals > 10 min. We suggest that this pattern of diving activity reflects variation in spatial distribution of prey rather than the effect of physiological constraints on diving ability.     

Foraging by little auks in the distant marginal sea ice zone during the chick-rearing period

During the chick-rearing period, little auks Alle alle adopt a bimodal foraging strategy, alternating long trips with several short ones. It has been postulated that they reach more remote areas during long feeding trips than during short ones. However, the range of their foraging flights has never actually been measured. The aims of this study were to find the exact location of the little auk feeding grounds and to investigate whether they reach remote areas during long foraging trips using miniature GPS and temperature loggers. The study was conducted in 2009 in Magdalenefjorden (79°34′N, 11°04′E), one of the main breeding grounds of little auks on Spitsbergen. The temperature logger records indicated that during short trips, little auks visit warmer waters (situated close to the colony) than during long ones. The tracks of two GPS-equipped birds indicated that during long trips little auks foraged in the distant, food-abundant marginal sea ice zone, at least 100 km away from the colony. During long trips, birds make several stops at sea, perhaps sampling the foraging area with respect to prey distribution. Since food conditions near the studied colony are usually suboptimal, little auks may be exploiting distant feeding areas to compensate for the poorer-quality food available at nearby foraging grounds. The extended duration of long foraging trips may enable birds to collect food for chicks on food-abundant, remote foraging grounds as well as acquire, process and excrete food needed for self-maintenance, reducing the costs of flight to the colony.     

An energetic correlate between colony size and foraging effort in seabirds, an example of the Adélie penguin Pygoscelis adeliae

Central-place foraging seabirds alter the availability of their prey around colonies, forming a "halo" of reduced prey access that ultimately constrains population size. This has been indicated indirectly by an inverse correlation between colony size and reproductive success, numbers of conspecifics at other colonies within foraging range, foraging effort (i.e. trip duration), diet quality and colony growth rate. Although ultimately mediated by density dependence relative to food through intraspecific exploitative or interference competition, the proximate mechanism involved has yet to be elucidated. Herein, we show that Adélie penguin Pygoscelis adeliae colony size positively correlates to foraging trip duration and metabolic rate, that the metabolic rate while foraging may be approaching an energetic ceiling for birds at the largest colonies, and that total energy expended increases with trip duration although uncompensated by increased mass gain. We propose that a competition-induced reduction in prey availability results in higher energy expenditure for birds foraging in the halo around large colonies, and that to escape the halo a bird must increase its foraging distance. Ultimately, the total energetic cost of a trip determines the maximum successful trip distance, as on longer trips food acquired is used more for self maintenance than for chick provisioning. When the net cost of foraging trips becomes too high, with chicks receiving insufficient food, chick survival suffers and subsequent colony growth is limited. Though the existence of energetic studies of the same species at multiple colonies is rare, because foraging metabolic rate increases with colony size in at least two other seabird species, we suggest that an energetic constraint to colony size may generally apply to other seabirds.     

Adjustments in the Time,Distance and Direction of Foraging in Dinoponera quadriceps Workers

We measured individual decisions regarding the adjustments of time, distance and direction of foraging in Dinoponera quadriceps. We observed two colonies in an area of secondary Atlantic Forest, FLONA-ICMBio, in Northeastern Brazil. The workers were individually marked. We recorded the displacement of workers, their returns to the nest with and without food, the time spent searching for food, maximum and total distance, inter-trip latency and direction of trips. The time spent searching for food, maximum distance and transport velocity did not vary with food size. The previous trip success reduced the latency between foraging trips and increased the percentage of success on the next trip. However, this previous success did not demonstrate a significant variation relative to the time spent searching on the next trip or direction of search. The workers maintained an individual directional fidelity during foraging. The adjustments of these foraging variables under individual control contributed to the efficiency at the colony level. D. quadriceps is compatible with the central place theory and risk sensitivity model of behavior.     

Foraging strategy of the little auk Alle alle throughout breeding season – switch from unimodal to bimodal pattern

Energy and time allocation differs between incubation and chick‐rearing periods, which may lead to an adjustment in the foraging behaviour of parent birds. Here, we investigated the foraging behaviour of a small alcid, the little auk Alle alle during incubation and compared it with the chick‐rearing period in West Spitsbergen, using the miniature GPS (in Hornsund) and temperature loggers (in Magdalenefjorden). GPS‐tracking of 11 individuals revealed that during incubation little auks foraged 8–55 (median 46) km from the colony covering 19–239 (median 120) km during one foraging trip. Distance from the colony to foraging areas was similar during incubation and chick‐rearing period. During incubation 89% of foraging positions were located in the zone over shallower parts of the shelf (isobaths up to 200–300 m) with sea surface temperature below 2.5°C. Those environmental conditions are preferred by Arctic zooplankton community. Thus, little auks in the Hornsund area restrict their foraging (both during the incubation and chick‐rearing period) to the area under influence of cold, Arctic‐origin water masses where its most preferred prey, copepod Calanus glacialis is most abundant. The temperature logger data (from 4 individuals) indicate that in contrast to the chick‐rearing period, when parent birds alternated short and long trips, during the incubation they performed only long trips. Adopting such a flexible foraging strategy allows little auks to alter their foraging strategy to meet different energy and time demands during the two main stages of the breeding.     

Effects of prey abundance on the foraging behaviour, diving efficiency and time allocation of breeding Guillemots Uria aalge

The foraging behaviour of Guillemots Uria aalge at sea was compared between 2 years of radically different food abundance. Radio telemetry was used to determine foraging locations and diving patterns. In the poor compared with the good food year, foraging trips were much longer, the birds foraged more than six times further from their breeding sites, they spent over five times as much time diving when at sea and their estimated energy expenditure was twice as great. Time spent foraging in the poor food year was at the expense of time spent sitting at the colony. The duration of a foraging trip was a poor indicator of distance travelled but a good indicator of the amount of time spent diving. Mean dive durations, surface pause durations and interbout periods did not differ between years, but individuals made more than four times as many dives per diving bout in the poor food year. Surface pause lengths did not vary with water depth in either year. In the poor food year, birds made shorter surface pauses for a dive of a gi^ven duration than in the good food year, possibly accepting a lactic acid debt in order to maximize searching time, The duration of the interbout period was positively related to the number of dives in the previous bout, and dives tended to get shorter in long diving sequences, suggesting possible exhaustion effects. These data demonstrate that breeding Guillemots have the capacity to adjust their foraging behaviour and time budgets in response to changes in food abundance, but this flexibility was not sufficient to compensate fully for the very low food abundance experienced by birds in this study.     

Female and male Leach's Storm Petrels (Hydrobates leucorhous) pursue different foraging strategies during the incubation period

Reproduction in procellariiform birds is characterized by a single egg clutch, slow development, a long breeding season and obligate biparental care. Female Leach's Storm Petrels Hydrobates leucorhous, nearly monomorphic members of this order, produce eggs that are between 20 and 25% of adult bodyweight. We tested whether female foraging behaviour differs from male foraging behaviour during the ~ 44-day incubation period across seven breeding colonies in the Northwest Atlantic. Over six breeding seasons, we used a combination of Global Positioning System and Global Location Sensor devices to measure characteristics of individual foraging trips during the incubation period. Females travelled significantly greater distances and went farther from the breeding colony than did males on individual foraging trips. For both sexes, the longer the foraging trip, the greater the distance. Independent of trip duration, females travelled farther, and spent a greater proportion of their foraging trips prospecting widely, as defined by behavioural categories derived from a hidden Markov Model. For both sexes, trip duration decreased with date. Sex differences in these foraging metrics were apparently not a consequence of morphological differences or spatial segregation. Our data are consistent with the idea that female foraging strategies differed from male foraging strategies during incubation in ways that would be expected if females were still compensating for egg formation.     

Adverse foraging conditions may impact body mass and survival of a high Arctic seabird

Tradeoffs between current reproduction and future survival are widely recognized, but may only occur when food is limited: when foraging conditions are favorable, parents may be able to reproduce without compromising their own survival. We investigated these tradeoffs in the little auk (Alle alle), a small seabird with a single-egg clutch. During 2005-2007, we examined the relationship between body mass and survival of birds breeding under contrasting foraging conditions at two Arctic colonies. We used corticosterone levels of breeding adults as a physiological indicator of the foraging conditions they encountered during each reproductive season. We found that when foraging conditions were relatively poor (as reflected in elevated levels of corticosterone), parents ended the reproductive season with low body mass and suffered increased post-breeding mortality. A positive relationship between body mass and post-breeding survival was found in one study year; light birds incurred higher survival costs than heavy birds. The results of this study suggest that reproducing under poor foraging conditions may affect the post-breeding survival of long-lived little auks. They also have important demographic implications because even a small change in adult survival may have a large effect on populations of long-lived species.     

Divergent diving behavior during short and long trips of a bimodal forager,the little auk Alle alle

The purpose of this study was to characterize for the first time seabird diving behavior during bimodal foraging. Little auks Alle alle, small zooplanktivorous Alcids of the High Arctic, have recently been shown to make foraging trips of short and long duration. Because short (ST) and long trips (LT) are thought to occur in different locations and serve different purposes (chick‐ and self‐feeding, respectively) we hypothesized that foraging differences would be apparent, both in terms of water temperature and diving characteristics. Using Time Depth Recorders (TDRs), we tested this hypothesis at three colonies along the Greenland Sea with contrasting oceanographic conditions. We found that diving behavior generally differed between ST and LT. However, the magnitude of the disparity in diving characteristics depended on local foraging conditions. At the study site where conditions were favorable, diving behavior differed only to a small degree between LT and ST. Together with a lack of difference in diving depth and ocean temperature, this indicates that these birds did not increase their foraging effort during ST nor did they travel long distances to seek out more profitable prey. In contrast, where local foraging conditions were poor, birds increased their diving effort substantially to collect a chick meal during ST as indicated by longer, more U‐shaped dives with slower ascent rates and shorter resting times (post‐dive intervals and extended surface pauses). In addition, large differences in diving depth and ocean temperature indicate that birds forage on different prey species and utilize different foraging areas during LT, which may be up to 200 km away from the colony. Continued warming and deteriorating near‐colony foraging conditions may have energetic consequences for little auks breeding in the eastern Greenland Sea.     

Inter-population variation in the behaviour of adult and juvenile Red-footed Boobies Sula sula

Early life is a critical phase of the life cycle of animals and is attracting increased attention because little information is available on the behaviour of young individuals during this period. Behaviour during early life is probably influenced by the environmental conditions encountered by young animals, but data on intraspecific variation between breeding sites during this crucial period of life are limited. Here we study variability in the foraging behaviour of juveniles and adults in three colonies of a pantropical seabird, the Red-footed Booby Sula sula. Both adults and juveniles were measured and fitted with GPS loggers in three remote islands: Genovesa (Galapagos, Eastern Pacific Ocean), Europa (Western Indian Ocean) and Surprise (New Caledonia, Western Pacific Ocean). Foraging behaviour was compared between age-classes, sex and colonies by examining trip characteristics, different behaviours at sea, potential associations between individuals and morphological characteristics. Compared with adults, juveniles conducted shorter trips that were restricted to around the colony, especially on Genovesa (max. range: 203.4 ± 125.1 km and 3.6 ± 3.1 km, respectively). Juveniles appeared more constrained by poor flight skills and experience rather than by their morphology. Adults travelled 45% of the time during at-sea trips, whereas juveniles spent a a lower proportion of time travelling but foraged more often using an ‘area-restricted search’ behaviour, potentially training to catch prey. Associations between juveniles were commonly detected in the three colonies and occurred mostly during foraging, suggesting that social learning is an important strategy. Variability of morphometric measurements in both adults and juveniles was high between sites, with larger birds found on Genovesa. These results suggest that adaptations to local environmental conditions are already visible in their early life. Future studies should continue to investigate the behavioural flexibility of juvenile birds to better understand the effect of local environmental conditions during this critical stage of life.     

VARIATION IN THE FORAGING LOCATION OF ANTARCTIC FUR SEALS (ARCTOCEPHALUS GAZELLA) AND THE EFFECTS ON DIVING BEHAVIOR

This study investigated how female Antarctic fur seals adapt their foraging behavior, over time scales of days, to spatial unpredictability in the distribution of their food. Lactating Antarctic fur seals are central-place foragers that feed on highly patchy but spatially and temporally dynamic food. We measured the foraging distribution of 28 fur seals to test whether variation in foraging trip durations was reflected in variation in the location of foraging and the diving behavior of seals at sea. Based on the maximum distance travelled from the breeding beach, three categories of foraging trips were denned: those to the continental shelf area ( n = 12, median = 71 km), to oceanic water ( n = 11, median =164 km), and to farther offshore oceanic waters ( n = 5, median = 260 km). Trip duration and mean surface speed were positively correlated with the maximum distance travelled from the breeding beach. Seals on longer trips spent proportionally less of their time submerged, but there was no significant difference in the total number of dives or the total time spent foraging by seals in relation to trip duration. Evidence from this study and previous work investigating energy gain suggests that an animal on a longer foraging trip could potentially have a higher mean energy return per dive than a similar animal on a shorter foraging trip. Evidence presented suggests that the type of foraging trip (near or far) is not predetermined by the animal but may be a simple response to the stochastic distribution of the resources available.     

Patterns of energy acquisition by a central place forager: benefits of alternating short and long foraging trips

In some seabirds, foraging trips have been defined as eitherlong or short, with the length of time spent traveling to theforaging area apparently a critical feature in determining foragingtrip length. Using logger technology, together with complimentarydata from published studies, we investigated traveling and foragingtimes in 18 free-living Adélie Penguins Pygoscelis adeliae,which were foraging for chicks. Most deep, foraging dives weredistributed around the center of the foraging trip. This centraltendency was particularly apparent if the cumulative amountof undulations in the depth profile (indicative of prey capture)was considered during deep dives; values started to increasebefore 20.9% and ceased after 67.2% of the dives had occurred.This concentration of the feeding activity in the middle ofthe foraging trip indicates that birds traveled to and froma prey patch whose location varied little over the birds' trips.These data form the basis for a simple model that uses travelingand foraging times together with projected rates of prey ingestionand chick and adult gastric emptying to determine that thereare occasions when, to optimize rates of prey ingestion whileat sea for both adults and chicks, birds should conduct foragingtrips of bimodal lengths.     

Foraging and provisioning strategies of black-browed albatrosses in relation to the requirements of the chick: natural variation and experimental study

We studied regulation of the food supply to black-browed albatrosschicks at Kerguelen by simultaneously recording the provisioningrates achieved by individual parents and satellite trackingforaging birds during two seasons, by studying changes in adultmass, and by experimentally manipulating the food requirementof chicks. In 1994 black-browed albatrosses had a higher breedingsuccess and produced heavier chicks that grew faster than in1995. They spent a similar time foraging but brought heaviermeals to their chick in 1994. Satellite tracking indicated thatin both seasons birds foraged in the same oceanographic area,250 km from the colony. Travel times to and from this area remainedunchanged, and similar times were spent foraging there. In ourstudy area, black-browed albatrosses appear to rely on a foodresource that is predictable in location, but whose availabilityvaries from one year to the next. The principal difference betweenyears of differing food availability was that birds broughtlarger meals when food was more abundant Costs of commutingto nearby feeding areas are probably low and allow the deliveryof energy to the chick at a high rate. A study carried out in1991 indicated that there was no relationship between the changesin adult mass from one trip to the next and the duration offoraging trips or feed mass, suggesting that adult body conditionhad little influence on the provisioning strategy of this species.An experiment whereby some chicks were deprived of food andothers received supplementary food showed that parents of underfedchicks spent the same time foraging and brought slightly largeramounts of food to their chicks as control parents. We suggestthat parents are searching for food to the maximum limits oftheir ability and thus cannot reduce further foraging time,but underfed chicks can swallow more food. Parents of overfedchicks delivered less food and increased the time between feeds.The reduction in provisioning frequency is interpreted as thecapacity of parents to modify their foraging behavior accordingto the nutritional status of the chick, but the reduction offeed mass is probably the result of chicks being close to theirmaximum assimilatory capacity. Comparison between Procellariiformspecies indicates extensive differences in the degree to whichparents can regulate the supply of food to their chicks. Neriticspecies like black-browed albatrosses appear to have a reducedability to regulate, and especially to increase provisioningrates, whereas more pelagic species may have a greater regulationability     

Foraging trip duration of honeybee increases during a poor air quality episode and the increase persists thereafter

Increased concentration of airborne particulate matter (PM) in the atmosphere alters the degree of polarization of skylight which is used by honeybees for navigation during their foraging trips. However, little has empirically shown whether poor air quality indeed affects foraging performance (foraging trip duration) of honeybee. Here, we show apparent increases in the average duration of honeybee foraging during and after a heavy air pollution event compared with that of the pre‐event period. The average foraging duration of honeybees during the event increased by 32 min compared with the pre‐event conditions, indicating that 71% more time was spent on foraging. Moreover, the average foraging duration measured after the event did not recover to its pre‐event level. We further investigated whether an optical property (Depolarization Ratio, DR) of dominant PM in the atmosphere and level of air pollution (fine PM mass concentration) affect foraging trip duration. The result demonstrates the DR and fine PM mass concentration have significant effects on honeybee foraging trip duration. Foraging trip duration increases with decreasing DR while it increases with increasing fine PM mass concentration. In addition, the effects of fine PM mass concentration are synergistic with overcast skies. Our study implies that poor air quality could pose a new threat to bee foraging.     

Corticosterone and foraging behavior in a pelagic seabird

Because endocrine mechanisms are thought to mediate behavioral responses to changes in the environment, examining these mechanisms is essential for understanding how long-lived seabirds adjust their foraging decisions to contrasting environmental conditions in order to maximize their fitness. In this context, the hormone corticosterone (CORT) deserves specific attention because of its major connections with locomotor activities. We examined for the first time the relationships between individual CORT levels and measurements of foraging success and behavior using satellite tracking and blood sampling from wandering albatrosses (Diomedea exulans) before (pretrip CORT levels) and after (posttrip CORT levels) foraging trips during the incubation period. Plasma CORT levels decreased after a foraging trip, and the level of posttrip CORT was negatively correlated with individual foraging success, calculated as total mass gain over a foraging trip. Pretrip CORT levels were not linked to time spent at sea but were positively correlated with daily distance traveled and maximum range at sea. In this study, we were able to highlight the sensitivity of CORT levels to variation in energy intake, and we showed for the first time that individual CORT levels can be explained by variation in foraging success. Relationships between pretrip CORT levels and daily distance traveled and maximum range were independent of pretrip body mass, suggesting that slight elevations in pretrip CORT levels might facilitate locomotor activity. However, because both foraging behavior and pretrip CORT levels could be affected by individual quality, future experimental studies including manipulation of CORT levels are needed to test whether CORT can mediate foraging decisions according to foraging conditions.     

Diurnal foraging routines in a tropical bird,the rock finch Lagonosticta sanguinodorsalis: how important is predation risk?

An animal's foraging decisions are the outcome of the relative importance of the risk of starvation and predation. Fat deposition insures against periods of food shortage but it also carries a cost in terms of mass dependent predation risk due to reduced escape probability and extended exposure time. Accordingly, birds have been observed to show a unimodal foraging pattern with foraging concentrated at the end of the day under conditions of predictable food resources and high predation risk. We tested this hypothesis in a tropical granivorous finch, the rock firefinch Lagonosticta sanguinodorsalis , in an outdoor aviary experiment during which food was provided ad lib and the risk of predation was varied by providing food either adjacent to, or 5 m away from cover. Rock firefinches showed a bimodal foraging pattern regardless of the risk of predation at which they fed. The results suggest that predation is relatively unimportant in shaping their daily feeding pattern despite mass gain during the day being similar to temperate birds. Foraging patterns closely follow diurnal temperature variation and this is suggested to be the main determinant of the observed bimodal pattern.     

The relationship between foraging behaviour and energy expenditure in Antarctic fur seals

By using time-depth recorders to measure diving activity and the doubly-labelled water method to determine energy expenditure, the relationship between foraging behaviour and energy expenditure was investigated in nine Antarctic fur seal females rearing pups. At-sea metabolic rate (MR) (mean of 6.34 ± 0.4 W. kg^-1; 4.6 times predicted BMR) was positively correlated to foraging trip duration (mean of 4.21 ± 0.54 days; r²= 0.5, P < 0.04). There were no relationships between MR and the total number of dives, the total time spent diving or the total vertical distance travelled during the foraging trip. There was, however, a close negative sigmoidal relationship (r²= 0.93) between at-sea MR and the proportion of time at sea spent diving. This measure of diving behaviour may provide a useful, inexpensive means of estimating foraging energy expenditure in this species and possibly in other otariids. The rate of diving (m.h^-1) was also negatively related to at-sea MR (r²= 0.69, P < 0.005). Body mass gain during a foraging trip had a positive relationship to the time spent at sea (r²= 0.58, P < 0.02) and the total amount of energy expended while at sea (r²= 0.72, P < 0.004) such that, while females undertaking long trips have higher metabolic rates, the energetic efficiency with which females gain mass is independent of the time spent at sea. Therefore, within the range of conditions observed, there is no apparent energetic advantage for females in undertaking foraging trips of any particular duration.     

Foraging and provisioning in Antarctic fur seals: interannual variability in time-energy budgets

This study examined three competing hypotheses to explain howlactating Antarctic fur seals (Arctocephalus gazella) respondto changes in the level of resource availability. Antarcticfur seals have episodic bouts of suckling (1-3 days), alternatingwith foraging trips (3-10 days). Foraging time budgets variedsignificantly (p <.001) among 8 consecutive years at BirdIsland, South Georgia. Foraging trip duration increased during periodsof relative food shortage. Time spent ashore was more consistentamong years than foraging trip duration but declined duringa year of particularly low food availability. In 4 of the 8years, there was a significant positive correlation betweentime spent ashore and foraging trip duration. In the other years,the relationship was close to statistical significance. Energydelivery to pups during suckling bouts followed an asymptoticpower function. Energy gain during foraging trips was estimatedfrom diving behavior, which suggested that the energy gain functionwas linear. Distance traveled during foraging trips was correlatedwith foraging trip duration, and long foraging trips were associatedwith reduced foraging intensity. There was support for the hypothesisthat lactating Antarctic fur seals compensate for reduced resources byincreasing the foraging trip duration rather than working harderand increasing their energy expenditure. However, there wasmost support for the hypothesis that lactating Antarctic furseals adjust time spent ashore as well as foraging trip duration,possibly to maximize the delivery of food to their offspring.Lactation appears to impose constraints on provisioning of offspringthat differ from those of seabirds foraging in the same environment andoften on the same prey.