Foraging trip decisions by the streaked shearwater Calonectris leucomelas depend on both parental and chick state

Parents of albatross and shearwater species employ a dual foraging strategy, feeding their chicks quickly in repeated short trips and then restoring their own fuel reserves during longer trips. A decline in parental body condition is believed to trigger longer trips, but chick body condition and age may also play a role. To investigate these factors in the little-studied streaked shearwater Calonectris leucomelas, we monitored the nest attendance of 17 pairs on Mikura Island in 2005 using an automated identification system. We also monitored body mass changes and meal masses of 5 of the 17 pairs using an automated weighing system. Although the birds did not show a clear dual foraging pattern, trip duration varied widely from 1 to 15 days. On average, the birds fed chicks 67.6 g during nighttime meals at 2.74-day intervals. Since meal mass did not depend on trip duration, feeding efficiency (meal mass delivered per unit trip duration) decreased as trip duration increased. Parents accumulated more energy reserves when they took longer trips. Parents appeared likely to initiate longer trips when their body condition declined or chick body condition recovered.     

Spatial and temporal variation in the provisioning behaviour of female rockhopper penguins Eudyptes chrysocome filholi

As seabirds are central place foragers during breeding, their provisioning behaviour and their ability to face variable energy demand from the chicks is expected to vary with environmental conditions. The provisioning behaviour of female rockhopper penguins Eudyptes chrysocome filholi was recorded over the chick‐rearing period at Kerguelen (KER) and Crozet (CRO) archipelagoes (two very distinct marine environments), using time‐depth recorders, or VHF transmitters coupled with an automatic recording station. No influences of the method have been found on the average foraging trip durations. Some previously undescribed short and multiple trips within a day were recorded using the automatic recording system. These multiple trips (6.8 h) were mostly performed with <5 days old chicks, a period during which feeding rates were the highest (1.1 meals per day), at both sites. During the brooding period, both KER and CRO females mainly performed daily trips of increasing duration (2 h longer at CRO) and at decreasing frequency. During the crèche compared to the brooding period, females from KER performed slightly fewer daily trips (0.6 per day) and more (<3 days) overnight trips, while females from CRO performed very few daily trips (0.1 per day) and more overnight trips, some of them being long trips lasting 5 to 29 days, mostly initiated during the transition between the brooding and the crèche periods. The result fit the hypothesis that long trips permit females to restore and/or maintain their body condition at more distant foraging places. It seemed that chick developement during the brooding period and environmental factors during the crèche period conditioned trip duration of females. Due to more long trips at CRO, the female feeding frequency was twice as high at KER than at CRO during the crèche period, while males participated in the feeding duties. Based on differences in female behaviour, we hypothesize that the male's contribution is likely to differ strongly from one site to another, and may buffer the possible decrease in female feeding frequency by feeding the chicks if food is less abundant.     

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.     

Biomechanical and age-related differences in balance recovery using the tether-release method.

This paper reviews experimental studies that have used the tether-release method to examine the biomechanical and age-related differences in the stepping response used after a simulated fall. The tether-release method has been used to create a repeatable perturbation that simulates the initial unbalanced configuration of the body during a trip or slip. In this technique, the test subject is held in an initial forward or backward inclined position by means of a horizontal tether or cable. To initiate a fall, the subject is released from this position after a short time delay. This review focuses on studies that have explored various biomechanical parameters in an effort to understand what attributes allow for successful balance recovery by stepping or stumbling. Strong associations between recovery ability and biomechanical parameters such as step length, step timing, and joint torques point to the importance of neuromuscular capacities that relate to lower extremity flexibility, reaction time, and strength. Therefore, the maintenance or enhancement of these necessary attributes should be considered when developing exercise-based fall intervention programs for older adults.     

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.     

Overnight foraging trips by chick‐rearing Nazca Boobies Sula granti and the risk of attack by predatory fish

Most tropical booby species complete breeding foraging trips within daylight hours, thus avoiding nights at sea. Nazca Boobies Sula granti are unusual in this respect, frequently spending one or more nights away from the nest. We used GPS dataloggers, time‐depth recorders, and changes in body weight to characterize foraging trips and to evaluate potential influences on the decisions of 64 adult Nazca Boobies to spend a night at sea, or to return to their chicks on Isla Española, Galápagos, in daylight hours. The tagged birds foraged east of Isla Española, undertaking both single‐day (2–15 h, 67% of trips) and overnight trips (28 h–7.2 days, 33%), and executing 1–19 foraging plunge‐dives per single‐day trip. Birds might forage longer if they are in nutritional stress when they depart, but body weight at departure was not correlated with trip length. Birds might be expected to return from longer trips with more prey for young, but they returned from single‐day and overnight trips with similar body weights, consistent with previous indications that Nazca Boobies forage until accumulating a target value of prey weight. Birds with a lower dive frequency during the first 5 h of a trip were more likely to spend the night at sea, suggesting that they might choose to spend the night at sea if prey capture success was low. At night, birds almost never dived and spent most of their time resting on the water’s surface (11.8–12.1 h, > 99% of the time between civil sunset and civil dawn). Thus, the night is an unproductive time spent among subsurface predators under low illumination. The birds’ webbed feet provided evidence of this risk: 24% of birds were missing > 25% of their foot tissue, probably due to attacks by predatory fish, and the amount of foot tissue lost increased with age, consistent with a cumulative risk across the lifespan. In contrast, other tropical boobies (Blue‐footed Sula nebouxii and Brown Boobies Sula leucogaster), which do not spend the night on the water, showed no such damage. These results suggest that chick‐rearing Nazca Boobies accept nocturnal predation risk on occasions of low prey encounter during a foraging trip’s first day.     

Development of foraging behavior in juvenile northern elephant seals

Rapid development of foraging ability is critical for phocids. In northern elephant seals Mirounga angustirostris , juvenile survivorship is low compared with adults and foraging difficulties are potentially associated with increased mortality. At Año Nuevo, California, foraging behavior of nine juvenile females during their third foraging migration and five juvenile females on their fourth foraging migration were documented using a variety of commercially available and custom time depth recorders. Foraging success, diving ability, time at depth, bouts of behavior and body composition changes were compared between trips to sea. There were no significant differences in foraging success measured as mass gain between the third and fourth trips to sea. There were differences in how energy was deposited between lean and adipose tissue compartments. Diving ability developed between trips to sea, reflected in significant increases in depth, dive duration and bottom time. Development also occurred within trips to sea. Depth, dive duration and bottom time increased with time at sea. Aerobic capacity appears to increase between the third and fourth trip, with a significantly increased percentage of total time submerged and a significantly lower diving rate. All juveniles on the fourth trip and four out of nine juveniles on the third trip followed marked diel patterns, foraging deep during the day and shallow at night. Like adults, juveniles appeared to stay primarily aerobic with surface intervals independent of dive durations. These results confirm that female juvenile northern elephant seals undergo important developmental changes in foraging behavior between the third and fourth trip, but these changes do not significantly impact foraging success.     

Diving pattern and performance in the macaroni penguin Eudptes chrysolophus

The pattern and characteristics of diving in two female macaroni penguins Eudyptes chrysolophus was studied, during the brooding period, using continuous-recording time-depth recorders, for a total of I8 days (15 consecutive days) during which the depth, duration and timing of 4876 dives were recorded. Diving in the first 11 days was exclusively diurnal, averaging 244 dives on trips lasting 12 hours. Near the end of the brooding period trips were longer and included diving at night. About half of all trips (except those involving continuous night-time diving) was spent in diving and dive rate averaged 14–25 dives per hour (42 per hour at night). The duration of day time dives varied between trips, and averaged 1.4–1.7 min, with a subsequent surface interval of 0.5–0.9 min. Dive duration was significantly directly related to depth, the latter accounting for 53% of the variation. The average depths of daytime dives were 20–35 m (maximum depth 11 5 m). Dives at night were shorter (average duration 0.9 min) and much shallower (maximum 11 m); depth accounted for only 6% of the variation in duration. Estimates of potential prey capture rates (3–5 krill per dive; one krill every 17–20 s) are made. Daily weight changes in chicks were directly related to number of dives, but not to foraging trip duration nor time spent diving. Of the other species at the same site which live by diving to catch krill, gentoo penguins forage exclusively diurnally, making longer. deeper dives; Antarctic fur seals, which dive to similar depths as macaroni penguins, do so mainly at night.     

Central place foraging in larvae of the charaxine butterfly,Polyura pyrrhus (L.): A case study in a herbivore

Central place foraging by larvae of the charaxine butterfly,Polyura pyrrhus, was studied. Larvae made foraging trips from the silken pads they constructed on leaflets of their foodplant,Acacia sp. A foraging trip sometimes involved complete depletion of a single patch of foodplant pinnules. Larvae which did not deplete a patch appeared to eat until they were satiated, whereas larvae which depleted a patch either visited another patch (multiple-patch foraging) or returned directly to the pad (single-patch foraging). If the food intake at the first patch was small a larva tended to make a “multiple-patch” decision, especially when the pinnule-patch was distant from the resting pad. The duration between successive foraging trips (resting time on the pad) was much longer than the round trip duration: on average about 3 h and 15 min, respectively. The resting time is suggested to be a handling time (i.e., digesting food in the gut) and was disproportional to the amount of food consumed, i.e., the handling efficiency was higher when the larva consumed a larger amount of food. This may be the reason why larvae usually ate until they were satiated. A food-intake-rate maximizing model was constructed to describe the decision rule as to whether a larva should make a single-patch or a multiple-patch foraging trip. One of the model's predictions (i.e., larvae should engage in multiple-patch foraging when the food intake at the first patch is small) qualitatively corresponds with data, however, the model does not explain the effect of travelling time on decision making in larvae. Several other factors which may influence the decision making of larvae are discussed.     

Flipper bands do not affect foraging‐trip duration of Magellanic Penguins

ABSTRACT Flipper bands are used to mark penguins because leg bands can injure their legs. However, concerns remain over the possible effects of flipper bands on penguins. We examined the effects of stainless‐steel flipper bands on the duration of foraging trips by Magellanic Penguins (Spheniscus magellanicus) at Punta Tombo, Argentina, using an automated detection system. We predicted that, if bands were costly and increased drag, flipper‐banded penguins would make longer foraging trips than those with small or no external markings. We tagged 121 penguins with radio‐frequency identification (RFID) tags and an additional external mark. We placed either a stainless‐steel band on the left flipper (N= 62) or a 2×10‐mm small‐animal ear tag in the outside web of the left foot (N= 59). We measured foraging‐trip durations (N= 376 trips) for 68 adult penguins with chicks from 15 December 2007 to 28 February 2008. Contrary to predictions, trip duration was similar for banded and web‐tagged penguins (P= 0.22) and for males and females (P= 0.52), with no interaction between tag type and sex (P= 0.52). No penguins marked in the 2007 breeding season and recaptured between 30 September and 30 November 2008 (N= 113) lost flipper bands or web tags, but three RFID tags failed between March and September 2008. Properly designed and applied flipper bands were a reliable marking method for Magellanic Penguins, had a lower failure rate than RFIDs, and did not affect foraging‐trip duration.     

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.     

SUMMER DIVING BEHAVIOR OF MALE WALRUSES IN BRISTOL BAY, ALASKA

Pacific walruses ( Odobenus rosmarus divergens ) make trips from ice or land haul-out sites to forage for benthic prey. We describe dive and trip characteristics from time-depth-recorder data collected over a one-month period during summer from four male Pacific walruses in Bristol Bay, Alaska. Dives were classified into four types. Shallow (4 m), short (2.7 min), square-shaped dives accounted for 11% of trip time, and many were probably associated with traveling. Shallow (2 m) and very short (0.5 min) dives composed only 1% of trip time. Deep (41 m), long (7.2 min), square-shaped dives accounted for 46% of trip time and were undoubtedly associated with benthic foraging. V-shaped dives ranged widely in depth, were of moderate duration (4.7 min), and composed 3% of trip time. These dives may have been associated with navigation or exploration of the seafloor for potential prey habitat. Surface intervals between dives were similar among dive types, and generally lasted 1–2 min. Total foraging time was strongly correlated with trip duration and there was no apparent diel pattern of diving in any dive type among animals. We found no correlation between dive duration and postdive surface interval within dive types, suggesting that diving occurred within aerobic dive limits. Trip duration varied considerably within and among walruses (0.3–9.4 d), and there was evidence that some of the very short trips were unrelated to foraging. Overall, walruses were in the water for 76.6% of the time, of which 60.3% was spent diving.     

Luxury in freshwater and stress at sea? The foraging of the Common Tern Sterna hirundo

We compared the foraging strategies of Common Terns Sterna hirundo in freshwater (Lake Jeziorsko, Brzeg, Poland) and marine environments (Minsener Oldeoog, German Wadden Sea). Body mass changes, nest relief and duration and number of feeding trips per day were studied by automatically weighing the adults, using electronic balances under the nests. At the freshwater site, adults were lighter both before and after feeding and gained less mass during a trip. in the Wadden Sea, single feeding trips lasted longer than at the freshwater site and the terns made fewer trips per day. To achieve the same mass gain per day as in birds in freshwater, trips at sea had to be longer and food intake per trip was higher. The daily duration of absence for feeding and the daily mass gain were about the same in both areas. The limnetic feeders finished foraging earlier in the evening than the terns foraging at sea. These differences are consistent with the hypothesis that limnetic prey availability was consistent, whereas the tides limited the availability of marine prey. In consequence, foraging over freshwater presents several advantages, such as higher colony attendance, better mate coordination and better parental care.     

Nest attendance and foraging movements of northern fulmars rearing chicks at Bjørnøya Barents Sea

We studied several aspects of the foraging ecology of fulmars rearing young chicks on Bjørnøya. To determine precisely the duration of foraging trips during the brooding period, we used an automated logging system that recorded the presence of fulmars fitted with transponders. We also tracked, with satellite transmitters, four parent fulmars during the brooding period, and two after the chick had been left alone. When brooding the chick, fulmars appeared to alternate very rapidly on the nest, with foraging trips lasting on average 8?h. This period appeared constraining for the birds since parents lost mass. The growth of chicks was dependent on the ability of the female (and not the male) to do short foraging trips. At this time birds are foraging at an average distance of 60?km from the colony, with birds concentrating on the shelf around Bjørnøya. They did not return from one trip to the next to the same foraging area. As the season progressed and the chicks were left alone on the nest, parents increased the duration and maximum range of foraging trips as well as the distance covered. However, they still perform a succession of relatively short foraging trips to the east of the Bjørnøya shelf but they interspersed these short trips with longer foraging trips. One bird returned twice to the same site along the Norwegian coast 570?km from Bjørnøya, the other foraged at 580?km in the mid-Barents Sea. Average flight speed including time spent on the water was 28?km/h and reached 70?km/h during bouts of more than 1?h when the bird was probably continuously in flight.     

The influence of food supply on the breeding ecology of Kittiwakes Rissa tridactyla in Shetland

We measured the breeding performance, body condition, time budgets and foraging ranges of Kittiwakes Rissa tridactyla at Sumburgh Head, Shetland, in two years of contrasting food availability. Kittiwakes in Shetland generally feed their young almost entirely on sandeels, and fisheries data indicated that stocks of sandeels in Shetland waters were at least ten times higher in 1991 than in 1990. Fledging success of Kittiwakes was nil in 1990 and 68% of eggs laid in 1991, although clutch-size and hatching success were no different between years. Post-hatching foraging trips in 1991 were of comparable duration to those recorded at other colonies in conditions of good food supply (2–3 h), while trips recorded during incubation or post-hatching in 1990 were approximately three times longer on average than at corresponding stages of the breeding season in 1991. Radio-tracking data indicated that adults generally stayed within 5 km of the colony in 1991 but flew more than 40 km from the colony on each trip in 1990. Eggs were apparently not left unattended in either year, despite the fact that this required adults to incubate for periods in excess of 44 h in 1990. The extent to which adults were able to increase trip durations, foraging ranges and incubation shift lengths between years, while maintaining hatching success, indicates the degree to which Kittiwakes are normally buffered against adverse feeding conditions during incubation. Reduced nest attendance and lower body-condition of adults post-hatching in 1990, in conjunction with complete post-hatching breeding failure, indicate that adults were beyond the limits of their buffering capacity during chick-rearing in 1990.     

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.     

Satellite tracking of Adélie penguins

Summary Female Adélie penguins (Pygoscelis adeliae) that take too long on their first post-laying foraging trip are a major cause of breeding failure, but in the ice-filled waters of Antarctica, determining where they go and why they are away so long has proved difficult. Here we describe the first successful attempt to track penguins at sea using satellite telemetry. Four females foraged in different locations, dispelling the notion of a common feeding ground. They moved up to 272 km from the rookery and covered from 551 to 1,121 km on their trips, swimming at minimum average speeds around 1.2 m/s. The birds were most likely to be in the water between 0630 and 1430 when light intensity, important for a visual predator, was greatest. Carrying the transmitters reduced rates of fat deposition (weight gain), increasing the duration of foraging trips of females, and suggested that they may forage until their fat depots reach a minimum threshold level. This has two implications: (i) durations of these postlaying foraging trips could potentially be used as an indicator of krill abundance (Euphausia sp), the almost exclusive food of Adélie penguins during this period, and (ii) any reduction in krill stocks caused by harvesting could increase foraging trip durations with a concomitant increase mi breeding failures.     

Infected honeybee foragers incur a higher loss in efficiency than in the rate of energetic gain

Parasites, by altering the nutritional and energetic state of their hosts, can significantly alter their foraging behaviour. In honeybees, an infection with Nosema ceranae has been shown to lower the energetic state of individual bees, bringing about changes in behaviours associated with foraging. Comparing the foraging trip times, hive times in between trips, and the crop contents of uninfected and infected foragers as they depart on foraging trips and return from them, this study examined how any differences in these variables influence alternative foraging currencies. The results show that infected bees take longer foraging trips, spend shorter time in the hive between successive trips and bring back less sugar from each trip. These changes have a stronger adverse effect on their efficiency of energetic gain as compared with their rate of energetic gain, which has important implications for individual and colony life history.     

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.     

Sex‐specific food provisioning in a monomorphic seabird,the common guillemot Uria aalge: nest defence,foraging efficiency or parental effort?

Sexual differences in food provisioning rates of monomorphic seabirds are well known but poorly understood. Here, we address three hypotheses that attempt to explain female-biased food provisioning in common guillemots Uria aalge : (1) males spend more time in nest defence, (2) females have greater foraging efficiency, and (3) males allocate a greater proportion of foraging effort to self-maintenance. We found that males spent no more time with chicks than females but made longer trips and travelled further from the colony. There was extensive overlap between sexes in core foraging areas, indicating that females were not excluding males from feeding opportunities close to the colony. However, as a result of their longer trips, the total foraging areas of males were much greater than those of females. There was no difference between sexes in overall dive rate per hour at sea, in behaviour during individual dives or in a number of other measures of foraging efficiency including the frequency, depth and duration of dives and the dive: pause ratio during the final dive bout of each trip, which was presumably used by both sexes to obtain prey for the chick. These data strongly suggest that sexes did not differ in their ability to locate and capture prey. Yet males made almost twice as many dives per trip as females, suggesting that males made more dives than females for their own benefit. These results support the hypothesis that female-biased food provisioning arose from a difference between sexes in the allocation of foraging effort between parents and offspring, in anticipation of a prolonged period of male-only post-fledging care of the chick, and not from differences in foraging efficiency or time spent in nest defence.