Interactions with Great Skuas Stercorarius skua as a factor in the long‐term decline of an Arctic Skua Stercorarius parasiticus population

Many of the UK’s seabird species have displayed high variation in breeding success since the 1980s, largely due to changes in the availability of Lesser Sandeels Ammodytes marinus, their main prey. During this time, Arctic Skuas Stercorarius parasiticus experienced a rapid decline in the UK and the species has subsequently been placed on the Red List of birds of conservation concern. Although shortage of Lesser Sandeels is likely to be an influential factor, the Arctic Skua’s breeding range overlaps with that of the Great Skua Stercorarius skua, a larger bird with a more varied diet, and interspecific interactions for nesting habitat may exert an additional pressure on Arctic Skua breeding populations. Results from four censuses, spanning 21 years, were used to model habitat use and analyse distributional change in nesting Arctic Skuas at a major colony located on Fetlar, Shetland, Scotland. The decline in Arctic Skuas was not uniform across the island and competition with Great Skuas for nest‐sites appears to have influenced localized breeding distribution. By 2006, Arctic Skuas had been almost entirely excluded from shrub heath, blanket bog and coastal heath habitats, which were identified as preferred habitat in 1986. In 2006, Arctic Skua breeding territories were mainly restricted to one core area of preferred habitat where over 90% nested in high density as this habitat became increasingly occupied by Great Skuas. The more generalist foraging habit of the Great Skua allowed the population to grow rapidly as numbers of the more specialist Arctic Skua decreased during times of low sandeel availability. Our model suggests that both interspecific competition for territories with Great Skuas and food limitation have played important roles in the decline of Arctic Skuas on Fetlar.     

Feeding territoriality and the reproductive consequences in brown skuas<Emphasis Type="Italic"> Catharacta antarctica lonnbergi</Emphasis>

In the maritime Antarctic, brown skuas (Catharacta antarctica lonnbergi) show two foraging strategies: some pairs occupy feeding territories in penguin colonies, while others can only feed in unoccupied areas of a penguin colony without defending a feeding territory. One-third of the studied breeding skua population in the South Shetlands occupied territories of varying size (48 to >3,000 penguin nests) and monopolised 93% of all penguin nests in sub-colonies. Skuas without feeding territories foraged in only 7% of penguin sub-colonies and in part of the main colony. Females owning feeding territories were larger in body size than females without feeding territories; no differences in size were found in males. Territory holders permanently controlled their resources but defence power diminished towards the end of the reproductive season. Territory ownership guaranteed sufficient food supply and led to a 5.5 days earlier egg-laying and chick-hatching. Short distances between nest and foraging site allowed territorial pairs a higher nest-attendance rate such that their chicks survived better (71%) than chicks from skua pairs without feeding territories (45%). Due to lower hatching success in territorial pairs, no difference in breeding success of pairs with and without feeding territories was found in 3 years. We conclude that skuas owning feeding territories in penguin colonies benefit from the predictable and stable food resource by an earlier termination of the annual breeding cycle and higher offspring survivorship.Research licence: Umweltbundesamt Bonn 13.4-94003-1/5-7.     

The influence of food availability on the breeding effort and reproductive success of Arctic Skuas Stercorarius parasiticus

Territorial attendance, chick growth rate and breeding success of Arctic Skuas Stercorarius parasiticus in Shetland were lowest in the late 1980s when recruitment of Sandeels Am-modytes marinus in the surrounding waters was poor. The relationships between both fisheries-based and avian indices of food availability and annual variation in Arctic Skua chick growth and breeding success between 1976 and 1994 were better described by a threshold effect rather than linear functions. Arctic Skuas conform to the model proposed by Cairns, which predicts the responses of seabirds to changes in prey availability. Skua clutch size, egg volume, hatching success and hatching date were not reliable indices of Sandeel availability. However, annual fluctuations in Arctic Skua breeding numbers may be a useful indicator of changes in prey abundance.     

Heat stress in a high-latitude seabird: effects of temperature and food supply on bathing and nest attendance of great skuas Catharacta skua

Birds such as great skuas Catharacta skua adapted for successful breeding at high latitudes may experience problems of heat dissipation in mild climates. Great skuas spend time bathing at freshwater sites close to breeding territories and here, we examine impacts of heat stress on bathing, foraging and nest attendance of adults during three breeding seasons with marked variation in the availability of prey (1-group sandeels Ammodytes marinus ). Adults exhibited diurnal variation in bathing activity that matched heat-stress conditions. Moreover more birds bathed on days of higher average heat stress, suggesting that bathing plays a role in thermoregulation. Bathing numbers were lower in years of poor food availability, when adult attendance at territories was low, probably because lower attendance reduced the opportunity for parents to bathe without leaving chicks unattended. Chicks are normally guarded by female parents and fed by males but under conditions of low food availability territorial attendance of breeding pairs was particularly low on days of high heat stress, with chicks regularly left unattended at air temperatures exceeding 14°C. Unattended chicks are at risk of being killed by neighbouring conspecifics and survival of chicks to fledging was low in the two years of low sandeel stocks. Our study indicates that for great skuas, indirect effects of climate change on prey stocks and direct effects on heat stress experienced by adults may be additive.     

The effects of changes in food availability on the breeding ecology of great skuas Catharacta skua in Shetland

Great skuas on Foula, Shetland have responded to a decline in the availability of sandeels since the late 1970s by increasing the proportion of other items in their diets. This change is correlated with the annual recruitment of sandeels in Shetland waters. Since 1983 there has been a 10-fold increase in predation by great skuas upon other seabirds, as Furness & Hislop (1981) suggested might occur in response to a low availability of sandeels. Changes in diet have been accompanied by a 50% reduction in adult territorial attendance as adults increased their foraging effort, such that between 1987 and 1989 breeding adults were probably working as hard as they were able to. Despite this, breeding success was less than 40% in 1987 and less than 15% in 1988 and 1989. The major cause of breeding failure was predation of unguarded chicks by adults from neighbouring territories. The willingness of adults to expose their chicks to high predation risk is probably maintained because of a positive correlation between chick pre-fledging growth and post-fledging survival, which is expressed up to the age of two years and which will place a strong pressure upon adults to feed their chicks as well as possible. The high expenditure of effort by adults in 1987 and 1988 did not affect the weights of those birds incubating eggs in 1988 and 1989, but there was a slight (3%) decrease in egg size between the late 1970s and the late 1980s. Changes in the age structure of the breeding population and the absence in 1989 of 28% of adults colour-ringed during incubation in 1988 suggest an increase in the rate of egress since the 1970s. These changes probably represent an increase in the long-term costs of reproduction to adults at this colony.     

Sex‐specific mass loss in chick‐rearing South Polar Skuas Stercorarius maccormicki– stress induced or adaptive?

Mass loss of chick‐rearing birds can be the direct consequence of physiological stress (reproductive stress hypothesis) or an adaptive mass adjustment in response to the increased demands on flight efficiency during the flight‐intensive chick‐rearing period (adaptive mass loss hypothesis). To test which of these hypotheses best explains mass loss in South Polar Skuas Stercorarius maccormicki rearing chicks, a food supplementation experiment was carried out in the austral summer 2000/01 at King George Island, Antarctica. Half of the breeding pairs were fed about 20% of the chick's daily energy demand every second day and chick growth and adult nest attendance were recorded. Parents were caught at the start and the end of chick‐rearing to calculate adult mass loss. Male parents of food‐supplemented pairs attended their nest territories more than control males but females kept their attendance constant. Chick growth was only minimally affected and the treatment probably had no fitness consequences. Male Skuas in control pairs had a higher deviation from the body size–mass regression at the end of chick‐rearing compared with the start, supporting the stress hypothesis, whereas female deviation remained unchanged. Males of food‐supplemented pairs were heavier than unsupplemented males at the end of the breeding cycle but not significantly so. Food‐supplemented females were lighter at the end, supporting the adaptive mass loss hypothesis. Adult mass loss is thus best explained by the reproductive stress hypothesis in males but by the adaptive mass loss hypothesis in females. However, the two hypotheses are not mutually exclusive and the results do not exclude the possibility that mass loss in females is stress‐induced but the amount of mass lost is an adaptive adjustment to the reliability of the food supply. The finding that members of a breeding pair may follow different strategies of mass adjustment has implications for the use of mass loss as an index of parental effort. Without knowing which strategy each sex has adopted it is of little use to compare mass loss between parents.     

THE SIGNIFICANCE OF KLEPTOPARASITISM DURING THE BREEDING SEASON IN A COLONY OF ARCTIC SKUAS STERCORARIUS PARASITICUS IN ICELAND

Breeding and kleptoparasitism of Arctic Skuas was studied at Vik, Iceland, in 1973. Hatching success was 88.9%; fledging success was 0.27 fledglings per pair. The heavy chick mortality occurred mostly in the first week after hatching. In the early part of the breeding season skuas fed by robbing kittiwakes of their food at sea and by eating arthropods at the breeding grounds. At the time of hatching of skua eggs, which coincided with the hatching of Puffin eggs, skuas switched their feeding activities to kleptoparasitism of Puffins and fish so obtained was the principal item of most skuas' diet thereafter. In the first week post-hatching, the energy balance of an average adult skua pirating Puffins was estimated to be negative, but changed to positive a week later, although later, coinciding with an influx of non-breeders it turned back to negative. Arctic Terns which normally breed at Vik, and are exploited by skuas, failed to breed in 1973, and the abnormally heavy chick loss is therefore attributed to this failure of the terns. The success of the skuas kleptoparasitizing Puffins depends on the proximity of the interactions to the cliff or to the ocean, where Puffins seck refuge. Skuas catch fish mostly in the air, especially if it is dropped from high. Fish dropped from low is mostly taken by competing gulls and Ravens, which mostly control the ground and lower airspace. Skuas chasing in groups enhance their success, but the average success per member decreased with group increase. However, the success of the group-member in the ‘best’ position was equal to that of a single skua. Puffins carry 1 (large)-6 (smaller) fish, an approximately equal load irrespective of number of fish. Skuas preferentially chase Puffins carrying ‘large’ fish, thus maximizing their yield per effort. Arctic Skuas responded to changes in the numbers of arriving Puffins with a functional response, but their monitoring of the food supply was far from perfect.     

The effect of age and year on the survival of breeding adult Great Skuas Catharacta skua in Shetland

The survival rates of breeding adult Great Skuas Catharacta skua were examined at Foula, the largest colony in the world, where numbers have been declining since the late 1970s. Resightings of colour-ringed breeding adults over a 12-year period were analysed using Cormack-Jolly Seber models to estimate survival rates. Annual survival rates averaged 0.89 but varied among years between 0.82 and 0.93, with annual variations being temporally associated with variations in sandeel abundance during the breeding season. Most birds appeared to die outside the breeding season and so it is possible that nutritional stress and reproductive costs of breeding in years of poor food supply affect survivorship on migration or in the wintering range. Survival rates of adult Great Skuas were affected by their age according to a quadratic equation, with survival increasing significantly with age from 0.73 in 5-year-old-birds to between 0.85 and 0.96 in birds from 7 to 22 years old, with a sharp decline to between 0.75 and 0.87 in birds over 22 years old. Year effects were evident when controlling for age, indicating that annual variations in survival rates are not explained by changes in age-composition of the marked population among years.     

The behaviour of seabirds foraging at fishing boats around Shetland

Among the different types of fishing vessels around Shetland, whitefish trawlers attract the largest numbers of scavenging seabirds and provide the most food. Offal was almost all consumed by seabirds, predominantly by Fulmars Fulmarus glacialis , which excluded other species by their aggression. Fulmars generally ignored discarded whole fish, which were mainly taken by Great Black-backed Gulls Larus marinus , Gannets Sula bassana and Great Skuas Catharacta skua . Although flatfish were usually ignored because seabirds found them difficult to swallow and they sank faster, most discarded roundfish were consumed. Herring Gulls L. argentatus , Lesser Black-backed Gulls L. fuscus and Kittiwakes Rissa tridactyla were rarely able to obtain offal or discards. Herring Gulls and Lesser Black-backed Gulls spent much time on the periphery of feeding flocks while Kittiwakes rarely attempted even to join these. Most of the birds at trawlers were in adult plumage, and it is suggested that the low proportion of immature birds present was a further reflection of the highly competitive feeding conditions at trawlers. We suggest that likely changes in fishing practice and seabird population sizes in the immediate future may result in Herring Gulls, Lesser Black-backed Gulls and Great Skuas finding feeding on waste around trawlers increasingly difficult, so they may be further displaced by Fulmars, Gannets and Great Black-backed Gulls.     

PREDATION AND KLEPTOPARASITISM BY SKUAS IN A SHETLAND SEABIRD COLONY

Feeding methods and relations of Great Skuas and Arctic Skuas to prey were studied in a seabird colony at Hermaness, Shetland. Great Skuas obtained food by kleptoparasitism, predation and scavenging. They induced Gannets to regurgitate by interfering with their flight; grasping the Gannet by the wing or tail or pushing it down with the feet on its back. Gannets tried to escape by descending to the surface, and regurgitated during 12% of the chases, most frequently when pursued by several birds. Great Skuas caught Puffins by swooping at flocks in the colony. Puffins flying with fish to their young were also chased, releasing food on one fifth of the attacks, or escaping down to the sea and diving. Great Skuas also took Kittiwake nestlings by hovering and grasping the chick with the bill, killing and eating it on the surface. Adult Kittiwakes from nearby nests took to the air, mobbing the predator. More Kittiwakes were engaged in mobbing at unsuccessful than at successful predation attempts, indicating that colonial breeding may be of selective value under such predation. Two different estimates pointed to a Kittiwake nestling predation of 0–12 and 014 young per pair. Fledging success of Kittiwakes was estimated at 0–87-1-06 young per pair, considerably lower than at English colonies where predators are absent. In spite of the predation, the Kittiwake colony showed no signs of decrease. Agonistic behaviour and other evidence indicate that Great Skuas defend feeding territories at the seabird colony. Skuas, gulls and Fulmars competed for food at carcasses. Fulmars dominated and chased away skuas. Arctic Skuas deprived Puffins of food. They patrolled the cliff, intercepting Puffins arriving with fish, snatching it from their victim's bill, or inducing them to release fish. Puffins continuing their inward flight lost food more often (30%) than birds descending to the sea (15%)—sometimes diving below. This opportunity to escape may explain the lower success of skuas at Hermaness than at a Puffin colony farther inland from the shore (Grant 1971). Other factors being equal, proximity to the sea may thus reduce the risk of kleptoparasitism.     

Skuas on the Eurasian tundra; relative occurrence of species, ages and colour phases

The breeding ranges of the three closely related skua species in the genus Stercorarius are highly sympatric on the Arctic tundra. During the Swedish-Russian Tundra Ecology Expedition, 1994, the ages and colour phases of Pomarine Skuas Stercorarius pomarinus , Arctic Skuas Stercorarius parasiticus and Long-tailed Skuas Stercorarius longicaudus were recorded at 15 sites along a transect across 140̀ of longitude from the Kola Peninsula in the west to Wrangel Island in the east. An index of the lemming numbers was also measured. Pomarine Skua comprised 52% of the 1587 skuas seen, with 38% Long-tailed Skua and 10% Arctic Skua. The Arctic Skua occurred at low densities all along the transect. The Pomarine Skua was most common at the northernmost sites in this lemming year. It was absent from all sites with low rodent densities but also from two more southerly sites with high lemming numbers. The Long-tailed Skua showed a distribution sympatric with that of the Pomarine Skua but occurred at lower densities, and it also bred at the more southerly sites. No skuas in second-year plumage were observed. Older subadult skuas were observed in increasing numbers with age. The proportion of subadults was markedly higher in the Pomarine Skua (10%) compared with the two other species. The proportion of dark Pomarine Skuas was about 8% all along the transect. In the Arctic Skua, there was an extremely marked shift from 64% dark birds on the Kola Peninsula to an almost complete dominance by the light phase in the rest of Arctic Russia. I suggest that dark skuas are more efficient kleptoparasites over the sea while the light phase is at an advantage hunting over the tundra.     

Incubation and brooding rhythm of the Cape Petrel Daption capense at Nelson Island, South Shetland Islands, Antarctica

Incubation and brooding performance of the Cape Petrel Daption capense at Nelson Island, South Shetland Islands, Antarctica, in the 1991 -1992 austral summer is reported in detail and compared with data from two other areas. There was an inverse relationship between the mean foraging trip length and mean peak weight of chicks. Variation in shift length throughout incubation resulted from a combination of seasonal effects (food supply) and behavioural adjustment to hatching. The length of the last incubation shift was independent of its number but decreased with date of initiation. The average weight of nestattending birds increased steadily during the incubation period and at the same rate in both sexes. Females attained seasonal peak body-weight before the egg hatched, whereas males just recovered their pre-breeding weight. Males took on a slightly larger share of incubation (52%, range 40–63%) and invested more time in mutual nest attendance. Length of foraging trips varied consistently in pairs and individuals, its repeatability being twice as high in females as in males. The incubation performance of individual females (length of the first and an average foraging trip) was correlated with the size of the egg they had laid and also to subsequent chick growth and fledging success. There was a tendency in successful pairs to spend more time together at the nest and to have shorter foraging trips. I suggest that variation in breeding performance among pairs was mainly a result of individual variation in female quality, with pairs where the female contributed more to incubation being more successful. There is little evidence that egg losses were caused by parental errors.     

ORNITHOLOGICAL TRANSECTS IN THE NORTH ATLANTIC

An analysis is presented of observations made in the North Atlantic from 14 August to 10 September 1966, and comparisons are made with previously published data relating to this area. The majority of the records were obtained during transects along latitudes 55° N. and 58° N., but other data were collected in Labrador and Newfoundland coastal waters. During the period spent at sea 30 species were recorded, including five passerine species that came aboard ship. Manx Shearwaters were seen in Newfoundland waters where there have been few previous records. The record of a Balearic Shearwater appears to be the most northerly to date. Some evidence of correlation was found between the numbers of Great Shearwaters seen and the surface water temperature, but there was little evidence of correlation in the case of the Fulmar. Other points of interest are the records of moult in the Great Shearwater, and the records of Grey Phalaropes and Sabine's Gulls. An analysis of the skua movements suggests that the Arctic Skuas seen originated from Scandinavia, the Great Skuas from Iceland and the Pomarine and Long-tailed Skuas from the Canadian arctic.     

Contrasting effects of GPS device and harness attachment on adult survival of Lesser Black‐backed Gulls Larus fuscus and Great Skuas Stercorarius skua

Telemetry has become an important method for studying the biology and ecology of animals. However, the impact of tracking devices and their method of attachment on different species across multiple temporal scales has seldom been assessed. We compared the behavioural and demographic responses of two species of seabird, Lesser Black‐backed Gull Larus fuscus and Great Skua Stercorarius skua, to a GPS device attached using a crossover wing harness. We used telemetry information and monitoring of breeding colonies to compare birds equipped with a device and harness, and control birds without an attachment. We assessed whether tagged birds have lower short‐term breeding productivity or lower longer term overwinter return rates (indicative of overwinter survival) than controls. For Great Skua, we also assessed whether territory attendance within the breeding season differed between tagged and control birds. As with previous studies on Lesser Black‐backed Gull, we found no short‐term impacts on breeding productivity or long‐term impacts on overwinter return rates. For Great Skua, there was no evidence for impacts of the device and harness on territory attendance or breeding productivity. However, as found by a previous study of Great Skuas using a different (body) harness design, there was strong evidence of reduced overwinter return rates. Consequently, a device attached using a wing harness was considered suitable for long‐term deployment on Lesser Black‐backed Gulls, but not on Great Skuas. These findings will inform the planning of future tracking studies.     

Consequences of foraging trip duration on provisioning behaviour and fledging condition of common murres Uria aalgae

We examine the provisioning constraints of a pursuit‐diving seabird in a cold ocean regime by comparing the behaviour of common murres Uria aalge rearing chicks at two colonies in the Northwest Atlantic during 1998‐2000. Funk Island is the largest (340,000–400,000 breeding pairs) and most offshore (60 km) colony of common murres in eastern Canada. Seventy‐five percent of the Northwest Atlantic population of common murres breeds on this island. Great Island is one island within the Witless Bay Ecological Reserve, which is the second largest breeding aggregation (100 000 breeding pairs) and is located near‐shore (2 km). The primary forage fish species in Newfoundland waters is capelin Mallotus villosus, which spawns on or near coastal beaches during summer. Therefore, the two study colonies differ in their distance to food resources and colony size. It is within this natural context that we compare: (1) prey types and frequency of delivery (amount of prey), (2) parental time budgets, and (3) the mass and condition (mass/wing length) of fledglings at both colonies. Similarly sized female capelin (100–150 mm) were delivered to chicks at both colonies. Foraging time per day per parent, a proxy of foraging effort, was similar at both colonies (Great Island: 5.1 h; Funk Island: 5.5 h), as was the percentage of time spent with mates (Great Island: 12.3%; Funk Island: 10.9%). Foraging trips, however, were longer at Funk Island (4.1 h) than at Great Island (2.9 h). This resulted in lower feeding rates of chicks (0.17 feeds per h) and poorer condition of fledglings (2.9 g/mm) at Funk Island compared to those at Great Island (0.22 feeds per h; 3.9 g/mm). We hypothesize that provisioning efforts are constrained at Funk Island by (1) distant food resources and increased competitor density, resulting in longer foraging trip durations and (2) the time spent paired with mates at the colony, which may reflect a minimum time required to maintain breeding sites due to higher breeding densities at Funk Island compared to Great Island. Demographic consequences of this poor fledgling condition at Funk Island are unknown, but fledglings may sufficiently accelerate growth at sea due to their closer proximity to an important nursery area. If fledgling survival is compromised, however, the lower potential for growth at Funk Island will impact the entire Northwest Atlantic population of common murres.     

Parental sex roles of Malaysian plovers during territory acquisition,incubation and chick-rearing

Shorebirds show high variability in parental care strategies among species, populations, and environments. Research on shorebird parental sex roles can help to understand the selective pressures that shape avian breeding strategies. Although several studies have examined parental care strategies in holarctic shorebirds, very little research has been conducted in the tropics. Here we examined parental sex roles during territorial defence, incubation, and chick-rearing in Malaysian plovers Charadrius peronii in the Gulf of Thailand. The costs and gains of particular parental behaviour may vary between the sexes and can be affected differently by environmental factors and chick age. Thus we also examined how temperature, prey availability, chick or embryo age, and time of day affected parental sex roles. Males spent more time defending territories and were further away from chicks whereas females spent more time incubating eggs. Both adults contributed to chick defence during disturbances throughout the entire chick-rearing period. Total nest attendance (sum of both sexes) was affected by the modelled temperature of an unincubated egg. Prey availability, embryo age, and time of day had no effect on total nest attendance. Males adjusted incubation effort in response to temperature only at high temperatures (>36°C) whereas females adjusted nest attendance at high and low temperatures. Chick age had no effect on the proportion of time adults spent defending territories or responding to disturbance. Pairs were more likely to fledge chicks if both the male and female spent more time defending territories. For Malaysian plovers, high cooperation between the sexes during parental care may help to achieve high quality breeding territories, maintain body conditions during hot days, protect offspring from predators and attacking conspecifics, and contribute to high lifetime reproductive success.     

Kleptoparasitic interactions of Arctic Skuas Stercorarius parasiticus and Black Guillemots Cepphus grylle in northeastern Hudson Bay, Canada

Kleptoparasitic behaviour of Arctic Skuas was studied at a Black Guillemot colony in northeastern Hudson Bay, Canada. Skuas procured fish in 7-2% and 19-9% of attacks, and Black Guillemots lost fish in 7-2% and 21-2% of chases in 1982 and 1983, respectively. In 1983 chase outcome was related to distance between birds at the end of a chase and number of skuas in pursuit. The skuas' foraging efficiency (energy intake/energy expended during hunting) was estimated at 45 for 1982 and 5–8 for 1983, and total daily energy expenditure was estimated at 2–4 and 2-1 × b.m.r. for these years. Energetic calculations suggested that skuas obtained sufficient food to meet their daily needs in 3–6 h of hunting in 1982 and 2–7 h in 1983. The absence of intensive kleptoparasitic activity by breeding skuas in North America is attributed to the paucity of colonies suitable for both feeding (due to available host species and colony topography) and nesting (due to presence of arctic foxes and/or polar bears).     

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.     

Relationship between reversed sexual dimorphism,breeding investment and foraging ecology in a pelagic seabird,the masked booby

Reversed sexual dimorphism (RSD) may be related to different roles in breeding investment and/or foraging, but little information is available on foraging ecology. We studied the foraging behaviour and parental investment by male and female masked boobies, a species with RSD, by combining studies of foraging ecology using miniaturised activity and GPS data loggers of nest attendance, with an experimental study where flight costs were increased. Males attended the chick more often than females, but females provided more food to the chick than males. Males and females foraged during similar periods of the day, had similar prey types and sizes, diving depths, durations of foraging trips, foraging zones and ranges. Females spent a smaller proportion of the foraging trip sitting on the water and had higher diving rate than males, suggesting higher foraging effort by females. In females, trip duration correlated with mass at departure, suggesting a flexible investment through control by body mass. The experimental study showed that handicapped females and female partners of handicapped males lost mass compared to control birds, whereas there was no difference for males. These results indicate that the larger female is the main provisioner of the chick in the pair, and regulates breeding effort in relation to its own body mass, whereas males have a fixed investment. The different breeding investment between the sexes is associated with contrasting foraging strategies, but no clear niche differentiation was observed. The larger size of the females may be advantageous for provisioning the chick with large quantities of energy and for flexible breeding effort, while the smaller male invests in territory defence and nest guarding, a crucial task when breeding at high densities. In masked boobies, division of labour appears to be maximal during chick rearing—the most energy-demanding period—and may be related to evolution of RSD.     

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.