Chick-rearing Period Research Articles

Geographical variation in carbon stable isotope signatures of marine predators: a tool to investigate their foraging areas in the Southern Ocean

Stable carbon isotope signatures (δ13C) are increasingly being used to determine the foraging habitats of consumers in the Southern Ocean. An underlying assumption is that a latitudinal gradient in δ13C values at the base of the food chain should also be reflected in organisms at higher trophic levels. Our main objective was to test that assumption by using penguin chicks (7 taxa) as predator models, because the feeding habits of provisioning adult penguins are well known during the chick-rearing period. As expected, a strong negative correlation was found between latitude and δ13C values of whole blood of penguin chicks. δ13C values ranged from –24.8 ± 0.5‰ in Adelie penguins living in Antarctica (67°S), to –19.5 ± 0.3‰ in northern rockhopper penguins living in the subtropics (38°S). Unlike δ13C values, stable nitrogen isotope signatures (δ15N) were not related to latitude but instead were strongly affected by penguins' diet, with fish-eaters having higher δ15N values than crustacean eaters. δ13C values also reflect more local spatial foraging segregation of penguins at a subantarctic archipelago (Kerguelen Islands), where they varied from –22.3 ± 0.2‰ for king penguins feeding in oceanic waters to –15.9 ± 0.3‰ for gentoo penguins foraging in an enclosed bay. Blood δ13C values moreover allowed differentiation of 2 sub-populations of gentoo penguins foraging in 2 different environments at Kerguelen. The method has 2 limitations related to the lack of longitudinal variations and to overlapping δ13C values due to different isotopic gradients (latitudinal and inshore/offshore) in the marine environment. The study nevertheless shows that the δ13C values of marine predators are efficient indicators of the foraging habitats at various spatial scales, both in terms of latitude in oceanic waters, and in terms of inshore/offshore gradients in waters surrounding subantarctic islands.

AGE-RELATED SURVIVAL AND BEHAVIOR OF SNOWY PLOVER CHICKS

Precocial young often experience high mortality prior to achieving flight, especially in the first days after hatching. We quantified relationships between chick age, survival, behavior, and response to natural and anthropogenic danger for the threatened Western Snowy Plover (Charadrius alexandrinus nivosus) in coastal northern California, USA. Plover chicks were most likely to die in the first three days of their 28-day prefledging period, especially on sandy ocean beaches compared to coarser substrates of river habitats. Chick survival in both habitats increased across the ,120-day chick-rearing period. Improved survival of older chicks coincided with an age-related reduction in brooding and increased distance from the tending parent, which was not related to season. Lower survival of younger chicks was correlated with a tendency to lie motionless when approached by humans; nearly all older chicks responded to human approach by running to evade danger. Chicks of all ages were more likely to lie motionless when potential avian predators flew nearby. Age-dependent survival of Snowy Plover chicks paralleled a trend of increasing development, thermal independence from adults, and capability of evading predators.

Seabird behavior as an indicator of food supplies: sensitivity across the breeding season

We used empirical data on the time allocation of common murres Uria aalge in relation to measures of local prey density to examine whether adults provisioning chicks are more sensitive to changes in prey density than birds that are incubating eggs. We hypothesized that seasonal differ- ences in food requirements of incubating and chick-rearing parents would affect the form of the rela- tionship between time spent at the colony and local food density. We found that the relationship did differ between the incubation and chick-rearing period in 3 important ways: (1) there was a strong non-linear relationship between food density and colony attendance during chick-rearing and a weaker relationship during incubation; (2) incubating birds were able to maintain relatively constant rates of attendance over a wider range of food densities than chick-rearing birds and only reduced colony attendance under extremely poor feeding conditions, if at all; and (3) incubating birds spent more time attending nest sites at the colony than provisioning birds. These differences confirmed that chick-rearing parents are more sensitive to changes in food density than incubating parents, and that measurements of time allocation during the incubation period would have limited value as an indicator of ecosystem change.

OCEANOGRAPHIC HABITAT OF AN ENDANGERED MEDITERRANEAN PROCELLARIIFORM: IMPLICATIONS FOR MARINE PROTECTED AREAS

Marine protected areas (MPAs) require ecologically meaningful designs capable of taking into account the particularities of the species under consideration, the dynamic nature of the marine environment, and the multiplicity of anthropogenic impacts. MPAs have been most often designated to protect benthic habitats and their biota. Increasingly, there is a need to account for highly mobile pelagic taxa, such as marine birds, mammals and turtles, and their oceanic habitats. For breeding seabirds foraging from a central place, particular attention should be paid to distant foraging grounds and movement corridors, which can often extend to hundreds of kilometers from breeding colonies. We assessed the habitat use by the most threatened Mediterranean seabird, the Balearic Shearwater, Puffinus mauretanicus, using vessel-based surveys during the chick-rearing period (May-June). We used a hierarchical modeling approach to identify those environmental variables that most accurately reflected the oceanographic habitat of this species by (1) delineating its foraging range using presence/ absence data and (2) identifying important foraging grounds where it concentrates in dense aggregations. The foraging range comprised the frontal systems along the eastern Iberian continental shelf waters (depth <200 m) and areas close to the breeding colonies in the Balearic Islands. Shearwaters aggregated in productive shelf areas with elevated chlorophyll a concentrations. Following the model of a core-buffer MPA, we envisioned those areas of dense aggregation (i.e., the area of influence of the Ebro River discharge and Cape La Nao regions) as the core regions deserving elevated protection and more stringent management. More diffuse protective measures would be applied within the larger buffer region, delineated by the foraging range of the species. Marine zoning measures can greatly benefit the conservation of the Balearic Shearwater and other far-ranging seabirds by extending protective measures beyond their breeding colonies during both the breeding and non-breeding seasons.

Male chicks are more costly to rear than females in a monogamous seabird, the Common Murre

Feeding rates and mass loss during chick rearing were compared for individually marked parents of male and female Common Murre (Uria aalge) chicks at Great Island, Newfoundland, Canada, from 1997–2001. Both parents in this socially monogamous seabird species share parental care duties until colony departure, after which the single chick is fed only by its father. Because murres provision their single chicks with one clearly visible fish per trip, it is possible to accurately determine whether parents differentially feed male and female chicks. Based on slightly greater mass of males in adulthood, possibly favored by sex differences in breeding roles, we predicted that male nestlings would be fed more than females. Fathers’ feeding rate to sons, but not daughters, increased with chick age, whereas maternal feeding rate increased with chick age for both sexes. When year-corrected feeding rates of pairs rearing both sexes were compared, both mothers and fathers fed their sons significantly more than their daughters in the later part of the chick-rearing period. Moreover, parents rearing male chicks lost mass at a significantly higher rate than those rearing females. There was no difference in fledging age for sons and daughters. These results indicate that differential parental allocation occurs and has measurable costs even in a species with only slight adult sexual dimorphism.

Prey-switching by Cassin’s auklet Ptychoramphus aleuticus reveals seasonal climate-related cycles of Euphausia pacifica and Thysanoessa spinifera

Euphausia pacifica and Thysanoessa spinifera comprise a substantial proportion of the diet of Cassin's auklet Ptychoramphus aleuticus in the Gulf of the Farallones, California. Ocean climate variability has been linked to fluctuations in euphausiid abundance and life-history parame- ters. We tested the hypothesis that seasonal patterns of prey use by auklets correspond to within- season variation in upwelling and sea surface temperature (SST), by comparing the proportion (by number) of E. pacifica and T. spinifera in auklet diet samples obtained during the chick-rearing period to SST and an upwelling index (UI) over 11 years. We found that the proportion of E. pacifica adults in auklet diet decreased over the chick-rearing period, and increased with increasing UI and decreasing SST. We detected curvature in the relationship between adult E. pacifica and UI, with E. pacifica increasing to a point and decreasing thereafter. T. spinifera in auklet diet showed an increase over the chick-rearing period, but no relationships were detected with UI or SST. Mass of individual adult E. pacifica increased with increasing UI and decreasing SST. We detected moderate curvature in the relationship between mass of individual adult T. spinifera and SST. The proportion of E. paci- fica juveniles in auklet diet did not change seasonally and was not related to UI or SST, while the pro- portion of T. spinifera juveniles in auklet diet increased seasonally and with increases in UI. Seasonal changes in the ingestion of euphausiids by auklets appeared to be affected by ocean climate variabil- ity; however, prey-selection via a shift in foraging location or competition with other marine preda- tors are alternative valid explanations. This study highlighted the utility of using top marine preda- tors to sample prey availability, thereby complementing traditional net sampling procedures for determining the distribution and abundance of euphausiids in marine systems.

Serum immunoglobulin G levels are positively related to reproductive performance in a long-lived seabird, the common tern (Sterna hirundo)

The evolution of longevity requires a low risk of mortality from extrinsic factors, relative to intrinsic factors, so that individuals that differentially invest in physiological self-maintenance and minimize their annual reproductive costs will maximize lifetime fitness through a prolonged reproductive lifespan. The trade-off between reproductive effort and self-maintenance, as measured by immune function, has been well documented in short-lived birds, but is difficult to demonstrate in long-lived birds. To assess self-maintenance in a long-lived seabird, we measured serum protein levels, including immunoglobulin G (IgG = IgY), in 30 breeding pairs of common terns (Sterna hirundo) and their first-hatched (A) chicks. Most parents were of known age from banding as hatchlings; our sample was selected to contrast young breeders (6-9 years) with very old birds (17-23 years). Body-mass of the parents declined by 5% during the chick-rearing period, while serum protein levels were stable. Serum IgG levels were higher in parents of offspring with faster growth rates, while IgG levels were lower in parents whose broods were reduced by starvation. A-chicks in broods of two had higher IgG levels than singleton chicks. Albumin levels were not related to reproductive performance. Thus, despite adequate statistical power, we could find no evidence for a trade-off between reproduction and self-maintenance in common terns, even in old age. The results are consistent with life-history predictions for long-lived vertebrates, in which selection favors sustained self-maintenance across the reproductive lifespan. The positive relationships between IgG levels and reproductive performance indicate that IgG can be used as an index of parental "quality."

FOOD AVAILABILITY AFFECTS ADULT SURVIVAL AS WELL AS BREEDING SUCCESS OF PARASITIC JAEGERS

Long-lived species are expected to reduce their current reproductive effort in adverse conditions in order not to jeopardize their future reproduction. Parasitic Jaegers (Stercorarius parasiticus) breeding in Shetland, United Kingdom, have declined substantially in numbers over the last 15 years, although the causes of this decline remain unclear. By carrying out a supplementary feeding experiment during the chick-rearing period, we tested the hypothesis that food availability has contributed to this decline. Data were collected on adult territory attendance, chick growth rates, and breeding success during the experimental season, and adult return rates, body condition, and laying dates during the following season. Differences in carbon and nitrogen isotope ratios in the natural and supplementary food, analyzed in the chicks' feathers, showed that chicks received one-fifth of their protein requirements from the supplementary food, but this amount varied between broods. Compared with controls, supplemented pairs showed higher attendance on the territory, improved breeding success, and higher return rate. This emphasizes that adult return rate is more sensitive to food availability than has been assumed previously. Control birds may have maintained too high a current reproductive effort at the expense of future reproduction, perhaps due to the unpredictability of their main food source, sandeels, which seemed abundant early in the season but scarce during chick rearing. This study suggests that the consequences of poor food availability are shared between offspring and parents in this species.

Foraging Ecology of Caspian Terns in the Columbia River Estuary, USA

Comparisons were made of the foraging ecology of Caspian Terns (Sterna caspia) nesting on two islands in the Columbia River estuary using radio telemetry and observations of prey fed to chicks and mates at each colony. Early in the chick-rearing period, radio-tagged terns nesting at Rice Island (river km 34) foraged mostly in the freshwater zone of the estuary close to the colony, while terns nesting on East Sand Island (river km 8) foraged in the marine or estuarine mixing zones close to that colony. Late in the chick-rearing period, Rice Island terns moved more of their foraging to the two zones lower in the estuary, while East Sand Island terns continued to forage in these areas. Tern diets at each colony corresponded to the primary foraging zone (freshwater vs. marine/mixing) of radio-tagged individuals: Early in chick-rearing, Rice Island terns relied heavily on juvenile salmonids (Oncorhynchus spp., 71% of identified prey), but this declined late in chick-rearing (46%). East Sand Island terns relied less on salmonids (42% and 16%, early and late in chick-rearing), and instead utilized marine fishes such as Anchovy (Engraulis mordax) and Herring (Clupea pallasi). Throughout chick-rearing, Rice Island terns foraged farther from their colony (median distance: 12.3 km during early chick-rearing and 16.9 km during late chick-rearing) than did East Sand Island terns (9.6 and 7.7 km, respectively). The study leads to the conclusion that Caspian Terns are generalist foragers and make use of the most proximate available forage fish resources when raising young.

Factors Affecting Chick Provisioning by Caspian Terns Nesting in the Columbia River Estuary

We investigated factors affecting chick provisioning by radio-tagged Caspian Terns (Sterna caspia) nesting in a large colony on East Sand Island in the Columbia River estuary during 2001. Caspian Tern predation on juvenile salmonids (Oncorhynchus spp.) in the estuary prompted resource managers to relocate ca. 9,000 pairs of terns nesting on Rice Island (river km 34) to East Sand Island (river km 8), where terns were expected to consume fewer salmonids in favor of marine forage fishes. This study investigated factors influencing foraging success, diet composition, and overall reproductive success at the managed Caspian Tern colony. Our results indicated that daytime colony attendance by nesting terns averaged 64% and decreased throughout the chick-rearing period, while duration of foraging trips averaged 47 min and increased during the same period; these seasonal changes were more strongly related to date than chick age. Average meal delivery rates to 2-chick broods (0.88 meals h-1) were 2.6 times greater than to 1-chick broods (0.33 meals h-1). Parents delivered more juvenile salmonids to chicks during ebb tides than flood tides, but meal delivery rates to the nest remained constant, suggesting diet composition tracks relative availability of prey species. Foraging trips resulting in delivery of juvenile salmonids averaged 68% longer than foraging trips for schooling marine forage fishes, indicating higher availability of marine prey near the colony. High availability of marine forage fish in the Columbia River estuary during 2001 was apparently responsible for high colony attendance, short foraging trips, high chick meal delivery rates, and high nesting success of Caspian Terns on East Sand Island.

Territorial defence and agonistic behaviour of breeding bearded vultures Gypaetus barbatus toward conspecifics and heterospecifics

Territoriality and agonistic behaviour against conspecifics and heterospecifics was recorded for nine breeding pairs of bearded vulture Gypaetus barbatus between 1992 and 1996 in the Pyrenees (NE Spain). For both sexes, the defence intensity increased from the pre-laying to the chick-rearing period. The intensity of nest defence was significantly higher in males during the pre-laying period but no sex differences were found during the incubation and chick-rearing periods. Competition for nest sites, food and the potential risk of predation may affect the bearded vulture's nest defence behaviour towards other species. Most agonistic interactions took place against Eurasian griffon vultures (the most abundant species and the most important competing for nest sites) and common ravens (the most likely potential predator and a kleptoparasitic species). Nest defence against conspecifics mainly took place during the pre-laying period and may be linked to sperm competition and food resource competition.

Inter-annual variation in the diet of female southern rockhopper penguin (Eudyptes chrysocome chrysocome) at Tierra del Fuego

Inter-annual variation in the diet of female southern rockhopper penguins (Eudyptes chrysocome chrysocome) at Staten Island was studied during the early chick-rearing period to investigate the components of the diet and highlight some points of the possible food web in the study area. Gregarious crustaceans, small juveniles of squid and octopus, fish larvae and juvenile fish dominated the diet. There was a high degree of variability in the relative contribution of the different prey taxa during the three seasons studied. Overall, crustaceans were by far the most abundant in terms of number. Cephalopods contributed less to numbers, while fish was the least represented of the prey item in 2 years. Inter-annual variation in the proportion of prey items consumed was apparent only for some prey species: Thysanoessa gregaria, Gonatus antarcticus, Themisto gaudichaudii, Harpagifer bispinis and Salilota australis. These data suggest variability in the prey resources at sea during the study period and a subsequent opportunism of this penguin species to exploit what is differentially available. In terms of the food web in the area, we suggest an apparent relationship in the availability between T. gaudichaudii and G. antarcticus, and between T. gaudichaudii and H bispinnis. These relationships emphasise the importance of understanding food web interactions, especially those involving multiple trophic levels, when determining the role of upper-trophic level predators in marine systems.

Annual Variation in Diet of Breeding Great Cormorants: Does it Reflect Varying Recruitment of Gadoids?

Abstract Great Cormorant (Phalacrocorax carbo) diet was studied during three years (2001-2003) in an area where Arctic Kelp (Laminaria hyperborea) is extensively distributed off the central Norwegian coast. A total of 608 diet samples, 378 (62.2%) chick regurgitations, 22 (3.6%) whole fish, and 208 (34.2%) pellets were collected from the colonies at regular intervals during the chick-rearing period. From these samples a total of 1,013 food items (after pairing the otoliths) were isolated, representing 18 fish species. Gadoids, mainly Cod (Gadus morhua) and Saithe (Pollachius virens) dominated the diet (75% numerically, 86% by biomass). During the first year of the study, Cod represented nearly 50% of the diet, but decreased to 13% in 2003. At the same time, the occurrence of Saithe in the diet increased from 23% to 65%. For Saithe age II-group fish dominated the diet in 2001, and I- and II- group dominated in 2002 and 2003. For Cod 0-group fish dominated the diet in 2001 and 0- and I-group fish dominated ...

The effect of recreational disturbance on an upland breeding bird, the golden plover Pluvialis apricaria

The use of the countryside for recreation has increased dramatically in recent years. This has led to concern amongst conservationists about the effects increased human disturbance may have on important wild animal populations. In the UK, recent legislation has widened the level of access to upland habitats, which support internationally important breeding bird populations. Determining the extent to which recreational disturbance affects upland breeding birds is therefore a conservation priority. We used data collected over 13 years to investigate the impact of recreational disturbance on the distribution and reproductive performance of golden plovers breeding in close proximity to the Pennine Way, an intensively used long-distance footpath. Importantly, the Pennine Way was resurfaced in 1994 to prevent further erosion of the surrounding vegetation. We were therefore able to examine if the response of golden plovers to recreational disturbance was influenced by changes in the intensity and extent of human activity resulting from the resurfacing work. Before the Pennine Way was resurfaced, golden plovers avoided areas within 200 m of the footpath during the chick-rearing period. At this time over 30% of people strayed from the footpath and the movement of people across the moorland was therefore widespread and unpredictable. Following resurfacing, over 96% of walkers remained on the Pennine Way, which significantly reduced the impact of recreational disturbance on golden plover distribution; golden plovers only avoided areas within 50 m of the footpath at this time. Despite the clear behavioural responses of golden plovers to the presence of visitors, there was no detectable impact of disturbance on reproductive performance. In many countries, a conflict arises between the use of the countryside for recreational purposes and the protection of habitats or species of high conservation value. However, this study suggests that the implementation of simple measures to influence visitor behaviour can dramatically reduce the impact of recreational disturbance on wild animal populations.

Flight energetics of the Marbled Murrelet,Brachyramphus marmoratus

We measured flight speeds (n = 3000) of Marbled Murrelet, Brachyramphus marmoratus (J.F. Gmelin, 1789), to determine whether flight speeds of an exceptionably fast bird coincide with the maximum-range speeds (Vmr) predicted by aerodynamic theory. The mean (±SE) speed of 22.6 ± 0.21 m·s–1was significantly higher than the Vmrpredicted by four models, using conventional values for the parasite drag coefficient (CDpar). In order for the Penny cuick model to predict a Vmrof 22 m·s–1, a CDparof 0.05, which is lower than any previously reported, is necessary; the other models would need to assume even lower values for CDpar. We concluded that the cruising speed of Marbled Murrelets exceeds Vmr. Marbled Murrelets may exceed Vmras a result of behavioural decisions, and we examined two behavioural hypotheses: that flight speeds exceed Vmrto (1) minimize predation rate and (2) maximize chick growth rate. However, there was no significant difference between flight speeds during high (daylight) and low (darkness) predation periods or between chick-rearing and non-breeding periods. Marbled Murrelets may also appear to fly at a speed that exceeds Vmrbecause the underlying aerodynamic theory is inaccurate for this species. To examine the reliability of aerodynamic theory for Marble Murrelets, we compared measured wingbeat frequencies (f) to those predicted by Pennycuick's model. The mean f was significantly lower than the frefpredicted by Pennycuick's model, and generally, f = 7.9m–0.22is a better model for auks than Pennycuick's model. In addition, the Strouhal number was particularly low (0.12 ± 0.02). We conclude that the current aerodynamic models are insufficient for an exceptionally fast-flying bird.

Inter-colony and sex differences in the effects of parental body condition and foraging effort on the brood growth of Ad�lie penguins

Among colonies with different foraging distances, central-place-foraging seabirds may change their foraging and reproductive efforts. We compared the body condition, meal frequency, and diving behavior of male and female Adelie penguins at two locations: Dumont d'Urville, where there was little sea ice and they foraged in open waters far from the colony; and Syowa, where there was heavy, fast sea ice and they foraged in ice cracks close to the colony. The parental mass decrease rate during the chick-rearing period was similar between colonies and between sexes. A large individual variation in meal frequency positively affected the brood growth rate, but daily underwater time did not. A weak but significant positive effect of body condition on brood growth rate was found only in males at Syowa. It was suggested that males work with better body condition than females. We propose the hypothesis that the regional difference in the distance to the feeding sites and the sex difference in body energy reserve might constrain the capacity to regulate reproductive effort.

The diet of black-browed albatrosses at the Diego Ramirez Islands, Chile

The diet of black-browed albatrosses was studied at Gonzalo Island, Diego Ramirez, Chile, during the early chick-rearing periods of 2000, 2001, and 2002. Diet composition was determined by sampling chick-stomach contents during January and February of each year. Reconstituted meal mass was similar throughout the study, with diet being dominated by fish in all 3 years. Overall, the main items taken were the fishes Macruronus magellanicus (66–89% by mass) and Micromesistius australis (2.6–3.7% by mass), which are both fisheries-related species, and the squid Martialia hyadesi (8–20% by mass). The distribution of the prey species indicates that black-browed albatrosses obtained the bulk of their food over the South American continental shelf, but also foraged at the Antarctic Polar Front. The prevalence in the diet of fish species discarded from fishing operations, and the presence of fish hooks and fish bait species, indicate a strong association with fisheries in southern Chile.

CAN SURVEILLANCE RADAR BE USED TO MONITOR THE FORAGING DISTRIBUTION OF COLONIALLY BREEDING ALCIDS?

Abstract We used surveillance radar to track flocks of colonially breeding alcids at Latrabjarg, northwest Iceland, that were returning from their feeding grounds. We compared the flight directions of incoming flocks during the chick-rearing period with alcid densities at sea during both the incubation and chick-rearing periods. The direction of incoming flocks was significantly related to alcid distribution at sea during the chick-rearing period. At-sea alcid counts suggested a temporal change in alcid use of foraging areas within a breeding season. Surveillance radar appears practical for monitoring preferred foraging areas of alcids within seasons and between years. Further, radar can provide an overall picture of alcid distribution near colonies, complementary to studies on foraging flights of individual birds. ?Pueden Utilizarse Radares de Vigilancia para Monitorear la Distribucion de Alcidos que se Reproducen en Colonias? Resumen. En Latrabjarg, noroeste de Islandia, utilizamos radares de vigilancia...

Dietary evidence for spatial foraging segregation in sympatric albatrosses ( Diomedea spp.) rearing chicks at Iles Nuageuses, Kerguelen

The food of three closely related and sympatric species of mollymawks (Diomedea spp.), the black-browed (BBA, D. melanophrys), grey-headed (GHA, D. chrysostoma) and yellow-nosed (YNA, D. chlororhynchos) albatrosses, was compared at Iles Nuageuses, Kerguelen, during the chick-rearing period. BBA preyed almost equally upon cephalopods (39% by fresh mass), fish (31%) and penguins (31%), while GHA fed more on squids (52%, 16% and 28%, respectively) and YNA fed more on fish and not on penguins (13%, 84% and 0%, respectively). Crustaceans were always a minor component of the diet (<3%). Patagonian toothfish was the main fish prey, and Todarodes sp. the main cephalopod prey for the community. Accumulated beaks emphasise the importance of juvenile ommastrephid squids in the diet of mollymawks, accounting for 81%, 71% and 55% of the total number of beaks in BBA, GHA and YNA samples, respectively. BBA preyed also upon a significant number of the octopod Benthoctopus thielei (12%) and of the cranchiid squid Galiteuthis glacialis (4%), while GHA fed more on G. glacialis (18%) and on the onychoteuthid Kondakovia longimana (8%). When feeding on the same prey, prey size was similar for the albatross species. Comparison of overall prey biogeography together with the presence/absence of prey species indicators of water masses indicates segregation through different foraging areas among the three mollymawks. BBA forage almost exclusively over the shelf and upper slope waters surrounding the Kerguelen Archipelago. By contrast, GHA and YNA feed mainly in oceanic waters, YNA favouring the warm subtropical waters, and GHA the cold Antarctic waters. It is thus remarkable that birds from the same breeding grounds forage over such a wide latitudinal range, from about 35–40°S to 60–65°S, encompassing the Subtropical Zone for YNA, the Antarctic Zone for GHA and the Antarctic Polar Frontal Zone (where Kerguelen is located) for the three species.

Conflict or co-existence? Foraging distribution and competition for prey between Adélie and chinstrap penguins.

Adelie (Pygoscelis adeliae) and chinstrap penguins (P. antarctica) are morphologically and ecologically very similar, have very similar diet and breed sympatrically in the Scotia Arc from the South Sandwich Islands to the Antarctic Peninsula. To investigate how these two species co-exist, their foraging distribution and diet were studied during the chick-rearing period at Signy Island, South Orkney Islands, during the breeding seasons of 2000 and 2001. Satellite tracking data from of 19 Adelie penguins and 24 chinstrap penguins were used to compare foraging distributions. In both years the diet of both species was exclusively Antarctic krill (Euphausia superba) of the same size range. In a year of low prey availability (2000), there was a statistically significant segregation of foraging areas between the two species, however, in a year of normal resource availability (2001) there was no such segregation. There was a significant difference in the foraging areas used by Adelie penguins between years but not for chinstrap penguins. Adelie penguins foraged significantly farther (mean 100 km) from the colony than chinstrap penguins (mean 58 km) in 2000 but not in 2001 (mean 58 km and 35 km respectively). In 2000, the breeding success of Adelie penguins was 51% lower than the long-term mean compared to 15% lower in chinstrap penguins. Both species achieved above average breeding success in 2001. The changes in foraging distribution and breeding success suggest that in years of low resource availability, chinstrap penguins may be able to competitively exclude Adelie penguins from potential inshore foraging areas. Current trends in climatic change and possible effects on ice distribution and krill abundance suggest that conditions could become less favourable for Adelie penguins than chinstrap penguins in areas where both species occur.