Temporal variation in the diet of gentoo penguins at the Falkland Islands

Knowledge of diet is critical in interpreting the ecological roles of marine top predators and provides information towards their conservation and management. The Falkland Islands hold the largest number of breeding gentoo penguins. Yet knowledge of gentoo penguin diet at the Falklands is limited to either broad taxonomic divisions of prey items or dietary samples collected only on a single day. This study is the first to investigate gentoo penguin diet at Cow Bay, Falklands, to the species level, over repeated sampling intervals during the breeding period. Through stomach content analysis, we determined diet over a large temporal scale (2002/2003/2004–2011/2012/2013) and between the guard and crèche periods of chick rearing. The principle prey item by reconstituted mass was rock cod fish Patagonotothen spp., for all periods (47–78 %) except that of the 2012/2013 crèche period (19 %) when Falkland herring Sprattus fugensis made up the bulk of the diet (52 %). Of the cephalopods recovered, Patagonian squid Doryteuthis gahi was prominent (1–24 %), while crustaceans contributed negligibly to gentoo penguin diet. Our findings revealed that gentoo penguins breeding at the Falkland Islands were primarily demersal foragers with an ability for pelagic feeding. Diet choice appears to reflect prey availability.     

The diet of the Imperial Shag Phalacrocorax atriceps at a colony on New Island, Falkland/Malvinas Islands combining different sampling techniques

The diet of the Imperial Shag Phalacrocorax atriceps was studied on New Island, Falkland/Malvinas Islands during the 2008/2009 breeding season, with some additional data from 2007/2008. The diet comprised a large variety of prey, mainly fish, crustaceans and squid. In contrast to other species of the blue-eyed shag complex, prey not only consisted of benthic organisms but also included pelagic prey. Different sampling techniques were combined in order to obtain a comprehensive overview of the diet. Pellets, regurgitations and stomach contents yielded different results. We discuss the causes for these variations including different sample availability over time. In particular, the stomach analyses seem to overestimate the importance of squid based on the occurrence of squid beaks. For the pellet analyses, lobster krill accounted for the majority of the prey remains except during the second half of December (i.e. when young chicks were being attended), when fish was more important.     

Relevance of forage fish in the diet of Magellanic penguins breeding in northern Patagonia,Argentina

We quantified the trophic niche of Magellanic penguins (Spheniscus magellanicus) breeding and moulting in Golfo San Jorge, Argentina, through conventional stomach content and stable isotope analysis. A total of 112 adults were flushed during the early and late chick stages of 2011 and 2012 at Isla Vernacci Norte, and at least 15 prey taxa were found, including fishes, cephalopods, crustaceans and polychaetes. Overall, Argentine anchovy (Engraulis anchoita) showed the highest contribution in terms of importance by mass (68.1–85.3%, depending on chick stage and year), except for the old chick stage in 2011 when the shortfin squid (Illex argentinus) was the main prey consumed (56.0%). Based on carbon and nitrogen isotopic values from a total of 256 blood samples, corresponding to young and old chicks and to adults of both sexes sampled throughout the incubation, chick and moult stages at the above mentioned colony and years, Bayesian mixing model outputs showed that Argentine anchovy was always the main prey (48–86%). Bayesian mixing model outputs obtained from adults of both sexes and their chicks during the late chick stage of 2013 at Isla Vernacci Norte, Isla Tova and Isla Leones also showed that Argentine anchovy was the main prey consumed. This is the first comprehensive assessment of Magellanic penguin diet composition in northern Patagonia, quantifying the relative contribution of prey in the diet of adults and chicks at different stages of the annual cycle and years, and confirms the relevance of a forage fish such as the Argentine anchovy in its trophic ecology.     

Associations between wintering avian predators and schooling fish in the Skagerrak-Kattegat suggest reliance on predictable aggregations of herring Clupeaharengus

We investigated the distributional dynamics of piscivorous seabirds in relation to the distribution of young fish outside the breeding season in the Skagerrak and Kattegat, using synoptic bottom trawl and seabird surveys. Kittiwake Rissa tridactyla , Common Guillemot Uria aalge and Razorbill Alca torda were significantly correlated with herring Clupea harengus abundance at coarse spatial scales (>20 km). The association between the three seabird species and other abundant fish species like sprat Sprattus sprattus , whiting Merlangius merlangus and Norway pout Trisopterus esmarki was considerably weaker. In the Skagerrak and Kattegat, the location of aggregations of immature herring during winter is relatively stable and predictable. The correlation with herring and the lack of correlation with the abundant, but less stable occurrence of sprat of similar caloric value suggest that the tendency among piscivorous seabirds to depend on predictable concentrations of prey during the breeding season is a common feature during the non-breeding season as well. The results are discussed in relation to ecological and winter habitat characteristics of wintering piscivorous seabirds in the eastern North Atlantic, and in relation to the management and conservation of nursering herring in the Skagerrak and Kattegat.     

The Foraging Ecology of the Endangered Cape Verde Shearwater,a Sentinel Species for Marine Conservation off West Africa

Large Marine Ecosystems such as the Canary Current system off West Africa sustains high abundance of small pelagic prey, which attracts marine predators. Seabirds are top predators often used as biodiversity surrogates and sentinel species of the marine ecosystem health, thus frequently informing marine conservation planning. This study presents the first data on the spatial (GPS-loggers) and trophic (stable isotope analysis) ecology of a tropical seabird—the endangered Cape Verde shearwater Calonectris edwardsii–during both the incubation and the chick-rearing periods of two consecutive years. This information was related with marine environmental predictors (species distribution models), existent areas of conservation concern for seabirds (i.e. marine Important Bird Areas; marine IBAs) and threats to the marine environment in the West African areas heavily used by the shearwaters. There was an apparent inter-annual consistency on the spatial, foraging and trophic ecology of Cape Verde shearwater, but a strong alteration on the foraging strategies of adult breeders among breeding phases (i.e. from incubation to chick-rearing). During incubation, birds mostly targeted a discrete region off West Africa, known by its enhanced productivity profile and thus also highly exploited by international industrial fishery fleets. When chick-rearing, adults exploited the comparatively less productive tropical environment within the islands of Cape Verde, at relatively close distance from their breeding colony. The species enlarged its trophic niche and increased the trophic level of their prey from incubation to chick-rearing, likely to provision their chicks with a more diversified and better quality diet. There was a high overlap between the Cape Verde shearwaters foraging areas with those of European shearwater species that overwinter in this area and known areas of megafauna bycatch off West Africa, but very little overlap with existing Marine Important Bird Areas. Further investigation on the potential nefarious effects of fisheries on seabird communities exploiting the Canary Current system off West Africa is needed. Such negative effects could be alleviated or even dissipated if the ‘fisheries-conservation hotspots’ identified for the region, would be legislated as Marine Protected Areas.     

Consumption of marine resources by seabirds and seals at Heard Island and the McDonald Islands

Summary The seabird and seal community at Heard Island and the McDonald Islands comprised an estimated total biomass of 27893 tonnes of which the 15 breeding species of seabirds made up 70%. The total annual consumption of marine resources was estimated to be approximately 521 000 t, of which 81% was consumed by seabirds Approximately 165 000 t of fish, 41 600 t of squid and 312 000 t of crustaceans are consumed annually by this seabird and seal community. The annual energy flux to this community was estimated to be 2.17·10¹² kJ and approximately 56 000 t of carbon are consumed annually. Breeding populations of King Penguins and Antarctic Fur Seals are increasing, that of the Southern Elephant Seal is decreasing; there are no data on the population trend for Macaroni Penguins, the predominant consumer species. Commercial fisheries are presently operating at the nearby Iles Kerguelen, and similar activities may prove to be commercially viable at Heard Island. The fishery is for Champsocephalus gunnari, a major prey species of penguins and Antarctic Fur Seals at Heard Island during the summer breeding season.     

Diet and prey consumption of Antarctic petrels Thalassoica antarctica at Svarthamaren, Dronning Maud Land, and at sea outside the colony

The diet of the Antarctic petrel Thalassoica antarctica was studied during two seasons at Svarthamaren, an inland colony in Dronning Maud Land, Antarctica, and in the pack ice off the coast of Svarthamaren. The most important food (wet mass) at Svarthamaren was crustaceans (67%), fish (29%) and squid (5%); however, individuals collected in the pack ice took mostly fish (87%). The prey composition and lengths of prey are comparable to what has been documented in other studies on this species. Estimates of food consumption by birds breeding at Svarthamaren (ca. 250,000 pairs) suggest that approximately 6500 tonnes of crustaceans, 2800 tonnes of fish and 435 tonnes of squid are consumed during the breeding season. The annual consumptions of these birds are estimated to be 34,100 tonnes of crustaceans, 14,700 tonnes of fish, and 2300 tonnes of squid. Satellite telemetry data indicate that Antarctic petrels from Svarthamaren may fly more than 3000 km during one foraging trip, and thus may cover a huge ocean area to obtain their prey. Received: 1 September 1997 / Accepted: 3 February 1998     

Seasonal and annual variation in the diet of breeding and non-breeding Wilson's storm-petrels on King George Island, South Shetland Islands

The diet of breeding and non-breeding Wilson's storm-petrels (Oceanites oceanicus) was analysed over four breeding seasons on King George Island, South Shetland Islands, in order to test if there are changes in diet composition within and between seasons. Krill was the most important food item, followed by myctophid fish and amphipods. Breeding birds showed a clear seasonal pattern of diet, with krill decreasing and alternative prey increasing from the incubation to the chick-rearing period. Non-breeders were not found to change their diet composition. The data suggest that Wilson's storm-petrels selectively choose alternative prey to krill in order to meet the nutrient demands of their chicks. In years of low food availability, Wilson's storm-petrels may have a limited option of prey choice.     

Variation in the diet of Genypterus blacodes(Ophidiidae) around the Falkland Islands

The diet change with size, season and area was investigated using the stomachs of 496 kingclip Genypterus blacodes collected around the Falkland Islands (south‐west Atlantic) between August 2001 and September 2002. The key prey species were rockcod Patagonotothen spp., benthic isopods and Patagonian grenadier Macruronus magellanicus . Kingclip <50 cm total length ( L _T) fed mainly on crustaceans and small fishes. With size the diet shifted away from crustaceans towards Patagonotothen spp. in kingclip 50–100 cm L _T, and finally towards larger fishes such as M. magellanicus and Micromesistius australis australis in kingclip >100 cm L _T. The niche breadth was highest in fish >100 cm L _T and the lowest in fish <50 cm L _T. The larger kingclip generally selected larger individuals of the same prey species, with the exception of the Patagonian squid Loligo gahi , where all ingested squid were of similar size, regardless of the predator length. The importance of the main prey species varied substantially between five consequent seasons studied, and appeared to follow the seasonal abundance and availability of prey. The spatial variability in the diet was found in kingclip caught in regions occupied by transformed temperate and sub‐Antarctic waters. The rockcod, which is available throughout the year around the Falkland Islands, was the most important prey in the kingclip diet. Kingclip takes advantage of other seasonally abundant prey species during their seasonal migrations ( e.g . L. gahi ) and also scavenge on discards from fishing vessels when available.     

Geographical and temporal variation in the diet of Bank Cormorants Phalacrocorax neglectus in South Africa

The Bank Cormorant Phalacrocorax neglectus is endemic to the Benguela upwelling ecosystem off southwest Africa and is classified as Endangered owing to a recent large reduction in its number. It is thought that food scarcity, including a decreased abundance of West Coast rock lobster Jasus lalandii, has been a major driver of the decrease, yet its diet in South Africa is poorly known. We collected 941 pellets regurgitated by Bank Cormorants, at 18 South African breeding colonies during 1975–1985, and 1 523 pellets at 17 colonies during 1995–2002. The species composition of the diet (% numbers) was significantly different between the two periods, with widespread decreases in proportions of rock lobster in the west and of octopus and cuttlefish Sepia spp. at most localities. These taxa were replaced in the diet by fish, including Gobiidae and Clinidae. The pelagic goby Sufflogobius bibarbatus, an important prey of Bank Cormorants in Namibia, was absent from pellets collected in 1975–1985 but common at northern localities from 1995–2002. Composition of the diet by frequency of occurrence was only determined for 1995–2002, when rock lobster was present in 67% of all samples collected, cuttlefish in 39%, and Clinidae in 32%. Data for 1975–1985 and 1995–2002 showed that carapace lengths of rock lobsters eaten by Bank Cormorants averaged 56 mm (range 22–82 mm) and 50 mm (range 22–75 mm), respectively, which compares to the minimum legal size of 75 mm for fisheries in South Africa. This energy- rich prey item was an important constituent of the diet in the winter breeding period.     

Herring and Sprat Abundance Indices Predict Chick Growth and Reproductive Performance of Common Terns Breeding in the Wadden Sea

Herring (Clupea harengus) and sprat (Sprattus sprattus) are the key prey resources of common terns (Sterna hirundo) breeding in the Wadden Sea. Breeding success of the terns has been below average since 2002, coinciding with exceptionally low herring recruitment and sprat abundance. Time series of herring and sprat abundance in the North Sea and in the Wadden Sea were analyzed to explain long-term breeding success and chick development at two common tern breeding colonies. North Sea herring recruitment and sprat abundance in the Wadden Sea explained the largest part of common tern breeding success, both as single variables and in a multiple regression approach. Breeding success showed stronger correlations with herring recruitment indices derived from the North Sea region compared to the Wadden Sea. Also, herring and sprat abundance data explained more variability in breeding success than of more directly responding measures such as growth rate and maximum weight of chicks. Despite spatial and temporal incoherences between fish surveys and the common tern breeding season, breeding success of common terns reflected the abundance of their key prey fish beyond their foraging range and breeding season. We argue that the ecological connectivity between large- and small-scale herring abundance and the responsiveness of common tern breeding success is strong enough to establish a fish–seabird indicator system to be potentially valuable in monitoring and conservation.     

Growth and decline of a penguin colony and the influence on nesting density and reproductive success

Colonial breeding is characteristic of seabirds but nesting at high density has both advantages and disadvantages and may reduce survival and fecundity. African penguins (Spheniscus demersus) initiated breeding at Robben Island, South Africa in 1983. The breeding population on the island increased in the late 1990s and early 2000s before decreasing rapidly until 2010. Before the number breeding peaked, local nest density in the areas where the colony was initiated plateaued, suggesting that preferred nests sites were mostly occupied, and the area used by breeding birds expanded. However, it did not contract again as the population decreased, so that nesting density varied substantially. Breeding success was related positively to the prey available to the breeding birds and negatively to local nest density, particularly during the chick-rearing period, suggesting a density-dependence operating through social interactions in the colony, possibly exacerbated by poor prey availability when the breeding population was large. Although nest density at Robben Island was not high, nesting burrows, which probably reduce the incidence of aggressive encounters in the colony, are scarce and our results suggest that habitat alteration has modified the strength of density-dependent relationships for African penguins. Gaining a better understanding of how density dependence affects fecundity and population growth rates in colonial breeders is important for informing conservation management of the African penguin and other threatened taxa.     

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.     

Blood fatty acids indicate inter- and intra-annual variation in the diet of Adélie penguins: Comparison with stomach content and stable isotope analysis

Adélie penguin (Pygoscelis adeliae) diet is an important indicator of prevailing environmental conditions and resource availability. In this study, dietary variation within and between years was studied with fatty acid signature analysis (FASA), stomach content analysis (SCA) and stable isotope analysis (SIA). We profiled the fatty acid (FA) composition of whole blood collected from adult penguins throughout the breeding season, and from chicks during the crèche period, in 2001 and 2002. Differences were detected in FA profiles between years, breeding stage and age (adults vs. chicks). These patterns broadly corresponded to those observed from SCA and SIA, with a mix of krill and fish consumed in the early part of the breeding season in both years, krill dominating the diet during the chick-rearing periods in 2001, and fish in 2002. Different metabolic and physiological demands between stages, and ages, may also influence FA profiles but warrants further investigation. In-situ calibrations of adult FA blood profiles were made using corresponding stomach samples to quantify diet composition. Using linear discriminate function analysis, we classified adult FA profiles into 3 meal-types: krill, fish or mixed. A higher proportion of adults had fish-like profiles during the arrival and guard periods. Krill-like profiles dominated during the incubation and crèche periods, although there were a relatively high proportion of fish-like and mixed profiles as well. These patterns corresponded to results from SCA and SIA. This study demonstrates that FASA has the potential to be integrated with other dietary tools to enhance diet monitoring studies, which are currently integral to ecosystem management and conservation measures. The in-situ calibration method used offers a simple and effective alternative to more rigorous calibration techniques developed elsewhere.     

Foraging location and range of White-chinned Petrels Procellaria aequinoctialis breeding in the South Atlantic

The foraging range and principal feeding areas of White‐chinned Petrels breeding at South Georgia were determined using satellite telemetry. Foraging trips during incubation lasted 12–15 days and covered 3000–8000 km and 2–11 days and 1100–5900 km during chick‐rearing. Adults covered less distance per day during chick‐rearing (71 km) than during incubation (91 km) but the proportion covered at night (47%) was the same. Mean (31–34 km/h) and maximum (80 km/h) flight velocities were similar during both periods of the breeding season and during day and night. Between incubation shifts, White‐chinned Petrels travelled to the Patagonian shelf; during chick‐rearing they foraged more extensively. Most locations were between 30° to 55°W and 52° to 60°W around South Georgia/Shag Rocks and south to the South Orkney Islands. Diet samples from known foraging locations suggested birds fed mainly on krill and squid. They caught the squid Brachioteuthis? picta and Galiteuthis glacialis around Shag Rocks/South Georgia and also at sites close to the South Orkney Islands; Illex argentinus on the Patagonian shelf. Dispersal of adults after breeding failure was south to the South Orkney Islands then west to the Falkland Islands. This study confirms that breeding White‐chinned Petrels are amongst the widest‐ranging of seabirds; they may minimise competition with other Procellariiformes in the South Atlantic by their more extensive foraging range. The nature and extent of their range also brings substantial risk of high mortality rate in South Atlantic long‐line fisheries.     

Diet composition of expanding breeding populations of the Magellanic Penguin

The Magellanic Penguin (Spheniscus magellanicus) is the most abundant and widely distributed seabird breeding on the Patagonian Coast of Argentina. We combined conventional stomach content and stable isotope analysis to assess Magellanic Penguin diet during the chick rearing stage at the two northernmost colonies in an area subject to fisheries. In 2011 and 2013, Thornfish (Bovichtus argentinus) was the main prey by mass at Complejo Islote Lobos (63.0% and 32.3%, respectively) and Argentine Anchovy (Engraulis anchoita) at Estancia San Lorenzo (85.2 and 63.3%, respectively). Magellanic Penguins from both colonies showed low isotopic niche overlap in 2011 (36%) and no overlap in 2013, suggesting a different use of prey species and/or foraging areas. Stable isotope mixing models showed that Argentine Anchovy (52.8%) and Thornfish (65.9%) were the main prey at Complejo Islote Lobos in 2011 and 2013, respectively, while Patagonian Redfish (Sebastes oculatus) (46.4%) and Squat Lobster (Munida gregaria) (50%) were the main prey at Estancia San Lorenzo in 2011 and 2013, respectively. Results show that in addition to Argentine Anchovy, previously recognized as main prey for breeding Magellanic Penguins in northern Patagonia, other juvenile or small sized fish are important diet items. Diet results suggest different scenarios of food conditions for each colony, despite the relative short distance between breeding locations. The low contribution of Argentine Hake (Merluccius hubbsi) and Argentine Shortfin Squid (Illex argentinus) suggests a low trophic overlap with commercial fisheries. The information provided is key to understand changes in the marine ecosystem and potential penguin-fishery interactions.     

How Much Is Too Much? Assessment of Prey Consumption by Magellanic Penguins in Patagonian Colonies

Penguins are major consumers in the southern oceans although quantification of this has been problematic. One suggestion proposes the use of points of inflection in diving profiles (‘wiggles’) for this, a method that has been validated for the estimation of prey consumption by Magellanic penguins (Spheniscus magellanicus) by Simeone and Wilson (2003). Following them, we used wiggles from 31 depth logger-equipped Magellanic penguins foraging from four Patagonian colonies; Punta Norte (PN), Bahía Bustamente (BB), Puerto Deseado (PD) and Puerto San Julián (PSJ), all located in Argentina between 42–49° S, to estimate the prey captured and calculate the catch per unit time (CPUT) for birds foraging during the early chick-rearing period. Numbers of prey caught and CPUT were significantly different between colonies. Birds from PD caught the highest number of prey per foraging trip, with CPUT values of 68±19 prey per hour underwater (almost two times greater than for the three remaining colonies). We modeled consumption from these data and calculate that the world Magellanic penguin population consumes about 2 million tons of prey per year. Possible errors in this calculation are discussed. Despite this, the analysis of wiggles seems a powerful and simple tool to begin to quantify prey consumption by Magellanic penguins, allowing comparison between different breeding sites. The total number of wiggles and/or CPUT do not reflect, by themselves, the availability of food for each colony, as the number of prey consumed by foraging trip is strongly associated with the energy content and wet mass of each colony-specific ‘prey type’. Individuals consuming more profitable prey could be optimizing the time spent underwater, thereby optimizing the energy expenditure associated with the dives.     

An impact of seasonal squid migrations and fishing on the feeding spectra of subantarctic notothenioids Patagonotothen ramsayi and Cottoperca gobio around the Falkland Islands

Seasonal changes in the diet of two abundant notothenioid fishes, rock cod Patagonotothen ramsayi (Nototheniidae) and frogmouth Cottoperca gobio (Bovichthidae) were studied on the Falkland Island shelf. The rock cod is a near-bottom browser, feeding mostly on gelatinous and crustacean plankton. The diet of juveniles [10–16 cm total length (TL)] includes mostly copepods in summer and comb jellies Mnemiopsis leydyi in winter. Adult rock cod (17–34 cm TL) in summer consumed mainly plankton, with comb jellies and salps being of primary importance. In winter they mainly took benthos, primarily ophiuroids and lobster krill ( Munida spp.), and the comb jelly. During the fishing season, rock cod also scavenge on fishing discards. The frogmouth is a benthic ambush predator; both small (11–30 cm) and large (31–80 cm) representatives feed on the rock cod throughout the year. During the seasonal offshore migration of the squid Loligo gahi (June–August), this prey predominated in the large frogmouth diet, with crab, Peltarion spinosulum , of secondary importance. The region is seasonally exploited by the squid trawl fishery. As the fishing season commences (August–September), discarded rock cod also becomes an important food item for the frogmouth. Seasonal offshore migrations of L. gahi into the common depth range of both notothenioids have a significant direct (change in the C. gobio diet) and indirect (scavenging on fishing discards and targeting bottom scavengers by P. ramsayi and C. gobio during the squid fishing) impact on their diets over the Falkland shelf. The niche breadth of rock cod varied from 2.63 to 3.21 and that of frogmouth varied from 2 to 6.29, depending on fish size and season.     

Intra-seasonal variation in zooplankton availability,chick diet and breeding performance of a high Arctic planktivorous seabird

Different phenological responses to climate changes by species representing preys and predators may lead to mismatch between functionally dependent components of an ecosystem, with important effects on its structure and functioning. Here, we investigate within-season variation in zooplankton availability, chick diet composition and breeding performance of a small planktivorous seabird, the little auk (Alle alle) in two large colonies in Hornsund and Magdalenefjorden, Spitsbergen, differing in synchrony of breeding (11-day vs. 22-day hatching period, respectively). Assuming similar zooplankton phenology and existing differences in duration of the little auk breeding period, we expected lower availability of the preferred food in the less synchronized colony in Magdalenefjorden and in consequence a negative effect on nestling body mass and survival. We found that in both colonies Calanus glacialis (copepodite stage CV) was the most important prey item in the chick diet making up 68–87 % of the biomass and energy of all prey items. The only exception was the end of the chick-rearing period in Magdalenefjorden, when contribution of this prey item was significantly lower (24–26 %). Thus, late breeders in Magdalenefjorden were apparently mismatched regarding C. glacialis CV availability. However, the hatching date did not affect birds fitness (reproductive output and chick pre-fledging mass) significantly. Results of our study indicate that little auks breeding on Spitsbergen can respond to a wide range of environmental conditions and prey availabilities through the plasticity of their foraging behaviour, which may help them to maintain their optimum fitness level in changing and unpredictable environments.     

Feather corticosterone reveals stress associated with dietary changes in a breeding seabird

Changes in climate and anthropogenic pressures might affect the composition and abundance of forage fish in the world's oceans. The junk‐food hypothesis posits that dietary shifts that affect the quality (e.g., energy content) of food available to marine predators may impact their physiological state and consequently affect their fitness. Previously, we experimentally validated that deposition of the adrenocortical hormone, corticosterone, in feathers is a sensitive measure of nutritional stress in seabirds. Here, we use this method to examine how changes in diet composition and prey quality affect the nutritional status of free‐living rhinoceros auklets (Cerorhinca monocerata). Our study sites included the following: Teuri Is. Japan, Middleton Is. central Gulf of Alaska, and St. Lazaria Is. Southeast Alaska. In 2012 and 2013, we collected “bill loads” delivered by parents to feed their chicks (n = 758) to document dietary changes. We deployed time–depth–temperature recorders on breeding adults (n = 47) to evaluate whether changes in prey coincided with changes in foraging behavior. We measured concentrations of corticosterone in fledgling (n = 71) and adult breeders' (n = 82) feathers to determine how birds were affected by foraging conditions. We found that seasonal changes in diet composition occurred on each colony, adults dove deeper and engaged in longer foraging bouts when capturing larger prey and that chicks had higher concentrations of corticosterone in their feathers when adults brought back smaller and/or lower energy prey. Corticosterone levels in feathers of fledglings (grown during the breeding season) and those in feathers of adult breeders (grown during the postbreeding season) were positively correlated, indicating possible carryover effects. These results suggest that seabirds might experience increased levels of nutritional stress associated with moderate dietary changes and that physiological responses to changes in prey composition should be considered when evaluating the effect of prey quality on marine predators.