Stercorarius Antarcticus Lonnbergi Research Articles

Non-breeding behaviour in the Brown Skua (Stercorarius antarcticus lonnbergi): insights from modelling moulting patterns and stable isotope analyses

ABSTRACT Long-term changes in the life history and behaviour of seabirds during the non-breeding season can reflect shifts in environmental conditions. However, long-term marine studies are scarce, particularly on southern hemisphere seabirds. Here, we used moult scores from 86 Brown Skuas (Stercorarius antarcticus lonnbergi), a large predatory seabird breeding on the Chatham Islands, Aotearoa/New Zealand to model both the timing and duration of primary feather moult. In addition, we analysed stable isotope values (δ13C and δ15N) from 62 modern (2014–16) and ten museum tail feathers. These data provide insights into the non-breeding behaviour of Brown Skua. Interestingly, our results show that the primary feather moult occurred prior to birds departing the colony, starting on average on 2 January ± 5 days (SE). The average start of primary feather moult occurred five days prior to the end of breeding (7 January ± 10 days (SD)) and 42 days before the birds departed the colony (13 February ± 11 days (SD)). The average duration of primary feather moult was 189 ± 14 days (SE). Importantly, low δ13C values in four females suggested that tail feather moult might also occur while skuas are at the colony. There was no difference in tail feather δ13C and δ15N values between any pairwise comparison of modern and museum years. However, values of δ15N from tail feathers sampled in 2014 were different from those sampled in 2015 and 2016. This large annual variation in δ15N values from tail feathers over such a short period makes long-term comparisons difficult to interpret, particularly between years with low sample sizes. While the stable isotope analyses of tail feathers are informative, we recommend future studies of skuas sample the primary coverts rather than tail feathers.

PELLETS AND PREY REMAINS AS INDICATORS OF THE DIET OF TWO SYMPATRIC SKUAS (AVES: STERCORARIIDAE) ON KING GEORGE ISLAND, ANTARCTICA

South Polar skua (Stercorarius maccormicki) and Brown skua (Stercorarius antarcticus lonnbergi) have opportunistic feeding habits and are the dominant predators in terrestrial Polar regions. These skuas exploit a wide range of food items, including marine organisms, other birds, and even garbage. In the present study, we compare the diets of these two skua species during the breeding season, using pellets and prey remains collected within their territories. The samples were collected at six sites in Admiralty Bay, on King George Island, Antarctica. We identified eight different items, which we classified as “penguin”, “flying bird”, “skua”, “fish”, “gastropod”, “krill”, “egg” and “marine debris”. In the first breeding season (2008/2009), penguins and flying birds were the food resources more abundant for both skua species, and their diet composition was similar. In the second breeding season (2010/2011) South Polar skua exploited more fish and flying birds than Brown skua; the latter exploited more eggs and penguins. Our findings corroborate those of previous studies, demonstrating that in sympatry South Polar skua exploit more fish than Brown skua. The diet of South Polar skua also varied between breeding seasons, reflecting the opportunistic foraging behavior of these skuas. As in other studies, we recorded that skua is a food resource for both skua species, but it was more common in the diet of South Polar skua. Marine debris was recorded only in the samples of Brown skua. Birds are important food items for both skuas, although significant differences were found in the diets of these sympatric species, with shifts in the composition of the diet probably reflecting fluctuations in the abundance of prey populations, which are known to be common at Admiralty Bay, although more data will be needed to confirm this link.

Bottom‐up effect of eradications: The unintended consequences for top‐order predators when eradicating invasive prey

Abstract The eradication of invasive species from islands yields significant conservation returns. However, novel challenges continue to arise as projects expand in their scope, complexity and scale. Prey‐loss and secondary poisoning were historically considered to have limited impact on native top‐order predators when planning eradications, but this has rarely been tested quantitatively. We used a 10‐year timeseries of Brown Skua Stercorarius antarcticus lonnbergi breeding surveys and isotopic dietary analysis on Macquarie Island to investigate how prey‐loss and secondary poisoning deaths resulting from the eradication of an abundant invasive prey species, European rabbits Oryctolagus cuniculus, affected a top‐order predator. Skua nest density declined from 7.14 nests/km2 (95% CI: 6.01–8.27) in the presence of rabbits (pre‐eradication) to 3.73 nests/km2 (95% CI: 2.96–4.51) in the first 3 years after the eradication of rabbits, before showing signs of recovery in the 4 years thereafter. However, breeding success dropped from 1.01 chicks/nest (95% CI: 0.76–1.26) to as low as 0.38 chicks/nest (95% CI: 0.23–0.53) with little evidence of recovery. Secondary poisoning affected a greater number of skuas than anticipated prior to the eradication, including skuas nesting in areas where rabbits were not typically hunted as prey. We highlight that invasive prey often replace native prey in the diet of native predators rather than provide an additional source of food, and rapid eradication of non‐native prey can have long‐term impacts for predators, particularly when recovery of native prey is slow. Synthesis and applications. Monitoring programmes that complement large‐scale eradication projects and address (a) trophic‐driven declines in predator populations and (b) population‐level impacts of secondary poisoning are integral to ensuring bottom‐up effects of eradications are anticipated and adequately quantified. If prey deficits caused by eradication of invasive prey are expected to be severe but short‐lived, supplementary feeding programmes may buffer against increased predation pressure on native prey and reduced breeding success of native predators. Alternatively, if the rapid recovery of native prey is not expected to occur naturally, breeding programmes and translocation of native prey prior to assist recovery of native predators should be considered to support ecosystem restoration.

Dynamics of circulating lipoproteins and lipids in Brown Skua (Stercorarius antarcticus lonnbergi) during the breeding cycle

In central-place foragers the breeding cycle is often the period with the highest energy cost, where dietary and stored lipids play a key role. Lipids are mobilized through blood lipoproteins providing fuel to tissues. Thus, the use of food and endogenous resources with high-energy fats is important to sustain individuals’ nutritional demands. To evaluate the physiological components associated to energetic demands and their variation regarding life history processes, we analysed the plasma circulating lipoprotein levels, the lipid classes and fatty acid (FA) composition in Brown Skua (Stercorarius antarcticus lonnbergi) in breeding stages with different energy requirements: incubation (In), early rearing (Er) and late rearing (Lr). We expected to find differences according to the energy demands of each stage and the sex. The level of Very Low Density lipoproteins was affected by the sex and the breeding stage, whereas Low Density lipoproteins only by the stage. Total plasma lipid content and circulating lipid classes were invariant among the stages studied; however, differences in total plasma FA and monounsaturated FA (oleic and nervonic) abundances were observed among stages. Besides, a decrease in the n-3:n-6 ratio was observed towards the Lr stage. The differences found in lipoproteins may be linked to changes in the energy demands throughout the breeding period. Moreover, the variation observed in FA abundance and n-3:n-6 ratio may be related to a preferential mobilization of FA from stores as fuel or, possibly, to a differential use of the available food resources.

Plastic ingestion risk is related to the anthropogenic activity and breeding stage in an Antarctic top predator seabird species

During the last decades plastic pollution has become a common issue in marine environments. Studies on seabirds have focused on species that ingest plastics mistaken for prey or indirectly through their preferred prey or, on how foraging strategy influences this behaviour. We evaluated plastic ingestion in relation to the proximity of nests to areas with different anthropogenic pressure, breeding status and breeding stage. We analyzed regurgitated pellets (n = 1001) from a seabird, the Brown Skua (Stercorarius antarcticus lonnbergi) at Esperanza/Hope Bay, Antarctic Peninsula. Plastics were found in 9% of pellets, only in breeders from an area with high antropogenic activity. The prevalence of plastic increased during the brooding of chicks stage, when skuas expand their feeding niche. Our results support previous work which demonstrated that seabirds with wider feeding niche show higher loads of plastics. Altogether, this provides insights into the dynamics of plastic transfer within the environment.

Diet of the Brown Skua (Stercorarius antarcticus lonnbergi) at Hope Bay, Antarctic Peninsula: differences between breeders and non-breeders

Fil: Borghello, Paloma. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Division Zoologia de Vertebrados. Seccion Ornitologia; Argentina

Declining health status of Brown Skua (Stercorarius antarcticus lonnbergi) parents and their offspring during chick development

During breeding, Brown Skuas (Stercorarius antarcticus lonnbergi) perform a great physical effort in the care and maintenance of the nest. Interestingly, the breeding colony on which this work was conducted is in steady decline in recent years. In order to understand the physiological background responsible for the trade off between reproductive effort and health status, in adults and the possible reasons for the colony decline, we evaluated the health status in Brown Skuas parents and growing chicks and blood samples were obtained in three different breeding stages from adults: In (incubation), Er (after egg hatching), and Lr (during chick rearing) and from developing chicks. Serum albumin, α-, β-. and γ-globulin fractions, IgY level and corticosterone were determined in adults, while plasma proteins concentration, electrolytes, and γ-globulins were determined in chicks. Differences in albumin, α-, and γ-globulins and IgY levels were observed in adults through the reproductive stages, indicating a decline in nutritional status and immunocompetence during breeding. In addition, a decline in body condition and immune status was observed in growing chicks, indicating a close relationship between the decrease of the health status and reproductive effort performed by adults and the physiological status of the nestlings during development.

Skuas (Stercorarius spp.) moult body feathers during both the breeding and inter‐breeding periods: implications for stable isotope investigations in seabirds

Seabirds are mostly thought to moult during the inter‐breeding period and the isotopic values of their feathers are often therefore assumed to relate to their assimilated diet during such periods. We observed Brown Skuas Stercorarius antarcticus lonnbergi and South Polar Skuas Stercorarius maccormicki moulting on a breeding site at King George Island, Antarctica. This raises concerns about the reliability of using stable isotopes in feathers to infer feeding localities of birds during the inter‐breeding period. We analysed the δ13C and δ15N values of growing and fully grown body feathers collected from the same individuals. For both species, δ13C values of growing feathers indicated feeding areas in the Antarctic zone (breeding grounds), whereas most fully grown feathers (100% for South Polar Skuas and 93.3% for Brown Skuas) could be assigned to northern latitudes (non‐breeding grounds). However, a few fully grown body feathers of Brown Skuas (6.7% of the feathers, belonging to two birds) showed isotopic values that indicated moult in the Antarctic zone. As the growth period of those feathers was unknown, they could not be used with confidence to depict the foraging behaviour of the birds during the non‐breeding period. Although precautions must be taken when inferring dietary information from feathers in seabirds where the moulting pattern is unknown, this study shows that if the development stage of a feather (growing/fully grown) is identified, then dietary information from both breeding and non‐breeding seasons can be obtained on the same individual birds.

Flexibility in foraging strategies of Brown Skuas in response to local and seasonal dietary constraints

The Brown Skua Stercorarius antarcticus lonnbergi is an opportunistic species that displays a high degree of flexibility in foraging tactics. We deployed glo- bal positioning system (GPS) and immersion (activity) loggers on breeding Brown Skuas of known sex, body size and condition at Admiralty Bay, King George Island with the aim to examine the impacts of spatial and seasonal fluctuations in prey availability on movement and foraging behavior. We also investigated whether reversed sexual size dimorphism (females larger than males) in this species leads to differences between sexes in foraging behavior and whether this or other factors contribute to variation in breeding success. Analysis of the GPS data highlighted the high degree of plasticity in foraging behavior among individuals. Although most Brown Skuas were flexible in their feeding tactics, this was not enough to ensure a suc- cessful breeding season, as few pairs fledged chicks. Dur- ing early chick rearing, Brown Skuas spent most of their time on land, feeding almost exclusively on penguin chicks. By late chick rearing, when the availability of penguins had diminished, Brown Skuas supplemented the food obtained on land by traveling to the ocean. All for- aging trips to sea occurred during daylight, mostly during the early morning. Despite marked sexual size dimorphism, we failed to find any difference in foraging tactics between males and females. Furthermore, although laying date affected the number of chicks hatched (earlier pairs were more successful), no relationship was found between breeding success and male or female body size, condition or degree of dimorphism within pairs.

Variation in diet composition during the breeding cycle of an Antarctic seabird in relation to its breeding chronology and that of its main food resource

This work had two aims related to the diet of brown skuas (Stercorarius antarcticus lonnbergi) breeding at Laurie Island (South Orkney Islands, Antarctic). The first aim was to explore whether there are changes throughout the breeding season. The second aim was to determine whether those changes relate to differences in food resource availability of their main prey, penguins, at different time periods of the penguins’ breeding cycles, or to different moments of the skuas breeding cycle, which may variably restrict the foraging activities of parents. Diet was analyzed from pellet samples grouped in two different ways. They were grouped in three periods defined for the skuas breeding cycle (laying and incubation; early parental care; later parental care), or the pellets were assigned to five periods based on the type of food resources available at the penguin colonies (eggs; eggs and small chicks; small and large chicks; large and fledged chicks; fledged chicks). A temporal variation in diet composition was evident from the analysis of contingency tables for both sample grouping methods. The more represented item in every period for both analyses was adult penguins, which may be related to the proposed cleaning function of the gut of penguin feathers. Both ways of grouping the samples suggest a relationship between the kind of resources available at the penguin colonies and the easiness of delivering them to the skuas chicks, reflected in a successive predominance of use of penguin eggs first and of penguin chicks and other birds later.

Over-representation of bird prey in pellets of South Polar Skuas

We report the first study to compare the contents of pellets and regurgitates of South Polar Skuas, Stercorarius maccormicki, at two breeding colonies on the Antarctic Peninsula. Samples were taken across years from identified breeding pairs at Potter Peninsula in sympatry with 30–40 breeding pairs of Brown Skuas, Stercorarius antarcticus lonnbergi, and from Cierva Point with 2 pairs of Brown Skuas. In contrast to the general consensus that penguins feature in the diet most prominently in the absence of Brown Skuas, we found a much more frequent occurrence of penguin remains in samples at Potter than Cierva, but only in pellets. At Cierva, penguin feathers were largely replaced in the pellets by a high frequency of non-food moss. The rare occurrence of penguins in regurgitates from both sites is consistent with scavenging rather than active predation. The high frequency of feathers in pellets at Potter is consistent with a gut-cleansing function similar to that performed by moss, which is abundant only at Cierva. We conclude that pellets over-represent penguins in the diet. For any species that consumes feathers, the evidence must consider the alternative possibility of a non-food function of ingesting feathers.

Trophic interactions between brown and south polar skuas at Deception Island, Antarctica

It is broadly accepted that the brown skua (Stercorarius antarcticus lonnbergi) competitively excludes the south polar skua (S. maccormicki) from penguin colonies when breeding sympatrically, forcing the latter to feed on marine resources. The purpose of this work was to examine the diets and trophic niche breadths of each species where they co-occur and to determine the degree of overlap. To this end, we analyzed 169 pellets of brown skuas, collected in two different areas (20 individuals), and 152 of south polar skuas, collected in three different areas (18 individuals), on Deception Island, South Shetland Islands, Antarctica, during the austral summer 2000. Pellet analysis often underestimates the amount of easily digestible prey, but allows for comparisons of the relative contributions of different items in the diet. South polar skuas at our study locations consumed seven different food items and had a trophic niche breadth of 0.133 compared to brown skuas that fed on 10 different items and had a trophic niche breadth of 0.078. The niche overlap between the species was 82.1%. Penguins were the principal food source of both species, however, brown skuas fed mostly on chicks, while south polar skuas fed on adults (carcasses). The use of different age classes of penguins as a food source offers an alternative to competitive exclusion, allowing the coexistence of these species on Deception Island.

Skeletal morphoanatomy of the Brown Skua Stercorarius antarcticus lonnbergi and the South Polar Skua Stercorarius maccormicki

The Brown Skua Stercorarius antarcticus lonnbergi and the South Polar Skua Stercorarius maccormicki, which breed sympatrically in the Southern Hemisphere, are difficult to identify in the field because of their similar coloration and external morphology. Although several morphological and biological aspects have been previously discussed, no data have been published about their skeletal anatomy. In this paper we make a comparative description of the cranial and appendicular skeleton of both skua species. As a result, and although the differences are small, it has been found that some characters of the skull and the thoracic and pelvic limbs, allow the distinction between S. antarcticus lonnbergi and S. maccormicki.