Conservation implications of variation in diet and dietary specialisation in great skuas

1. Great skuas Catharacta skua are the only member of the genus Catharacta in the northern hemisphere. The UK holds around 60% of the World population with 8,000 breeding pairs. 2. Sustained population growth of the great skua in Scotland during the last century appears attributable to an abundant supply of discards (from commercial fisheries) and sandeels Ammodytes marinus. In addition great skuas may scavenge or predate other seabirds and their chicks when other prey is in short supply. 3. Following a decline in sandeels in the 1980s there is some evidence that an increase in predation of seabirds by great skuas may be affecting seabird populations. Future measures to reduce the amounts of fish discarded may result in a further increase in predation by great skuas. This highlights the need to quantify the current impact of great skua predation on seabird populations, and monitor any future change. 4. Accurate assessment of great skua diet is fundamental to this type of research. Many studies have used pellets of indigestible prey to assess diet in skuas and gulls, but have not quantified this technique. Captive great skuas were fed a range of fish and birds to try and understand more fully how pellets reflect diet. Feeding trials showed that skuas fed on a diet of birds produced more pellets than when feeding on fish. Fish species strongly influenced the number of pellets produced as well as the proportion and size of otoliths recovered. The numbers of pellets cast also varied significantly among differing species of bird meals. Field trials revealed that only a small proportion of pellets produced are being sampled. This study highlights the need to carefully validate the use of pellets to assess diet, particularly in a species of conservation concern. 5. The diet of great skuas was estimated based on five different techniques (pellets, prey remains, spontaneous regurgitates, observed feeds and water off-loading) and the results compared. The diet composition based on five sampling techniques in a single year generally showed a good correlation with one another. However comparing the proportion of the three main prey types estimated by four sampling techniques over three years revealed a significant interactive effect of year and sampling technique on the diet composition. While estimates of diet using different sampling techniques may be broadly comparable, technique dependent biases mean that the advantages and disadvantages of each sampling technique need to be borne in mind before conducting diet studies. A small proportion of great skuas breeding at Hermaness, Shetland exhibit distinct dietary specialisation, feeding almost exclusively upon seabird prey. Around half of these "bird-specialists" defend feeding territories within a section of seabird colony, the remainder foraging away from breeding territories. "Bird-specialists" retained their feeding habit and, if present, territory, between years. Time-budgets revealed that "bird-specialists" with feeding territories spent less time foraging than "bird-specialists" without a feeding territory or skuas feeding predominantly on fish. Results of radiotracking great skuas for the first time suggest that "bird-specialists" have smaller home ranges than "others". In all years "bird-specialists" show similar productivity to "others", but earlier hatching dates (a good measure of quality in great skuas). While we do not know whether high quality skuas feed on seabirds or that feeding on seabirds advances laying date, hatching early is likely to confer an advantage to "bird- specialists". Non-specialist great skuas experienced a reduction in clutch volume and chick condition during 1999, compared with 1998 - presumably due to a reduction in food availability. "Bird-specialists" did not experience a similar decline in clutch volume and chick condition between years, and showed higher clutch volume and chick condition than "others" in 1999. In addition to changes in clutch volume and chick condition, adult non-specialists showed reduced annual survival, compared with "bird-specialists" over the same period. These results suggest that "bird-specialists" not only have earlier hatching dates in all years, but in certain years also gain an advantage in terms of improved chick condition and adult survival that may have implications for lifetime reproductive success (LRS). Apparent fitness benefits derived from specialising in bird predation may have conservation implications for seabirds colonies in Shetland

Social information use and collective foraging in a pursuit diving seabird

Individuals of many species utilise social information whilst making decisions. While many studies have examined social information in making large scale decisions, there is increasing interest in the use of fine scale social cues in groups. By examining the use of these cues and how they alter behaviour, we can gain insights into the adaptive value of group behaviours. We investigated the role of social information in choosing when and where to dive in groups of socially foraging European shags. From this we aimed to determine the importance of social information in the formation of these groups. We extracted individuals’ surface trajectories and dive locations from video footage of collective foraging and used computational Bayesian methods to infer how social interactions influence diving. Examination of group spatial structure shows birds form structured aggregations with higher densities of conspecifics directly in front of and behind focal individuals. Analysis of diving behaviour reveals two distinct rates of diving, with birds over twice as likely to dive if a conspecific dived within their visual field in the immediate past. These results suggest that shag group foraging behaviour allows individuals to sense and respond to their environment more effectively by making use of social cues

Stable isotopes demonstrate intraspecific variation in habitat use and trophic level of non‐breeding albatrosses

The non‐breeding period is critical for restoration of body condition and self‐maintenance in albatrosses, yet detailed information on diet and distribution during this stage of the annual cycle is lacking for many species. Here, we use stable isotope values of body feathers (δ13C, δ15N) to infer habitat use and trophic level of non‐breeding adult Grey‐headed Albatrosses Thalassarche chrysostoma (n = 194) from South Georgia. Specifically, we: (1) investigate intrinsic drivers (sex, age, previous breeding outcome) of variation in habitat use and trophic level; (2) quantify variation among feathers of the same birds; and (3) examine potential carry‐over effects of habitat use and trophic level during the non‐breeding period on subsequent breeding outcome. In agreement with previous tracking studies, δ13C values of individual feathers indicate that non‐breeding Grey‐headed Albatrosses from South Georgia foraged across a range of oceanic habitats, but mostly in subantarctic waters, between the Antarctic Polar Front and Subtropical Front. Sex differences were subtle but statistically significant, and overlap in the core isotopic niche areas was high (62%); however, males exhibited slightly lower δ13C and higher δ15N values than females, indicating that males forage at higher latitudes and at a higher trophic level. Neither age nor previous breeding outcome influenced stable isotope values, and we found no evidence of carry‐over effects of non‐breeding habitat use or trophic level on subsequent breeding outcome. Repeatability among feathers of the same individual was moderate in δ13C and low in δ15N. This cross‐sectional study demonstrates high variability in the foraging and migration strategies of this albatross population

Three dimensional tracking of a wide-ranging marine predator: flight heights and vulnerability to offshore wind farms

1. A large increase in offshore wind turbine capacity is anticipated in the next decade, raising concerns about possible adverse impacts on birds as a result of collision risk. Birds’ flight heights greatly influence this risk yet height estimates are currently available only using methods such as radar or ship-based observations over limited areas. 2. Bird-borne data-loggers have the potential to provide improved estimates of collision risk and here, we use data from GPS-loggers and barometric pressure-loggers to track the three-dimensional movements of northern gannets rearing chicks at a large colony in SE Scotland (Bass Rock), located < 50km from several major wind farm developments with recent planning consent. We estimate the foraging ranges and densities of birds at sea, their flight heights during different activities and the spatial variation in height during trips. We then use these data in collision-risk models to explore how the use of different methods to determine flight height affects the predicted risk of birds colliding with turbines. 3. Gannets foraged in and around planned wind farm sites. The probability of flying at collision- risk height was low during commuting between colonies and foraging areas (median height 12m) but was greater during periods of active foraging (median height 27m), and we estimate that ~1500 breeding adults from Bass Rock could be killed by collision with wind turbines at two planned sites in the Firth of Forth region each year. This is up to 12 times potential mortality predicted using other available flight height estimates. 4. Synthesis & Applications: The use of conventional flight height estimation techniques resulted in large underestimates of the numbers of birds at risk of colliding with wind turbines. Hence we recommend using GPS and barometric tracking to derive activity-specific and spatially-explicit flight heights and collision risks. Our predictions of potential mortality approached levels at which long-term population viability could be threatened, highlighting a need for further data to refine estimates of collision risks and sustainable mortality thresholds. We also advocate raising the minimum permitted clearance of turbine blades at sites with high potential collision risk from 22m 51 to 30m above sea level

Gannets are not attracted to fishing vessels in Iceland-potential influence of a discard ban and food availability

BLC was supported by a NERC GW4+ Doctoral Training Partnership studentship from the Natural Environment Research Council [NE/L002434/1]. We thank Ólafur Torfason, Niall Tierney, and Rachel Stroud for fieldwork assistance in Skrúður, and Mamma-Rósa for food and housing in Vestmannaeyjar. We thank the Hellisey hunting club for the use of cabin and assistance with boat trips to Hellisey. We thank Filipa Samarra, Miguel Neves, Gary Haskins, and team members in the Icelandic Orca Project for boat trips to Hellisey. We thank Lucy Hawkes, David Pascall, Alice Williams, Richard Phillips, Brendan Godley and all reviewers for constructive comments on the manuscript. The GPS tracking data are available through the BirdLife International Seabird Tracking Database (http://www.seabirdtracking.org).Peer reviewedPublisher PD

A bird's eye view of discard reforms: bird-borne cameras reveal seabird/fishery interactions.

notes: PMCID: PMC3590202types: Journal Article; Research Support, Non-U.S. Gov'tCopyright: © 2013 Votier et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.Commercial capture fisheries produce huge quantities of offal, as well as undersized and unwanted catch in the form of discards. Declines in global catches and legislation to ban discarding will significantly reduce discards, but this subsidy supports a large scavenger community. Understanding the potential impact of declining discards for scavengers should feature in an eco-system based approach to fisheries management, but requires greater knowledge of scavenger/fishery interactions. Here we use bird-borne cameras, in tandem with GPS loggers, to provide a unique view of seabird/fishery interactions. 20,643 digital images (one min(-1)) from ten bird-borne cameras deployed on central place northern gannets Morus bassanus revealed that all birds photographed fishing vessels. These were large (>15 m) boats, with no small-scale vessels. Virtually all vessels were trawlers, and gannets were almost always accompanied by other scavenging birds. All individuals exhibited an Area-Restricted Search (ARS) during foraging, but only 42% of ARS were associated with fishing vessels, indicating much 'natural' foraging. The proportion of ARS behaviours associated with fishing boats were higher for males (81%) than females (30%), although the reasons for this are currently unclear. Our study illustrates that fisheries form a very important component of the prey-landscape for foraging gannets and that a discard ban, such as that proposed under reforms of the EU Common Fisheries Policy, may have a significant impact on gannet behaviour, particularly males. However, a continued reliance on 'natural' foraging suggests the ability to switch away from scavenging, but only if there is sufficient food to meet their needs in the absence of a discard subsidy.EU INTERREG project CHARM-IIINatural Environment Research CouncilAssociation for the Study of Animal Behaviour research gran

Stable isotope values in South American fur seal pup whiskers as proxies of year-round maternal foraging ecology

Natural selection should favour strategies that maximise reproductive success. Females may use different resources during progressive stages of reproduction according to energetic demands, behavioural constraints and prey availability. We used South American fur seal, Arctocephalus australis australis, pup whisker isotope values as proxies for maternal diet and habitat use to determine how resource use (1) changes throughout pup development from in utero growth to mid-end of lactation and (2) how it differs among individuals. The longest whisker was cut from 5 male and 5 female fur seal pups (of approximately 8 months of age) at Bird Island, Falkland Islands, in 2018, and δ15N values and δ13C values were analysed every 5 mm along the length of each whisker. Patterns in δ13C values indicated that mothers used different habitats during the annual cycle, likely coinciding with seasonal shifts in prey availability or distribution. The individual specialisation index based on δ13C values was 0.34, indicating that adult females used different habitats, which could reduce intra-specific competition and ultimately enhance pup growth and survival. An increase in δ15N values occurred along every pup whisker from pup birth to mid-end of lactation, which likely reflected trophic enrichment related to suckling and fasting by pups, overriding the maternal isotopic signature. Pup whisker stable isotopes are useful proxies of maternal foraging ecology. However, physiological processes complicate interpretations by altering δ15N values. Interpreting these values therefore requires additional knowledge of the species’ ecology and physiology

Intra-specific niche partitioning in Antarctic fur seals, Arctocephalus gazella

Competition for resources within a population can lead to niche partitioning between sexes, throughout ontogeny and among individuals, allowing con-specifics to co-exist. We aimed to quantify such partitioning in Antarctic fur seals, Arctocephalus gazella, breeding at South Georgia, which hosts ~95% of the world’s population. Whiskers were collected from 20 adult males and 20 adult females and stable isotope ratios were quantified every 5 mm along the length of each whisker. Nitrogen isotope ratios (δ15N) were used as proxies for trophic position and carbon isotope ratios (δ13C) indicated foraging habitat. Sexual segregation was evident: δ13C values were significantly lower in males than females, indicating males spent more time foraging south of the Polar Front in maritime Antarctica. In males δ13C values declined with age, suggesting males spent more time foraging south throughout ontogeny. In females δ13C values revealed two main foraging strategies: 70% of females spent most time foraging south of the Polar Front and had similar δ15N values to males, while 30% of females spent most time foraging north of the Polar Front and had significantly higher δ15N values. This niche partitioning may relax competition and ultimately elevate population carrying capacity with implications for ecology, evolution and conservation

Risk exposure trade-offs in the ontogeny of sexual segregation in Antarctic fur seal pups

Sexual segregation has important ecological implications, but its initial development in early life stages is poorly understood. We investigated the roles of size dimorphism, social behavior, and predation risk on the ontogeny of sexual segregation in Antarctic fur seal, Arctocephalus gazella, pups at South Georgia. Beaches and water provide opportunities for pup social interaction and learning (through play and swimming) but increased risk of injury and death (from other seals, predatory birds, and harsh weather), whereas tussock grass provides shelter from these risks but less developmental opportunities. One hundred pups were sexed and weighed, 50 on the beach and 50 in tussock grass, in January, February, and March annually from 1989 to 2018. Additionally, 19 male and 16 female pups were GPS-tracked during lactation from December 2012. Analysis of pup counts and habitat use of GPS-tracked pups suggested that females had a slightly higher association with tussock grass habitats and males with beach habitats. GPS-tracked pups traveled progressively further at sea as they developed, and males traveled further than females toward the end of lactation. These sex differences may reflect contrasting drivers of pup behavior: males being more risk prone to gain social skills and lean muscle mass and females being more risk averse to improve chances of survival, ultimately driven by their different reproductive roles. We conclude that sex differences in habitat use can develop in a highly polygynous species prior to the onset of major sexual size dimorphism, which hints that these sex differences will increasingly diverge in later life

Stable isotopes reveal the importance of seabirds and marine foods in the diet of St Kilda field mice

Introduced mammals have devastated island nesting seabird populations worldwide. Declines in breeding seabirds on St Kilda, UK, have been linked to climate change and predation from great skuas Stercorarius skuas, but the introduced St Kilda field mouse Apodemus sylvaticus hirtensis may also play a role by feeding on adults, chicks or eggs. Here, we use stable isotopes in St Kilda mouse blood and potential dietary items to investigate their foraging ecology, specifically focussing on the importance of seabirds and marine foods in their diet. Mice were seasonally sampled at three sites on Hirta, St Kilda over three consecutive years (2010–2012). The δ13C and δ15N ratios were used in analyses, including isotope niche and dietary source mixing models, to examine foraging behaviour among locations and between seabird breeding seasons. Mice sampled in Carn Mor – where the majority of the island’s seabirds nest - had consistently higher δ13C than other locations throughout the year, with δ15N also being significantly higher for all but one comparison. The isotopic niche width (SEAs) of Carn Mor mice in each season were distinct from the other locations, and became smaller during the seabird breeding season. Dietary mixing models revealed that seabirds made up a large proportion of the diet for mice from Carn Mor, particularly during the seabird breeding season. In conclusion, our work reveals that seabird-derived foods are likely to form a significant part of the diet of St Kilda mice populations located in and around breeding colonies. It is unclear however, whether this is from scavenging or predation of seabirds, or through their discarded food items. Given that mice have had significant effects on seabird populations elsewhere, it is important to carry out further work to determine whether mice are a significant cause of seabird mortality in this island ecosystem