The influence of meal size on prey DNA detectability in piscivorous birds

Molecular methods allow noninvasive assessment of vertebrate predator–prey systems at high taxonomic resolution by examining dietary samples such as faeces and pellets. To facilitate the interpretation of field‐derived data, feeding trials, investigating the impacts of biological, methodological and environmental factors on prey DNA detection, have been conducted. The effect of meal size, however, has not yet been explicitly considered for vertebrate consumers. Moreover, different noninvasively obtained sample types remain to be compared in such experiments. Here, we present a feeding trial on abundant piscivorous birds, Great Cormorants (Phalacrocorax carbo), to assess meal size effects on postfeeding prey DNA detection success. Faeces and pellets were sampled twice a day after the feed of large (350–540 g), medium (190–345 g) and small (15–170 g) fish meals contributing either a large (>79%) or small (<38%) share to the daily consumption. Samples were examined for prey DNA and fish hard parts. Molecular analysis of faeces revealed that both large meal size and share had a significantly positive effect on prey DNA detection rate postfeeding. Furthermore, large meals were detectable for a significantly longer time span with a detection limit at ~76 hr and a 50% detection probability at ~32 hr postfeeding. In pellets, molecular methods reliably identified the meal consumed the previous day, which was not possible via morphological analysis or when examining individual faeces. The less reliable prey DNA detection of small meals or meal shares in faeces signifies the importance of large numbers of dietary samples to obtain reliable trophic data.     

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.     

Flexible foraging techniques in breeding Cormorants Phalacrocorax carbo and Shags Phalacrocorax aristotelis: benthic or pelagic feeding?

A total of 8772 dive durations were recorded during 117 diving bouts in five Cormorants Phalacrocorax carbo and five Shags Phalacrocorax aristotelis breeding at the Chausey Islands, France. Diet of the birds was assessed by analysis of 526 pellets containing 13,016 otoliths. Radio-tracking data indicated that Cormorants fed exclusively on pelagic fish during social fishing (5% of the trips) and executed 11% pelagic and 60% benthic dives during the remaining 95% of the trips. In Shags, 44% of all trips were pelagic, and the remaining 56% included 9% pelagic and 67% benthic dives. The proportions of benthic to pelagic dives varied widely between dive sequences of single birds and between individuals and sexes in both species. The prey spectrum of the Cormorants contained both pelagic (29%) and benthic fish (67%) and confirmed considerable flexibility in foraging. In Shags, birds may adjust their diving patterns to accommodate the behaviour of their main prey, sandeels Ammodytidae (87% of all prey). We propose that the wetability of plumage may explain this flexibility.     

Diet analysis in piscivorous birds: What can the addition of molecular tools offer?

In trophic studies on piscivorous birds, it is vital to know which kind of dietary sample provides the information of interest and how the prey can be identified reliably and efficiently. Often, noninvasively obtained dietary samples such as regurgitated pellets, feces, and regurgitated fish samples are the preferred source of information. Fish prey has usually been identified via morphological analysis of undigested hard parts, but molecular approaches are being increasingly used for this purpose. What remains unknown, however, is which dietary sample type is best suited for molecular diet analysis and how the molecular results compare to those obtained by morphological analysis. Pellets, feces, and regurgitated fish samples of Great Cormorants (Phalacrocorax carbo sinensis) were examined for prey using both morphological hard part analysis and molecular prey identification. The sample types and methods were compared regarding number of species detected (overall and per sample) as well as the prey species composition and its variability among individual samples. Via molecular analysis, significantly higher numbers of prey species were detected in pellets, feces, and fish samples. Of the three sample types, pellets contained the most comprehensive trophic information and could be obtained with the lowest sampling effort. Contrastingly, dietary information obtained from feces was least informative and most variable. For all sample types, the molecular approach outperformed morphological hard part identification regarding the detectable prey spectrum and prey species composition. We recommend the use of pellets in combination with molecular prey identification to study the diet of piscivorous birds.     

OBSERVATIONS ON THE NESTING SITES AND NESTING BEHAVIOUR OF THE KITTLITZ'S SANDPLOVER CHARADRIUS PECUARIUS

Jackson, S. 1984. Predation by Pied Kingfishers and Whitebreasted Cormorants on fish in the Kosi estuary system. Ostrich 55:113-132.

Identification of otoliths from the regurgitated pellets of Pied Kingfishers Ceryle rudis and Whitebreasted Cormorants Phalacrocorax carbo from the Kosi estuary system provides information on the relative proportions of fish species in the diets of the birds. This information can be related to the feeding habits, distribution and abundance of their prey. It is also an indication of the feeding range of the birds. There is little overlap between both the size classes and the species of fish taken by the two predators. This is because of the difference in size and fishing techniques of C. rudis and P. carbo, and of differences in their feeding ranges. Competition for food between the two populations of birds studied is minimized by these differences.     

Relationships between bird morphology and prey selection in two sympatric Great Cormorant Phalacrocorax carbo subspecies during winter

Variation in bird morphology (notably sex size dimorphism) has been suggested to contribute to differences in food use between individuals. We explore the hypothesis of food partitioning (diet overlap and prey size selection) in two sympatric subspecies of the Great Cormorant Phalacrocorax carbo with respect to bird morphology (subspecies and sex) in inland French waters. These areas represent a recent and increasing contact zone used as a common non-territorial winter feeding area by the continental P. c. sinensis and the marine P. c. carbo subspecies. A high dietary overlap between subspecies and sexes was found at the major sites studied. Prey size selection was found to be site-specific and generally related to a gradient of structural size and body mass (male P. c. carbo  > male P. c. sinensis  > female P. c. carbo  > female P. c. sinensis ). With respect to bird morphology, differences in prey size consumed by Great Cormorants were more pronounced between sexes in freshwater habitats than between subspecies. This was reinforced by the fact that P. c. carbo birds entering inland areas were smaller than birds on the coast. These results also suggest that bird morphology is an important determinant of dietary differences. Our study demonstrated that P. c. carbo is an efficient generalist forager in continental areas, and food partitioning in type and size of prey between the two subspecies is low.     

Social Reversal of Sex‐Biased Aggression and Dominance in a Biparental Cichlid Fish (Julidochromis marlieri)

In biparental species, aggression, dominance, and parental care are typically sexually dimorphic. While behavioral dimorphism is often strongly linked to gonadal sex, the environment—either social or ecological—may also influence sex‐biased behavior. In the biparental cichlid fish Julidochromis marlieri, the typical social environment for breeding pairs consists of large females paired with smaller males. While both sexes are capable of providing territory defense and parental care, the larger female provides the majority of defense for the pair, while the smaller male remains in the nest guarding their offspring. We examine the contributions of sex and relative mate size to these sex‐biased behaviors in monogamous J. marlieri pairs. Both female‐larger and male‐larger pairs were formed in the laboratory and were observed for territorial aggression (against conspecifics and heterospecifics), dominance, and parental care. In female‐larger pairs, territorial aggression and intra‐pair dominance were female‐biased, while in male‐larger pairs this bias was reversed. For both pairing types, the presence of an intruder amplified sex differences in territorial aggression, with the larger fish always attacking with greater frequency than its mate. Though less robust, there was evidence for plasticity of sex‐bias for some egg care related behaviors in the inverse direction. Our study suggests that relative mate size strongly influences the sex bias of aggression and dominance in J. marlieri and that this aspect of the social environment can override the influence of gonadal sex on an individual's behavior. The remarkable plasticity of this species makes Julidochromis an exciting model that could be used to address the relationship between proximate and ultimate mechanisms of behavioral plasticity.     

Size‐selective fishing affects sex ratios and the opportunity for sexual selection in Alaskan sockeye salmon Oncorhynchus nerka

Selective exploitation can cause adverse ecological and evolutionary changes in wild populations and also affect sex ratios but few studies have empirically documented skewed sex ratios in exploited fishes (other than species with extreme sexual size dimorphism, SSD). To investigate the possibility of sex‐selective fishing on Alaskan sockeye salmon Oncorhynchus nerka, we assessed sex ratios in fish at two spatial scales: within each of five fishing districts and among 13 breeding populations in one of these districts. We predicted that populations’ sex ratios would vary based on the average size of fish and SSD because size affects vulnerability to fishing. At the larger scale, we found a small but significant bias in fish returning to four of the five fishing districts (average = 52% females), and in four of the five districts males were caught at significantly higher rates than females. At the finer scale there was marked variation in sex ratio on the breeding grounds, ranging from 36% to 47% males. Populations with fish of intermediate sizes experienced the greatest sex ratio biases; the greater vulnerability of males than females to fishing resulted from a combination of larger SSD and different harvest rates between the sexes associated with the fishery size‐selectivity curve shape. Skewed sex ratios may change competition and behavior on the breeding grounds, relaxing selection on male traits associated with mate choice by females or intra‐sexual competition and altering demographic and evolutionary pressures on the fish. Assessment of the size selectivity of fishing gear and the population's SSD can help to illuminate if and how exploitation can affect sex ratios. Future studies examining size‐selective fishing should also evaluate the consequences for sex ratios, as this might help explain changes in harvested population structure and sustainability.     

Great Cormorants Phalacrocorax carbo feed on larger fish in late winter

Capsule: Great Cormorants Phalacrocorax carbo prey on larger fish in late winter compared to early winter.

Aims: This study aimed to describe Cormorant diet and to discover whether prey size changed during winter.

Methods: The diet of Cormorants was studied using regurgitated pellets collected from roosting places at the upper Elbe River, Czech Republic, during winter (from December to March).

Results: The diet consisted of 24 fish species from 6 fish families. Roach Rutilus rutilus dominated in the diet (over 50% of biomass). Size of fish in the diet increased over time during the whole winter. Except for European Chub Squalius cephalus, the increased size applied for the majority of the most frequently consumed fish species: Roach, European Perch Perca fluviatilis and White Bream Abramis brama. Cormorants consumed mostly fish species of lower commercial and angling value.

Conclusion: Fisheries management should reflect on the fact that competition for larger-sized fish is the highest in late winter and in early spring. For that reason, stocking of potentially vulnerable fish could be delayed to the time when overwintering birds leave the area and recreational fishing could be restricted in late winter.     

Spawning and reproductive patterns of six exploited finfish species from the Arabian Sea, Sultanate of Oman

The spawning periods, sex ratio and length at first maturity were determined for six commercially important finfish species collected from Oman's industrial trawl fishery. During 1 year of monthly sampling, biological data were collected on 5586 fish. ancova was used to compare the length–weight relationships between sexes. There were sex‐specific differences in length‐weight relationships for three of the six species examined. One serranid, Epinephelus diacanthus, and a sparid, Pagellus affinis, had sex ratios significantly different from the expected ratio of 1 : 1. Size–frequency data revealed more male fish in the larger size classes for E. diacanthus, P. affinis and Nemipterus japonicus. Data on the development of female gonads and Gonado‐somatic Index (GSI) revealed that the six species spawned at different times of the year. With the exception of female E. diacanthus, all fish had low GSI values during the onset of the summer monsoon period in June. The proportion of immature vs mature fish in the sample differed among species. For two species, Lethrinus nebulosus and Cheimerius nufar, more than 40% of the fish sampled (both sexes) were immature. Approximately 33% female and 41% male P. affinis were also immature. Management options such as temporal and area closures are discussed in light of the large number of immature fish caught by the fishery.     

PREY SELECTION AND TEMPORAL VARIATION IN THE DIET OF SUBANTARCTIC SKUAS AT INACCESSIBLE ISLAND,TRISTAN DA CUNHA

Ryan, P.G., & Moloney, C.L. 1991. Prey selection and temporal variation in the diet of Subantarctic Skuas at Inaccessible Island, Tristan da Cunha. Ostrich 62:52-58.

More than 2 500 prey items of Subantarctic Skuas Cuthuructa anturcricu were identified from regurgitated pellets collected at a roost of non-breeding skuas during summer 1989–90 at West Point, Inaccessible'Is-land. Most prey items (96,5%) were birds, primarily burrow-nesting procellariiforms. Sixteen bird species were recorfed in the diet, but five secies accounted for 94,6% of identifiable prey remains: Whitebellied Storm Petrel Fieettu grullariu (53,5%), Whitefaced Storm Petrel Pelugodrorna marina 15,1%), Broad-billed Prion Pacaptilu vittutu (14,0%), Great Shearwater Puffinus gruvis (7,3%), and Iommon Diving Petrel Pelecunoids urinutrix (4,7%). Petrel chicks were important in the diet, particularly Great Shearwaters. The main non-avian prey were fish and goose barnacles Lepas sp., but their importance may have been underestimated. The remains of a rat Rattus sp. presumably were derived from the neighbouring island of Tristan da Cunha; rats are not known to occur on Inaccessible Island. Prey size affected seabird availability to skuas, and dietary composition vaned throughout the summer in relation to seabird breeding cycles. Both these factors reduce the value of skua diet as an indicator of the relative abundances of burrow-nesting petrels. There was no correlation between skua diet and estimates of breeding bird abundance, but this may be a consequence of studying non-breeding skuas. Subantarctic Skua diet in 1989–90 was similar to that recorded in previous years, with consistent seasonal trends between years.     

Die Nahrung von Kormoranen (Phalacrocorax carbo sinensis) in Bayern

Zusammenfassung Die wachsende Zahl von Kormoranen (Phalacrocorax carbo sinensis) in Bayern führte in den letzten Jahren zu erheblichen Konflikten zwischen Fischerei und Naturschutz. Deshalb wurde die Zusammensetzung der Kormorannahrung an verschiedenen Gewässertypen untersucht: Zwei große Voralpenseen (Chiemsee, Ammersee), drei künstlich angelegte Gewässer (Altmühlsee, Baggersee Ochsenanger bzw. Breitengüßbacher Baggersee), zwei freifließende bzw. staugeregelte große Fließgewässer (Donau, Inn) und ein kleineres Fließgewässer (Alz). Insgesamt wurden 4697 Speiballen untersucht. Diese enthielten die Hartteile von 9587 Fischindividuen.Von 50 in den Untersuchungsgewässern vorkommenden Fischarten wurden 24 in den Speiballen der Kormorane angetroffen. Allerdings machten nur 6–10 Arten 89–95% aller gefressenen Fischindividuen aus, wobei in den einzelnen Gewässern lediglich 2–3 Arten mit 57–85% die Hauptbeute darstellten. Am häufigsten wurden Weißfische (Fam. Cyprinidae) in den Speiballen nachgewiesen. An den großen Voralpenseen bildete die Renke (Coregonus spec.) zeitweise einen weiteren wichtigen Nahrungsbestandteil. An Ammersee, Altmühlsee, Donau und Inn wurden häufig auch Barsche (Perca fluviatilis) gefressen, an der Donau auch Kaulbarsche (Gymnocephalus cernua) bzw. Schrätzer (G. schraetzer). Dagegen spielten an der schneller fließenden Alz Äschen (Thymallus thymallus) und Salmoniden (Fam. Salmonidae) eine größere Rolle. Außer an der Alz dominierten an allen Untersuchungsgewässern häufige Schwarmfische. Die Nahrung wies an den Voralpenseen und an der Donau die größte Diversität auf.Die Nahrungszusammensetzung variierte häufig von Monat zu Monat. Diese Fluktuationen weisen auf Verhaltensweisen bzw. horizontale und vertikale Wanderbewegungen der einzelnen Fischarten hin. Häufig zeigen sie auch einen Wechsel der von den Vögeln befischten Habitate auf.Meist wurden Fische mit Körperlängen von 9–28 cm erbeutet. An der schneller fließenden Alz wurden im Mittel größere Fischen gefangen als in allen übrigen Gewässern. Am Ammersee, dem einzigen Gewässer das ganzjährig untersucht werden konnte, wurden in den Sommermonaten kleinere Fische erbeutet als im Winter. Nur wenige Jungfische wurden gefressen. So wiesen lediglich 32 Speiballen (1%) mehr als 20 Fische auf.

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The food of Cormorants (Phalacrocorax carbo sinensis) in Bavaria

Summary In Bavaria, an increase in Cormorant (Phalacrocorax carbo sinensis) numbers has lead to increasing conflict between fishermen and conservationists. The diet of cormorants was assessed by examining the contents of oral pellets from seven study sites, thought to be representative of Bavarian waters. These were two large pre-alpine lakes (Chiemsee and Ammersee), a reservoir (Altmühlsee), two gravel pits (Ochsenanger and Breitengüßbacher Baggersee), two uncontrolled or controlled large rivers (Danube and Inn), and a smaller river (Alz). The sample of pellets comprised 2944 collected from these sites in the years 1991–1994 and 1758 from an earlier study (winter 1990/91). Altogether. 9587 individual prey items were found. Of 50 fish species inhabiting the study waters, 24 were found in the pellets of the cormorants, but only 6–10 species accounted for 89–95% by numbers. Moreover. within each site 2 or 3 species predominated by number (57–85%).Most fish remains in pellets were from cyprinids (Cyprinidae) but at times on pre-alpine lakes whitefish (Coregonus spec.) were consumed in numbers. At the lakes Ammersee and Altmühlsee, as well as on the rivers Danube and Inn, cormorants also fed commonly on Perch (Perca fluviatilis), and along the Danube also on Ruffe (Gymnocephalus cernua) and Striped Ruffe (G. schraetzer), respectively. Only at the River Alz, with its faster flow, were Grayling (Thymallus thymallus) and salmonids (Salmonidae) important as cormorant prey. In general, common shoaling fish species dominated the diet of the birds, an exception being the River Alz again. The diet showed highest diversity at the pre-alpine lakes and at the River Danube.There was some seasonal variation in the prey species composition. This was probably caused by fish behaviour and fish movements, which perhaps influenced their vulnerability to cormorant predation. Some variation also suggested birds switched between feeding sites. Cormorants used a wide range of the available prey sizes. Mostly, fish of 9–28 cm in length were taken. At the River Alz, the fish sizes consumed were larger than at all other sites. At Lake Ammersee, the only water studied year-round, larger prey were taken in winter than in summer. In general, few fry were eaten, as only 32 pellets (1%) contained more than 20 individual fish.

     

Differences in food delivered to chicks by males and females of little auks ( Alle alle ) on South Spitsbergen

Sex differences in the quality and quantity of food loads brought to little auk (Alle alle) chicks were investigated in a large colony in Hornsund (South Spitsbergen). Adults returning to the colony were caught in mist-nets and food loads were taken from their gular pouch. The sex of each bird was determined by means of molecular methods. Females brought significantly more food per single load than males in terms of wet weight (30% more on average), number of prey items (46%) and energy contents (39%). However, there was no difference between the sexes in the size of prey taken. Energy-rich Calanus glacialis, originating from cold Arctic waters, was the most frequent prey item and made up the majority of food loads brought by both males and females (75 and 72%, respectively). This indicates that both sexes forage mainly in Arctic waters. However, differences in the proportion of cold water Calanus species (C. glacialis and C. hyperboreus), warm water Calanus species (C. finmarchicus), as well as other taxa, between males and females may suggest different ways of exploiting the feeding area.     

The diet of Herring Gulls (Larus argentatus) during winter and early spring on the lower Great Lakes

In the Great Lakes, the Herring Gull (Larus argentatus) is a prominent member of the aquatic bird community, and has been used to monitor spatial and temporal trends in contaminant levels. To understand more fully contaminant loading outside the breeding season, we analysed the contents of 1298 freshly regurgitated pellets and 179 fresh faeces, collected in March and early April 1978–83, and between late December and late February 1990–91, from the vicinity of breeding colonies in Lakes Ontario and Erie, the Niagara River, Detroit River, and south-eastern parts of Lake Huron. Most adult Herring Gulls from the Great Lakes population winter in these areas, but there is no published account of their food habits other than during the breeding season. Most pellets from colonies close to large urban centres contained remains of garbage, as well as various fish species. Small mammals, notably Deer Mice (Peromyscus maniculatus) dominated the early spring diet at Lake Huron colonies near agricultural areas. At all other sites fish predominated in pellets and faeces, but garbage items were also recorded regularly. The species of fish consumed varied regionally, probably reflecting local availability. In Lake Ontario, Rainbow Smelt (Osmerus mordax) and Alewife (Alosa pseudoharengus) occurred most frequently in samples, whereas Freshwater Drum (Aplodinotus grunniens) was the main fish prey in Lake Erie and the Detroit River. Dietary differences were apparent between years, within seasons, and amongst areas. While these may have reflected some real differences in food availability, interpretation of these results was confounded by various biases inherent in the sampling of pellets and faeces to determine diet in such an opportunistic species. Therefore, it would be unwise to draw rigid conclusions as to regional or seasonal differences in the diets of piscivorous birds, based upon analyses of diet from only a small sample of sites or years. Herring Gulls appear to feed mainly on fish and garbage in winter and early spring on the lower Great Lakes (much as during the breeding season), but any locally abundant food source is probably exploited opportunistically.     

Variation of diet and food consumption of the scaldfish Arnoglossus laterna(Walbaum, 1792)

Although Arnoglossus laterna (Walbaum, 1792) is a common benthic fish in Portugal, several aspects of its feeding ecology remain incomplete. In this study, diet was examined and the food consumption estimated on inshore waters in the central coast of Portugal. The diet of the scaldfish included mainly mysids, amphipods and polychaetes, while the decapod Philocheras bispinosus Hailstone was the most frequent prey. Variation in the diet according to fish size (two size‐classes), sex and spawning/non‐spawning seasons was examined. Significant differences in diet composition were found between seasons, whereby the most frequently consumed prey in the non‐spawning season were crustaceans, while polychaetes and nemerteneans comprised most of its diet in the spawning season. The average value of the vacuity index was very similar between seasons (24.7% non‐spawning season and 25.0% spawning season). Diet overlap between size‐classes and sexes was high, but low between the two seasons. Food consumption of A. laterna was estimated based on diet characterization and evacuation rate. The estimate of the daily food consumption was 1.15 mg dry weight for the non‐spawning season and 1.67 mg dry weight for the spawning season. Significant differences in consumption rates were found between size‐classes and sexes. Larger individuals showed a higher consumption rate than smaller individuals. Females consumed more food than males in the non‐spawning season, while in the spawning season males consumed more than females.     

Seasonal and annual differences in the foraging ecology of two gull species breeding in sympatry and their use of fishery discards

Niche segregation between similar species will result from an avoidance of competition but also from environmental variability, including nowadays anthropogenic activities. Gulls are among the seabirds with greater behavioural plasticity, being highly opportunistic and feeding on a wide range of prey, mostly from anthropogenic origin. Here, we analysed blood and feather stable isotopes combined with pellet analysis to investigate niche partitioning between Audouin's gull Larus audouinii and yellow‐legged gull Larus michahellis breeding in sympatry at Deserta Island, southern Portugal, during 2014 and 2015. During the breeding season there was considerable overlap in the adults’ diet, as their stable isotope values of blood and primary feather (P1) did not differ, and their pellets were comprised mainly by marine fish species. However, Audouin's gulls presented higher occurrences of pelagic fish, while yellow‐legged gulls fed more on demersal fish, insects, and refuse. SIAR mixing models also estimated a higher proportion of demersal fish in the diet of yellow‐legged gulls. We also found differences between the two gull species in chicks’ feathers, suggesting that Audouin's gull adults selected prey with lower carbon isotope values to feed their young. Secondary feather (S8) of Audouin's gull presented higher isotope values compared to yellow‐legged gulls, indicating different foraging areas (δ¹³C) and/ or trophic levels (δ¹⁵N) between the two species in the non‐breeding season. During both the all‐year and non‐breeding periods the yellow‐legged gull showed a broader isotopic niche width than Audouin's gull in 2013, and in 2014 the two gull species exhibited different isotopic niche spaces. Our study suggests that both gull species foraged in association with fisheries during the breeding season. In this sense, a discard ban implemented under the new European Union Common Fisheries Policy may lead to a food shortage, therefore future research should closely monitor the population dynamics of Audouin's and yellow‐legged gulls.     

Sex‐biases in distribution and resource use at different spatial scales in a migratory shorebird

In migratory species, sexual size dimorphism can mean differing energetic requirements for males and females. Differences in the costs of migration and in the environmental conditions occurring throughout the range may therefore result in sex‐biases in distribution and resource use at different spatial scales. In order to identify the scale at which sexual segregation operates, and thus the scale at which environmental changes may have sex‐biased impacts, we use range‐wide tracking of individually color‐ringed Icelandic black‐tailed godwits (Limosa limosa islandica) to quantify sexual segregation at scales ranging from the occupation of sites throughout the non‐breeding range to within‐site differences in distribution and resource use. Throughout the range of this migratory shorebird, there is no evidence of large‐scale sex differences in distribution during the non‐breeding season. However, the sexes differ in their selection of prey types and sizes, which results in small‐scale sexual segregation within estuaries. The scale of sexual segregation therefore depends on the scale of variation in resource distribution, which, in this system, is primarily within estuaries. Sexual segregation in within‐site distribution and resource use means that local‐scale anthropogenic impacts on estuarine benthic prey communities may disproportionately affect the sexes in these migratory shorebirds.     

Adjustment of pre‐moult foraging strategies in Macaroni Penguins Eudyptes chrysolophus according to locality,sex and breeding status

The annual moult creates the highest physiological stress during a penguin's breeding‐cycle and is preceded by a period of hyperphagia at sea. Although crucial to individual survival, foraging strategies before moult have been little investigated in keystone marine consumers in the Southern Ocean. The Macaroni Penguin Eudyptes chrysolophus demonstrates how individuals may adjust their foraging strategies during this period in line with constraints such as potential intraspecific competition between localities, foraging ability between dimorphic sexes and timing at sea between breeding and non‐breeding population components. We recorded pre‐moult behaviour at sea for 22 Macaroni Penguins from Crozet and Kerguelen Islands (southern Indian Ocean) during 2009 and 2011, using light‐based geolocation and stable isotope analysis. Penguins were distributed in population‐specific oceanic areas with similar surface temperatures (3.5 °C) south of the archipelagos, where they foraged at comparable trophic levels based on stable isotopes of their blood. Bayesian ‘broken stick’ modelling with concurrent analysis of seawater temperature records from the animal‐borne devices showed that within each population, females remained 6 days longer than males in the colder waters before heading back towards their colonies. Finally, 17 other non‐breeding individuals that moulted earlier had a higher mean blood δ¹⁵N value than did post‐breeding birds, meaning that early moulters probably fed more on fish than did late moulters. Our findings of such adjustments in foraging strategies developed across locality, sex and breeding status help understanding of the species' contrasted pre‐moult biology across its range and its ecology in the non‐breeding period.     

Microchemical provenancing of prey remains in cormorant pellets reveals the use of diverse foraging grounds

Piscivorous birds in aquatic ecosystems exert predation pressure on fish populations. But the site-specific impact on fish populations, including stocked and commercially used fish species, remains disputed. One of the key questions for the management of piscivorous birds and fish is determining the origin of prey and thus which fish populations are targeted by the birds. We addressed this question by provenancing otoliths (earstones) of fish obtained from regurgitated pellets of piscivorous birds by otolith microchemistry analysis. We retrieved otoliths from regurgitated pellets of great cormorants (Phalacrocorax carbo sinensis) collected every 2 weeks for 2 years from breeding and roosting colonies at Chiemsee in Bavaria, Germany, and classified them according to family or species. We collected water samples from Chiemsee and potential surrounding foraging grounds. We measured the strontium (Sr) ⁸⁷Sr/⁸⁶Sr isotope ratio and Sr mass fraction of water and otoliths using (laser ablation) inductively coupled plasma-mass spectrometry. We assigned otoliths from regurgitated pellets to habitat clusters of origin by comparing the Sr isotopic and elemental composition of otoliths and waterbodies. In 36% of cormorant pellets collected at Chiemsee, prey was assigned to waterbodies distinct from Chiemsee. Furthermore, cormorants used different foraging sites during 1 day. Microchemical provenancing of prey remains can contribute to identifying foraging sites of piscivorous birds and to what extend the birds switched among foraging sites.     

Evolution of prey holding behaviour and large male body size in Ninox owls (Strigidae)

The hawk owl genus Ninox is unique among raptorial birds in that it includes three species in which males are substantially larger than females. This is a reversal of the normal pattern observed in both diurnal and nocturnal raptorial birds in which females are larger. Interestingly, these three Ninox species also are both the largest of the 22 species in the genus and the only species that exhibit the striking behaviour of ‘prey holding’ in which large (> 600 g) mammalian or avian prey is captured at night and held with body parts intact, and draped below a roost for the entire day without being consumed. Because explanations of the evolution of large male size suggest that it results from competition among males, the adaptive significance of prey holding was studied in a wild population of powerful owl Ninox strenua. Prey holding is largely confined to breeding males and its occurrence varies significantly across the breeding cycle, being most frequent during incubation and brooding. The study did not clearly resolve whether prey holding is a form of food storage or territorial display; however, both functions can select for large male body size and therefore play a significant role in the evolution of nonreversed size dimorphism. Although female‐only incubation and brooding is typical of Ninox owls and other owl species, prey holding appears to occur only in the large Ninox species because of the unique combination of large body size, large prey size, separate sex roles, and obligate cavity nesting. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 95 , 284–292.