The behaviour of seabirds foraging at fishing boats around Shetland

Among the different types of fishing vessels around Shetland, whitefish trawlers attract the largest numbers of scavenging seabirds and provide the most food. Offal was almost all consumed by seabirds, predominantly by Fulmars Fulmarus glacialis , which excluded other species by their aggression. Fulmars generally ignored discarded whole fish, which were mainly taken by Great Black-backed Gulls Larus marinus , Gannets Sula bassana and Great Skuas Catharacta skua . Although flatfish were usually ignored because seabirds found them difficult to swallow and they sank faster, most discarded roundfish were consumed. Herring Gulls L. argentatus , Lesser Black-backed Gulls L. fuscus and Kittiwakes Rissa tridactyla were rarely able to obtain offal or discards. Herring Gulls and Lesser Black-backed Gulls spent much time on the periphery of feeding flocks while Kittiwakes rarely attempted even to join these. Most of the birds at trawlers were in adult plumage, and it is suggested that the low proportion of immature birds present was a further reflection of the highly competitive feeding conditions at trawlers. We suggest that likely changes in fishing practice and seabird population sizes in the immediate future may result in Herring Gulls, Lesser Black-backed Gulls and Great Skuas finding feeding on waste around trawlers increasingly difficult, so they may be further displaced by Fulmars, Gannets and Great Black-backed Gulls.     

Intra-population variation in the diet of an avian top predator: generalist and specialist foraging in Great Black-backed Gulls Larus marinus

ABSTRACT

Capsule: Great Black-backed Gulls Larus marinus breeding on Skokholm, UK, fed predominantly on seabirds, rabbits, refuse, and marine prey, with the majority of pairs being dietary generalists, but with some specialist pairs.

Aims: To understand the significance of Great Black-backed Gulls as top predators on a small offshore island with internationally important numbers of breeding seabirds (Skokholm, UK) by quantifying their diet and to determine how this varies within the breeding season, to test for pair-level dietary specialization and to examine the consequences of dietary differences for reproductive performance.

Methods: Regurgitated pellets were collected and analysed from 26 breeding pairs on Skokholm during 2017 and related to breeding success.

Results: Analysis of 1035 pellets revealed that, overall, Great Black-backed Gulls fed on seabirds (48% – mostly Manx Shearwaters Puffinus puffinus), mammals (38% – mostly European Rabbits Oryctolagus cuniculus), anthropogenic waste (7%), and marine prey (7%). Diet varied among pairs with 18 (73%) generalist pairs and 7 (27%) specialist pairs (of which, 5 were bird specialists and 2 were mammal specialists). Diet also varied seasonally, but pair-level dietary diversity was repeatable through the breeding season. Dietary diversity did not covary with breeding success.

Conclusion: Great Black-backed Gulls are top predators on Skokholm. Variation in diet among pairs emphasizes that not all individuals contribute equally in terms of predation. Understanding the incidence of this variation has important ecological implications, particularly where apex predators may exert a strong top-down influence.     

A comparison of Herring Gull Larus argentatus and Lesser Black-backed Gull Larus fuscus nest sites: their characteristics and relationships with breeding success

Nest sites used by Herring Gulls and Lesser Black-backed Gulls in a mixed colony were compared. Correlations between features surrounding a nest and breeding success at different stages of the breeding cycle are presented. Lesser Black-backed Gulls nested on more vegetated and flatter areas than did Herring Gulls, even though the latter species had a lower hatching success at less vegetated sites. The difference in the general topography of nest sites between the species suggests that the Lesser Black-backed Gull, through an individual defence strategy, may be better adapted to use sites which are accessible to ground predators than the Herring Gull. The fledging success of Lesser Black-backed Gulls tended to increase with an increased proportion of relatively tall vegetation close to the nest. The Lesser Black-backed Gull may therefore be able to leave its young unattended and rely on their concealment for protection against predators. In contrast, the Herring Gull may rely more on parental vigilance to protect young. More frequent attendance by adult Herring Gulls at the nest site during chick rearing compared with Lesser Black-backed Gulls supports this hypothesis.     

SEABIRDS OF THE SENEGAL UPWELLING AND ADJACENT WATERS

Seabird distributions were observed off Senegal during the upwelling season there, in February-March 1976, and are interpreted against an oceanographic background. Gannets Sula bassana , Pomarine Skuas Stercorarius pomarinus , Great Skuas Catharacta skua , Lesser Black-backed Gulls Larus fuscus , Black-headed Gulls L. ridibundus , Sandwich Terns Sterna sandvicensis and Royal Terns S. maxima were the dominant species on the shelf and slope adjacent to the inshore upwelling zone. Grey Phalaropes Phalaropus fulicarius were the only seabirds associated with an oceanic 'front' farther offshore; it is suggested that this and similar boundary zones are important feeding areas during the pelagic phase of this species' annual cycle.
Leach's Storm-Petrels Oceanodroma leucorhoa were seen east to 20W, but the predominant storm-petrel of the waters between 23W and the continental slope was the Madeiran O. castro. It is suggested that the hatching period of Madeiran Storm-Petrels on the Cape Verde Islands is timed to take advantage of the arrival there of young fish in the surface waters, spawned off the African coast and drifted offshore. However Madeiran Storm-Petrels make no direct use of the coastal upwelling; nor do Little Shearwaters Puffinus assimilis or White-faced Storm-Petrels Pelagodroma marina , the other winter-breeding Cape Verdes seabirds, which probably feed close to the Islands.
The Senegal upwelling is underexploited by seabirds, in contrast to comparable upwelling systems elsewhere. It is suggested that this is due to the seasonal timing of the upwelling, the scarcity of secure nest-sites for breeding seabirds, and the competing attractions of the Benguela upwelling in the South Atlantic.     

Effects of cleaning a tidal river of sewage on gull numbers: a before-and-after study of the River Tyne,northeast England

For most of the 20th century, the tidal reaches of the River Tyne in northeast England were used as an open sewer for organic wastes from almost a million people. This produced anaerobic conditions in the river, but supplied an appreciable source of food for five species of gulls. Observations on the number of gulls using the river were made in 1969/70 to establish the situation before a sewage treatment scheme was implemented. By 1994, the volume of untreated sewage discharged into the river had been reduced by 97% and by 86% in the study area, and a second survey was carried out to evaluate the effect of the clean-up on gull numbers. Overall, gull numbers showed a significant decrease of 37% between the two study periods. The decrease in winter was 38% and was also significant, while the decline in summer was 33%, but not significant. Considering individual species, numbers of Common and Great Black-backed Gulls declined by 93% and 91% respectively. In both these cases this is presumed to be due to the decline in available food. Black-headed Gull numbers did not change significantly and it appears they have been able to find alternative feeding sites such as mud flats exposed at low tide and the outfall where treated effluent is discharged. Numbers of Lesser Black-backed Gulls and Kittiwakes rose dramatically as a result of increases in breeding numbers in the area. Neither species feeds along the river. Numbers of Herring Gulls did not change significantly — it is likely that the effect of the decline in food availability was masked by the increase in breeding numbers in towns along the river banks. The different response of each of the species of gulls indicates the differences in their ecology and breeding status in the area.     

Marine mammal and seabird summer distribution and abundance in the fjords of northeast Cumberland Sound of Baffin Island, Nunavut, Canada

Critical baseline population knowledge is required to properly assess the status of marine mammal and bird populations in the Canadian Arctic and the effects of climate trends on them. To address this need for one significant Arctic region, a boat-based marine mammal and seabird transect survey was conducted in Cumberland Sound fjords during summer 2008. During 173 km effort (20 h), 959 birds were recorded representing at least nine species which were dominated by Common Eiders (Somateria mollissima borealis), Iceland or Glaucous Gulls (Larus glaucoides or Larus hyperboreus), and Black Guillemots (Cepphus grylle), in addition to less common birds including Red-throated and Common Loons (Gavia stellata and Gavia immer), Northern Fulmars (Fulmarus glacialis), and Great or Lesser Black-backed Gulls (Larus marinus or Larus fuscus). Of these, 480 birds were observed on the water in one event consisting of eiders and gulls which may have biased encounter rates. Of 101 marine mammal sightings, four species were represented: 73 harp seals (Pagophilus groenlandicus), 13 beluga whales (Delphinapterus leucas), nine bowhead whales (Balaena mysticetus), five ringed seals (Pusa hispida), and one unidentified pinniped. A pod of four killer whales (Orcinus orca) was observed off-effort in Pangnirtung Fjord during the survey period. This pilot study provided the first estimates of relative abundance for marine mammals and seabirds in the study area to aid in developing future surveys.     

Summer diet of seabirds feeding in sea-ice-covered waters near Svalbard

Summary We describe the summer diets of four seabird species. Kittiwake (Rissa tridactyla), Little Auk (Alle alle), Black Guillemot (Cepphus grylle) and Brünnich's Guillemot (Uria lomvia) collected in sea-ice-covered waters near Svalbard. Birds collected in an area filled with young sea-ice, within the seasonal sea-ice zone, were compared with birds collected from the perennial sea-ice zone dominated by multiyear ice. Pelagic Crustacea and fish dominated the diet of birds feeding in young ice, while sympagic Crustacea and fish were most important in the diet of birds feeding in multiyear ice. Boreogadus saida was the most important fish food item. B. Saida was present in the ice in both areas, while sympagic Crustacea were lacking in the area filled with young ice. Important food items in young ice were B. saida (Black Guillemots and Kittiwakes), Cajanus sp. (Little Auks) and Pandalus borealis (Brünnich's Guillemot). B. saida (Black Guillemots, Kittiwakes, Brünnich's Guillemots), Gammarus wilkitzkii (Brünnich's Guillemot, Black Guillemot) and Apherusa glacialis (Little Auk) were most important in multiyear ice. In general, Black Guillemots and Kittiwakes fed on fish, whereas Brünnich's Guillemots fed on the larger, and Little Auks on the smaller, Crustacea. The importance of sympagic species in the diet of seabirds is thought to be closely related to the age and history of the ice in the feeding area.     

INTRASPECIFIC PREDATION AND COLONIAL BREEDING IN LESSER BLACK-BACKED GULLS LARUS FUSCUS

The breeding biology of Lesser Black-backed Gulls was studied on Skokholm Island, Pembrokeshire, where the number of breeding pairs has been increasing at about 20% per annum since 1963. Laying was found to be synchronous within small groups. Clutch size and breeding success showed seasonal declines over the spread of breeding. The loss mainly of eggs, but to a lesser extent of chicks also, caused this overall decline in success. Hide observations indicated that the bulk of these losses arose through predation by nesting adult Lesser Black-backed Gulls on their nearest neighbours. Not infrequently the protagonist had lost its own clutch shortly before turning predator, and such a chain-sequence should lead to a steady build-up of aggressive failed breeders and so account for the observed seasonal increase in egg loss. Attentiveness to the clutch decreased with season but this was unlikely to have been important in precipitating the predation. Intermediate plant cover was associated with highest nest density and also with highest chick survival. In addition, nest density was directly correlated with chick survival. Whether plant cover and nest density separately affect chick production remains unresolved. Nevertheless, the increasing nest densities in this colony caused by the growth of the gull population are thought to be responsible for the widespread intraspecific predation, the intensity of which is probably a new feature of the gulls' breeding behaviour. The implications of this 'internal' predation for laying synchrony and aggregated nesting are discussed; these two factors of the breeding pattern probably evolved largely to combat 'external' predators. Not only are they no protection against inter-neighbour predation but appear to facilitate it. It remains to be seen whether this kind of predation will significantly affect breeding patterns with further increases in nest density.     

SPECIES ISOLATION BETWEEN THE HERRING GULL LARUS ARGENTATUS AND LESSER BUCK-BACKED GULL L. FUSCUS

There is a large, mixed colony of the two “ring” species, the Herring Gull Larus argentatus and Lesser Black-backed Gull L. fuscus, on Walney Island, northwest Lancashire. These birds are nesting at the very high density of one nest/40 square yards, or more, but although they defend their territories against both species indiscriminately, there is effectively no hybridization. This paper discusses the nature of the species isolation mechanism, and its function. Since the two species can produce fertile hybrids, the mechanism must be of an ecological/behavioural nature, rather than morphological incompatibility. It is shown that there are slight differences in breeding season and habitat, but these do not seem to be great enough to account for the high degree of isolation. It is likely that species isolation depends primarily on the female's choice of a mate. It is suggested that, as specific cues, she uses the differences in call-note tones, and the colour of the back (and perhaps also of the eye-ring), or both. Herring Gulls and Lesser Black-backs are adapted to slightly different niches. The overlap is so great, however, that any hybrid is unlikely to be at a disadvantage; but the overlap can only have arisen very recently, as a result of both species taking advantage of the increased availability of human refuse. It is possible that the isolation mechanism was evolved to cope with earlier conditions, when food was more limited, and the species' niches more sharply defined.     

Timing of primary moult in adult Herring Gulls and Lesser Black-backed Gulls

Summary Timing of primary moult in relation to the breeding cycle is presented for 113 actively moulting adult Herring Gulls and 79 Lesser Black-backed Gulls from Walney Island, England. Moult in both species occurred about when the eggs hatched in mid-May. The entire Herring Gull population began to moult the primaries within a period of 50 days. Lesser Black-backed Gulls started to moult 10 days later than Herring Gulls but many birds were not yet moulting as late as 5. August when I left the study area. It is suggested that the population of Lesser Black-backed Gulls consists of residents and migrants and that the former begin to moult earlier than the latter.

---

Der zeitliche Ablauf der Handschwingenmauser von Silber- und Heringsmöwe

Zusammenfassung Bei 133 ad. Silber- und 79 ad. Heringsmöwen von Walney Island, England, wurde der Beginn der Handschwingenmauser in Beziehung zum Brutzyklus untersucht. Bei beiden Arten setzte die Mauser ungefähr Mitte Mai zum Zeitpunkt des Schlüpfens der Jungen ein. Alte Silbermöwen begannen innerhalb von 50 Tagen mit der Handschwingenmauser. Die Heringsmöwe fing etwa 10 Tage später an, doch hatten manche Individuen noch bis Anfang August nicht mit der Mauser begonnen. Die Heringsmöwen-Population auf Walney besteht vermutlich aus Stand- und Zugvögeln; erstere scheinen früher zu mausern.

     

Intermittent breeding in the Herring Gull Larus argentatus and the Lesser Black-backed Gull Larus fuscus

Thirty-three percent and 37% of adult Herring Gulls Larus argentatus and 34% and 40% of adult Lesser Black-backed Gulls Larus fuscus , all with some previous breeding experience, failed to breed in 1993 and 1994, respectively. About half of these intermittent breeders of both species failed to breed in both years. Common amongst the intermittent breeders were smaller individuals, birds with low survival chances and relatively unsuccessful breeders.     

Recent changes in colony sizes of two seabird species in Scotland are not strongly influenced by Birds Directive status of the colony

Special Protection Areas (SPAs) designated under the EU Birds Directive have improved the conservation status of many terrestrial bird species in Europe, but protecting breeding sites may be less effective for highly mobile birds such as seabirds. Colony census data for Great Skuas Stercorarius skua and Great Black-backed Gulls Larus marinus in Scotland show that breeding numbers have fared no better in sites where these species are SPA breeding features and, counter-intuitively, the evidence indicates better performance in non-SPA colonies, most likely because non-SPA colonies are generally smaller so are less subject to density-dependent competition. The main drivers of population change are widespread rather than colony-based in these two species with recent reductions in carrying capacity. Many other seabird species are vulnerable to similar widespread pressures so seabird conservation strategy needs to focus on mitigating these pressures, as designation of seabird breeding sites as SPAs is not enough to ensure effective seabird conservation.     

BREEDING SUCCESS AND POPULATION GROWTH IN A COLONY OF HERRING AND LESSER BLACK-BACKED GULLS LARUS ARGENTATUS AND L. FUSCUS

This paper describes the results of investigations into the factors affecting breeding success of the Herring and Lesser Black-backed Gulls Larus argentatus and L. fuscus, in the large colony on Walney Island, northwest Lancashire, between 1962 and 1965. These investigations were concerned with the incubation period, and the first ten days after hatching. The survival of chicks to ten days is 67% in Herring Gulls, and 56% in Lesser Black-backs. Most of these losses occur in the period just after hatching and are due to “cannibalism” by other gulls. This form of predation does not appear to be masking any effects from starvation or disease. The following factors contribute to egg or chick mortality:breeding too late (and, to a much smaller extent, too early); breeding in the open, as opposed to amongst cover; the facts that eggs in small clutches have a lower hatching success than those in large ones and that Herring Gull (but not Lesser Black-back) chicks in small broods are less likely to survive to ten days than are those in large broods. Chick mortality after the first ten days is not certainly known. About 30% of the eggs laid gave rise to fledged young— or about one fledged chick per pair. In the Herring Gull, the average clutch size (2.56) is lower than that of the Lesser Black-back (2.76). Both species show a seasonal decline in clutch size—this occurs earlier in the Herring Gulls than in the Lesser Black-backs. The Walney population, which stood at about 700 pairs in 1950, had reached 12,000 in 1957, and is at present about 18–19,000 pairs. It is suggested that this increase may be linked to the greater availability, or exploitation, of human garbage in the Morecambe Bay area. The population explosion between 1950 and 1957 must have been partly due to massive immigration and could not have come about through natural increase alone. The possible influences of the gulls' behaviour on the population growth are discussed. There is no evidence of any “shock disease”, although the Walney colony is very crowded. “Cannibalism” is regarded, not as evidence of a failing food supply, but as an extension of the normal hunting behaviour of these omnivorous gulls; it will be an economical means of obtaining food only in a large, dense colony, such as Walney. It may be offset by increased breeding efficiency due to social factors.     

Summer in the city: behaviour of large gulls visiting an urban area during the breeding season

Capsule: Large numbers of Herring Gulls Larus argentatus and Lesser Black-backed Gulls Larus fuscus from a traditional colony in the Netherlands visit an urban area for food in the chick rearing period, causing nuisance while doing so.

Aims: To assess the potential contribution of large gulls breeding in a traditional colony to gull–human conflicts in cities.

Methods: Colour-ringed gulls from a nearby colony were counted in the study area throughout the 2014 breeding season. The average numbers observed daily per species and sex were compared between different breeding phases.

Results: Fluctuations in numbers of both species could be explained by breeding phase. Numbers of females of both species and male Lesser Black-backed Gulls dropped significantly during laying and incubation. Numbers peaked post-hatching, coinciding with increased engagement in nuisance events and more frequent displacements within the study area.

Conclusion: Large gulls from a traditional colony frequently visited the urban study area, especially when food demand for chick provisioning was high, suggesting that city visits are motivated by accessibility of urban food. The proportion of rooftop breeding gulls in this area is low. Therefore, measures to avoid nuisance should focus on reducing food availability rather than controlling gull breeding.     

GPS tracking data of Lesser Black-backed Gulls and Herring Gulls breeding at the southern North Sea coast

In this data paper, Bird tracking - GPS tracking of Lesser Black-backed Gulls and Herring Gulls breeding at the southern North Sea coast is described, a species occurrence dataset published by the Research Institute for Nature and Forest (INBO). The dataset (version 5.5) contains close to 2.5 million occurrences, recorded by 101 GPS trackers mounted on 75 Lesser Black-backed Gulls and 26 Herring Gulls breeding at the Belgian and Dutch coast. The trackers were developed by the University of Amsterdam Bird Tracking System (UvA-BiTS, http://www.uva-bits.nl). These automatically record and transmit bird movements, which allows us and others to study their habitat use and migration behaviour in great detail. Our bird tracking network is operational since 2013. It is funded for LifeWatch by the Hercules Foundation and maintained in collaboration with UvA-BiTS and the Flanders Marine Institute (VLIZ). The recorded data are periodically released in bulk as open data (http://dataset.inbo.be/bird-tracking-gull-occurrences), and are also accessible through CartoDB and the Global Biodiversity Information Facility (GBIF).     

Assimilation efficiency of adult Kittiwakes and Brünnich's Guillemots fed Capelin and Arctic Cod

The mean assimilation efficiencies of 10 adult Kittiwakes (Rissa tridactyla) and 10 Brünnich's Guillemots (Uria lomvia) fed on Capelin (Mallotus villosus) were 77.5% and 74.4%, respectively. When fed on Arctic Cod (Boreogadus saida) they were 83.1% and 78.2%, respectively. After correction for nitrogen retention, the assimilation efficiencies decreased to 72.2%, 70.6%, 81.2% and 74.7%, respectively. Kittiwakes and Brünnich's Guillemots seem to have the same ability to utilize the energy of the different food items. The differences in assimilation efficiencies when fed two fish species was mainly related to the fat content of the fish.     

The cost of egg production: increased egg production reduces future fitness in gulls

Measurements of costs of reproduction are essential for our understanding of the evolution of reproductive effort. While in birds the effects of increased chick-rearing effort on subsequent survival and fecundity have been relatively well studied experimentally, costs associated with increased egg-production effort have received relatively little attention. We experimentally increased the egg-production effort of individually marked Lesser Black-backed Gulls Larus fuscus and followed their breeding performance in the next year. In the season following increased egg production, females, but not males, were less likely to be resighted in the study plot and those that did return were less likely to produce a clutch compared to control birds. It is unclear whether the observed effect on local return rate represents differential survival, differences in breeding propensity or differences in dispersal between experimental and control females. In any event, all of these would adversely affect the fitness of experimental females. In addition, those experimental females that did breed invested less in egg production the following season, which again is likely to affect breeding performance. Thus, this study provides evidence that there is an inter-brood trade-off between current egg-production effort and future fitness in birds.     

Riding the tide: intriguing observations of gulls resting at sea during breeding

Adult Lesser Black‐backed Gulls Larus fuscus were followed during the breeding season using a high‐resolution GPS tracking system. Little is known about the foraging movements of these birds and what they do when they are away from the colony. The study revealed intriguing yet infrequent behavioural patterns showing that birds would sit on the sea surface drifting passively with the tidal current for several hours, resting at sea rather than in the breeding colony. The potential function of the observed patterns is discussed as well as the relevance of these observations to other species.     

Lesser Black-backed Gulls Larus fuscus thriving on a non-marine diet

Capsule: Lesser Black-backed Gulls Larus fuscus breeding 30?km from the coast in the Netherlands focussed entirely on terrestrial food sources and reached relatively high breeding success.

Aim: To gain insight in the foraging ecology, habitat use and breeding performance of inland-breeding Lesser Black-backed Gulls.

Methods: We received data from seven birds fitted with global positioning system (GPS) loggers. The colony was frequently visited to collect pellets and boluses and to monitor reproductive success, mortality and growth rate of chicks.

Results: The GPS data revealed that mainly terrestrial habitats were used, 98% of these GPS positions were within 25?km of the colony. Refuse dumps were the most preferred sites, but also agricultural fields and freshwater bodies were often visited. Only two of the 710 recorded trips were directed to the North Sea. The pellet and bolus analyses confirmed the GPS data: no marine food remains were found. Breeding success of birds in the enclosure was relatively high, with 90% of eggs hatched and 51% of chicks fledged (1.6 chicks/pair).

Conclusions: Relying on terrestrial food is feasible when sources are available in the vicinity of the colony. We conclude that Lesser Black-backed Gulls could theoretically shift towards inland breeding after a fishery discards ban.