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I present the first record for Honduras of Western Gull, Larus occidentalis Audubon, 1839, a species found on the Pacific coast of southern Canada, the United States and northern Mexico. An adult was present for at least two weeks at an estuary in the Gulf of Fonseca in southern Honduras. This observation represents the third record of this species for Central America.

Methylmercury concentrations vary widely across geographic space and among habitat types, with marine and aquatic-feeding organisms typically exhibiting higher mercury concentrations than terrestrial-feeding organisms. However, there are few model organisms to directly compare mercury concentrations as a result of foraging in marine, estuarine, or terrestrial food webs. The ecological impacts of differential foraging may be especially important for generalist species that exhibit high plasticity in foraging habitats, locations, or diet. Here, we investigate whether foraging habitat, sex, or fidelity to a foraging area impact blood mercury concentrations in western gulls (Larus occidentalis) from three colonies on the US west coast. Cluster analyses showed that nearly 70% of western gulls foraged primarily in ocean or coastal habitats, whereas the remaining gulls foraged in terrestrial and freshwater habitats. Gulls that foraged in ocean or coastal habitats for half or more of their foraging locations had 55% higher mercury concentrations than gulls that forage in freshwater and terrestrial habitats. Ocean-foraging gulls also had lower fidelity to a specific foraging area than freshwater and terrestrial-foraging gulls, but fidelity and sex were unrelated to gull blood mercury concentrations in all models. These findings support existing research that has described elevated mercury levels in species using aquatic habitats. Our analyses also demonstrate that gulls can be used to detect differences in contaminant exposure over broad geographic scales and across coarse habitat types, a factor that may influence gull health and persistence of other populations that forage across the land-sea gradient.

A mortality event involving at least 14 Western Gulls (Larus occidentalis) was observed on 10 October 2019 on Huntington State Beach, Orange County, California, US. Clinical signs of affected gulls included generalized weakness and difficulty standing and flying. Six additional Western Gulls with similar clinical signs were admitted for rehabilitation between 24 October and 7 November, including birds from Newport Beach and Laguna Beach, south of Huntington Beach. Eleven carcasses were submitted for postmortem examination, including nine gulls collected on 10 October from Huntington Beach, one collected on 24 October from Laguna Beach, and one collected on 6 November from Newport Beach. Six of seven gulls tested were positive for Clostridium botulinum toxin type A by mouse bioassay, including five collected on 10 October from Huntington Beach and one from Laguna Beach, approximately 23 km south, on 24 October, suggesting the toxin was available to scavenging birds for nearly 2 wk following the original exposure. Botulism type C, and less commonly type E, are most frequently documented in wild birds, including waterfowl and fish-eating birds, respectively. In contrast, botulism type A is the most common cause of foodborne botulism in humans, acquired from food contaminated with C. botulinum spores, but it has not previously been associated with mortality in free-ranging wild birds.

Our study investigated the influence of bottom-up and top-down drivers on the declining fledging success at a once thriving breeding colony of Caspian terns (Hydroprogne caspia). Situated at the mouth of the Columbia River, OR, East Sand Island (ESI) is home to the largest Caspian tern breeding colony in North America. Since 2001, the decline in fledging success of Caspian terns at ESI has been associated with a significant increase in average river discharge during May and June. During the years 2001–2011, the abundance of forage fish available to terns in the estuary was inversely related to river discharge. This relationship also apparently affected the reliance of nest predators on the tern colony as a food source, resulting in increased disturbance and decreased fledging success at the tern colony in years of higher river discharge. There was a significant longitudinal increase in disturbance rates by bald eagles (Haliaeetus leucocephalus) during June for terns nesting at the ESI colony, and eagle disturbance rates were positively associated with May river discharge. We also found a significant increase in kleptoparasitism rates of terns by hybrid glaucous-winged/western gulls (Larus glaucescens x Larus occidentalis) since 2001, and Caspian tern fledging success at ESI decreased with increasing average annual rates of gull kleptoparasitism. Our results support the hypothesis that the decline in Caspian tern fledging success at this large estuarine colony was primarily driven by the interaction of bottom-up and top-down factors, influencing tern fledging success through the food supply and triggering potential predators to identify the tern breeding colony as an alternative source of prey.

In most avian species, egg-turning behavior during incubation is vital for proper embryonic development and hatching success. However, changes in turning behaviors are rarely studied across different temporal scales (e.g., day–night or across incubation phases), though the timing of incubation behaviors affects reproductive success. We used data loggers encapsulated in artificial eggs to measure turning rates and angle changes of eggs in Western Gull (Larus occidentalis) and Laysan Albatross (Phoebastria immutabilis) nests. We examined diurnal and daily cycles in egg-turning behaviors across early, middle, and late incubation phases. Our results indicate that (1) egg-turning behaviors remain similar throughout incubation, resulting in a consistent environment for developing chicks; (2) egg-turning rates and angle changes vary according to diurnal cycles and day length in each species; and (3) egg-turning rates, but not angle changes, were similar between species. Egg-turning behaviors may vary am...

Studies of avian nest success often focus on examining influences of variation in environmental and seasonal factors. However, in-depth evaluations can also incorporate variation in individual incubation behaviour to further advance our understanding of avian reproductive ecology. We examined these relationships in colonially nesting Black-crowned Night-Herons Nycticorax nycticorax using intensive video-monitoring methods to quantify incubation behaviours. We modelled nest survival as a function of both extrinsic factors and incubation behaviours over a 3-year period (2010–12) on Alcatraz Island, USA. Model-averaged parameter estimates indicated that nest survival increased as a function of greater incubation constancy (% of time spent incubating eggs within a 24-h period), and average daily precipitation throughout the nesting stage. Common Ravens Corvus corax are the only known nest predator of Night-Herons on Alcatraz Island, as on many other coastal Pacific islands. We also investigated the effects of heterospecific nesting of California Gulls Larus californicus and Western Gulls Larus occidentalis in a mixed-species colony with Night-Herons, based on nesting proximity data collected over a 2-year period (2011–12). This second analysis indicated that, in addition to incubation behaviours, nesting heterospecifics are an important factor for explaining variation in Night-Heron nest survival. However, contrary to our original expectation, we found that Night-Herons experienced increased nest survival with increasing distance from gull colony boundaries. These results may apply to other areas with multiple colonial nesting species and similar predator communities and climatic patterns.

Avian species host diverse communities of microorganisms which have important roles in the life of birds, including increased metabolism, protection from disease, and immune system development. Along with high human populations and a diversity of human uses of coastal zones, anthropogenic food sources are becoming increasingly available to some species, including gulls. Anthropogenic associations increase the likelihood of encountering foreign or pathogenic bacteria. Diseases in birds caused by bacteria are a substantial source of avian mortality; therefore, it is essential to characterize the microbiome of seabirds. Here, we determined both core and environmentally derived microbial communities of breeding western gulls (Larus occidentalis) from six colonies in California and Oregon. Using DNA extracted from bacterial swabs of the bill, cloaca, and feet of gulls, 16S rRNA gene sequencing was performed targeting the V4 region. We identified a total of 8542 operational taxonomic units (OTUs) from 75 gulls. Sixty-eight OTUs were identified in gulls from all six colonies with the greatest representation from phyla’s of Firmicutes, Proteobacteria, Bacteroidetes, and Actinobacteria. Overall, microbial richness based on Chao’s Abundance-based Coverage Estimator (ACE) index was similar for all colonies (mean = 2347 OTUs) with the smallest coastal colonies having the highest richness (mean = 2626 OTUs) and the largest colonies, located farther off-shore, having the lowest (mean = 2068 OTUs). This survey represents the most in-depth assessment to date of microbes associated with western gulls, and the first study to identify both species-specific and environmentally derived bacteria across multiple populations.

Over the last century, human activities have altered coastal ecosystems by fishing through the marine food web and increasing anthropogenic resources (e.g. landfills), both of which may alter native predator–prey interactions. We conducted a 100-year retrospective stable isotope analysis to investigate temporal shifts in relative resource use and individual variation of a generalist seabird (Western Gull, Larus occidentalis ) and the implications of gulls' shifting resource use on one of their native prey—threatened steelhead populations ( Oncorhynchus mykiss ). We applied a Bayesian mixing model (MixSIAR) to historical (early 1900s) and modern (early 2000s) populations of generalist gulls and compared changes in resource use to a piscivorous seabird population (Brandt's Cormorant, Phalacrocorax penicillatus ) in Monterey Bay (California, USA). δ 15 N significantly declined for both seabird species, suggesting shifts to lower trophic-level marine prey in the last 100 years. The shift in δ 15 N was significantly larger for Western Gulls, suggesting a shift in gull resource use to prey not in the marine environment. Mixing models suggest anthropogenic resources (e.g., landfills) comprise the majority of modern gull diet (0.31; 0.18–0.44 95% CI), whereas it contributed relatively little to gull diet in the early 1900s (0.10; 0.01–0.26 CI). Furthermore, we found although steelhead contribute relatively less to overall modern gull diet, increasing gull populations and simultaneous steelhead population decline likely results in increased per capita predation risk to modern steelhead populations—our best estimate is that modern predation risk is ~ 2.4 times higher than historically, but this estimate depends on parameter values and overlaps with zero. This study highlights the impact of human activities on coastal predators and the potential consequences for native imperiled prey.

Predation by marine birds has resulted in substantial losses to runs of Pacific salmon Oncorhynchus spp., in some cases necessitating management action. Recovery of PIT tags on a seabird breeding colony (Ano Nuevo Island) indicated that western gulls Larus occidentalis prey upon federally listed Coho Salmon Oncorhynchus kisutch and steelhead O. mykiss in central California. Whereas salmonid populations in central California have decreased in recent decades, the western gull population on Ano Nuevo Island has increased. We observed gulls Larus spp. within estuaries to document predation and used radiotelemetry to examine gull movement in relation to the availability of salmonids. During 2008 and 2009, observed predation events of out-migrating salmonids by gulls were rare; 21 events occurred during 338 h of observations at two estuaries. During the prehatch and chick-rearing phases of breeding, which coincided with migration of salmonids from fresh to salt water, 74% of the detections of radio-tagg...

Background Plasticity in foraging behavior among individuals, or across populations may reduce competition. As a generalist carnivore, western gulls (Larus occidentalis) consume a wide range of marine and terrestrial foods. However, the foraging patterns and habitat selection (ocean or land) of western gulls is not well understood, despite their ubiquity in coastal California. Here, we used GPS loggers to compare the foraging behavior and habitat use of western gulls breeding at two island colonies in central California. Results Gulls from offshore Southeast Farallon Island (SFI; n = 41 gulls) conducted more oceanic trips (n = 90) of shorter duration (3.8 ± 3.3 SD hours) and distance (27.1 ± 20.3 km) than trips to the mainland (n = 41) which were nearly 4 times longer and 2 times farther away. In contrast, gulls from coastal Año Nuevo Island (ANI; n = 20 gulls) foraged at sites on land more frequently (n = 103) but trip durations (3.6 ± 2.4 h) and distances (20.8 ± 9.4 km) did not differ significantly from oceanic trips (n = 42) where trip durations were only slightly shorter (2.9 ± 2.7 h) and equidistant (20.6 ± 12.1 km). Gulls from both colonies visited more sites while foraging at sea but spent significantly longer (3–5 times) durations at each site visited on land. Foraging at sea was also more random compared to foraging trips over land where gulls from both colonies visited the same sites on multiple trips. The total home range of gulls from SFI (14,230 km2) was 4.5 times larger than that of gulls from ANI, consistent with greater resource competition resulting from a larger abundance of seabirds at SFI. Conclusions Population-level plasticity in foraging behavior was evident and dependent on habitat type. In addition, gulls from SFI were away foraging longer than gulls from ANI (22% vs. 7.5%, respectively), which impacts the defense of territories and attempts at nest predation by conspecifics. Our results can be used to explain lower chick productivity at SFI, and can provide insight into increased gull activity in urban areas.

Seabirds are often cited as sentinels of the marine environment, but are rarely used in traditional ocean and coastal contaminant monitoring. Four classes of persistent organic pollutants (POPs, n = 68) and three trace elements (mercury, selenium, and arsenic) were measured in the eggs of California least terns (Sterna antillarum browni), caspian terns (Hydroprogne caspia), double-crested cormorants (Phalacrocorax auritus), and western gulls (Larus occidentalis) that nest in the Southern California Bight. Building on a periodic five year regional monitoring program, we measured contaminant exposure and assessed the utility of seabirds as regional contaminant biomonitors. We found that the eggs of larger, more piscivorous species generally had the highest concentrations of POPs and trace elements while California least terns had the lowest concentrations, except for mercury which was higher in least terns. As expected, DDT concentrations were elevated near the Palos Verdes Superfund site. However, we also detected a previously unknown latitudinal pattern in PBDE concentrations in least terns. POP congener profiles also confirmed differences in contamination in urban least tern colonies closest to urban centers. Though toxicants were at detectable levels across species and sites, concentrations were below those known to cause adverse effects in avian taxa and are steady or declining compared to previous studies in this region. Our results suggest that regional seabird monitoring can inform site-specific remediation and support management and protection of regionally-threatened wildlife and coastal systems. Integration of seabird contaminant data with traditional sediment, water, bivalve and fish monitoring is needed to further our understanding of exposure pathways and food web contaminant transfer.

Tidal cycles are important cues for many marine and near-shore animals. Birds that typically feed at the water's edge during daytime low tides are rarely present on beaches at night. However, we found that several species of diurnal birds reliably attended and preyed upon the nocturnal spawning runs of a marine fish, Leuresthes tenuis, the California Grunion (Teleostei: Atheriopsidae) on the shores of Malibu Lagoon State Beach (MLSB), Los Angeles County, California, USA. Avian hunters of California Grunion included Black-crowned Night Heron Nycticorax nycticorax, Great Blue Heron Ardea herodias, Snowy Egret Egretta thula, and Western Gull Larus occidentalis. Spawning runs of California Grunion are synchronized by tides within a narrow window of time, two hours following the semilunar high tides of full and new moons. These silverside fish aggregate en masse predictably, and further increase their vulnerability by fully emerging from the water while spawning. This allows the birds to capture Calif...

ResumenSe documenta la presencia de aberraciones de pigmentos de melaninas en la coloración de las plumas en 4 especies acuáticas en Baja California Sur: Podiceps nigricollis, Egretta rufescens, Puffinus opisthomelas y Larus occidentalis. Estas observaciones representan los primeros registros de leucismo en México para estas especies. La documentación de este tipo de casos puede ayudar a entender la frecuencia de las aberraciones y su variación entre los diferentes grupos de aves.AbstractWe report the occurrence of plumage pigment aberrations in 4 species of waterbirds in Baja California Sur: Egretta rufescens, Larus occidentalis, Podiceps nigricollis, Puffinus opisthomelas. These observations represent the first leucistic records for these species for Mexico. The documentation of such cases can help to understand the impact of chromatic aberrations, and their variation between different groups of birds.

Cannibalism occurs regularly across a broad range of taxa with a variety of ecological and evolutionary consequences. Rises in sea surface temperature (SST) have been linked to increased cannibalism in some species, including polar bears (Ursus maritimus), Peruvian anchovy (Engraulis ringens), and Peruvian hake (Merluccius gayi peruanus), and might be expected in birds that depend on marine food webs for sustenance. Increased SSTs are associated with lowered ocean thermoclines and weakened upwellings. These changes, in turn, lead to decreased productivity in surface water and movement of surviving forage fish to deeper water, thereby food-stressing surface feeders such as gulls, diminishing energy intake and lengthening foraging bouts. While controlling for a suite of other environmental factors, we tested whether egg cannibalism and hatching success were independent of rises in local SST at a colony of Glaucous-winged Gulls (Larus glaucescens) and Glaucous-winged × Western Gull (L. glaucescens ×...

Glaucous-winged Gulls (Larus glaucescens) and Western Gulls (Larus occidentalis) are found along the Pacific Coast of North America. Where their breeding ranges overlap in Oregon and Washington they hybridize, producing a continuum of phenotypic variation. Whereas most colonies containing these hybrids are found on islands along the coast, several, including a large colony at Protection Island National Wildlife Refuge, WA, USA, are located in the Salish Sea. In 2010, we scored the phenotypes of 81 gull pairs at the Protection Island colony using an index based on plumage melanism and bare-part coloration. We also monitored the breeding success of these pairs. Gulls from the entire range of phenotypes in the L. glaucescens-occidentalis complex bred on the colony, although most gulls appeared more like L. glaucescens. Significant assortative mating based on colorimetric phenotype occurred on the colony, but a low number of L. occidentalis-type pairs appeared primarily responsible for this trend. Although the mass of the 3rd egg in the clutch was significantly smaller for pairs with increasingly L. occidentalis-like males, we observed no significant difference in hatching success or clutch size across the range of phenotypes.

Egg mass at laying is an important predictor of hatchling mass in birds (Deeming and Birchard 2007; Alabi and others 2012). Hatchling mass, in turn, affects reproductive success; smaller eggs produce smaller chicks, and smaller chicks are less likely to survive (Parsons 1975). Egg mass thus serves as an important measure of parental investment and fitness. For colonial seabirds, egg mass can be determined using a portable scale carried through the colony during nest surveys. In order to receive an accurate reading, however, the scale must rest on a level surface, remain sheltered from the wind, and be recalibrated at each new site. Furthermore, because eggs lose moisture over time and get progressively lighter, an egg should be weighed on the day that it is laid (Romanoff and Romanoff 1949). It is not always possible or convenient to fulfill these requirements for measuring egg mass. In contrast, egg length and width, which remain constant from laying to hatching, can be measured rapidly and accurately in the field with calipers. Several equations for estimating egg mass or volume from length and width have appeared in the literature. Hoyt (1979) estimated fresh egg mass M from length L and width W with the model M~kMLW , in which kM is a speciesspecific parameter. He provided kM values for 26 species, including Western Gulls (Larus occidentalis), for which kM was calculated to be 0.000531 g/mm (n 5 8). For chicken eggs, Narushin (2005) used geometric considerations to obtain the volume model V~ a{bW ð ÞLW, and Abanikannda and others (2007) used a model of the form M~azbLz cWzd(W=L). In this study, we tested these 3 alternative models as well as 3 allometric models for Glaucous-winged Gulls (L. glaucescens). In particular, we: (1) parameterized the models with data collected from a Glaucous-winged Gull colony; (2) selected the best model(s) using the Akaike Information Criterion (AIC); and (3) validated the selected model(s), without refitting, on a data set that was not used for parameter estimation. We collected data during the 2009 and 2010 breeding seasons at Protection Island National Wildlife Refuge (486079400 N, 122655900 W), Jefferson County, Washington, USA. Violet Point, a gravel spit extending to the southeast of the main island, contained a breeding colony of 3000 (in 2009) and 2469 (in 2010) pairs of Glaucous-winged Gulls (Cowles and others 2012, and unpubl. data). We selected 5 rectangular sample subareas of the Violet Point colony as shown in Figure 1 of Henson and others (2010; Plots A–E), with a combined sample area of 4205 m. Plot A bordered the beach on the north-central edge of the colony; Plot B was landlocked in the center of the colony; Plot C was at the west end of the colony and bordered the north beach of a small marina closed to the public; Plot D was in the center of the colony and also bordered the north beach of the marina; Plot E was in the center of the colony and bordered the south beach of the marina. Daily, throughout the 2009 and 2010 laying seasons (late May–early July), we monitored all nest cups in the 5 sample areas. When clutch initiation occurred in a nest, we placed a numbered wooden stake near the nest. Each staked nest was checked daily for new eggs. The mass of each egg was measured on the day it was laid using a 400-g capacity Ohaus Scout Pro SP401 portable electronic balance, and each egg was measured with vernier calipers at its widest and longest points. One egg, a ‘dwarf’ whose length (55.9 mm) and width (38.6 mm) were GENERAL NOTES

International audience; We investigated the role of selection in generating and maintaining species distinctness in spite of ongoing gene flow, using two zones of secondary contact between large gull species in Europe (Larus argentatus and Larus cachinnans) and North America (Larus glaucescens and Larus occidentalis). We used the pattern of neutral genetic differentiation at nine microsatellite loci (F(ST)) as an indicator of expected changes under neutral processes and compared it with phenotypic differentiation (P(ST)) for a large number of traits (size, plumage melanism and coloration of bare parts). Even assuming very low heritability, interspecific divergence between L. glaucescens and L. occidentalis in plumage melanism and orbital ring colour clearly exceeded neutral differentiation. Similarly, melanism of the central primaries was highly divergent between L. argentatus and L. cachinnans. Such divergence is unlikely to have arisen randomly and is therefore attributed to spatially varying selection. Variation in plumage melanism in both transects agrees with Gloger's rule, which suggests that latitude (and associated sun and humidity gradients) could be the selective pressure shaping differentiation in plumage melanism. We suggest that strong species differentiation in orbital ring colour results from sexual selection. We conclude that these large gull species, along with other recently diverged species that hybridize after coming into secondary contact, may differ only in restricted regions of the genome that are undergoing strong disruptive selection because of their phenotypic effects.

Nesting chronology, habitat use, subcolony use, and hatchability were documented for the Black-crowned Night Heron (Nycticorax nycticorax) nesting at Alcatraz Island, San Francisco Bay, California during 1990-2002. Reproductive success was estimated using the Mayfield method and compared among years. Totals of monitored nests per year ranged from 68 in 2001 to 341 in 1996, with a trend of declining numbers since 1996. An increase in numbers of the Western Gull (Larus occidentalis), the Black-crowned Night Heron’s primary competitor, occurred during the same period. Overall reproductive success of the Black-crowned Night Heron at Alcatraz Island was below the 13-year average of 56.4% since 1996. During the study, the average number of chicks fledged per nest each year ranged from 0.46 to 1.27, which is less than the two chicks per nest suggested as a requirement for a sustained population. Embryos in five of 187 failed Black-crowned Night Heron eggs were deformed. In 1990 and 1991, eggs were analy...

Summary 1 The population size of seabirds is often difficult to estimate from surveys at breeding colonies because of factors such as burrow nesting and nocturnal breeding. The reliability of alternative surveys conducted at sea has not previously been validated. Sample-based estimates from at-sea surveys could be biased and have poor precision due to non-random survey design and the uneven distribution of seabirds at sea. 2 We conducted at-sea surveys of western gulls Larus occidentalis and common guillemots Uria aalge of the Farallon Islands, California, USA, and waved albatrosses Phoebastria irrorata of the Galapagos. The observed counts were modelled using generalized additive models (GAMs), with a correction factor for bird movement relative to the ship included in an offset. The models were used to produce estimates of the size of the seabird populations at sea, which were compared with independent colony-based estimates, adjusted to account for the number of non-breeders. 3 Gull and guillemot populations were estimated separately for each of 10 survey years. Temporal trends were estimated by smoothing through the annual values. The albatross data from 7 survey years were pooled to obtain a single estimate of average population size. 4 The coefficients of variation (CVs) of the annual estimates were approximately 10%, 15–20% and 15–45% prior to smoothing for the albatross, gull and guillemot, respectively. The CVs of the smoothed estimates were about 10% for the gull and 15% for the guillemot. These represent substantial improvements in precision over previous sample-based estimates from at-sea surveys. 5 The colony-based estimates usually lay within the 95% confidence intervals (CIs) of both the annual and smoothed sea-based estimates, showing that the sea-based methods worked well. 6 Synthesis and applications. We conclude that GAMs of at-sea seabird survey data, collected under suitable protocols and corrected for bird movement, can accurately estimate population size. Given sufficient demographic information, these methods can provide a valuable tool for the management of populations that are difficult to census at the breeding colony.

We studied the effects of low-frequency climate change on the reproductive performance of 11 species of marine bird in the southern California Current system, 1969–1997. Reproductive performance of Brown Pelican (Pelecanus occidentalis) and Double-crested Cormorant (Phalacrocrax auritus) in southern California demonstrated an increase in the 1970s and early 1980s, attributable to recovery from organochlorine contamination (primarily DDE). Brandt’s Cormorant (Phalacrocorax penicillatus) in central California was the only species to demonstrate a secular increase in performance through time, a pattern that remains unexplained. Ashy Storm-petrel (Oceanodroma homochroa ) and Pelagic Cormorant (Phalacrocorax pelagicus) demonstrated curvilinear patterns of change, with decreasing reproductive performance in the past decade. All other species including Western Gull (Larus occidentalis), Pigeon Guillemot (Cepphus columba), Xantus’s Murrelet (Synthiloboramphus hypoleucus ), Common Murre (Uria aalge), Cassin’s Auklet (Ptychoramphus aleuticus) and Rhinoceros Auklet (Cerorhinca monocerata ) showed diminishing reproductive performance through time. Patterns of change for the murre and auklets were not significant, presumably because of a lack of reproductive variation for these species, which display a conservative breeding effort (i.e. single-egg clutches). Changes in the birds’ abilities to provision young and maintain chick survival during May–July each year appeared most closely related to overall changes in reproductive performance. Dietary change indicated a decline in use of juvenile rockfish ( Sebastes spp.) by marine birds in central California. There was also significant interannual variability in consumption of juvenile rockfish and the euphausiid Thysanoessa spinifera. Patterns of change in marine bird reproductive performance were generally concordant between southern and central California after considering the period of recovery for Brown Pelican and Double-crested Cormorant. The decline in reproductive performance and changes in diet composition do not appear directly related to the polarity reversal of the Pacific Decadal Oscillation

Numerous studies reveal strong, positive skews in long-term breeding performance among free-living animals, yet few studies explore the mechanisms underlying such variation. We examine the results of a 12-yr study of a population of Western Gulls, Larus occidentalis. Of 112 pairs for which we have either long-term (≥5 yr) or lifetime reproductive output, 44% bred for only a single year, and an additional 25% bred for only 2–3 yr. A few pairs bred successfully for 6 –12 yr and showed higher average clutch size, hatching success, and fledging success within any single season than did less successful breeders. The principal trait influencing both survival and reproduction was individual diet, which consisted of a mix of human refuse and fish. A strong, positive relationship existed among the amount of fish taken, breeding life-span, and reproductive performance. Birds with short life-spans took little or no fish on an annual basis. Birds with breeding life-spans >10 yr and high breeding success took >60% fis...

The genetic mating system of western gulls Larus occidentalis breeding on Southeast Farallon Island, California, was determined using multilocus DNA fingerprints of 33 chicks from 22 broods. No extra-pair paternity (EPP) was found, despite extra-pair copulations (EPCs) occurring. This suggests that paternity guards are effective, and that females gain few genetic benefits from EPCs. The EPP in western gulls concurs with that of other seabirds, reinforcing the idea that seabirds generally have a monogamous genetic mating system.

1. Natal dispersal distance (NDD; the distance between natal site and recruitment site) was studied in relation to proximal factors including sex, cohort, recruitment age, nesting density, nest site quality; and also within-colony movements of prospecting prebreeders among 133 recruits of two cohorts (1979–80) in a large, dense colony of western gulls Larus occidentalis (Audubon) on South-east Farallon Island, California. NDD was also studied in relation to ultimate factors, assessed from total number of chicks fledged to 1994, survival of breeding adults, and fitness-related life history traits, including brood size and hatching date. The Farallones colony is an unusual case in that it includes 50% of the species population and 90% of all western gulls breeding within a radius of 1000 km. Hence, potential recruits could choose between locations within the Farallones colony, but had limited choice to recruit at other colonies. 2. Among prebreeders, distance from natal site to prospecting site increased with age among gulls aged 2 to 4 years, and stabilized in 4- to 8-year-olds. Most 2-year-olds prospected on their natal sites. 3. Compared to other bird species, natal philopatry was strongly developed in both sexes, but was significantly greater in males (median distance between natal and recruitment site among males, 17 m; median distance among females, 50 m). 4. More philopatric males nested in areas of higher density and with a tendency for more nest cover (P = 0·080, when controlling for the effect of density). A non-linear relation between NDD and recruitment age among males resulted from shorter NDD in 4-, 5- and 6-year-olds compared to 3-year-olds and males older than 6 years. There was no relation between NDD and recruitment age in females. 5. Males who had hatched early in the season (individuals who were, on average, more dominant, with higher prebreeder survival and recruitment probability; Spear & Nur 1994) had shorter NDD than males who hatched later. 6. Fitness costs associated with natal philopatry were detected in both sexes. More philopatric males survived significantly less well than did less philopatric males. Average number of chicks fledged among experienced females (6–11 years breeding experience) was significantly lower among more philopatric compared to less philo patric individuals, as was cumulative number of chicks fledged for all females (as of 1994). 7. Thus, whilst philopatry was well developed in these gulls, the trait appeared maladaptive. This paradoxical result may be related to an extended period of poor food supply (1989–94) in the Gulf of the Farallones. We hypothesize that fitness costs associated with philopatry reflect different life history strategies where high philopatry may be part of a high effort strategy characterized by higher reproductive effort and lower survival. These results are consistent with the view that relative advantages and disadvantages between life history strategies could lead to selective equilibrium, depending on environmental conditions.

Survey of morphometric, colorimetric, and allozymic variation in the Glaucous-winged Gull (Larus glaucescens), the Western Gull (L. occidentalis) and their hybrids is based on 706 specimens taken from 33 colony areas located throughout the breeding range of both species. Whereas most morphometric characters overlap between taxa, colorimetric characters exhibited significant intraspecific and interspecific clinal variation. Canonical discriminant function analysis of colorimetric characters clearly separated pure and hybrid morphotypes and confirmed the intermediacy of character expression in hybrids. Plumage melanism of the mantle and primary tips are among the best discriminators of hybrids. Twenty-five of 32 presumptive genetic loci were polymorphic. Although there were no fixed differences between taxa, gene frequencies exhibited significant heterogeneity across geography. Despite moderately high gene flow (Nm = 2.8 individuals per generation), populations of the L. glaucescens-occidentalis complex are not panmictic, and the hybrid zone forms a partial barrier to gene flow. Hybrids are genetically more similar to L. glaucescens than to L. occidentalis. The hybrid zone extends from Juan de Fuca Strait, Washington, south to Coos Bay, Oregon. A broad zone of introgression extends from the Queen Charlotte Islands in British Columbia south to Coos Bay, Oregon. The hybrid zone appears to have expanded in recent years, yet its midpoint near Grays Harbor, Washington, has remained stable. Hybridization appears to be responsible for skewed introgression and elevated genic diversity in L. glaucescens. The fact that L. glaucescens shows high levels of introgression while maintaining its genetic identity lends support to the hypothesis that hybridization in birds may serve as a mechanism for increasing or restoring genetic diversity in forms that are adapted to local environments.

We examined the relationships among parental age, hatching asynchrony, egg size, and the "third-chick disadvantage" of Western Gulls (Larus occidentalis) on Southeast Farallon Island, California, during the 1984-1990 breeding seasons. Third chicks showed lower survival than first or second chicks for all parental age classes combined. Fledging success of first- and second-hatched chicks was related to parental age in a parabolic fashion, whereas fledging success of the third chick increased sigmoidally with increasing age; the probability of fledging of first-, second-, and third-hatched chicks converged for birds greater than 15 years old, negating the third-chick disadvantage for the oldest parents. Egg volume increased with increasing age for females 4 to 13 years old, and levelled off or declined slightly thereafter. For older females, the size disadvantage of third-laid eggs was relatively less than for younger females. Egg volume did not affect fledging success when parental age and year were considered in a multiple logistic (binomial) regression analysis. Hatching asynchrony decreased slightly with increasing parental age. Fledging success of third-hatched chicks decreased significantly with increasing hatching asynchrony. The greater hatching synchrony in broods of older parents may increase the competitive ability of third chicks. The third-chick disadvantage, often viewed as an adaptive response to variable food supplies, may be a nonadaptive effect of resource availability governed, in part, by the foraging

Accumulation of vertebrate remains in recent and fossil deposits by avian and mammalian predators is well documented (e.g., Moon, 1940; Mellett, 1974; Mayhew, 1977; Dodson and Wexlar, 1979; Emslie, 1988; Hoffman, 1988). Seabirds with this behavior include albatrosses and skuas (Clarke et al., 1981; Ryan and Fraser, 1988), the latter of which can accumulate up to a hundred carcasses of petrels in a concentrated midden (Furness, 1987). Cormorants, murres and other diving seabirds that prey on molluscivorous fish can transport deep-water molluscs into shallow-water deposits and may account for anomalous occurrences of molluscs in the fossil record (Lindberg and Kellogg, 1982). Western Gulls (Larus occidentalis) are large larids (similar in size to Glaucous-winged and Herring Gulls) that also accumulate bones of fish, birds, mammals and shell or other hard parts from invertebrates on their breeding colonies (Moore, 1975). This behavior can result in mixed assemblages of vertebrate and invertebrate remains that include species from local and distant environments. We studied pellet and bone accumulation by Western Gulls on Southeast Farallon Island, California, with the objectives to (1) determine the rate of bone accumulation by gulls, (2) identify patterns in bone representation and breakage in gull pellets, and (3) document the percent of non-native vertebrate remains that occur in regurgitations and pellets as a result of long-distance foraging by gulls. Southeast Farallon Island is located 42 km west of San Francisco and is the southernmost of three island groups in the Gulf of the Farallones. It was formed of uplifted granitic rock and has an area of 44 ha that includes flat marine terrace and numerous cliffs and hills; the highest peak has an elevation of 109 m. Vegetation is dominated by the endemic Farallon Weed (Lasthenia minor maritima) and at least 38 other native and introduced species of grasses and plants (Coulter, 1971). Southeast Farallon Island has the largest breeding colony of seabirds in the continental United States, which includes 12 species (3 cormorants, 5 alcids, 2 stormpetrels, 1 gull and 1 oystercatcher). Because the Gulf of the Farallones is an upwelling system, annual variation in the marine environment and ocean productivity affects prey selection and breeding success of these seabirds (Ainley and Boekelheide, 1990). The population of Western Gulls approximates 25,000 breeding adults, or 40% of the world population (Spear, 1988). Breeding territories occur across a multitude of habitats on the island, ranging from exposed level marine terrace to rocky slopes and cliffs, so that most available nesting areas are used. Gulls attend breeding territories nearly year round with lowest attendance in the fall after breeding. At this time, gulls disperse along the Pacific coast in correlation with the period of lowest food availability in the Gulf of the Farallones (Spear, 1988). Territory attendance increases in winter and spring and egg laying commences in late April (Ainley and Boekelheide, 1990). Gulls consume vertebrate prey whole but, like most avian predators, are unable to digest bone. Pellets containing feathers, bone and other hard parts are regurgitated 24-48 hours after consumption and may include parts from more than one meal. Gulls also have an expanded esophageal sac (crop) in which food is stored and then regurgitated to feed chicks, mates or to break apart and reconsume. Relatively large prey items can be stored in this sac including complete adult Cassin's Auklets (Ptychoramphus aleuticus). This nocturnal species (average weight 170 g) is the most abundant bird on Southeast Farallon Island, where there are up to 105,000 breeding adults (Manuwal, 1974); it excavates burrows or occupies rock crevices for nesting, and is frequently killed and eaten by Western Gulls (Fig. 1A, B). Diets of Western Gulls on Southeast Farallon Island were studied by Ainley and Boekelheide (1990) who collected and analyzed pellets and feces. Gulls feed primarily on marine fish (60-90%), but also on invertebrates (5-7%), birds and human garbage. In addition, gulls can be diet specialists and select one prey species consistently over others (Pierotti and Annett, 1987). On Southeast Farallon Island, specialists on Cassin's Auklets or gull chicks are rare but obvious (Fig. 1 A, B). The ground surface on Southeast Farallon Island is presently littered with thousands of bones of fish, birds and mammals (Fig. lA). In some areas of the marine terrace bone density is as high as 1000 or more bones per m2, but density varies considerably throughout the breeding colony. The most apparent, and probably most abundant, taxa represented by these bones are domestic animals (chicken, turkey, cattle) whose remains were brought to the island primarily by Western Gulls, the only seabird on Southeast Farallon Island that feeds on garbage. Bones of cattle include fragments of vertebrae, ribs and limb bones that were butchered (with obvious saw and cut marks) for human consumption. It is unlikely that these bones were left from earlier human occupation of the island fresh regurgitations containing paper and other garbage and bones of domestic animals that still have tissue adhering to them are observed frequently near gull territories during the breeding season. To investigate this bone accumulation, we surveyed 51 breeding territories of gulls during a 40-44 day period from mid June to late July, 1989. Most territories we surveyed were occupied by gulls with chicks; a few were incubating eggs or had lost their chicks

Salmonella enterica is a zoonotic pathogen that has been isolated from free-ranging marine mammals throughout the world, with animals in the Channel Islands of California (USA) showing the highest prevalence. The goal of this study was to determine prevalence, antimicrobial sensitivity and genetic similarity using pulsed-field gel electrophoresis (PFGE) of Salmonella in sev- eral non-domestic animal species on San Miguel and San Nicolas Islands. Fecal samples were col- lected from 90 California sea lion Zalophus californianus pups, 30 northern elephant seal Mirounga angustirostris pups and 87 western gulls Larus occidentalis in the Channel Islands and 59 adult male sea lions in Puget Sound, WA (USA). Salmonella were isolated, identified and serotyped, followed by antimicrobial susceptibility testing and PFGE. Of the California sea lion pups that were sampled on the islands, 21% (n = 19) were positive for Salmonella, whereas no adults males in Puget Sound were positive. Of the northern elephant seal pups sampled, 87% (n = 26) were harboring Salmonella. Only 9% (n = 8) of western gulls were shedding Salmonella, with one of these gulls harboring the only antimicrobial resistant isolate. The serotypes found in these animals were Enteritidis, Montevideo, Newport, Reading, and Saint Paul. The only serotype that showed variation on PFGE was Newport. The pinnipeds of the Channel Islands harbor Salmonella at a higher prevalence than pinnipeds from other geographic areas observed in previous studies. Researchers and veterinarians should exercise increased caution when working with these animals due to the zoonotic potential of Salmonella.

PERIODIC SURVEYS WERE CONDUCTED FROM NOVEMBER 1993 TO JUNE 1994 IN ESTERO RANCHO BUENO, A MANGROVE LAGOON ENVIRONMENT ON THE WEST COAST OF BAJA CALIFORNIA SUR. WE FOUND 5,65 BIRDS (56 SPECIES). THE AVIFAUNA INCLUDED 31 RESIDENT, 22 MIGRATORY, AND 3 SPECIES OF UNDETERMINATED MIGRATORY STATUS. MOST OF THE SPECIES WERE COASTAL AND ESTUARINE BIRDS; HOWEVER THERE WERE SOME TERRESTIAL AND WIDE-RANGING BIRDS. THERE IS EVIDENCE THAT AT LEAST 2 SPECIES BREED IN THE AREA; 4 SPECIES ARE PROTECTED BY MEXICAN LAW AND 5 SPECIES HAVE COMMERCIAL VALUE. DIFFERENT INDICES SHOW THAT THE BIRD COMMUNITY DIVERSITY IS LOWER BY THE END OF SPRING AND THE DOMINANCE OF THE BROWN PELICAN (PELECANUS OCCIDENTALIS), THE WESTERN GULL (LARUS OCCIDENTALIS), AND THE RING-BILLED GULL (L. DELAWARENSIS) INCREASES IN THIS TIME OF THE YEAR. COMPARISON OF SPECIES RICHNESS BETWEEN AREAS WITH AND WITHOUT MANGROVE HABITAT INDICATES THAT EQUITY IS HIGHER IN AREAS WITH MANGROVE.

Fecal samples from 212 selected marine mammals, marine birds, and raptors were cultured for Salmonella spp. on arrival at rehabilitation centers in California from May 1999 through July 2000. Salmonella spp. were cultured from nine (4%) animals, and seven serotypes were isolated: Johannesberg, Montevideo, Newport, Ohio, Saint Paul, Enteritidis Group D, and 4,5,12:1 Monophasic. One western gull (Larus occidentalis) had two serotypes. Antibiotic susceptibilities and chromosomal fingerprints were evaluated for Salmonella isolates. Some isolates were resistant to gentamicin, amoxicillin–clavulanic acid, and ampicillin. Chromosomal fingerprints with XbaI and XhoI restriction enzymes differed between serotypes but not between individuals carrying the same serotype of Salmonella.

In recent years, a question has arisen as to whether male and female investigators view natural phenomena in the same way. There could be differences in the research questions asked, the type of data collected, or the way in which a particular datum or observation is interpreted (Fox-Keller, 1985), leading to the possibility that male domination of certain fields leads to a biased view of nature. Lawton, Garska, and Hanks (this volume) have suggested that male-biased narratives have become so pervasive that both male and female researchers are trained to emphasize this perspective, such that some female investigators use a male-centered perspective in their research.

Generalist predators can contribute to extinction risk of imperiled prey populations even through incidental predation. Quantifying predation on small populations is important to manage their recovery, however predation is often challenging to observe directly. Recovery of prey tags at predator colonies can indirectly provide minimum estimates of predation, however overall predation rates often remain unquantifiable because an unknown proportion of tags are deposited off-colony. Here, we estimated overall predation rates on threatened wild juvenile steelhead (Oncorhynchus mykiss) by generalist adult Western Gulls (Larus occidentalis) in six central California (USA) watersheds. We estimated predation rates by gulls from the recapture of PIT (passive integrated transponder) tags that were originally inserted into steelhead and were subsequently deposited at a Western Gull breeding colony, Ano Nuevo Island (ANI). We combined three independent datasets to isolate different processes: (1) the probability a tag...

Historically, islands off the Northwestern coast of Baja California hosted large breeding colonies of seabirds. Three decades ago DDT and DDE achieved such high levels in the region as to cause reproductive failure of brown pelicans (Pelecanus occidenta/is; Jehl 1973, Anderson et el. 1975) and double-crested cormorants (Phalacrocorax auritus; Gress et el. 1973). In 1981-82 pintails (Anas acute) collected in San Quintin, Baja California, and terns (Sterna elegans and S. caspia) at San Diego Bay still had detectable levels of pp'-DDE (More et el. 1987; Ohlendorf et el. 1985), but the mobility of these species did not allow for precise determination of the origin of the pesticides. Since the early 1970's the level of bioaccumulation of DDT and its metabolites in birds in Northwestern Mexico has been unknown. To document the current accumulation of pesticide residues in the region, we decided to study a species that: (a) did not have any current conservation problems, (b) reproduced widely throughout the study area, and (c) whose eggs could be used to monitor local conditions. The Western gull is abundant in the region, has breeding colonies on many of the islands, and does not face any conservation problems. Individuals of this species remain near an area where food and breeding conditions are optimum (Spear 1988) and exhibit strong nesting-area and group fidelity (G. Hunt pets. comm. and L. Spear pets. comm.). The objective of this work was to determine the concentration of DDT and its metabolites in eggs of Western Gulls (L. occidentalis), from the southern coast of California and the Northwestern coast of Baja California.

Gulls (Laridae) are well known for foraging in a wide range of habitats and for using a broad diversity of behaviors for obtaining food (Burger 1988; Burger and Gochfeld 1996). One of the less-commonly observed foraging behaviors is foot paddling (Tinbergen 1960; Burger 1988), a method sometimes used when gulls seek prey in meadows, mudflats, and sandy intertidal zones. Apparently this foraging behavior has not previously been reported for western gulls, Larus occidentalis (Pierotti and Annett 1995). At 15:30 on 2 August 2005, during ebb tide on the northern tip of Cape Lookout (45025'N, 123059'W), in Tillamook County, Oregon, our attention was drawn to a western gull in 2ndyear plumage that began foot paddling on the sandy beach near tide line. The gull stood in 1 place and rapidly trampled the wet sand, alternating the feet in up-and-down movements of its legs. It quickly created a water-filled pit into which it sank, which has been termed the "quicksand effect" (Sparks 1961). During a continuous bout of foot paddling, lasting about 10 s, the gull frequently looked down at the forming water-filled pit and pecked at the water surface at a rate of 2 to 3 pecks/s, apparently capturing numerous invertebrates too small for us to see with binoculars at a distance

There has been considerable debate in the study of hybrid zones as to whether hybrids may be superior to parental types within the area of contact (bounded hybrid superiority). In birds, naturally occurring hybridization is relatively common, and hybridization within this group always involves mate choice. If hybrids are superior, females choosing heterospecific mates should be expected to show higher fitness under the conditions prevalent in the hybrid zone. Hybrid superiority under these circumstances would reduce reinforcement and thereby help to maintain the hybrid zone. To examine this issue, we studied reproductive performances of hybrids and parental species of gulls (Larus occidentalis and Larus glaucescens) at two colonies within a linear hybrid zone along the west coast of the United States. This hybrid zone contains predominantly gulls of intermediate phenotype. Previous studies indicated that hybrids were superior to one or both parental types, but provided no data on possible mechanisms that underlie this hybrid superiority. Using a hybrid index designed specifically for these species, we identified to phenotype more than 300 individuals associated with nests, including both individual males and females within 73 pairs in the central portion of the hybrid zone and 74 pairs in the northern portion of the hybrid zone. There was little evidence of assortative mating, and what little there was resulted solely because of pairings within intergrades. In the central hybrid zone, females paired with hybrid males produced larger clutches and hatched and fledged more chicks compared with females paired to western gull males. This was a result of heavy predation on eggs in sand habitat, where male western gulls established territories. In contrast, many hybrid males established territories in vegetated cover that was less vulnerable to predation. In the northern part of the hybrid zone, clutch size did not differ among pair categories, however, there were differences in hatching and fledging success, with females paired to hybrid males showing better success compared to females paired to glaucous-winged gull males. Hybrids showed better hatching and fledging success in the north because hybrids are more like western gulls than glaucous-winged gulls in foraging behavior, taking a higher percentage of fish in their diet, which enhances chick growth and survival. This is believed to be the first documentation of bounded hybrid superiority that delineates the mechanisms that underlie hybrid superiority.

We surveyed six Common Murre (Uria aalge) colonies in the vicinity of Elephant Seal Cove Beach (ESCB), Point Reyes National Seashore, Marin County, California from 1995 through 1997 to investigate the effects of disturbance on local murre populations in the wake of a small shipwreck and to establish seasonal and diurnal attendance patterns of coastal murre populations. Following the shipwreck in 1995, one colony (Cliff Rock West) was abandoned, and three others (Flattop Rock, Mid Rock, and East Rock) experienced reduced productivity due to disturbance and predation by Brown Pelicans (Pelecanus occidentalis), Western Gulls (Larus occidentalis), and Common Ravens (Corvus corax). A fifth colony, Face Rock, farthest from the grounding site, was apparently not affected. Murres attended Cliff Rock West only early in 1996, and not at all in 1997. A new colony formed on another seastack (Northwest Rock) after murres left the Cliff Rock West colony in 1996. However, no eggs were observed at Northwest Rock in 1996 and this colony was abandoned early in 1997. Seasonal attendance among colonies on Flattop Rock, Mid Rock, and East Rock followed a similar trend in both 1996 and 1997. Murre attendance fluctuated widely until the onset of egg-laying in early May, but then stabilized and increased somewhat untilJuly, when numbers began to drop at the end of the fledging period. Both air temperature and the upwelling index (UI) were significantly correlated with murre attendance in May and June during the incubation and nestling periods. Mean attendance increased 18-30% at these colonies between 1996 and 1997. Diurnal attendance in 1996 and 1997 varied both within and between colonies and days, yet there was a significant downward trend in numbers throughout the day, as well as an effect of colony disruption by other marine species. Given that seasonal and diurnal attendance patterns of Flattop, East, and Mid Rocks in 1996 and 1997 appear similar, both between years and to other studies, we assume that these colonies recovered quickly following the 1995 shipwreck. However, interspecific interactions and predation continue to affect diurnal attendance patterns and productivity of these colonies. Received 5 April 1998, accepted 11 October 1998, final acceptance 27 March 1999. Waterbirds 22(1): 130-139, 1999 Seasonal and diurnal attendance has been examined for a number of Common Murre populations (Lloyd 1975; Birkhead 1978a; Slater 1980; Harris et al. 1983; Hatch and Hatch 1989; Takekawa et al. 1990). During the incubation and nestling periods, the number of birds on a colony may be influenced by ambient weather; numbers may be lower during periods of high winds (Birkhead 1978a; Gaston and Nettleship 1981) or warm temperatures (Murphy and Schauer 1994). Few studies have investigated attendance patterns in relation to disturbance. Aside from human disturbance, disruption to murre colonies may be caused by predators and competitors; such effects have been recorded in studies of murre behavior, survivorship, productivity, and the costs and benefits of coloniality (Birkhead 1977; Spear 1993; Sydeman 1993; Parrish 1995; Gaston and Elliot 1996; Schauer and Murphy 1996). On 10June 1995, the vessel Wayfarer, a 42' "Grand Banks" pleasure-craft, ran aground ne r Elephant Seal Cove Beach, Point Reyes Headlands, Marin County, California (Fig. 1). As a result of the wreck, engine oil, debris, and approximately 200 gallons of diesel fuel were released into coastal waters (Point Reyes National Seashore Case/Incident Record #950136). During the event, a U.S. Coast Guard helicopter was dispatched to the scene to rescue the one-man crew.

We examined survival and recruitment (into the breeding population) in three cohorts of western gulls Larus occidentalis (Audubon) in relation to brood size at hatching, hatching order and hatching date. We divided the pre-breeding period into four stages: (i) the natal stage ― «pre-fledging» from hatching to independence; (ii) the juvenile stage ― the period from independence to age 1 year; (iii) the subadult stage ― the second and third years of life; and (iv) the recruitment stage ― the period when adults breed for the first time. Hatching date and hatching order (in order of importance) each had a strong, negative effect on survival in the natal stage. Brood size had little effect on natal survival; however, after controlling for hatching date there was a significant negative effect

In 1981-83, an estimated 2.2% of the male western gulls Larus occidentalis (Audubon) breeding on South-east Farallon Island, California, maintained feeding territories within breeding colonies of guillemots Uria aalge (Pont.) and Brandt's cormorants Phalacrocorax penicillatus (Brandt). These «specialist» gulls feed by means of scavenging, piracy and predation. This paper investigates specialist's survival and productivity, fidelity to feeding territories, foraging dynamics, energy consumption, and the effect of predation by these gulls on the breeding success of guillemots and cormorants