Western Gulls Larus Occidentalis Research Articles

Short-billed Dowitcher Limnodromus griseus killed by a Western Gull Larus occidentalis in western Washington, USA

Short-billed Dowitcher Limnodromus griseus killed by a Western Gull Larus occidentalis in western Washington, USA

Reducing agricultural loss and food waste: how will nature fare?

Reducing agricultural loss and food waste: how will nature fare?

Nest survival is influenced by parental behaviour and heterospecifics in a mixed‐species colony

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.

Assessing the Relationship between Gulls Larus spp. and Pacific Salmon in Central California Using Radiotelemetry

AbstractPredation 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 (Año 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 Año 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‐tagged western gulls occurred within 25 km of Año Nuevo Island, suggesting that the relative susceptibility of predation by western gulls using Año Nuevo Island decreased with distance from the island. Western gull presence at creek mouths was greatest during daylight hours (91% of detections), while juvenile salmonids were present predominantly at night (65% of detections). The greatest overlap between western gulls and salmonids occurred at dusk, and predation of out‐migrating salmonids was likely opportunistic. Deterring gulls from creek mouths when overlap between predator and prey might otherwise occur may buffer out‐migrating salmonids from predation. Our results will inform management strategies to most effectively reduce the impacts of gull predation on central California salmonids.Received May 8, 2014; accepted March 11, 2015

Avian Predators Target Nocturnal Runs of the Beach-Spawning Marine Fish, California Grunion,Leuresthes tenuis(Atherinopsidae)

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 California Grunion terrestrially. On nights when spawning runs of California Grunion could potentially occur, birds were present on MLSB, often before the fish began to run onto shore. Number of birds was high for the nights of likely runs whether after full or new moons, whether or not the fish appeared, but not on other nights. We suggest that birds rely on tidal cues to anticipate spawning runs of California Grunion, not the amount of moonlight or the actual presence of fish on shore. Shorebirds were more likely to forage at night at sites close to their upland roosting and nesting areas than at sites with less upland habitat. Observers on nearby Will Rogers State Beach and Topanga State Beach saw comparable spawning runs of California Grunion but reported significantly fewer avian predators. At MLSB, when tide conditions were right, avian predators appeared on the beach with even greater predictability than did their potential prey.

Assessing Avian Predation on Juvenile Salmonids using Passive Integrated Transponder Tag Recoveries and Mark–Recapture Methods

AbstractMany populations of coho salmon Oncorhynchus kisutch and steelhead O. mykiss are listed under the U.S. Endangered Species Act. Until recently, the role of avian predation in limiting recovery of coho salmon and steelhead in central California coastal watersheds has been overlooked. We used recoveries of passive integrated transponder (PIT) tags from Año Nuevo Island (ANI), a breeding site for several species of piscivorous seabirds, to estimate predation rates on juvenile salmonids and identify susceptible life stages and species responsible for predation. A total of 34,485 PIT tags were deployed in coho salmon and steelhead in six watersheds in San Mateo and Santa Cruz counties. Tags were deposited on ANI by predators after ingestion of tagged fish. Because tags were not removed from the island and were detected on multiple sampling occasions, we were able to use mark–recapture models to generate a corrected minimum predation estimate. We used POPAN, a variation of the Jolly–Seber model, to generate an estimate of gross population abundance, which accounted for tags deposited on the island but not detected during surveys. Detections of 196 tags from surveys conducted between autumn 2006 and spring 2009 were incorporated into the model, producing a gross population estimate of 242 tags (SE = 9.8). Addition of tags detected between autumn 2009 and 2010 to the abundance estimate from POPAN produced a new minimum estimate of 362 tags on ANI. Western gulls Larus occidentalis probably were the primary predator depositing tags on ANI. Minimum predation estimates ranged from 0.1% (Soquel Creek) to 4.6% (Waddell Creek) of outmigrating coho salmon and steelhead smolts. Predation was potentially greater given still unquantified deposition of tags off‐colony and destruction of tags during digestive processes of predators. Finally, avian predators targeted estuary‐reared fish, which contributed disproportionately to adult populations, further impacting imperiled salmon populations.Received August 4, 2011; accepted August 27, 2012

Prevalence and characterization of Salmonella spp. among marine animals in the Channel Islands, California

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 several non-domestic animal species on San Miguel and San Nicolas Islands. Fecal samples were collected 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.

Validating the use of generalized additive models and at‐sea surveys to estimate size and temporal trends of seabird populations

Summary 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. 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 Galápagos. 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. 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. 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. 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. 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.

No evidence for extra‐pair paternity in the western gull

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.

Natal dispersal in the western gull: proximal factors and fitness consequences

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.

Factors affecting recruitment age and recruitment probability in the Western Gull Larus occidentalis

We studied recruitment age (i.e. the age that a gull bred for the first time) and the probability that an adult reproduced during its life in three cohorts of Western Gulls Larus occidentalis breeding on southeast Farallon Island. We examined the effects of sex, hatching date, hatching order, brood size and annual variation in food supply during the recruitment period. Modal recruitment ages of males and females were 4 and 5 years, respectively, a difference that was significant. None of the factors examined, including hatching date, hatching order and brood size, had a significant effect on recruitment age. The probability of breeding in males who experienced low food supply in their fourth and fifth years was lower than that of males who experienced favourable food supply in their fourth and fifth years. Recruitment probability (the proportion of the sample group that recruited among those available to recruit during a given year) of 4‐ and 5‐year‐old males was lower in the food‐poor 1983 El Nino than in years of favourable food supply. The only variable affecting recruitment probability of females was age: recruitment probability was greater among 5‐year‐olds than 4‐year‐olds. Recruitment age in males was more sensitive to annual variation in food supply, probably because females have more difficulty recruiting due to an excess of adult females in the population and because of the different roles in reproduction in which males are primarily responsible for provisioning chicks and maintaining the territory.

Brood Size, Hatching Order and Hatching Date: Effects on Four Life-History Stages from Hatching to Recruitment in Western Gulls

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

Dynamics and Effect of Western Gulls Feeding in a Colony of Guillemots and Brandt's Cormorants

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

Human disturbance in Western Gull Larus occidentalis livens colonies and possible amplification by intraspecific predation

Indirect evidence is presented that human disturbances are having a profound effect on reproductive efforts of Larus occidentalis livens at several colonies in the Gulf of California. Breeding adults that lose their eggs or chicks apparently practise conspecific predation whether or not humans are present, thus increasing effects of human intrusions. These combined effects could lead to a severe decline in numbers or even pose a threat to the survival of this endemic population, if human disturbance is widespread. Attempts to assess breeding success throughout the Gulf seem warranted and, if necessary, some action to regulate human contact may be essential.