California Gulls Research Articles

Spatial aggregations of seabirds and their prey on the continental shelf off SW Vancouver Island

We investigated the spatial scales at which seabirds aggregate and associate with prey over the continental shelf off southwest Vancouver Island, British Columbia. Bird densities and hydroacoustic measures of prey abundance were recorded in all seasons from 1993 to 1995 from a vessel moving along fixed strip transects (mean distance 93 km; minimum spatial units 250 m). We used the neighbour K analysis to determine the spatial scale (patch radius) and number of birds (crowding) associated with aggregations of birds. Birds were grouped into 3 guilds: divers (dominated by common murres Uria aalge and other alcids), surface-feeders (dominated by California gulls Larus californicus and other gulls) and shearwaters (mainly sooty shearwaters Puffinus griseus). Flying birds occurred in smaller aggregations spread over a wider area than birds on the water. For birds on the water, patch radii were usually 2 to 8 km, and crowding averaged 574, 143 and 50 birds per patch for surface-feeders, shearwaters and divers, respectively. Patch radii showed few significant differences among bird species and guilds, and remained relatively constant throughout the year for most species, despite large seasonal changes in density and mean crowding. Abundance of prey (small schooling fish and euphausiids) was highest in the upper 10 m, declining progressively with deeper depths and showed marked seasonal trends (mean scores 4 to 5x higher in summer than in winter). Significant associations between birds and prey were usually within patch radii of 2 to 8 km. The appropriate scale to map and monitor seabirds and seabird-prey associations, for oil spill assessments and other reasons, would therefore be 1 to 10 km.

Pentosidine as a Measure of Chronological Age in Wild Birds

AbstractBirds have few reliable indicators of aging. Pentosidine is a product of nonenzymatic glycation that accumulates in tissues of an animal over its lifespan. The intent of this study was to determine if accumulation of skin pentosidine in birds of known ages changed as a function of time. Skin samples were obtained from the breast of 45 birds of various species obtained from the national aviary. In addition, foot webbing samples were obtained from 17 California Gulls (Larus callifornicus) of known ages. Collagen was measured by a hydroxypro-line spectrophotometric method and pentosidine was quantified using reverse phase high-performance liquid-chromatography. Pentosidine concentration in the skin and foot webbing increased linearly with age (P < 0.001). Hydroxyproline concentrations from the foot webbing were comparable to that measured in the skin; however, pentosidine concentrations were approximately one-fourth of that in skin. Knowledge regarding longevity of birds could provide information for species survival programs and insight into variations in longevity of an entire population.

PENTOSIDINE AS A MEASURE OF CHRONOLOGICAL AGE IN WILD BIRDS

Abstract Birds have few reliable indicators of aging. Pentosidine is a product of nonenzymatic glycation that accumulates in tissues of an animal over its lifespan. The intent of this study was to determine if accumulation of skin pentosidine in birds of known ages changed as a function of time. Skin samples were obtained from the breast of 45 birds of various species obtained from the national aviary. In addition, foot webbing samples were obtained from 17 California Gulls (Larus callifornicus) of known ages. Collagen was measured by a hydroxypro-line spectrophotometric method and pentosidine was quantified using reverse phase high-performance liquid-chromatography. Pentosidine concentration in the skin and foot webbing increased linearly with age (P < 0.001). Hydroxyproline concentrations from the foot webbing were comparable to that measured in the skin; however, pentosidine concentrations were approximately one-fourth of that in skin. Knowledge regarding longevity of birds could provide information fo...

Host specificity and colonization by Zachvatkinia caspica, an analgoid feather mite of Caspian Terns

The relationships between feather mites and their avian hosts have great potential as subjects for studies of evolution and ecology. However, we must first achieve a better understanding of the ecological roles of feather mites (mutualistic versus parasitic) as well as their degree of host specificity before we can search for broad generalities at work in bird/feather-mite systems. I investigated host switching and feeding ecology in Zachvatkinia caspica, an analgoid feather mite that lives among the feathers of Caspian Terns (Sterna caspia). My approach involved imping (i.e., transplanting) mite-free feathers from California Gulls (Larus californicus) and Caspian Terns onto mite-infested Caspian Tern wings and quantifying the extent to which mites colonized the newly introduced feathers. This approach allowed me to expose the mites to both host and non-host feathers as well as to the presence or absence of preen oils collected from the two bird species. Mites "incubated" on tern wings showed no obvious avoidance of gull feathers or preen oil. This colonization of gull feathers suggests that some mite species have the potential to occupy a number of host species and that host switching in nature may be limited by infrequent opportunities to colonize nontraditional hosts.

Colony Size and Diet Composition of Piscivorous Waterbirds on the Lower Columbia River: Implications for Losses of Juvenile Salmonids to Avian Predation

We investigated colony size and diet composition of piscivorous waterbirds (gulls, terns, and cormorants) nesting on the lower Columbia River from the mouth (river km 0) to the head of McNary Pool (river km 553) in 1997 and 1998. The study was prompted by concern that avian predation might constitute a significant source of mortality to juvenile salmonids Oncorhynchus spp. during out-migration. The diet of California gulls Larus californicus and ring-billed gulls L. delawarensis nesting in colonies above The Dalles Dam (river km 308) included few fish and very few juvenile salmonids. The sole exception was a small colony of California gulls in which salmonids accounted for 15% (by mass) of the diet. Juvenile salmonids were, however, an important component of the diet of colonial waterbirds nesting in the Columbia River estuary. On Rice Island (river km 34), salmonids accounted for 74% (by mass) of the diet of Caspian terns Sterna caspia, 46% for double-crested cormorants Phalacrocorax auritus, and 11% for glaucous-winged−western gulls L. glaucescens × L. occidentalis. Juvenile salmonids were especially prevalent in the diets of colonial waterbirds on Rice Island during April and May. By comparison, juvenile salmonids were significantly less prevalent in the diet of cormorants and gulls nesting lower in the estuary on East Sand Island (river km 8), presumably due to the greater availability of marine forage fishes. Our results indicate that avian predation on juvenile salmonids in the lower Columbia River is more prevalent in the estuary than near the large upriver gull colonies. Furthermore, the high incidence of salmonids in the diets of Caspian terns, cormorants, and gulls nesting on Rice Island suggests that the impact of avian predation on survival of smolts may be reduced by discouraging piscivorous birds from nesting there, while encouraging nesting on East Sand Island and other sites nearer to marine foraging areas.

Enhanced Success of California Gulls Nesting in Enclosures

-Gulls breeding in enclosures tend to have slightly lower success than those nesting elsewhere in a colony. Yet, in several recent years, California Gulls (Larus californicus) nesting in enclosures at Mono Lake, California, produced far more young than those breeding outside. In each of those years, colony-wide productivity was extremely low, due to predation on eggs and chicks by conspecifics. Birds in enclosures probably fared better because the fence walls reduced trespassing. Because rates of intraspecific aggression and predation vary annually, enclosure effects should be expected to vary between years. Conditions throughout a colony need to be evaluated before data from enclosures are used as an index of overall production. Received 31 August 2000, accepted 4 November 2000.

Field evaluation of a visual barrier to discourage gull nesting

Expanding gull populations along the Columbia River have been implicated in depredations to threatened and endangered migrating salmon smolt. We tested a visual barrier made of woven black polypropylene fabric to discourage gull nesting. The barrier was installed on Upper Nelson Island, Benton County, Washington, in parallel rows spaced 5 m apart. Gulls used 87% of the 7.9 ha island as nesting habitat and we estimated &gt;21 000 gull nests, 80% Ring-billed Gull Larus delawarensis and 20% California Gull L. califomicus nests. The zone with fencing had 84% fewer nests than the control zone. Silt fencing showed potential as a nonlethal bird management technique.

Seasonal Movements, Migration, and Range Sizes of Subadult and Adult Bamforth Lake California Gulls

--We investigated seasonal migration patterns of three age classes of California Gulls (Larus californicus). Using band recovery data and reported sightings of patagially marked gulls, we constructed location maps for fledglings, one to two-year-old gulls, and breeding-age adult gulls during five time periods: spring migration, breeding season, early and late fall migration, and winter. Using repeated observations, we also produced a location map with directional vector plots. Gulls followed a triangular pattern of movements. At the conclusion of breeding, gulls migrated west and northwest to the Pacific coast, distributing themselves mainly between San Francisco and British Columbia. During winter, gulls moved south along the Pacific coast and by spring were concentrated in southern California and northern Mexico. The range size was largest among fledglings which provided the northernmost and southernmost observations for the population. Few subadult gulls migrated to the breeding colony. During the breeding season, a substantial portion of breeding-aged adults remained on the Pacific coast and throughout the intermountain west but were not observed at other California gull colonies. While fledglings moved directly toward the Pacific coast at the end of the breeding season, many adults lingered near the colony site and at aquatic habitats south of the colony. We suggest that adults remain longer in the area to feed and to rejuvenate before attempting migration. Received 13 November 1997, accepted 2 April 1998.

Anatomy and histochemistry of spread-wing posture in birds. 2. Gliding flight in the California gull, Larus californicus: a paradox of fast fibers and posture.

Gliding flight is a postural activity which requires the wings to be held in a horizontal position to support the weight of the body. Postural behaviors typically utilize isometric contractions in which no change in length takes place. Due to longer actin-myosin interactions, slow contracting muscle fibers represent an economical means for this type of contraction. In specialized soaring birds, such as vultures and pelicans, a deep layer of the pectoralis muscle, composed entirely of slow fibers, is believed to perform this function. Muscles involved in gliding posture were examined in California gulls (Larus californicus) and tested for the presence of slow fibers using myosin ATPase histochemistry and antibodies. Surprisingly small numbers of slow fibers were found in the M. extensor metacarpi radialis, M. coracobrachialis cranialis, and M. coracobrachialis caudalis, which function in wrist extension, wing protraction, and body support, respectively. The low number of slow fibers in these muscles and the absence of slow fibers in muscles associated with wing extension and primary body support suggest that gulls do not require slow fibers for their postural behaviors. Gulls also lack the deep belly to the pectoralis found in other gliding birds. Since bird muscle is highly oxidative, we hypothesize that fast muscle fibers may function to maintain wing position during gliding flight in California gulls.

Anatomy and histochemistry of spread‐wing posture in birds. 2. Gliding flight in the California Gull, Larus californicus: A paradox of fast fibers and posture

Anatomy and histochemistry of spread‐wing posture in birds. 2. Gliding flight in the California Gull, Larus californicus: A paradox of fast fibers and posture

Brood Ecology of Mallards and Gadwalls Nesting on Islands in Large Reservoirs

Duckling survival is a critical component of waterfowl recruitment rates, yet estimates of survival are seldom available for ducklings on large lakes or reservoirs attracting dense concentrations of island-nesting waterfowl. We studied brood ecology of 51 radiotagged mallard (Anas platyrhynchos) and 24 radiotagged or marked gadwall (A. strepera) females nesting on islands in 2 lakes composing the Garrison Reservoir of North Dakota, from 1985 to 1987. Duckling survival to fledging varied widely by year and reservoir, ranging from 0 to 30% for mallards and 0 to 38% for gadwalls nesting on Mallard Island in Lake Sakakawea, versus from 28 to 35% for mallards and 0 to 41% for gadwalls nesting on islands in Lake Audubon. We used logistic regression to evaluate ecological factors predicting the survival of 606 ducklings in 69 broods of these species. Duckling survival varied (all effects P < 0.001) by year, species, reservoir of origin, hatching date, initial rearing site, and several interaction terms. Survival was lower for ducklings hatched in: (1) 1985 and 1987 when reservoir water levels were low in Lake Sakakawea, (2) mallard versus gadwall broods, (3) broods of both species hatched on Mallard Island, (4) broods produced after the median hatching date of 22 June, and (5) gadwall broods moved to off-island wetlands for rearing. When water conditions allowed, mallards used shallow wetlands with ≥50% emergent cover for rearing broods and moved broods farther (x = 5.1 km ± 0.53 [SE]) (P < 0.005) to reach these wetlands than gadwalls (x = 2.5 ± 0.36) which typically used permanent wetlands with less emergent cover. Inter-related factors potentially contributing to high duckling mortality in these large reservoirs included: (1) oligotrophic water conditions, fluctuating reservoir levels, and resulting lack of food and hiding cover in emergent vegetation along shorelines; (2) long travel distances needed to reach productive brood-rearing wetlands; (3) exposure to severe weather and wave action; and (4) predation by mink (Mustela vison), California gulls (Larus californicus), and other predators.

Predation at Intermountain West Fish Hatcheries

During 1993-94, we observed predators and surveyed managers at Intermountain West fish hatcheries to quantify the extent and species responsible for depredation losses. Our field study confirmed hatchery managers' impressions that great blue herons (Ardea herodias), black-crowned night herons (Nycticorax nycticorax), ospreys (Pandion haliaetus), and California gulls (Larus californicus) were the most important predators. At 2 intensively monitored hatcheries, we calculated losses to avian predation to be 7.0 and 0.6% of annual production, in contrast to the managers' estimates of 15% at each hatchery. Feeding mostly on dead or moribund fish, feral cats (Felis catus), raccoons (Procyon lotor), and striped skunks (Mephitis mephitis) had little effect on fish production. Private hatchery managers estimated higher (13%) total depredation losses than did managers of state owned hatcheries (5%). Hatchery managers viewed netting to be the only effective nonlethal control measure.

Mark-resighting analysis of a California gull population

California gulls ( Larus californicus ) of known age and sex were censused on their breeding colony in 1979, 1980 and 1984 through 1993. Ages of 235 males and 196 females ranged from 4 to 27 years. Age classes used in the analysis were limited to 17, 4 through 19, and 20 or more as a final age category because data on gulls over 20 were sparse. Survival declined with age in a way that was parsimoniously modelled with a quadratic function. Other factors, sex and time, did not explain any variation in survival. Resighting depended on age, sex and time. Younger adults skipped breeding more frequently than did older adults, and females skipped breeding more frequently than did males. There was also good evidence for time dependence in resighting probability, but its inclusion in the model occurred at the expense of interpretability and precision. In a data set such as this, resighting probability may assume more importance than a mere 'nuisance parameter'. In this study, resighting history measured attendance...

Offspring growth in the California gull: reproductive effort and parental experience hypotheses

Measures of adult feeding and foraging behaviour in the California gull, Larus californicus, were related to the growth of their offspring. Offspring showed significantly higher growth when average feeding interval, a measure of the time interval between feedings, and feeding latency following foraging decreased. The amount of time parents foraged was positively related to offspring growth and negatively correlated with average feeding interval. Either (1) increased foraging efficiency with parental age, or (2) increased reproductive effort with age, could explain age-related differences in patterns of feeding behaviour and their impact on offspring growth. However, data on foraging time support only the hypothesis of increased reproductive effort with parental age.

Offspring growth in the California gull: reproductive effort and parental experience hypotheses

Offspring growth in the California gull: reproductive effort and parental experience hypotheses

Probable epizootic chlamydiosis in wild California (Larus californicus) and ring-billed (Larus delawarensis) gulls in North Dakota.

During the summer of 1986, more than 400 California gulls (Larus californicus) and ring-billed gulls (Larvus delawarensis), primarily fledglings, died on an island in Lake Sakakawea near New Town, North Dakota (USA). Mortality was attributed largely to chlamydiosis. Necropsy findings in nine carcasses included splenomegaly (n = 9), hepatomegaly (n = 4), and pericarditis (n = 1). Livers from three California gulls and two ring-billed gulls, and spleens from the same five birds plus a third ring-billed gull were positive for Chlamydia psittaci by the direct immunofluorescence test. Chlamydia psittaci was isolated from separate pools of liver and spleen from one California gull and one ring-billed gull. This is believed to be the first record of epizootic chlamydiosis in gulls and the second report of epizootic chlamydial mortality in wild birds in North America.

Absence of Nest Density Effects in a Growing Colony of California Gulls

Nest density has been considered to affect the success of gull colonies. From 19841993 I studied the California Gull Larus californicus colony at Mono Lake, California, which has increased 30-fold since 1916 and attained nest densities that are among the highest reported for the species. I found that the Upper Critical Density (UCD) approximated 50 nests/100 m2, but even at 77 nests/100 m2 there was no effect on egg size, clutch size, or fledging success. Shorter (6-8 yr) sampling periods, however, would have indicated that either positive or negative effects had occurred. Gulls evidently disperse after the UCD is attained, because concentrations above the UCD did not persist into subsequent breeding seasons. Density effects are sometimes inferred from comparing long-term changes in population size (as an index to density) with various population parameters. The role of nest spacing, however, is best investigated by comparing the performance of birds nesting at different densities in the same year. Because productivity is influenced by factors that act away from as well as within a colony, studies of nest density provide little information on factors affecting population changes. Apparent effects of nest density in two long-term studies of Herring Gulls L. argentatus deserve re-evaluation.

Nest-Site Selection and Reproductive Success of Ring-Billed Gulls at Sprague Lake, Washington

AssTRAcr-Nest-site selection and reproductive success are reported for ring-billed gulls nesting at Sprague Lake, Washington, in 1981. Gulls nesting in tall grass and rocky beach habitats began laying significantly earlier than other gulls. Incubation times also varied by location, with birds in peripheral areas spending over three days longer to incubate their eggs than those nesting in the main part of the colony. Reproductive success was highest for birds nesting in tall grass, followed by those in rocky beach, sparse vegetation, low dense herb, and peripheral areas, respectively. Nest densities by habitat were determined in 1979, 1980, and 1981. Densities were highest in tall grass, followed by those in rocky beach, sparse vegetation, and low dense herb, respectively. Associated California gulls nested almost exclusively in sparse vegetation. Nests with supernormal clutches (5-6 eggs) were more common in sparse vegetation, low dense herb, and peripheral areas than in tall grass and rocky beach and accounted for 0.9% of the ring-billed gull nests in the colony.

A developmental constraint on the fledging time of birds

We examined the hypothesis that the rate of bone growth limits the minimum fledging time of birds. Previous observations in California gulls indicate that linear growth of wing bones may be the rate limiting factor in wing development. If bone growth is rate limiting, then birds with relatively long bones for their size could be expected to have longer fledging periods than birds with relatively short bones. We tested this by comparing the length of wing bones, relative to body mass, to the relative length of fledging periods among 25 families. The results support the hypothesis. A strong correlation exists between relative fledging period and relative bone length. Species which have relatively long bones for their body size tend to take longer to fly. In contrast, parameters that influence flight style and performance, such as size of the pectoralis muscle and wing loading, show little or no correlation with fledging time. The analysis also indicates that, when altricial and precocial species are considered together, bone length is more highly correlated with fledging time than is body mass or rate of increase in body mass during growth. These observations suggest that linear growth of bones does limit the growth of avian wings and that it is one of the factors that influences the fledging time of birds.

Alternate reproductive strategies in the California gull

We analysed 6 years of reproduction data for 176 California gulls (Larus californicus) surviving from 1980 to 1988. Using a statistical model adapted from Rao's (1958) and Tucker's (1966) generalized growth curve analysis, we reconstructed the reproductive patterns of gulls aged from 0 to 26 years. Individuals were highly consistent in following one of two patterns of reproduction. In a primary pattern employed by most gulls, individuals skipped breeding less frequently and laid larger clutches as they aged. Clutch size increased to a plateau and remained at high levels throughout remaining life. In an alternate pattern employed by a smaller subset of the sample, clutch size also increased to a plateau. However, as a result of frequent skipping of breeding and smaller clutches, this plateau was considerably lower compared to that of gulls adopting the primary reproductive pattern. Data on fledging success from 1980 and 1984 were consistent with the finding of two reproductive patterns. Gulls adopting the alternate reproductive pattern produce fewer offspring per breeding attempt but survive longer than gulls adopting the primary pattern. The frequency of gulls employing the alternate pattern will increase with age relative to gulls employing the primary pattern. The alternate pattern, and not senescence, may explain why several cross-sectional studies on seabirds report declines among the oldest breeders in measures of clutch size, egg mass, hatching success, and fledging success.