Larus Glaucescens Research Articles

THE BIRDS OF ST. MATTHEW ISLAND, BERING SEA

St. Matthew Island (60° 24′ N, 172° 42′ W) and its small nearby satellites, Hall Island and Pinnacle Rock, are isolated in the northcentral Bering Sea. This infrequently visited location occupies a geographic position with a deep Bering Land Bridge history and is in an area of interdigitation of the Old World, New World, and Beringian avifaunas. It is known for its three Beringian endemics, a bird (McKay’s Bunting, Plectrophenax hyperboreus), a small mammal, and a plant. This level of endemism is striking for a high-latitude island. The only previous summary of the avifauna of St. Matthew island (Hanna 1917) included 37 species. Our report considers more than 125 species and synthesizes data on presence and absence, abundance, and phenology. Because visits have been infrequent and concentrated during summer, our understanding of migration in this region remains poor, but the area is clearly affected by both the Old and New world migration systems. There is sufficient evidence to show that some profound changes among the island’s breeding birds have occurred during the past century. In particular, the breeding range of Glaucous-winged Gulls (Larus glaucescens) has been extended north to include St. Matthew, a change that is correlated with a northward shift in the extent of sea ice (Maslanik et al. 1996). King and Common eiders (Somateria spectabilis and S. mollissima) also have shown substantial changes in summer abundance. Other changes in the summer avifauna (e.g., among shorebirds) may reflect the dynamics of edge-of-range phenomena. Because of its central position in a region undergoing profound climate change and its demonstrated track record in showing avifaunal shifts, St. Matthew Island may represent an important bellwether for monitoring the biological effects of further climate change in the northern Bering Sea.

A vectorized method of importance sampling with applications to models of mutation and migration

An importance-sampling method is presented for computing the likelihood of the configuration of population genetic data under general assumptions about population history and transitions among states. The configuration of the data is the number of chromosomes sampled that are in each of a finite set of states. Transitions among states are governed by a Markov chain with transition probabilities dependent on one or more parameters. The method assumes that the joint distribution of coalescence times of the underlying gene genealogy is independent of the genetic state of each lineage. Given a set of coalescence times, the probability that a pair of lineages is chosen to coalesce in each replicate is proportional to the contribution that the coalescence event makes to the probability of the data. This method can be applied to gene genealogies generated by the neutral coalescent process and to genealogies generated by other processes, such as a linear birth–death process which provides a good approximation to the dynamics of low-frequency alleles. Two applications are described. In the first, the fit of allele frequencies at two microsatellite loci sampled in a Sardinian population to the one-step mutation model is tested. The one-step model is rejected for one locus but not for the other. The second application is to low-frequency alleles in a geographically subdivided population. The geographic location is the allelic state, and the alleles are assumed to be sufficiently rare that their dynamics can be approximated by a linear birth–death process in which the birth and death rates are independent of geographic location. The analysis of eight low-frequency allozyme alleles found in the glaucous-winged gull, Larus glaucescens, illustrates how geographically restricted dispersal can be detected.

Common Garter Snake Predation on Ring-Billed Gull Chicks in Washington

SHERROD SK, WHITE CM, WILLIAMSON FSL. 1976. Biology of the bald eagle on Amchitka Island, Alaska. Living Bird 15:143-182. STALMASTER MV. 1987. The bald eagle. New York, NY: Universe Books. 227 p. STALMASTER MV, PLETTNER RG. 1992. Diets and foraging effectiveness of bald eagles during extreme winter weather in Nebraska. Journal of Wildlife Management 56:355-367. THOMPSON SP. 1989. Observations of bald eagles eating glaucous-winged gull eggs in western Washington. Northwestern Naturalist 70:13-14. TODD CS, YOUNG LS, OWEN RB, JR, GRAMLICH FJ. 1982. Food habits of bald eagles in Maine. Journal of Wildlife Management 46:636-645. VERMEER K. 1963. The breeding biology of the glaucous-winged gull on Mandarte Island, B.C. Occasional Papers of the British Columbia Provincial Museum, No. 13. 104 p. WATSON JW, GARRETT MG, ANTHONY RG. 1991. Foraging ecology of bald eagles in the Columbia River Estuary. Journal of Wildlife Management 55: 492-499.

Population trends of nesting Glaucous-winged Gulls, Larus glaucescens, in the southern Strait of Georgia, British Columbia

Population trends of nesting Glaucous-winged Gulls, Larus glaucescens, in the southern Strait of Georgia, British Columbia

TERRITORY QUALITY AND REPRODUCTIVE SUCCESS OF BLACK OYSTERCATCHERS IN BRITISH COLUMBIA

Abstract I investigated the relationship between the physical characteristics of breeding territories and the reproductive success of Black Oystercatchers (Haematopus bachmani) in British Columbia. I compared measures of the threat of egg and brood depredation, conspecific competition, and food supply with measures of reproductive success on 38 oystercatcher breeding territories during 1996 and 1997. Oystercatchers breeding on territories near Glaucous-winged Gull (Larus glaucescens) colonies had smaller first clutches than pairs free of neighboring gulls. Oystercatcher pairs hatched and consequently produced more young on shallow sloping intertidal shoreline sites compared to steep-sloped islets and shorelines.

Predation on Pacific herring (Clupea pallasi) spawn by birds in Prince William Sound, Alaska

AbstractWe examined bird diets in areas with Pacific herring (Clupea pallasi) spawn at northern Montague Island in Prince William Sound, Alaska. Diets of the five most abundant bird species consisted primarily of herring spawn. Using a bioenergetics model, we estimated that in spring 1994 the five‐bird species ate 857.1 metric tons (mt), representing 31% of the estimated spawn deposition. The two most numerous consumers, glaucous‐winged gull (Larus glaucescens) and mew gull (Larus canus) consumed 26% and 3%, respectively, of the estimated spawn deposition. Surf scoters (Melanitta perspicillata), surfbirds (Aphriza virgata), and black turnstone (Arenaria melanocephala) together consumed 2% of the spawn deposition. In years with low spawn biomass, such as 1994, the number of herring larvae produced could be significantly affected by normal rates of avian predation. The high consumption by gulls, shorebirds, and surf scoters underscores the importance of herring spawn in the annual cycle of these species and requires further investigation.

Size- and Sex-Selective Mortality of Adult Sockeye Salmon: Bears, Gulls, and Fish Out of Water

Abstract Pacific salmon Oncorhynchus spp. exist as reproductively isolated breeding populations and display marked variation in size and age at maturity within and among populations. We conducted a detailed study of mortality on adult sockeye salmon O. nerka in Hansen Creek, a small stream in southwestern Alaska, to test the prediction that predation by brown bears Ursus arctos and death by stranding during upstream migration would selectively remove males and large (old) individuals. During the 3 years of this study, 13.7% of the mature salmon died in shallow water at the creek's mouth before reaching the spawning grounds. Males had proportionally higher mortality at the mouth than females (17.6% versus 10.8%), and the fish of both sexes that died at the mouth were larger and older than those that survived to ascend the creek. Of the fish that died at the mouth, females were more often attacked (chiefly scavenged) by glaucous-winged gulls Larus glaucescens than males, and the gulls tended to attack fish ...

Does Kleptoparasitism by Glaucous-Winged Gulls Limit the Reproductive Success of Tufted Puffins?

AbstractInterspecific kleptoparasitism (food stealing) occurs in many seabird species and can sometimes significantly affect host individuals and populations. We investigated effects of kleptoparasitism by Glaucous-winged Gulls (Larus glaucescens) on a population of Tufted Puffins (Fratercula cirrhata) in which almost all chicks die before fledging in some years. Rates of kleptoparasitism were estimated during two seasons and compared with several spatial and temporal factors that have been predicted to influence kleptoparasitism rates both within and among seasons. Kleptoparasitism rates increased from 2% in the first year to 19% in the second, whereas puffin fledging success declined precipitously from 81% in the first year to 11% in the second. Within years, kleptoparasitism rates increased with number of puffins returning with fish, decreased during fair weather, and were unaffected by tidal height. Spatially, those rates increased with puffin burrow density and declined with angle of the nesting slope. Gull kleptoparasitism success increased during foul weather and at higher puffin nesting elevations. Despite those associations, survival of puffin chicks was not influenced by kleptoparasitism activity near their nests although it increased with a combination of low elevation and high slope at the nesting burrow. Neither kleptoparasitism nor predation by gulls were exceptional relative to other seabird colonies and they were unlikely to cause reproductive failures that characterize puffins at that site. Other factors, particularly food shortages at sea, merit further attention in explaining those failures.

Does Kleptoparasitism by Glaucous-Winged Gulls Limit the Reproductive Success of Tufted Puffins?

Does Kleptoparasitism by Glaucous-Winged Gulls Limit the Reproductive Success of Tufted Puffins?

Passive Interference Competition by Glaucous-Winged Gulls on Black-Legged Kittiwakes: A Cost of Feeding in Flocks

AbstractWe analyzed data from two independent studies of foraging Black-legged Kittiwakes (Rissa tridactyla) in Prince William Sound, Alaska. Our purpose was to determine if Glaucous-winged Gulls (Larus glaucescens) hindered prey capture by kittiwakes. At tightly aggregated feeding flocks, gulls remained on the water directly over the prey and foraged by making brief hop-plunges or surface-seizes. Kittiwakes, in contrast, fed by diving from the air into open spots in the flock or around its periphery. Data from both studies indicated that kittiwakes made fewer feeding attempts in flocks that had greater numbers of gulls. However, kittiwake success rate per feeding attempt did not change as the number of gulls increased. Kittiwakes were more likely to avoid flocks that had a greater number of Glaucous-winged Gulls. Gulls successfully pirated less than 1% of fish captured by kittiwakes. Our findings suggest that passive interference may be costly for smaller birds that feed in multispecies feeding flocks.Competencia por Interferencia Pasiva de Larus glaucescens sobre Rissa tridactyla: El Costo del Forrajeo en GrupoResumen. Analizamos datos de dos estudios independientes sobre forrajeo de Rissa tridactyla en Prince William Sound, Alaska. El propósito de nuestro estudio fue determinar si Larus glaucescens impedía la captura de presas por parte de R. tridactyla. Las gaviotas permanecieron sobre el agua directamente sobre sus presas en bandadas de forrajeo altamente agregadas y forrajearon realizando zambullidos breves o capturas de superficie. En contraste R. tridactyla forrajeó lanzándose desde el aire a espacios abiertos entre la bandada o en su periferia. Los datos de ambos estudios indicaron que R. tridactyla realizó menos intentos de forrajeo en bandadas que tenían mayor número de gaviotas. Sin embargo, la tasa de éxito por intento de forrajeo no cambió con el aumento del número de gaviotas. R. tridactyla presentó una mayor probabilidad de evitar las bandadas con mayor número de gaviotas. Las gaviotas cleptoparasitaron exitosamente menos del 1% de los peces capturados por R. tridactyla. Nuestros resultados sugieren que la interferencia pasiva puede ser costosa para las aves más pequeñas que forrajean en bandadas multiespecíficas.

An Evaluation of Marine Bird Population Trends Following the ExxonValdez Oil Spill, Prince William Sound, Alaska

We examined post-spill trends (1989–1998) of marine bird populations in Prince William Sound (PWS) following the Exxon Valdez oil spill (EVOS) to evaluate recovery of injured taxa. Two criteria were employed. First, we examined population trends of injured taxa only in the oiled area of PWS using regression models. Second, we examined population trends of injured taxa in the oiled area relative to the unoiled area using homogeneity of the slopes tests. We considered a population recovering if there was a positive trend using either criteria. We considered a population not recovering if there was no trend using either criteria or a negative trend in the oiled area. A significant negative trend in the oiled area relative to the unoiled area was considered a continuing and increasing effect. Most taxa for which injury was previously demonstrated were not recovering and some taxa showed evidence of increasing effects nine years after the oil spill. Four taxa (loons Gavia spp, Harlequin Duck Histrionicus histrionicus, Bufflehead Bucephala spp, and North-western Crow Corvus caurinus) showed weak to very weak evidence of recovery. None of these taxa showed positive trends in both winter and summer. Nine taxa (grebes Podiceps spp, cormorants Phalacrocorax spp, Black Oystercatcher Haematopus bachmani, Mew Gull Larus canus, Glaucous-winged Gull Larus glaucescens, terns Sterna spp, murres Uria spp, Pigeon Guillemot Cepphus columba, and murrelets Brachyramphus spp) showed no evidence of recovery during summer or winter. Four taxa (scoters Melanitta spp, mergansers Mergus spp, goldeneyes Bucephala spp, and Black-legged Kittiwake Rissa tridactyla) showed evidence of continuing, increasing effects. We showed evidence of slow recovery, lack of recovery, and divergent population trends in many taxa which utilize shoreline and nearshore habitats where oil is likely to persist. Potential lingering spill effects and natural variability appear to be acting in concert in delaying recovery of many PWS bird populations.

DOES KLEPTOPARASITISM BY GLAUCOUS-WINGED GULLS LIMIT THE REPRODUCTIVE SUCCESS OF TUFTED PUFFINS?

Abstract Interspecific kleptoparasitism (food stealing) occurs in many seabird species and can sometimes significantly affect host individuals and populations. We investigated effects of kleptoparasitism by Glaucous-winged Gulls (Larus glaucescens) on a population of Tufted Puffins (Fratercula cirrhata) in which almost all chicks die before fledging in some years. Rates of kleptoparasitism were estimated during two seasons and compared with several spatial and temporal factors that have been predicted to influence kleptoparasitism rates both within and among seasons. Kleptoparasitism rates increased from 2% in the first year to 19% in the second, whereas puffin fledging success declined precipitously from 81% in the first year to 11% in the second. Within years, kleptoparasitism rates increased with number of puffins returning with fish, decreased during fair weather, and were unaffected by tidal height. Spatially, those rates increased with puffin burrow density and declined with angle of the nesting slo...

PASSIVE INTERFERENCE COMPETITION BY GLAUCOUS-WINGED GULLS ON BLACK-LEGGED KITTIWAKES: A COST OF FEEDING IN FLOCKS

We analyzed data from two independent studies of foraging Black-legged Kittiwakes (Rissa tridactyla) in Prince William Sound, Alaska. Our purpose was to determine if Glaucous-winged Gulls (Larus glaucescens) hindered prey capture by kittiwakes. At tightly aggregated feeding flocks, gulls remained on the water directly over the prey and foraged by making brief hop-plunges or surface-seizes. Kittiwakes, in contrast, fed by diving from the air into open spots in the flock or around its periphery. Data from both studies indicated that kittiwakes made fewer feeding attempts in flocks that had greater numbers of gulls. However, kittiwake success rate per feeding attempt did not change as the number of gulls increased. Kittiwakes were more likely to avoid flocks that had a greater number of Glaucous-winged Gulls. Gulls successfully pirated less than 1% of fish captured by kittiwakes. Our findings suggest that passive interference may be costly for smaller birds that feed in multispecies feeding flocks. Competencia por Interferencia Pasiva de Larus glaucescens sobre Rissa tridactyla: El Costo del Forrajeo en Grupo Resumen. Analizamos datos de dos estudios independientes sobre forrajeo de Rissa tridactyla en Prince William Sound, Alaska. El propósito de nuestro estudio fue determinar si Larus glaucescens impedía la captura de presas por parte de R. tridactyla. Las gaviotas permanecieron sobre el agua directamente sobre sus presas en bandadas de forrajeo altamente agregadas y forrajearon realizando zambullidos breves o capturas de superficie. En contraste R. tridactyla forrajeó lanzándose desde el aire a espacios abiertos entre la bandada o en su periferia. Los datos de ambos estudios indicaron que R. tridactyla realizó menos intentos de forrajeo en bandadas que tenían mayor número de gaviotas. Sin embargo, la tasa de éxito por intento de forrajeo no cambió con el aumento del número de gaviotas. R. tridactyla presentó una mayor probabilidad de evitar las bandadas con mayor número de gaviotas. Las gaviotas cleptoparasitaron exitosamente menos del 1% de los peces capturados por R. tridactyla. Nuestros resultados sugieren que la interferencia pasiva puede ser costosa para las aves más pequeñas que forrajean en bandadas multiespecíficas.

Nine Years After the Exxon Valdez Oil Spill: Effects on Marine Bird Populations in Prince William Sound, Alaska

Abstract We compared post Exxon Valdez oil-spill densities of marine birds in Prince William Sound from 1989–1991, 1993, 1996, and 1998 to pre-spill densities from 1984–1985. Post-spill densities of several species of marine birds were lower than expected in the oiled area of Prince William Sound when compared to densities in the unoiled area. These negative effects continued through 1998 for five taxa: cormorants, goldeneyes, mergansers, Pigeon Guillemot (Cepphus columba), and murres. Black Oystercatchers (Haematopus bachmani) and Harlequin Ducks (Histrionicus histrionicus) exhibited negative effects in 1990 and 1991. Loons showed a weak negative effect in 1993. Black-legged Kittiwakes (Rissa tridactyla) showed relative decreases in 1989, 1996, and 1998 which may have been caused by shifts in foraging distribution rather than declines in populations. Glaucous-winged Gulls (Larus glaucescens) showed positive effects in most post-spill years. Murrelets and terns showed relative increases in 1993, 1996, and 1998. Generally, taxa that dive for their food were negatively affected, whereas taxa that feed at the surface were not. Effects for some taxa were dependent upon the spatial scale at which they were analyzed. Movements of birds and the mosaic pattern of oiling reduced our ability to detect oil-spill effects, therefore our results may be conservative. Several marine bird species were negatively affected at the population level and have not recovered to pre-spill levels nine years after the oil spill. The reason for lack of recovery may be related to persistent oil remaining in the environment and reduced forage fish abundance.

Bounded hybrid superiority in an avian hybrid zone: effects of mate, diet, and habitat choice.

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.

NINE YEARS AFTER THE EXXON VALDEZ OIL SPILL: EFFECTS ON MARINE BIRD POPULATIONS IN PRINCE WILLIAM SOUND, ALASKA

Abstract We compared post Exxon Valdez oil-spill densities of marine birds in Prince William Sound from 1989–1991, 1993, 1996, and 1998 to pre-spill densities from 1984–1985. Post-spill densities of several species of marine birds were lower than expected in the oiled area of Prince William Sound when compared to densities in the unoiled area. These negative effects continued through 1998 for five taxa: cormorants, goldeneyes, mergansers, Pigeon Guillemot (Cepphus columba), and murres. Black Oystercatchers (Haematopus bachmani) and Harlequin Ducks (Histrionicus histrionicus) exhibited negative effects in 1990 and 1991. Loons showed a weak negative effect in 1993. Black-legged Kittiwakes (Rissa tridactyla) showed relative decreases in 1989, 1996, and 1998 which may have been caused by shifts in foraging distribution rather than declines in populations. Glaucous-winged Gulls (Larus glaucescens) showed positive effects in most post-spill years. Murrelets and terns showed relative increases in 1993, 1996, and...

Marbled Murrelets as Initiators of Feeding Flocks in Prince William Sound, Alaska

-I sought to determine which seabird species initiated small, ephemeral, multispecies feeding flocks in Prince William Sound, AK (PWS). I observed 47 feeding flocks at five sites duringJune 1996 and determined the initiating species at 34. All of the latter flocks were initiated by pursuit divers, of which 76.5% were Marbled Murrelets (Brachyramphus marmoratus), the most abundant seabird in PWS. Formation of feeding flocks followed either of two scenarios: 1) larids were attracted to a feeding location by the presence or activity of Marbled Murrelets (26.9%), or 2) both larids and murrelets were present and flock feeding began after the murrelets dove from the surface (50.0%). Other principal participants were Black-legged Kittiwakes (Rissa tridactyla) and Glaucous-winged Gulls (Larus glaucescens). Received 14 April 1999, accepted I June 1999.

The Ecophysiological Role of the Avian Lower Gastrointestinal Tract

In birds, lower intestinal Na-absorptive capacity is quantitatively important (compared to Na-intake rates or to body Na pools) in ostriches ( Struthio camelus), emus ( Dromaius novaehollandiae), chukars ( Alectoris chukar), and desert quails ( Callipepla gambeli), all species lacking nasal salt glands and having an ecophysiological need for Na retention; it is also important for ducks ( Anas platyrhynchos) on low-Na intake. However, Schmidt-Nielsen's hypothesis (that marine birds may conserve urinary water loss via Na-linked water absorption in the lower intestine and desalinate the absorbate via the salt glands) is not supported by studies on ducks and glaucous-winged gulls ( Larus glaucescens) maintained under hyperosmotic conditions. Evidence for regulation of lower intestinal Na transport in response to Na status is confined to chukars amongst these species, but regulated suppression of urinary reflux is inferred to occur in chukars, sand partridges ( Ammoperdix heyi), desert quails, house sparrows ( Passer domesticus), ducks, and glaucous-winged gulls when urine osmolality exceeds that from which lower intestinal transport can make net water gains against serosal-mucosal osmotic losses. Consequently, lower intestinal water absorption seems important in hydrated terrestrial birds, in which its principal role may be net absorption of water (particularly from intestinal fluid) after the small intestine has recovered water secreted into-intestine more anteriorly.

Hybridization and Reproductive Performance in Gulls of the Larus glaucescens-occidentalis Complex

The Glaucous-winged Gull (Larus glaucescens) and the Western Gull (L. occidentalis occidentalis) hybridize along the Pacific Coast from Cape Flattery, Washington, south through central Oregon. A hybrid index, based on plumage and soft part colors, was used to study the distribution of morphotypes (e.g., pure and hybrid birds), mating patterns and reproductive performance at eight sympatric and two allopatric colonies. Mate choice appears to be weakly positive-assortative. In 1989 in the hybrid zone, L. o. occidentalis experienced significantly greater reproductive performance, measured as clutch size and egg volume difference, than either L. glaucescens or mixed pairs containing at least one hybrid individual. Although the hybrid zone has expanded somewhat in recent times, the midpoint of the zone and the relative proportion of morphotypes has remained stable. The hybrid zone is situated on a marine-ecotone. Within the hybrid zone, relative reproductive performance of L. o. occidentalis and hybrids may shift between years, whereas L. glaucescens may experience poor reproductive performance in all years. The selection-hybridization balance hypothesis may best explain the L. glaucescens-occidentalis hybrid zone.

ESTIMATES AND TESTS OF PER CAPITA INTERACTION STRENGTH: DIET, ABUNDANCE, AND IMPACT OF INTERTIDALLY FORAGING BIRDS

Predicting the dynamics of natural food webs requires estimates of the strength of interactions among species. The ability to estimate per capita interaction strength from observational data is desirable because of the logistical difficulty of using experimental manipulations to obtain such measures for all species within complex natural communities. In this paper, I derive observational measures of per capita interaction strength having units matching those of dynamic food web models (per capita consumption and assimilation rates). I also highlight the difference between per capita interaction strength (a parameter used in theoretical models) and species impact (empirical measures of total species effect). I then use behavioral observations and population censuses in a rocky intertidal community to estimate both per capita interaction strengths and species impacts on invertebrate prey of Glaucous-winged Gulls (Larus glaucescens), American Black Oystercatchers (Haematopus bachmani), and Northwestern Crows (Corvus caurinus). Estimated per capita interaction strengths exhibited a skewed distribution with many weak interactions and few strong interactions: mean ± 1 sd of log10(interaction strength) = −1.95 ± 1.40 (bird-day/m of shore)−1. Per capita interaction strength correlated poorly (r2 = 0.152–0.157) and nonlinearly with both consumption rates and percentage contribution of a prey species to the diet. Using my observational estimates of per capita interaction strengths, I predicted the species impact of bird predation on different prey taxa. Predictions included strong effects of birds on goose barnacles (Pollicipes polymerus), limpets (Lottia and Tectura spp.), sea urchins (Strongylocentrotus spp.), and large starfish (Pycnopodia helianthoides and Solaster stimpsoni), but little effect on mussels (Mytilus californianus and M. trossulus), dogwhelk snails (Nucella spp.), and acorn barnacles (Semibalanus cariosus). I compared nine of the predictions with 126 results of experimental manipulations of birds. The predictions agreed both qualitatively and quantitatively with the experimental results. These findings suggest that observational measures of interaction strength that have units matching those of dynamical food web models may be reasonable to use in estimating those found in natural communities.