This Special Publication of Waterbirds is the result of a symposium on the decline of some North Atlantic gull populations held in Wilhelmshaven, Germany, in October 2013 as part of the 37th Annual Meeting of the Waterbird Society. Here, we focus on the rise and subsequent decline in the 20th century of Herring Gull (Larus argentatus) and Great Black-backed Gull (L. marinus) populations in eastern North America with reference to other regions. In addition to survey reports, the Special Publication includes several papers on closely related species (Lesser Black-backed Gull, L. fuscus; Ring-billed Gull, L. delawarensis; and Kelp Gull, L. dominicanus) with contrasting population trends, and papers related to breeding biology, diet and predation, movement, demographics and contaminants.

After an increase from the 1930s through the mid-1990s, the populations of the Herring Gull (Larus argentatus), the Lesser Black-backed Gull (L. fuscus fuscus), and the Great Black-backed Gull (L. marinus) in Finland are currently declining at rates of 0.5%, 2.7%, and 1.5% per annum, respectively. Although now declining in numbers, the Herring Gull is still the dominant gull species on Finnish coasts. The nominate Lesser Black-backed Gull, formerly the most abundant large gull in the Baltic Sea, is now considered endangered over its entire range. The Great Black-backed Gull has largely followed the trajectories of the other two species, but in much lower numbers. The decline of the Lesser Black-backed Gull may have been caused by severe reproductive failures due to pollutants and predation, while the factors behind the declines of the other two species are largely unknown.

Between-population variation of changes in numbers can provide insights into factors influencing variation in demography and how population size or density is regulated. Here, spatio-temporal patterns of population change of Herring Gull (Larus argentatus), Lesser Black-backed Gull (L. fuscus) and Great Black-backed Gull (L. marinus) in the British Isles are described. The aim of this study was to test for density-dependence and spatial variation in population trends as two possible, but not mutually exclusive, explanations of population changes with important implications for the understanding of these changes. Between 1969 and 2013, the three species showed different population trends with Herring Gulls showing a strong decline, Lesser Black-backed Gulls an increase until 2000 but then a decline since, and Great Black-backed Gulls showing no clear pattern. Population changes in Herring Gulls varied between different regions of the British Isles with decreases in the northern and western parts of the British Isles and no clear trends elsewhere. Population changes were density-dependent in the Herring Gull, and Lesser Black-backed Gulls showed faster population increases at lower Herring Gull densities. Herring Gulls seemed to seek refuge in urban environments, whereas Lesser Black-backed Gulls expanded their range into the urban environment. The large declines in hitherto abundant species create a dilemma for conservation bodies in prioritizing conservation policies. The spatial variation in population changes and the differences between species suggest that there is no single cause for the observed changes, thus requiring region and species-specific conservation management strategies.

Great Black-backed (Larus marinus), Lesser Black-backed (L. fuscus) and Herring (L. argentatus) gulls have all shown population increases and range expansion in Greenland in recent decades, but in very different ways. Great Black-backed Gulls increased from at least the 1960s from an established, local population. Lesser Black-backed Gulls immigrated from Iceland in the 1980s, spread rapidly over most of the southern part of Greenland and are common and abundant today. Herring Gulls also immigrated in the 1980s, but only established a small, scattered, and, at many sites, irregular breeding population. Here, we present the most recent information on the status and trends of Great Black-backed Gull, Lesser Black-backed Gull and Herring Gull populations in Greenland.

Information on the status, trends and dynamics of large gulls nesting in northern areas is limited. Herring (Larus argentatus) and Great Black-backed (L. marinus) gull breeding colonies were surveyed in coastal Labrador, Canada, from 1998 to 2003 to assess short-term population trends and colony site dynamics. Between 1998 and 2003, 117 islands distributed across three major archipelagos were surveyed on two or more occasions, documenting 1,054 gull clutches. Nesting was noticeably uniform across islands in the archipelagos, with mild aggregations noted in only one region. Incidence (the proportion of islands occupied by Herring and Great Black-backed gulls) declined over the study period, and there were fewer colonization events than abandonment events (also suggesting declines). Abundance declined in one region and overall at 2.9% per year over the 6-year period. Colony abandonment rates did not detectably decline with increasing colony size, and island isolation, indexed by the number of islands within 5 km, was not related to colonization rates. Herring and Great Black-backed gulls in Labrador showed positive incidence and abundance relationships at two spatial scales, indicating that more islands were occupied at larger local population sizes. This study revealed that colony-level metrics can be combined with population trend analysis to allow a richer assessment of the dynamics of colonial species.

Sable Island is the most isolated seabird colony site in eastern Canada and the United States, offering a unique opportunity to study the population dynamics of terns and gulls in an area removed from human activities. Sable Island likely supported tens of thousands of terns prior to 1900, but the population declined during the first half of the 20th century, coinciding with colonization by breeding gulls. An island-wide census of terns and gulls was conducted in 2012 and 2013, and those results were compared with surveys conducted over the previous 45 years to assess changes in population abundance and distribution. The current island-wide population of Common (Sterna hirundo) and Arctic (S. paradisaea) terns was approximately 6,500 and 4,200 combined breeding pairs in 2012 and 2013, respectively, down from about 9,000 pairs estimated in 2008/2009, but higher than all estimates between 1970 and 2006. Population growth of these tern species has been concentrated at two large colonies, each with over 2,000 breeding pairs, whereas the number of small colonies on the island has been in decline since 1998. Fewer than six pairs of Roseate Terns (S. dougallii) have nested on Sable Island since 1993, down from counts of more than 100 individuals in the 1970s. Estimates of breeding pairs ranged from 744 to 951 Herring Gulls (Larus argentatus) and 398 to 472 Great Black-backed Gulls (L. marinus) during 2012 and 2013. A survey of gulls documented 33 to 50% fewer Herring Gulls and 23% fewer Great Black-backed Gulls than in 1970. Sable Island's tern and gull populations have fluctuated asynchronously over the past 100 years, and may be returning to a previous ecosystem state when gulls were absent and terns were abundant, but mechanisms underlying these trends are unknown.

The status and trends of Maine's island-nesting Herring Gull (Larus argentatus) and Great Blackbacked Gull (L. marinus) populations have changed dramatically over the last century. Aerial photographs were used to count nesting Herring and Great Black-backed gulls at all colonies along the coast of Maine, USA, in 2008 and 2013. Population trends were assessed by comparing current survey data to a previous coastwide survey in 1977. The breeding population of Herring Gulls in Maine was estimated at 24,302 pairs nesting on 180 islands during 2008 and 21,488 pairs nesting on 180 islands during 2013. This represents an annual decline of 2.3% in the number of nests in Maine from 2008 to 2013. The breeding population of Great Blackbacked Gulls in Maine was estimated at 10,094 pairs nesting on 197 islands during 2008 and 6,934 pairs nesting on 191 islands during 2013. This represents an annual decline of 6.3% in the number of nests in Maine from 2008 to 2013. Nesting populations for both species appeared to peak in the 1990s. Between 1977 and 2013, the number of Herring Gull nests in Maine declined by 17%, and the number of nesting islands declined by 19%. Great Black-backed Gull populations also declined between 1977 and 2013, with a 30% decline in the number of nests and a 14% decline in the number of islands supporting nesting. The reason for the decline is unknown, but we speculate that these declines may be related to changing food availability around colonies and increased predation rates by Bald Eagles (Haliaeetus leucocephalus) and mammals.

A number of colonial waterbird species have been documented nesting on roof-tops throughout Europe and North America. The most common hypothesis explaining why gulls (Laridae) select roof-tops for nesting has been that population growth rates are higher than territory vacancy rates in traditional (island) habitat, suggesting that roof-tops are a non-preferred habitat. Roof-top habitat may actually be equal to or higher quality than island habitat as anthropogenic food is abundant and lower nest density may lead to lower intraspecific aggression and predation. During 2011–2012, reproductive effort and success was monitored in a regionally declining population of Herring Gulls (Larus argentatus) nesting on roof-tops in Portland, Maine, USA, and a nearby island-nesting colony on Appledore Island, Maine. Clutch size was lower in the roof-top colony, but egg volume did not differ between sites. Herring Gulls in the roof-top colony had lower hatching success but greater survival to day 30 for chicks that hatched than for those breeding in the island colony. The average number of chicks per nest to reach day 30 was 0.72 on the roof-tops and 0.84 on the island. This shows, therefore, that roof-top nesting may be an adaptive reproductive strategy even under scenarios with reduced competition for nesting territories on traditional nesting islands.

During the 20th century, gull populations in North America experienced considerable changes in abundance and geographic ranges. The objective of this study was to describe population trends of Herring Gulls (Larus argentatus) and Great Black-backed Gulls (L. marinus) in the New York Bight, USA, over a 40-year period (1974–2013). A variety of data sources using different survey methods provided estimates of the number of breeding pairs for both species. In the Long Island portion of the New York Bight, overall Herring and Great Black-backed gull nesting populations appear to have fluctuated considerably in size during this time period, and the largest numbers of breeding individuals of these two species occurred in the 1980s. In coastal New Jersey, the Herring Gull nesting population has remained relatively constant, whereas the Great Black-backed Gull nesting population has increased. Individual nesting colonies are dynamic and can vary in size considerably during even short time periods. Several factors, including sea-level changes and the availability of anthropogenic food sources (i.e., at landfills and fisheries by-catch), likely have strongly influenced individual colonies and the overall Herring and Great Blackbacked gull breeding populations in the New York Bight.

Ring-billed Gulls (Larus delawarensis) were rare at the beginning of the 20th century, possibly because of intensive exploitation. Once they became protected in 1916, their numbers increased throughout eastern North America, reaching a maximum of 875,000 breeding pairs around 1990. Since then, an overall decline of 19% has been recorded in the Great Lakes and the St. Lawrence River while their numbers tripled in Atlantic Canada. The largest concentrations are still found on the Great Lakes with 38% and 42% of the birds breeding in Canada and the USA, respectively. The remaining individuals breed along the St. Lawrence River (15%), on Lake Champlain (2%), and in the Gulf of St. Lawrence including the Atlantic Provinces and Saint-Pierre-et-Miquelon (3%). In 2009, a study was undertaken in the largest colony located on Île Deslauriers near Montreal, Québec, to determine the factors that regulate the number of breeding birds. This colony supported 52,000 pairs in 2000 and 44,000 in 2012. Ring-billed Gulls had a lower reproductive output in 2010–2012 compared to the late 1970s attributed to reduced chick survival. The number of gull-days at the nearest landfill declined from 906,000 in 1995 to 40,000 in 2012 following the implementation of a falconry deterrence program. Limited access to an abundant food supply located near the colony may have reduced chick survival. How this affects annual survival and population size remains to be determined. In the long term, policies that aim to reduce input of organic matter at landfills and discourage citizens from feeding gulls could further contribute to the decline of Ring-billed Gulls in eastern North America.

South Africa's population of Kelp Gulls (Larus dominicanus vetula) numbered about 10,000 pairs during 1976/1977–1980/1981, increased to 21,000 pairs from 2000/2001–2004/2005 and then decreased to 17,500 pairs from 2009/2010–2013/2014. The increase in the late 20th century and the decrease in the early 21st century were mainly attributable to large increases and decreases in numbers breeding at islands off the west coast of South Africa. The increases followed cessation of controls on Kelp Gulls at the islands in the 1970s and were associated with supplementary food provided by fisheries and landfill sites, whereas the decreases were influenced by substantial predation of chicks by Great White Pelicans (Pelecanus onocrotalus). The decreases of Kelp Gulls at islands off the west coast of South Africa in the early 21st century were offset to some extent by an increase in numbers breeding on mainland sites, especially around greater Cape Town and along the south coast. The proportion of Kelp Gulls breeding on the south coast increased from 15% in 2000/2001–2004/2005 to 44% in 2009/2010–2013/2014. As there are fewer islands off the south coast than the west coast of South Africa, the proportion of Kelp Gulls breeding at mainland sites as opposed to island localities increased from 12% in 2000 to 31% in 2014. Mainland colonies are more susceptible to disturbance by humans and predation by mainland carnivores, but, in spite of this, the generalist Kelp Gull has performed better in recent years than several specialist, endemic seabirds. At Dyer Island and Bird Island (Algoa Bay), where Kelp Gull numbers increased, predation by Kelp Gulls on eggs and chicks of threatened seabirds is thought to have contributed to decreases in these seabirds, and controls on Kelp Gulls have been reintroduced.

In South America, Kelp Gulls (Larus dominicanus) breed from Chilean Tierra del Fuego north to Río de Janeiro, Brazil, on the Atlantic coast and to Piura, Peru, on the Pacific coast. This review presents the first synthesis of information on the distribution and size of Kelp Gull colonies at the country level in coastal environments of Uruguay, Chile and Peru; provides an update on the breeding situation of the species in Brazil and Argentina; and allows the first evaluation of the overall coastal breeding population in South America. The breeding population in South America is now estimated to number at least 160,000 pairs. The largest population (at least 106,000 breeding pairs) is found in Argentina. Colonies of more than 1,000 pairs are uncommon. Population trends differed among coastal sectors, and important increases in numbers and formation of new colonies were only observed in Argentina. Available information suggests that predictable and abundant anthropogenic food subsidies, such as fishery discards and urban waste, are key factors contributing to the population growth in some coastal sectors. There is no strong evidence that Kelp Gulls are currently expanding their breeding range. However, given the population expansion in some areas and the potential conflicts with humans, ongoing monitoring efforts and population evaluations are necessary as they will provide the information required to support management decisions.

Large-scale relationships between changes in abundance of coastal breeding Herring (Larus argentatus) and Great Black-backed (L. marinus) gulls and commercial fisheries landings of bottom-dwelling groundfish spanning 28 years in four of Canada's east coast Provinces were investigated. Herring and Great Black-backed gull abundance data were compared between survey periods prior to (1986–1990) and following (2002–2006 and 2010–2014) the widespread reduction of groundfish fishing activities due to a moratorium that began in 1992. Regionwide declines in the number of breeding Herring and Great Black-backed gulls were observed between the 1986–1990 and 2002–2006 survey periods (Herring Gull: -3.7% per year; Great Black-backed Gull: -3.6% per year) and between the two periods following the moratorium (Herring Gull: -1.6% per year; Great Black-backed Gull: -4.1% per year). Total groundfish landings reported for the study area declined by 76% between the 1990–1992 and 2002–2006 fishing periods, and declined by an additional 25% between the two periods following the moratorium. A positive relationship was found between Province-wide groundfish landings and the number of breeding Great Black-backed Gulls corrected for coastline length. These results support the hypothesis that the moratorium reduced the availability of discards, which in turn played a role in the region's widespread decline of breeding Herring and Great Black-backed gull populations. In addition to continued declines in available discards, additional factors are likely influencing recent regional breeding population trends, including declines in available refuse and forage fish and increases in novel food sources such as expanding American mink (Neovison vison) farms.

Environmental conditions in eastern Newfoundland have changed considerably since the 1970s, as both bottom-up oceanographic and anthropogenic influences on seabird populations have fluctuated considerably. The diet, reproductive success, and presumably survival of gulls are intrinsically linked to these processes, and breeding populations have declined considerably through the 1980s and 1990s. To assess the populations of breeding large gulls in the Witless Bay Ecological Reserve in eastern Newfoundland and Labrador, Canada, nests were surveyed and clutch size determined for Herring Gulls (Larus argentatus) and Great Black-backed Gulls (L. marinus) breeding on Great, Gull, and Pee Pee Islands in 2011–2012. The total number of breeding gulls of these two species combined decreased by 41% on Gull Island, 78% on Great Island and 51% on Pee Pee Island since 2000. However, the declines differed among habitat type, with modest declines on puffin slopes (-15% to -52%) and the steepest declines in meadows (-70% to -88%), suggesting that large-scale causative factors are not solely responsible for changes in population size. Clutch size did not differ from that in 2000. Differential recruitment among highly philopatric gulls stemming from bottom-up diet-related variation in breeding success may be responsible for different changes in populations among different habitats.

The diets of gulls (Laridae) can have consequences for reproductive success, chick growth, and survival, yet there have been no quantitative assessments in eastern Newfoundland since the early 1970s. The diet of Herring Gulls (Larus argentatus) was examined through regurgitated prey items and pellets on Gull Island, Witless Bay, Newfoundland and Labrador, Canada, in 2012, and compared with similar data from 1970–1971. There was a significant shift in Herring Gull diet composition from blue mussels (Mytilus edulis) and capelin (Mallotus villosus) in the 1970s to garbage and Common Murre (Uria aalge) eggs in 2012. Delays in capelin spawning and the large increase in breeding Common Murres on Gull Island are likely factors influencing Herring Gull diet. Garbage, which includes human food scraps as well as plastic debris, now constitutes the single largest diet item for Herring Gulls, corresponding with a global increase in plastic pollution. The consistently low contribution of fisheries discards suggests that changes in fishing practices and availability of discards are only one possible factor in the Herring Gull decline in Witless Bay.

Herring Gulls (Larus argentatus) have been banded in large numbers in eastern North America for over a century, and much has been learned about their movements and non-breeding distributions. To see if changes have occurred in the non-breeding distribution of Herring Gulls, encounters of birds banded as chicks at the Witless Bay Ecological Reserve in eastern Newfoundland, Canada, were examined. Extensive banding activities occurred during 1966–1978 and most recently during 1999–2013 at this site. Overall, the patterns of encounter distributions have not changed greatly between the two time periods, in spite of the changes to the climate, marine ecosystems and land-use practices in the last 40 years (1966–1978 to 1999–2013). It appears that the migration habits of Herring Gulls breeding in Newfoundland are well established, and a northward shift in the non-breeding distribution, like that seen in several coastal bird species, was not apparent.

Following the discovery of widespread adverse reproductive effects in fish-eating colonial waterbirds nesting in the Canadian Great Lakes, Environment Canada started monitoring contaminants in Herring Gull (Larus argentatus) eggs in 1974. Current and historical concentrations and rates of decline of legacy contaminants (Polychlorinated Biphenyls [PCBs], 2,3,7,8-tetrachlorodibenzo-p-dioxin [TCDD] and organochlorine pesticides) in Herring Gull eggs from 15 Great Lakes colonies over 40 years are reported here. Large declines in contaminant concentrations were found in all colonies from the first year of reporting to 2013, with mean percent declines ranging from 72.7% for Σ chlordane to 95.2% for Mirex, indicating reduced availability of contaminants to wildlife. First-order exponential decay regressions indicated that rates of decline in eggs varied among compounds. Herring Gulls from Strachan Island (St. Lawrence River), for example, had the highest rates of decline for Dieldrin and Hexachlor Epoxide, whereas those from Middle Island (Lake Erie) had the lowest rates of decline for TCDD and PCBs, and those from Gull Island (Lake Michigan) had the lowest rates of decline for HCB and Mirex. Exponential rates of decline in Herring Gulls mirrored those previously reported for Great Lakes fish, thus demonstrating that Herring Gulls are a good fish-eating indicator species.

In 2001–2002, Great Black-backed Gull (Larus marinus) eggs from Lake Ontario had significantly higher concentrations of organochlorine contaminants than Herring Gull (L. argentatus) eggs. This study investigated whether higher contaminants in Great Black-backed Gulls affected immunocompetence, packed cell volume (PCV) and body condition in chicks, endocrine disruption and body condition in adults, and reproductive success. Variables were compared between species at two Lake Ontario sites (one in Ontario, Canada, and one in New York, USA), and with a control site in the Bay of Fundy, Canada. For Herring Gull chicks, body condition and total antibody titers to sheep red blood cells were greater at one contaminated site; PCV, phytohemagglutinin-P stimulation and reproductive output did not differ among sites. For Great Black-backed Gull chicks, PCV was significantly lower at one contaminated site; there were no other significant differences among sites. Between species, Herring Gull chicks had significantly higher antibody responses to sheep red blood cells at two sites and higher PCV at one site. Vitellogenin was not detected in the plasma of any adult males. Body condition of adult male Herring Gulls was significantly greater on Lake Ontario than in the Bay of Fundy control site. High contaminant concentrations in Great Black-backed Gull eggs from Lake Ontario were not associated with differences in phytohemagglutinin-P stimulation or body condition in chicks, nor with differences in reproductive output, compared with Herring Gulls, or with Great Black-backed Gulls at a control site.

Comprehensive understanding of the potential threats faced by seabirds requires information on their distribution and abundance in the marine environment where they spend a significant part of their life cycle. Data collected from shipboard seabird surveys from 2006–2014 were examined to identify high density marine areas in eastern Canada for Herring (Larus argentatus) and Great Black-backed (L. marinus) gulls during the nonbreeding season. The relative importance of these offshore areas was also compared to those used by Herring and Great Black-backed gulls in previous decades based on data collected from 1965–1992. Results show that both Herring and Great Black-backed gulls were broadly distributed over shelf waters throughout Atlantic Canada during the non-breeding season, as well as in the southern Gulf of St. Lawrence and the maritime St. Lawrence Estuary. These two species of gulls were also found to use the deeper slope waters, beyond the shelf break. The relative importance of different offshore areas for both species has changed over time, with decreases observed around the island of Newfoundland and on parts of the Scotian Shelf. An assessment of the overall vulnerability of Herring and Great Black-backed gull populations outside the breeding season will require continued monitoring at sea, integration of multiple datasets, and a comprehensive analysis of human activities offshore.

While the breeding ecology of gulls (Laridae) has been well studied, their movements and spatial organization during the non-breeding season is poorly understood. The seasonal movements, winter-site fidelity, and site persistence of Ring-billed (Larus delawarensis) and Herring (L. argentatus) gulls to wintering areas were studied from 2008–2012. Satellite transmitters were deployed on Ring-billed Gulls (n = 21) and Herring Gulls (n = 14). Ten Ring-billed and six Herring gulls were tracked over multiple winters and > 300 wing-tagged Ring-billed Gulls were followed to determine winter-site fidelity and persistence. Home range overlap for individuals between years ranged between 0–1.0 (95% minimum convex polygon) and 0.31–0.79 (kernel utilization distributions). Ringbilled and Herring gulls remained at local wintering sites during the non-breeding season from 20–167 days and 74–161 days, respectively. The probability of a tagged Ring-billed Gull returning to the same site in subsequent winters was high; conversely, there was a low probability of a Ring-billed Gull returning to a different site. Ring-billed and Herring gulls exhibited high winter-site fidelity, but exhibited variable site persistence during the winter season, leading to a high probability of encountering the same individuals in subsequent winters.

The increase in gull (Laridae) populations through the 20th century, largely due to an upsurge in anthropogenic food sources, has raised concerns about the effects of gulls on sympatric populations of other seabirds. In Newfoundland and Labrador, Canada, a reduction in fisheries discards due to the collapse of cod (Gadus morhua) populations and a phenological delay in the early 1990s and early 2000s in spawning capelin (Mallotus villosus) has supposedly resulted in increased seabird predation by gulls. Accordingly, the diet of Great Black-backed Gulls (Larus marinus) was quantified at two colonies (Gull Island, Witless Bay, Newfoundland, and Gannet Islands, Labrador), and the total mortality on sympatric breeding seabirds at each site was extrapolated. At the Gannet Islands, Great Black-backed Gulls primarily kleptoparasitized Atlantic Puffins (Fratercula arctica) bringing sandlance (Ammodytes sp.) to their chicks, whereas at Gull Island, seabirds formed the bulk of the gulls' diet. Great Blackbacked Gulls preferred murre (Uria spp.) eggs at both sites, selecting them in far greater proportion than their abundance, consuming up to 40% of Common Murre (U. aalge) eggs laid on Gull Island. Great Black-backed Gulls also targeted Black-legged Kittiwake (Rissa tridactyla) eggs, chicks, and adults at Gull Island. Great Black-backed Gulls clearly selected certain seabird prey disproportionately to their availability, but likely have not had significant effects on the populations of other sympatric seabirds, with the possible exception of Black-legged Kittiwakes, which declined significantly from the 1970s to the early 2000s.

Morphology, body condition and survival of Great Black-backed Gull (Larus marinus) chicks raised in broods of different size in southern New Brunswick, Canada, were compared. The relationship between age and body size measurements taken from 68 chicks, between 0 and 56 days of age, was analyzed. The logistic model provided the best fit of the morphological measurements (culmen, tarsus, and wing) to age, and the Gompertz model provided the best fit of body mass to age. These models were used to make linear transformations of body size variables that were used in subsequent analysis. There was no difference in the morphology of chicks raised in broods of different size. The first principle component (PC1) from the correlation matrix of the three transformed morphological variables was used as covariate in subsequent analysis to remove variation in body mass that was caused by differences in body size. All chicks were in the same relative body condition at hatch. Body condition of chicks raised in one-chick broods improved with age, that of chicks raised in two-chick broods did not change, but body condition of chicks from broods of three declined with age. In contrast, chicks from larger broods were more likely to survive the nestling period. Although body condition was lower in chicks from larger broods, other factors unrelated to body condition, such as differences in parental quality, led to higher survival for chicks in broods of three.

Herring Gull (Larus argentatus) and Great Black-backed Gull (L. marinus) morphometric data from various eastern North American locations was collected to examine the sources of variation in body size within and among geographic regions. For Herring Gulls, significant differences in all commonly taken measurements at local and regional scales were found. However, most of the variation in measurements was due to sex differences and the natural variance seen within local populations. Herring Gulls breeding in the Arctic did not show any evidence of being morphologically different from other groups. A discriminant function derived from a Newfoundland, Canada, breeding population of Herring Gulls successfully assigned the sex of birds in Atlantic Canada and Nunavut, Canada, further emphasizing that most of the variation seen is between sexes and not among local or even regional populations. It also indicates that the evitable variation introduced by inter-individual differences in measurements was insufficient to compromise the utility of the discriminant function. The correct classification rate was lower for Great Lakes breeding Herring Gulls, indicating that these birds have different morphologies than those of populations in easterly regions. In contrast, few differences and no clear geographic patterns were found in measurements for Great Black-backed Gulls. These results were consistent with recent genetic information, suggesting an older west to east radiation of Herring Gulls across North America and a lack of isolation among Great Black-Backed Gull populations.

Distinguishing among eggs of large gull species in mixed colonies is difficult because egg size is variable, size ranges overlap and colors are similar. Regional and yearly differences in egg size of Herring Gulls (Larus argentatus) were compared among three regions (Bay of Fundy, Newfoundland, and low Arctic). In two of these regions (Newfoundland and Bay of Fundy), eggs of Herring Gulls and Great Black-backed Gulls (L. marinus) were measured and discriminant analysis models were created to distinguish between the eggs of these two species. Egg dimensions of Herring Gulls decreased from low Arctic (largest) to Bay of Fundy to Newfoundland (smallest). In both species, where a = first-laid egg, b = second-laid, and c = third-laid, a- and b-eggs were of similar size, but c-eggs were significantly smaller; measurements of a- and b-eggs were pooled. The only annual differences were in a- and b-eggs (treated separately) in Newfoundland; there were no annual differences in c-eggs or in a/b-eggs combined. There were regional differences in a/b-eggs combined, but not in c-eggs. Three separate discriminant function models were created for Newfoundland a/b-eggs, Bay of Fundy a/b-eggs, and Newfoundland/Bay of Fundy c-eggs. Models discriminated 90% or more of the eggs. Length and diameter differ between species and must both be measured to discriminate between Herring and Great Black-backed gull eggs; diameter alone is not reliable. Future application of such models will improve identification of clutches in field situations and lead to more accurate gull population estimates.

In response to human activity and ecosystem changes, large gulls in eastern North America have shown a variety of population trends over the last 100 years. Seven years of capture-mark-recapture data were analyzed for Herring (Larus argentatus) and Great Black-backed (L. marinus) gulls breeding in eastern Newfoundland, Canada to estimate apparent survival and survival rates. Estimated survival was 0.864 for Great Black-backed Gulls and 0.837 for Herring Gulls. For both species, the survival rate calculated using live and dead encounters was higher than the apparent survival rate calculated from resighting data only and added almost an additional year of expected life. While these values are in the range of survival rates reported from other studies along the eastern coast of North America, the values for Herring Gulls were lower than survival rates reported elsewhere in North America and Europe. The lower survival rates seen in eastern North American Herring Gulls may be attributable to characteristics of the migration and wintering habitat, including poorer habitat quality (i.e., highly developed and industrialized east coast) and possibly gull (Laridae) control programs.