Common Guillemot Uria Aalge Research Articles

Behaviour and survival of Common Guillemot Uria aalge chicks at departure from a nest site in the Baltic Sea

Common Guillemots Uria aalge often breed high up in cliffs, forcing the juveniles to jump down to the beach or sea when leaving their nests. We studied survival of Common Guillemot chicks at time of nest departure on the island Lilla Karlsö in the Baltic Sea. All jumps were conducted either together with one of the parents, or, more commonly, the parent flew down shortly before the chick jumped. At this point, the parent was always found waiting for the chick directly underneath the nest, either on the beach or in the water, and never farther out than five meters if there was no beach below the cliff. If separated, all observed parents and juveniles reunited within one minute and then swam close together out to sea. Juvenile mortality was very low, only 0.5% in 2011, with 2 of 426 chicks dying. In both cases hitting a lower cliff ledge caused the mortality. No case of predation was observed. The high survival rates are most likely due to the chicks’ close proximity to their male parent at all times.

Reconciling policy with ecological requirements in biodiversity monitoring

Many of the breeding seabird populations in Britain and Ireland are of international importance; consequently, there is a statutory duty to protect these populations, as part of national biodiversity strategies and under Article 4 of the EU’s Directive on the Conservation of Wild Birds (EC/79/409). As part of this process, populations have been monitored annually at a sample of colonies since the mid-1980s and (near) complete surveys have been undertaken twice. Results of this monitoring are currently reported regionally, in an effort to reflect the impact of spatially varying environmental drivers of change; however, there is concern that these regions reflect policy requirements rather than ecological relevance, particularly for mobile species. We used the monitoring data to identify a series of ecologically coherent regions in which trends in abundance and breeding success varied in a consistent fashion and examined how closely the annually sampled data matched the change quantified by the whole population surveys. The number of ecologically coherent regions identified varied from 2 for the northern gannet Morus bassanus and common guillemot Uria aalge to 7 for the great cormorant Phalacrocorax carbo. Trends imputed for ecologically coherent regions more closely matched those observed between whole population censuses and were more consistent than those identified for more policy-driven monitoring regions. By accounting for ecology in the design of monitoring regions, population variation in mobile species can be more accurately represented, leading to the design of more realistic monitoring regions.

Optimal foraging in chick-raising Common Guillemots (Uria aalge)

The Norwegian population of the Common Guillemot Uria aalge has declined by >95% since the 1960s, and is classified as critically endangered in the Norwegian Red List. Much of the recent decline has been attributed to reduced food availability, but without extensive documentation of adult diet. Instead, chick diet has been considered a proxy of adult diet during the breeding season in many Norwegian studies. However, central-place foraging theory, especially for single-prey loaders, predicts that this may not be so, and this study directly compares the diet of adult and chick Common Guillemots during the breeding season at a colony in NE Norway. Whereas chicks were fed mainly capelin (Mallotus villosus) and sandeels (Ammodytes sp.), most of the adult diet consisted of the two youngest year classes of Gadidae, probably cod (Gadus morhua) and haddock (Melanogrammus aeglefinus). A successful ecosystem management of seabirds is dependent on a full understanding of how prey quality, abundance and availability influence seabird populations and their viability, such that knowing the true diet of adult Common Guillemots has important implications in the modelling and management of the Norwegian populations.

Recent decline in body condition of departing Common Guillemots Uria aalge at Hornøya, North Norway

Since 1980, there has been large variation and a recent decline in the mass and body condition of Common Guillemot Uria aalge chicks departing from the nest site in NE Norway. This may be related to deterioration in the feeding conditions off the colony and is alarming considering the critically endangered status of the species in Norway. To measure body condition, earlier studies have caught, weighed and measured guillemot chicks either while still on the breeding site or as they leave the colony. This study emphasises the importance of choosing and defining which method to use as they give different results.

Changes in the timing of egg-laying of a colonial seabird in relation to population size and environmental conditions

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 393:225-233 (2009) - DOI: https://doi.org/10.3354/meps08258 Changes in the timing of egg-laying of a colonial seabird in relation to population size and environmental conditions Stephen C. Votier1,*, Ben J. Hatchwell2, Meghann Mears2, Tim R. Birkhead2 1Marine Biology & Ecology Research Centre, University of Plymouth, Plymouth PL4 8AA, UK 2Department of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, UK *Email: stephen.votier@plymouth.ac.uk ABSTRACT: An emerging consequence of global climate change is its potential effect on the timing of seasonal biological events. Analysis of long-term datasets reveals a high degree of plasticity in the nature of phenological responses both within and among species, and understanding these differences is central to understanding the mechanisms and implications of climate-related change. We investigated factors influencing timing of breeding (median laying date) in a colonial nesting seabird, the common guillemot Uria aalge, over 23 breeding seasons between 1973 and 2008. There was a trend for earlier laying over this period, and earlier laying was associated with higher average breeding success. Multiple regression models (with de-trended explanatory variables to control for linear trends over time) indicate that the timing of breeding is positively correlated with a wide-scale climatic driver, the winter North Atlantic Oscillation (WNAO), and negatively correlated with population size: guillemots lay later in years with high WNAO indices and earlier in years with larger populations. Responses to environmental conditions are probably related to indirect effects on timing or abundance of food availability, direct effects of weather or both. The mechanism(s) leading to a possible relationship between laying date and population size are less clear. They may be related to Allee-type effects associated with social stimulation, improved foraging efficiency or a density-dependent increase in breeding site quality. Given the correlative nature of these results, we are cautious about the role of non-climatic (intrinsic) factors, but we cannot exclude that they play a role alongside climatic (extrinsic) factors in influencing reproductive phenology. KEY WORDS: Phenology · Climate change · Allee effect · Coloniality · NAO · Density dependence Full text in pdf format PreviousNextCite this article as: Votier SC, Hatchwell BJ, Mears M, Birkhead TR (2009) Changes in the timing of egg-laying of a colonial seabird in relation to population size and environmental conditions. Mar Ecol Prog Ser 393:225-233. https://doi.org/10.3354/meps08258 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 393. Online publication date: October 30, 2009 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2009 Inter-Research.

Long‐term changes in breeding phenology at two seabird colonies in the western North Sea

There is compelling evidence that the breeding phenology of many species has changed substantially in recent decades. However, taxonomic and spatial variation in the direction and rate of change is still not well understood. We explored these issues by analysing a dataset containing information on first egg dates of 10 species of seabird at two major breeding colonies (86 km apart) in the western North Sea over a period of 35 years. Within a species, timing of breeding was positively correlated between the two colonies, suggesting that factors affecting the phenology of these species operated at a regional rather than a colony scale. Comparison of time trends among the species revealed contrasting patterns, with some showing no systematic change, others becoming earlier and others later. The clearest species groupings appeared to be among the terns with arrival and/or first egg dates becoming earlier in Arctic Terns Sterna paradisaea, Common Terns Sterna hirundo and Sandwich Terns Sterna sandvicensis, and among the auks (Common Guillemot Uria aalge, Razorbill Alca torda and Atlantic Puffin Fratercula arctica) and Black‐legged Kittiwake Rissa tridactyla where the trend was in the opposite direction towards later breeding. This general trend towards later breeding in the latter group of species contrasts with correlational evidence from many other organisms indicating that breeding phenology is advancing in response to climate change.

Common Guillemot Uria aalge chick diet and breeding performance at Sumburgh Head, Shetland in 2007–09, compared to 1990–91

Chick diet of Common Guillemots Uria aalge was studied by direct observation at Sumburgh Head, Shetland during 2007–09. Lesser Sandeels Ammodytes marinus comprised 55% of prey items, the remainder being Gadidae, including Whiting Merlangius merlangus and Saithe Pollachius virens, and a few Snake Pipefish Entelurus aequoreus. This contrasts with 1990–91 when chick were fed 80% sandeels, the remainder mostly being unidentified Gadidae. When this shift from a diet dominated by sandeels took place is unknown, but a similar shift in chick diet at Fair Isle (40 km southwest of Sumburgh Head) seems to have occurred some time during 2000–02. Since 2003, weights of chicks near fledging have been lower than during the 1990s, and average breeding success has been reduced.

Sex‐specific food provisioning in a monomorphic seabird, the common guillemotUria aalge: nest defence, foraging efficiency or parental effort?

Sexual differences in food provisioning rates of monomorphic seabirds are well known but poorly understood. Here, we address three hypotheses that attempt to explain female‐biased food provisioning in common guillemotsUria aalge: (1) males spend more time in nest defence, (2) females have greater foraging efficiency, and (3) males allocate a greater proportion of foraging effort to self‐maintenance. We found that males spent no more time with chicks than females but made longer trips and travelled further from the colony. There was extensive overlap between sexes in core foraging areas, indicating that females were not excluding males from feeding opportunities close to the colony. However, as a result of their longer trips, the total foraging areas of males were much greater than those of females. There was no difference between sexes in overall dive rate per hour at sea, in behaviour during individual dives or in a number of other measures of foraging efficiency including the frequency, depth and duration of dives and the dive: pause ratio during the final dive bout of each trip, which was presumably used by both sexes to obtain prey for the chick. These data strongly suggest that sexes did not differ in their ability to locate and capture prey. Yet males made almost twice as many dives per trip as females, suggesting that males made more dives than females for their own benefit. These results support the hypothesis that female‐biased food provisioning arose from a difference between sexes in the allocation of foraging effort between parents and offspring, in anticipation of a prolonged period of male‐only post‐fledging care of the chick, and not from differences in foraging efficiency or time spent in nest defence.

Temporal variation in atmospheric ammonia concentrations above seabird colonies

Recent studies have shown that seabirds are an important source of ammonia (NH 3) emissions in remote coastal ecosystems. Nesting behaviour, which varies between seabird species, is likely to be a major factor in determining the proportion of excreted nitrogen (N) volatilised to the atmosphere as NH 3. A long-term NH 3 monitoring programme was implemented at a Scottish seabird colony with a range of species and associated nesting behaviours. The average monthly NH 3 concentration was measured at 12 locations over a 14-month period, to infer spatial (i.e. species-specific) and temporal (seasonal) changes in NH 3 emissions from different seabird species. An emissions model of seabird NH 3, based on species-specific bioenergetics and behaviour, was applied to produce spatial estimates for input to a dispersion model. Atmospheric NH 3 concentrations demonstrated spatial variability as a result of differing local populations of breeding seabirds, with the highest concentrations measured above cliff nesting species such as Common guillemot Uria aalge, Razorbill Alca torda and Black-legged kittiwake Rissa tridactyla. NH 3 concentrations above a colony of burrow nesting Atlantic puffin Fratercula arctica were low, considering the high number of birds. Emission of NH 3 from excreted N exhibits a time lag of approximately a month. It is likely that all excreted N is lost from the colony by volatilisation as NH 3 or surface run-off between breeding seasons. Modelled NH 3 emissions and concentrations correlated with measured concentrations, but were much higher, reflecting uncertainties in the local turbulent characteristics. The results allow multi-species seabird population data to be used for the calculation of regional and global NH 3 emission inventories, whilst improving understanding of N budgets of remote coastal ecosystems.

Reproductive Senescence in a Long‐Lived Seabird: Rates of Decline in Late‐Life Performance Are Associated with Varying Costs of Early Reproduction

Evolutionary theories of senescence predict that rates of decline in performance parameters should be shaped by early-life trade-offs between reproduction and somatic maintenance. Here we investigate factors influencing the rate of reproductive senescence in a long-lived seabird, the common guillemot Uria aalge, using data collected over a 23-year period. In the last 3 years of life, individual guillemots had significantly reduced breeding success and were less likely to hold a site or attempt to breed. Females senesced at a significantly faster rate than males. At the individual level, high levels of reproductive output earlier in life were associated with increased senescence later in life. This trade-off between early- and late-life reproduction was evident independent of the fact that as birds age, they breed later in the season. The rate of senescence was additionally dependent on environmental conditions experienced earlier in life, with evidence that harsh conditions amplified later declines in breeding success. Overall, individuals with intermediate levels of early-life productivity lived longer. These results provide support for the antagonistic-pleiotropy and disposable-soma theories of senescence and demonstrate for the first time in a wild bird population that increased rates of senescence in reproductive performance are associated with varying costs of reproduction early in life.

The diet of European Shag Phalacrocorax aristotelis, Black-legged Kittiwake Rissa tridactyla and Common Guillemot Uria aalge on Canna during the chick-rearing period 1981–2007

Chick diet of European Shags Phalacrocorax aristotelis, Black-legged Kittiwakes Rissa tridactyla and Common Guillemots Uria aalge at Canna was investigated over a 27- year period. The diet was mainly composed of Sprats Sprattus sprattus, Lesser Sandeels Ammodytes marinus and members of the Gadidae (a variety of species but mainly Trisopterus spp. and Whiting Merlangius merlangus). Other groups (ten families of fish, crustaceans, cephalopod molluscs and polychaete worms) were of minimal importance. Lesser Sandeels dominated the diet of young Black-legged Kittiwakes and European Shags, Sprats the diet of young Common Guillemots, whereas gadid otoliths were by far the commonest items found in pellets regurgitated by older European Shags. There were few significant temporal changes in species composition or the size of prey taken over the 27 years and the results confirm earlier findings that gadids are a normal and important part of the diet of seabirds at this colony.

Within‐ and between‐year variation in the juvenile survival of Common GuillemotsUria aalge

We studied juvenile survival of 20 cohorts of Common GuillemotUria aalgechicks colour‐ringed on the Isle of May, Scotland, using both live observations at the colony and dead recoveries, allowing estimation of fidelity to the colony as well as survival. In this seabird, chicks leave the colony when only partly grown and are cared for by the male parent for several weeks afterwards. First‐year survival varied strongly between cohorts, with a mean of 56% (range 30–91%). We did not identify any covariates which could explain this variation, whether relating to climate, population size or prey density. Survival was low during two regime shift episodes in the North Sea (1987–90 and 2000 onwards). Early hatched chicks were substantially more likely to survive than those hatching later in most years, whereas body condition at ringing had no detectable effect. Ringing recoveries indicated that mortality was highest in mid‐winter, i.e. well after the cessation of paternal care. These results do not support the hypothesis that variation in prey quantity or energy content before fledging is a primary driver of variation in juvenile survival. Rather, it seems that chicks of high‐quality parents are more likely to survive, as high‐quality females tend to lay earlier in the season, and high‐quality males presumably are better able to prepare their chicks to survive their first winter at sea. Very few (4%) Guillemots emigrated permanently before age 3 years, but from age 5 onwards 25–30% of birds annually left the colony or otherwise became unobservable.

Bioaccumulation of polonium 210Po in marine birds

The aim of this work was to determine the 210Po content in marine birds which permanently or temporally live in the Polish part of the Baltic Sea. We chose 11 species of sea birds: three species permanently residing at southern Baltic Sea, four species of wintering birds and three species of migrating birds. The results show that the polonium is non-uniformly distributed in the marine birds. The highest activities of 210Po were observed in feathers, muscles and liver and the lowest in skin and skeleton. Species of birds that eat crustaceans, molluscs, fish and plants (long-tailed duck Clangula hyemalis, white-winged scoter Melanitta fusca) accumulated more polonium than species that eat mainly fish (great cormorant Phalacrocorax carbo, common guillemot Uria aalge) or plants (tufted duck Aythya fuligula). Moreover, about 63% of the 210Po that was located in feathers of razorbil ( Alca torda) and long-tailed duck ( C. hyemalis) was apparently adsorbed, suggesting an external source such as the air. It means that the adsorption of 210Po on the feather surface may be an important transfer from air to water.

Determinants of quality in a long‐lived colonial species

1. In many animal populations a small proportion of individuals produce the majority of surviving offspring, but the underlying mechanisms are unclear. Behaviour may be an important determinant of variation in fitness: 'high-quality' individuals may have enhanced abilities in foraging or predator and parasite avoidance. 2. The role of behaviour in determining variation in quality was examined using the common guillemot Uria aalge, a monogamous seabird with biparental care. Using a novel mixed model approach, we analysed binary data on breeding success of each pair attempting to breed in each year with variables critical to breeding success (timing of breeding; inferred age; breeding experience and success; number of nest sites and partners) as fixed effects. Random effects for year, male, female and each distinct pairing of a male and a female were included in the model, allowing a quality estimate to be derived for each individual and pair. A range of behaviours associated with breeding were examined in relation to these quality estimates. 3. Breeding success declined with timing of breeding, and increased initially with age before declining in old age. It increased with previous successful experience, not breeding experience per se, until senescence effects became apparent. For males, breeding success declined with increasing numbers of mates. 4. The most important behavioural determinants of quality operated at the level of the pair, with the time mates spent together at the site and chick feeding rates both positively related to quality. At the individual level, trip durations and feeding rates were associated with female but not male quality, suggesting that pair quality was operating principally through the female. However, removal of laying date, the most important component in the binomial model, confirmed that the pair effect was much larger than the female effect. 5. This study demonstrates the potential of mixed modelling to determine quality estimates based on long-term breeding histories. The probability of a successful reproductive attempt was explained by the timing of breeding, age, successful breeding experience and number of mates. Behaviour was an important proximate mechanism underlying quality, in particular the foraging abilities of the pair, and the female's contribution to offspring provisioning. In species with biparental care, behavioural correlates of quality operate most strongly at the scale of the breeding pair, because contributions from both individuals are required for a successful outcome.

Evaluating the power of monitoring plot designs for detecting long‐term trends in the numbers of common guillemots

Summary In recent years concerns have been raised regarding the status of the common guillemot Uria aalge in the UK. Numbers have declined in several regions, highlighting the need for continued monitoring of this internationally important population. However, the extent to which the current monitoring scheme is capable of detecting declines and options for improving efficiency has received little attention. We investigated the power of different monitoring design options for detecting long‐term trends in abundance at a colony of guillemots. The ability to detect trends in abundance was reduced by the large temporal and spatial variability in colony attendance. Taking a linear mixed model approach, we obtained details on the sources and sizes of the variance components using count data collected from monitoring plots on the Isle of May, Scotland, and assessed how best to allocate sampling effort in the light of the count variability structure. Our results indicated that trend detection will be improved by counting birds in more plots rather than by increasing the number of counts at existing plots. The revisit pattern of counts at the monitoring plots during the seasonal counting period had little effect on trend detection power. However, given the practical issues associated with counting guillemots, alternative revisit patterns to the current approach are preferred. For a fixed number of visits per plot, power is strongly influenced by the choice of revisit design if the day‐to‐day variation in colony attendance is increased. Synthesis and applications. Aspects of the UK seabird monitoring scheme can be improved. Changes to the allocation of sampling effort and the plot‐revisit pattern will improve both the statistical power to detect long‐term trends and the efficiency of conducting the survey. We stress the importance of considering the structure and magnitude of the count variation in a power analysis because judicious design decisions depend on the relative magnitude of these variance components.

Fish, seabirds and trophic cascades in the Baltic Sea

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 323:233-238 (2006) - doi:10.3354/meps323233 Fish, seabirds and trophic cascades in the Baltic Sea Henrik Österblom1,*, Michele Casini2, Olof Olsson3, Anders Bignert4 1Stockholm University, Department of Systems Ecology, 106 91 Stockholm, Sweden 2Swedish Board of Fisheries, Institute of Marine Research, PO Box 4, 453 21 Lysekil, Sweden 3Mistra, Gamla Brogatan 36-38, 111 20 Stockholm, Sweden 4Swedish Museum of Natural History, Contaminant Research Group, PO Box 50007, 10405 Stockholm, Sweden *Email: henriko@ecology.su.se ABSTRACT: In the relatively simple Baltic Sea ecosystem, zooplankton-feeding sprat Sprattus sprattus is a major food source for breeding seabirds and piscivorous fish, and an important resource for commercial fisheries. Large-scale and long-term ecosystem changes resulting mainly from over fishing and recruitment failure of cod Gadus morhua, which is the main fish predator of sprat, have affected natural-history patterns in a piscivorous seabird, the common guillemot Uria aalge, in a complex way. As the sprat stock increased, leading to lower energy content of fish, common guillemot chick body mass at fledging decreased. However, chick fledging body mass recovered in recent years as the sprat stock diminished, which brought about corresponding increases in sprat weight-at-age and energy content. The cod and sprat fishery affect the common guillemots in the Baltic Sea, but the effects differ depending on the management strategy. KEY WORDS: Cod · Baltic Sea · Fisheries management · Gadus morhua · Common guillemot · Sprat · Sprattus sprattus · Trophic cascade · Uria aalge · Junk-food hypothesis Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 323. Online publication date: October 05, 2006 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2006 Inter-Research.

Population genetics of the common guillemot, Uria aalge, in the North Atlantic: geographic impact of oil spills

The population genetic structure of a species can be an important conservation tool informing us about the potential for genetic loss and the capacity for species recovery. Depending on the nature of population subdivision, it can also provide a means for assessing the source population of dead or injured individuals of unknown geographic origin. This type of information can be partic- ularly useful in instances of large-scale environmental accidents, such as oil spills. Following the wreck of the 'Erika' oil tanker in the Bay of Biscay in December 1999, more than 80 000 seabirds were washed ashore along the west coast of France. The most heavily affected species (80% of all birds) was the common guillemot Uria aalge, a widespread long-lived colonial seabird. In an attempt to evaluate the 'true' geographic extent of this accident, we carried out population genetic analyses using 6 microsatellite markers on samples from 22 breeding colonies in the North Atlantic and on individuals collected after the oil spill. A pattern of isolation by distance was detected among com- mon guillemot populations, but populations were only weakly structured, even at large spatial scales. The low level of genetic differentiation between colonies prevented clear assignments of oiled birds to their population of origin using only the genetic information. The weak genetic structure suggests that little genetic variability was lost during the oil spill and implies a high potential for population recovery via dispersal. However, current gene flow among extant colonies may overestimate the capacity of locally extinct colonies to recover due to the behavioural processes involved in recruit- ment. Clearly, the mortality of tens of thousands of high-latitude seabirds due to oil pollution war- rants ongoing scientific scrutiny and conservation effort.

Rapid-response recorders reveal interplay between marine physics and seabird behaviour

In the marine environment, the distribution of primary producers and higher trophic levels is strongly dictated by the physical characteristics of the water column. Typically, life concen- trates in regions where there are strong horizontal or vertical gradients in temperature or density ('frontal regions'). Top predators, such as diving mammals and birds, target fronts and their associ- ated high biomass of prey. Until now, the study of the interaction between diving predators and the physical environment has been hindered by the lack of adequate instrumentation. The response times of the available animal-borne temperature-depth recorders have been too slow to accurately measure the external environment where the animals are foraging. This limitation has been over- come by the development of a lightweight, rapidly responding temperature-depth logger. The instru- ment was successfully deployed on 2 diving seabird species (the common guillemot Uria aalge and the European shag Phalacrocorax (Stictocarbo) aristotelis) in the North Sea. A method for correcting temperature data for the time lag in the response of the temperature sensor is presented. The find- ings show the potential of this technology in 2 important ways: in providing unparalleled information on the interaction between predators and the marine environment, and as a low-cost method of obtaining high-quality oceanographic data.

Increase in frequency of the bridled morph of the Common Guillemot Uria aalge on the Isle of May, 1946–2000: a return to former levels?

Bridling in the Common Guillemot Uria aalge is a classic example of a stable ratio‐cline polymorphism. Between 1946 and 2000 the frequency of the bridled morph of the Common Guillemot colony on the Isle of May, south‐east Scotland, increased significantly from 3.5% to 5.9%. Demographic data collected between 1982 and 2000 indicated that the average breeding success of a pair including at least one bridled bird was 83.8%, significantly greater than 79.5% in unbridled pairs. Over the same period the average overwinter survival was 96.3% for bridled birds and 94.7% for unbridled birds, but the difference was not statistically significant. A population model showed that the increase in frequency of bridling could be accounted for as a return to an equilibrium level close to the 5.04% observed in 1936. Differences in breeding success contributed relatively little to the increase, which could be explained without recourse to differential selection pressure. The latitudinal variation in bridling suggests that bridled morphs are more cold tolerant than unbridled ones. However, the increase in the frequency of bridled birds on the Isle of May was not associated with any decrease in sea or air temperatures over the period.

Changes in feeding behaviour and reproductive success in the Common Guillemot Uria aalge on the island of Stora Karlsö

We studied the feeding behaviour and reproductive success in Common Guillemots Uria aalge at the island of Stora Karlsö to reveal effects of an increased density of the prey fish Sprat Sprattus sprattus. Feeding rates were almost twice as high in 1998 (4.3 feeds/chick/day) as in 1975 (2.2 feeds/chick/day), and feeding trips were short. The adult birds spent a large proportion of the day together at the site during the chick–rearing period, indicating that the birds were not under significant food stress. Contrary to what we had expected, reproductive success was lower in 1998 and 1999 (0.671, SE = ±0.045, n = 2) than in 1974–1977 (0.803, SE = ±0.010, n = 4) and chick survival and age at fledging was unchanged despite the increase of available prey. We speculate that the condition (nutritional value), rather than the abundance of Sprat, has been important in changing the Guillemots feeding behaviour. The study indicates that the reason for the observed lower reproductive success was lower egg survival, most likely an effect of an increased predation pressure from Herring Gulls Larus argentatus.