Seabird breeding populations have been experiencing change over the last 40 years with low reproductive success being associated with poor foraging conditions. A comparison of the breeding ecology of Arctic and Common Terns showed significant differences in reproductive strategy and output associated with differences in their sensitivities to changing conditions. Both clutch size and productivity were consistently lower in Arctic Terns than Common Terns. Chicks of both species hatched asynchronously with mortality increasing down the brood hierarchy. Younger sibling survival was impacted both by seasonal conditions and by elder sibling survival, although to varying degrees depending on species and hatching order. Arctic Terns foraged at a higher rate but on less energy rich prey and on a less varied diet. Both species increased provisioning and decreased parental attendance at the nest as chicks grew older, corresponding with the changes in chicks’ thermoregulatory abilities and energy requirements. Provisioning of chicks was primarily increased by selection of higher energy prey rather than through increased provisioning rate. Diurnal rhythms in either provisioning rate or diet were seen in both species, primarily associated with changing behaviour of prey fish. Temperature and precipitation affected provisioning, but adults maintained energy delivery to chicks. Increase in wind speed negatively impacted chick provisioning despite increased foraging effort. Chick mortality was linked to weather and feeding conditions, with high mortality linked to windy conditions and low provisioning. Yearly differences in productivity and chick mortality were associated with provisioning, and low parental attendance indicated poor foraging conditions. The reproductive output of both species is sensitive to declining foraging conditions and increased severity of weather conditions. The data implies that the sensitivity of Arctic Terns is greater than that of Common Terns, and that conditions around Coquet Island are sub-optimal for Arctic Tern breeding.

The principal object of this present study was to examine and compare the biology of four species of terns occurring in the same geographical region, and often in close proximity.

In this thesis I explore the importance of using different habitat types in winter for a migratory shorebird, the Icelandic Black-tailed godwit Limosa limosa islandica. Godwits use a mixture of estuarine and freshwater wetland habitats across all major wintering regions. The Irish wintering population of black-tailed godwits makes up c, 20% of the flyway population, and these birds use coastal mudflats and adjacent grassland habitats throughout the winter. In addition, very large numbers congregate on inland wetland ‘callows’ in late winter and spring. The Icelandic godwit population is currently increasing, however, numbers of godwits wintering in Ireland have not increased as rapidly as elsewhere in the winter range over the last 40 years. Using detailed behavioural studies, I show that grasslands in Ireland provide essential resources for godwits, as prey consumption rates on the mudflats are insufficient to meet energetic requirements. Despite their importance, grasslands receive virtually no protection. Surveys of field selection by godwits throughout southern Ireland indicate that small, enclosed fields are avoided but that godwits use large, open fields in both urban and rural dominated areas. Levels of urbanisation also have little impact on the use of resources on mudflats or grasslands by godwits. Candidate fields for inclusion within protected areas would therefore ideally be large, open fields situated throughout urban and rural areas. Finally, I explore the evidence for increasing use of grassland habitats by godwits wintering in the east of England, where rates of population increase have been particularly high and annual survival has been estimated to be lower than other winter regions. Count information and records of individually marked godwits indicate a rapid shift in habitat use since the 1990s, with inland grasslands now being used by large numbers of godwits. These sites are also being used progressively earlier in the winter, which may indicate that estuarine prey resources are also limited in the east of England. Thus, while there is evidence that estuarine habitats are preferred by godwits in winter, grasslands are also essential for maintaining a large proportion of the Icelandic black-tailed godwit population, and improving protection of these habitats should thus be a priority.

Ringed Plovers Charadrius hiaticula L. were studied on South Uist, Outer Hebrides, from 1.11.83 to 30.12.86. Internationally important numbers were present throughout the year. Winter to summer residency was approximately 68%. The winter population was male-biased, and concentrated on 'feeding beaches' near rocky features of the coast. Site fidelity was high. The feeding beach invertebrate fauna was of low diversity, but exceptionally high density and biomass. Ringed Plover weights showed a variable midwinter peak. Individuals which disappeared between winters were lighter per unit body size than those which did not. Spacing behaviour varied from non-aggressive 'free-feeding' to the sharing of territories by individuals which then bred together inland. The probabilities of individuals holding territories varied with sex, age, size and environmental conditions. Fewer long-term to intermittently territorial males disappeared between winters than would have been expected from their proportion in the population.

The utilisation of urban resources by the Herring Gull was studied between 1973 and 1976 in Northeast England. This involved a study of refuse tips as a winter food supply, and the use of inhabited buildings as nesting sites. The local breeding population is small, and there is an influx of gulls from east Scottish and Norwegian colonies into Northeast England during the winter months. Herring Gulls caught during winter could be sexed by bill depth alone; wing length was used as an indicator of geographical origin. Gulls fed at refuse tips in the study area throughout the winter months. There was a predominance of adults over immatures at refuse tips in winter, while immatures predominated over adults around inshore fishing vessels in the same area. There was considerable fluctuation in the number of gulls present at tips on different days, related to weather factors and feeding conditions elsewhere. Immature Herring Gulls were less constant to a particular feeding area than adults, and adult females less so than adult males. Individual Herring Gulls did not feed at refuse tips every day, and refuse did not constitute the major or necessarily the only food source for these birds in winter. There were two different feeding areas used by gulls on the refuse tips which differed in the abundance and availability of food: individual birds consistently used one or other area, and proportionally more adult females than males used the secondary area. The number of nesting pairs/ pairs and nest sites used by Herring Gulls nesting on buildings in Sunderland and South Shields were monitored. Their breeding success was higher than in more "natural" colonies, possibly due to the nature of the nesting sites. A national census of gulls nesting on buildings in Britain and Ireland was organised in 1976, to measure the growth and spread of rooftop nesting: the number of Herring Gulls nesting on buildings was found to be doubling within 5 years, and the number of Lesser Black-backs within 3 years. The potential public health hazard presented by gulls nesting on buildings was reviewed.

Aspects of the breeding biology of Lapwing, Oyster- catcher and Redshank were studied on a dry, cattle-grazed saltmarsh, Rockcliffe Marsh, in Cumbria. The vegetation of the marsh was predominantly graminoid, due to the influence of grazing and trampling by cattle. There was a halosere from the landward Lolio-cynosuretum to the seaward Puccinellietum. Invertebrate abundance and biomass declined across the halosere, as did grazing intensity, which was indicated by cowpat density. Cowpat density was positively correlated with the abundance and biomass of Diptera and total invertebrates. The proportion of dung-associated invertebrates varied across the halosere, but over 80% of Diptera in each vegetation type were dung-associated. The proportion of eggs plus chicks of each species which was trampled was positively correlated with cowpat density, indicating that cowpat density was a valid measure of grazing intensity. Each wader species nested at a higher than average cowpat density where the mean cowpat density was low, to maximise food availability, and at a lower than average cowpat density where the mean cowpat density was high, to minimise the risk of trampling. Lapwing nest density was positively correlated with cowpat density (proximate factor) and total invertebrate biomass (ultimate factor). The main prey of adult and chick waders were dung-associated invertebrates. The proximate factors involved in breeding area and nest-site selection by the fore-mentioned wader species and Dunlin and Ringed Plover were elucidated by a multivariate comparison of nest and non-nest samples. The proximate factors were typically related to those features associated with a grazed habitat, e.g. tussock abundance, cowpat density, and with the avoidance of inundation, e.g. distance to nearest creek and plateau edges. The proximate and ultimate factors were discussed with reference to their implications for breeding wader habitat management.

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Breeding systems encompass a diverse range of mating and parental care strategies that develop in relation to reproductive success. However, which factors determine the reproductive strategy that will achieve success, in terms of Darwinian fitness, is still a controversial issue in theoretical and empirical behavioural ecology. I argue in this dissertation that animal reproductive strategies depend on variation in the social environment, which is influenced by the number of available mates in a population. The purpose of this thesis was therefore to investigate the role of mating opportunities, in breeding system evolution by testing current theoretical models. First, I studied different contributory factors that have been proposed to influence variation in mating opportunities, including, brood attendance (chapter two) courtship (chapter three) and pair bond stability (chapter three) in four closely related plover species of genus Charadrius: Kentish plover, C. alexandrinus; Kittlitz’s plover, C. pecuarius; Madagascar plover, C. thoracicus; and white-fronted plover, C. marginatus. Second, I investigated whether mating opportunities differed between these plover species that exhibit different mating and parental care patterns yet have similar life histories and ecology (chapter three). Third, I also studied the spatial movements of plovers when searching for a mate (chapter four). Consistent with theoretical models, mating opportunities were highly variable between species suggesting that mating opportunities are an important component of the social environment. Since mating opportunities are linked to operational sex ratios (OSR, the ratio of sexually active males to females), this suggests that OSR exhibits substantial variation among closely-related species that may influence the direction and intensity of courtship and mate searching behaviour (chapter three and four) and whether to care for or desert offspring (chapter two). These results from wild populations suggest that a demographic property of the populations, OSR, influences mating opportunities, and in turn, facilitate different intensities of sexual selection and parental care patterns.