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Great tits (Aves: Passeriformes; Parus major) and blue tits (Parus caeruleus) nested in boxes in Wytham Woods near Oxford. The breeding densities of both species were limited by the availability of nest sites. The larger great tits were dominant in obtaining nest boxes. This was most important where breeding sites were scarce. The two species did not maintain interspecific territories or interfere with interspecific nest site spacing beyond the immediate vicinity of the nest. Blue and great tit numbers fluctuated in parallel where nest sites were not limiting resources. Annual changes in breeding numbers of great tits were negatively related to blue tit breeding density but great tit density did not seem to affect changes in the blue tit population. Overlap in the feeding sites of blue and great tits was greatest during the summer and interference competition was lowest at this time. The nestling diets of the two species were very similar. Despite an apparent abundance of food for nestlings, adults were pressed to feed large broods. Food for nestlings was probably a limiting and depletable resource. The date of clutch initiation of great tits, but not blue tits, was retarded at high densities of blue or great tits. The clutch sizes of both species were probably negatively affected by high breeding densities of congeners but the results were not clear cut. Heavy great tit fledglings are most likely to survive to breed. Great tit fledglings were heavier at low densities of blue tits. An experiment in which blue tit young were removed from a section of the Woods, showed that great tit nestlings were heavier and developed faster, and that female condition was better, than in a control section or section where blue tit broods were supplemented. In terms of resource competition, blue tits were the better scramble competitors and great tits the better interference competitors.

Chapter 1 is purely introductory and gives a brief account of the taxonomic status of the subject of this thesis, the Manx Shearwater Puffinus puffinus. The main study area, Skokholm Island, Pembrokeshire is described and a general account of what is already known of the breeding biology of the Manx Shearwater is provided as a background to the more detailed studies described in the present work which was continued over five breeding seasons, 1973-1977. In Chapter 2 I demonstrate that male and female Manx Shearwaters differed in the length of their bills and tarsi but not in wing length. However most of the chapter is concerned with the weights and measurements of shearwaters, most of which were of known age but unknown sex, caught at the colony by night. In 1973 and 1974 the weight of all age groups was highest in March and then declined to a minimum in June and July. Weight increased slightly in August. It was generally true that the older a bird was, the heavier it was in any particular month, and this effect appeared to hold good until the birds were 8-10 years old. Unlike weight, bill and wing length did not alter with age. The implications of these results are discussed in the light of current hypotheses concerning the delayed onset of breeding shown by many seabirds including the Manx Shearwater. In the pre-laying period, covered by Chapter 3, both male and female Manx Shearwaters lost weight up until about two weeks before laying. Males lost weight more rapidly than females and this was related to the fact that males visited the burrow more regularly. In the two weeks prior to laying the male continued regularly to visit the burrow at night but the female was virtually absent from the colony; it appears that she may travel into the Bay of Biscay to feed during this period of absence. In the pre-laying period the weight of breeding birds was not different from the weight of birds which have bred formerly but which were not known to be breeding during the current season. However, breeding birds tended to be heavier than birds which started to breed in a future year. To test the possibility that young birds may be prevented from breeding by a shortage of burrows artificial burrows were dug, and some were occupied by young birds, probably breeding for the first time. A burrow-blocking experiment was also carried out. The possibility that competition for burrows was greater in an area of higher as opposed to lower burrow density was investigated by comparing the pre-laying attendance pattern of breeders in the two areas. No difference was found. Chapter 4 shows that the breeding success of newly-formed pairs was lower than that of established pairs, mostly because newly-formed pairs were less successful at incubation. The lower success of new pairs was not due to the new pairing per se but to the fact that such new pairs tended to include birds without previous breeding experience. Thus experienced birds may avoid the disadvantageous consequences (to breeding success) of forming a new pair if they mate with another experienced bird, and this they did. Divorce and change of breeding burrow were both more likely after a breeding failure than a success. Both the laying date and egg volume of individual female Manx Shearwaters varied little from year to year, once the first few years of breeding were passed. I am unable to reconcile this finding with Perrins' (1970) suggestion that the laying date of the female Manx Shearwater is determined by the difficulties she may encounter early in the season in building up sufficient food reserves to form the egg. Instead I propose that, although early laying would be advantageous from the point of view of chick survival (Perrins 1966), the shearwaters do not lay earlier because of the difficulties that would be encountered in successfully incubating an early egg. Evidence supporting this idea is presented. In each of the four study years the fledging weight of chicks declined as the season progressed, as described in Chapter 5. Various lines of evidence, including an egg-swapping experiment, support the view that this decline was mostly due to a deterioration of feeding conditions late in the season, rather than to a tendency for parents less proficient at rearing heavy young to breed later. It seems that date of fledging and weight at fledging may both influence the fledgling's chances of survival but I am unable to determine the relative importance of these two factors. Different pairs of shearwaters differed in their ability to feed chicks, but chick-feeding performance was not related to age or breeding experience. Chapter 6 evaluates the parameters necessary for the construction of a life table. Of the chicks which fledge from Skokholm at least 25andnbsp;% survive to breed on Skokholm, whilst adult survival is about 90andnbsp;%. About 20andnbsp;% of those adults known to be alive and to have bred previously do not breed in any one year- The age of first breeding, currently about six, has increased over the past ten or fifteen years. Among the birds which have been ringed as chicks on Skokholm and which bred there during the study period there was a 2:1 ratio of males to females. I suggest that about half the females fledging from Skokholm settle to breed in other colonies. The body measurements (used as an indicator of sex) and abundance of Skokholm-ringed birds on nearby Skomer Island support this hypothesis. The Manx Shearwater life table is therefore constructed to take account of immigration to and emigration from the Skokholm colony. Recruitment to the breeding population and loss by mortality are roughly equal. Chapter 7 shows that the calls given by male and female Manx Shearwaters were different. The response of other shearwaters to these calls was investigated by means of playback experiments. Females recognized the calls of their male mates but I am unable to show a selective response of males to the calls of their female mates. This difference is considered to be related to the different roles of the two sexes and to be associated with the fact that most calls heard from the ground were given by males whilst most calls uttered in flight were probably given by females. There is no evidence that nestlings can recognize the calls of their parents. The value of colonial breeding is considered in the concluding Chapter 8. It seems that Manx Shearwaters in the dense Main Colony experienced a lower rate of predation, but they did not have greater reproductive success than those breeding in areas of lower burrow density elsewhere on the island. Although nesting habitat on Skokholm is not fully utilised there may be a limited supply of breeding burrows available. This would create competition for burrows which, together with competition for food, is suggested as an important influence on the breeding biology of the Manx Shearwater. There are four appendices. The first shows that birds first caught in the colony at two years old were caught earlier in the year when three years old than those which were caught for the first time at three. Similarly, birds which have been caught when two or three years old were caught earlier in the year when four years old than those birds which were caught for the first time at four. These differences in time of return to the colony appear not to be associated with sex. Appendix 2 discusses the relationship between the body size of offspring and their parents. In the Manx Shearwater it appears that about three-quarters of the phenotypic variance of body size is due to genetic causes, and may therefore be inherited. Appendix 3 describes a simple experiment designed to test the possibility that vision may be one sense used by Manx Shearwaters attempting to locate their breeding burrow. The result of the experiment was positive but more extensive tests would be required to assess the relative roles of vision and any other senses that may be employed in burrow location. Appendix 4 describes an unsuccessful visit to the Basque coast of northern Spain to assess the status of the Manx Shearwater in the south-east corner of the Bay of Biscay in the pre-laying period, late April. I tentatively suggest that it is only in exceptional circumstances that many shearwaters feed south of about 46° N.

The marine birds comprising the order Procellariiformes are an ancient and diverse assemblage. A large proportion are both nocturnal and burrow-nesting at their breeding colonies, where in sharp contrast to their behaviour at sea, they are highly vociferous. Virtually nothing is known regarding the adaptive features of this process. Various aspects of vocal behaviour have therefore been investigated from 1981 to 1983 for seven species at breeding stations in both the boreal and sub-tropical North Atlantic Ocean. In addition, the problem of how these birds return to their correct burrows at night has been considered. A detailed study was conducted on the Manx Shearwater Puffinus puffinus. Various approaches showed that immatures contribute most to the calling heard. Males establish and defend burrows, but both sexes partake in aerial calling. Calling at ground level serves both sexual and territorial functions, whereas aerial calling is probably mainly concerned with sexual advertisement. Some males are more silent in flight than others. These probably represent birds as yet without burrows, perhaps the youngest age classes. Flighting activity probably expedites the acquisition of burrows and mates in these birds which are awkward on land, and aerial calling probably improves signalling efficiency in attracting mates. Six other species (Bulweria bulwerii, Calonectris diomedea, Puffinus assimilis, Hydrobates pelagicus, Oceanodroma castro, Pelagodroma marina) were studied. As with Puffinus puffinus, sexual differences in voice exist for all except Bulweria and Pelagodroma, which also lack aerial calls. Thus a functional link exists between flight calls and their sexual divergence. Selection probably favours such divergence in species where males leave burrows to display in flight; the sexual identity of those species whose males do not is unambiguous as they remain in burrows and call. The calls of Puffinus puffinus and Hydrobates pelagicus were compared at local and regional levels. Divergence exists between but not within islands. Vocal drift in Puffinus puffinus is also apparent after several years. The calls of the nocturnal Procellariiformes are reviewed and discussed in relation to their systematics. The potential use of calls in petrel systematics is also evaluated and shown to be useful. Observations on Puffinus puffinus showed that olfactory and auditory cues are not used for burrow homing. Experiments also confirmed this, and point to sufficient visual development in this species, although other senses may be emphasised in different ecological situations.

Following a large kill of Black Guillemots in the Esso Bernicia oil spill in Shetland, this study was initiated to determine the population size and distribution of the species at different times of year and to investigate its ecology. The results will enable oil spill contingency plans to be made for this species and provide a base-line against which future work can be compared. This is the first major study of Black Guillemots in the UK. There were large diurnal and seasonal fluctuations in colony attendance patterns which were investigated in relation to various factors. Reliable census techniques were devised and suitable stretches of coastline were established as monitoring units. Black Guillemots rarely foraged in water more than 40-50 m deep, and seldom moved more than 3 km from the nest during the breeding season. The proximity of suitable breeding sites to shallow water feeding grounds accounted for the fairly regular breeding dispersion. On islands with mammalian predators only nest sites in inaccessible cliffs were used. In many areas breeding numbers were limited by nest site availability. The distribution at other times of year was more irregular. After breeding adults congregated in traditional moulting areas where they became flightless for 4-5 weeks. These flocks of up to c.600 adults constitute vulnerable "hot spots" at which the effect of oil pollution would be most severe. In winter there was some movement away from exposed coasts, resulting in concentrations in more sheltered, inshore waters. A wide range of fish and invertebrates were eaten by adults, largely reflecting the diversity of inshore benthic communities. There appears to be a greater dependence on invertebrates during the winter months, when fish are scarcer. Chicks were fed almost exclusively on fish: their diet composition was determined primarily by local availability, although some adults specialised on particular species. When large gadoid fish became available in late summer adults were able to provide more energy per feed and consequently chicks were heavier at fledging. Breeding parameters were quantified and analysed in relation to various factors. Breeding success and chick growth compared very favourably with results of studies elsewhere, indicating that conditions for breeding were good for Black Guillemots in Shetland. In a year of more favourable environmental conditions a greater proportion of inexperienced birds were able to reach the condition threshold for breeding. These birds tended to lay single-egg clutches, in sub-optimal sites at which egg predation was heavy and breeding success low. Some pairs of Arctic Skuas benefit ed considerably from kleptoparasitism of chick feeds at larger colonies, but Black Guillemot breeding success was not affected and chick growth was only depressed at high rates of piracy. Overall, conditions in Shetland are favourable throughout the year for this seabird, whose ecology is moulded to the exploitation of an inshore feeding niche.

This thesis aims to fill gaps unfilled by the recent upsurge in Chough studies. These are: the classification and functional analysis of aspects of their behaviour, examining feeding preferences in relation to land management, finding how literary attitudes towards them developed and obtaining quantitative insights into their past distribution to apply to possible explanations for their status changes. Three pairs near Llangranog, Dyfed were observed and findings were complemented with analysis of RSPB film offcuts. All observed behaviours were defined and classified according to social context and frequency of use. Of 45 behaviours seen, 21 had not previously been recorded. Single-link cluster diagrams were presented to illustrate observed postures and comparable corvid postures. Three aspects of their social behaviour were further examined: Food-begging by juveniles and females was related to time since fledging and copulation date respectively. Begging success depended on technique repertoire, which increased with time; increased foraging group size decreased the individual's vigilance time spent, with an earlier detection of potential tourist disturbance; wing-flicking did not occur with short flights nor after flights due to disturbance. It was frequent after landing while as frequent before take-off as during foraging excepting tourist disturbed flights. Ritualised forms of wing-flicking and bill-wiping formed the basis of Chough communication. Flow diagrams and canal models describing the functional organisation of these behaviours are presented. Choughs feed on short, mainly grass swards maintained by grazing, exposure and occasionally, cutting. When given the Jackdaw's old name, Choughs acquired their thieving reputation and were also regarded as fire-raisers until the 1830's. Distribution and abundance from 1750 to 1982 were estimated at county and national level. Their status was not significantly affected by climate. Grazing artificially boosted their numbers, agricultural changes and persecution from 1750 until 1940 caused their decline.

Following a large kill of Black Guillemots in the Esso Bernicia oil spill in Shetland, this study was initiated to determine the population size and distribution of the species at different times of year and to investigate its ecology. The results will enable oil spill contingency plans to be made for this species and provide a base-line against which future work can be compared. This is the first major study of Black Guillemots in the UK. There were large diurnal and seasonal fluctuations in colony attendance patterns which were investigated in relation to various factors. Reliable census techniques were devised and suitable stretches of coastline were established as monitoring units. Black Guillemots rarely foraged in water more than 40-50 m deep, and seldom moved more than 3 km from the nest during the breeding season. The proximity of suitable breeding sites to shallow water feeding grounds accounted for the fairly regular breeding dispersion. On islands with mammalian predators only nest sites in inaccessible cliffs were used. In many areas breeding numbers were limited by nest site availability. The distribution at other times of year was more irregular. After breeding adults congregated in traditional moulting areas where they became flightless for 4-5 weeks. These flocks of up to c.600 adults constitute vulnerable "hot spots" at which the effect of oil pollution would be most severe. In winter there was some movement away from exposed coasts, resulting in concentrations in more sheltered, inshore waters. A wide range of fish and invertebrates were eaten by adults, largely reflecting the diversity of inshore benthic communities. There appears to be a greater dependence on invertebrates during the winter months, when fish are scarcer. Chicks were fed almost exclusively on fish: their diet composition was determined primarily by local availability, although some adults specialised on particular species. When large gadoid fish became available in late summer adults were able to provide more energy per feed and consequently chicks were heavier at fledging. Breeding parameters were quantified and analysed in relation to various factors. Breeding success and chick growth compared very favourably with results of studies elsewhere, indicating that conditions for breeding were good for Black Guillemots in Shetland. In a year of more favourable environmental conditions a greater proportion of inexperienced birds were able to reach the condition threshold for breeding. These birds tended to lay single-egg clutches, in sub-optimal sites at which egg predation was heavy and breeding success low. Some pairs of Arctic Skuas benefit ed considerably from kleptoparasitism of chick feeds at larger colonies, but Black Guillemot breeding success was not affected and chick growth was only depressed at high rates of piracy. Overall, conditions in Shetland are favourable throughout the year for this seabird, whose ecology is moulded to the exploitation of an inshore feeding niche.