Sampling strategies for species with high breeding-site fidelity: A case study in burrow-nesting seabirds

Sampling approaches used to census and monitor populations of flora and fauna are diverse, ranging from simple random sampling to complex hierarchal stratified designs. Usually the approach taken is determined by the spatial and temporal distribution of the study population, along with other characteristics of the focal species. Long-term monitoring programs used to assess seabird population trends are facilitated by their high site fidelity, but are often hampered by large and difficult to access colonies, with highly variable densities that require intensive survey. We aimed to determine the sampling effort required to (a) estimate population size with a high degree of confidence, and (b) detect different scenarios of population change in a regionally important species in the Atlantic, the Manx shearwater (Puffinus puffinus). Analyses were carried out using data collected from tape-playback surveys on four islands in the North Atlantic. To explore how sampling effort influenced confidence around abundance estimates, we used the heuristic approach of imagining the areas sampled represented the total population, and bootstrapped varying proportions of subsamples. This revealed that abundance estimates vary dramatically when less than half of all plots (n dependent on the size of the site) is randomly subsampled, leading to an unacceptable lack of confidence in population estimates. Confidence is substantially improved using a multi-stage stratified approach based on previous information on distribution in the colonies. In reality, this could lead to reducing the number of plots required by up to 80%. Furthermore, power analyses suggested that random selection of monitoring plots using a matched pairs approach generates little power to detect overall population changes of 10%, and density-dependent changes as large as 50%, because variation in density between plots is so high. Current monitoring programs have a high probability of failing to detect population-level changes due to inappropriate sampling efforts. Focusing sampling in areas of high density with low plot to plot variance dramatically increases the power to detect year to year population change, albeit at the risk of not detecting increases in low density areas, which may be an unavoidable strategy when resources are limited. We discuss how challenging populations with similar features to seabirds might be censused and monitored most effectively

Some aspects of bill morphology in relation to ecology in the great tit Parus major

The study concluded that bill size was highly adaptive in that it was related to fitness (in the broad sense) or to some correlate of fitness. The results are considered in terms of existing niche theory and especially of "character displacement" and the "variable niche hypothesis"

The Encyclopaedia of Birds

Oxford475 p. ; Illus. ; 28 c

Breeding biology and feeding ecology of Black guillemots

Since 1974 a population study of Black Guillemots has been conducted on Flatey Island, NW Iceland. The main aspects are summarized below. Population numbers and distribution. The population has been censused several years, also those of 20 islets near Flatey. Information was collected on past status. Great changes have occurred since turn of the century. Last major change began in 1967; the population started increasing of such a scale that it can only have resulted from massive immigration. These changes are thought to be due to rats and Mink. Attendance and population structure. Data was gathered on timing of spring return to nesting areas, status of attending birds, sex ratio, area and nest-site fidelity, pair-bond maintenance, and post-breeding departure. Special attention has been given to the problem of censusing birds attending nesting areas. Breeding biology. Breeding biology variables were quantified and studied on a seasonal basisj;timing of laying, clutch size, interval between eggs, egg size, reproductive output, lengths of incubation and nestling periods. Supernormal clutch and brood experiments were conducted. I looked at length of replacement time of lost clutches, factors influencing timing of laying, its effect and that of clutch and egg size on breeding performance. Egg and chick losses were analyzed. Many interesting population phenomena seem to have been associated with the unusually rapid population increase. Feeding ecology. Analyses were made of prey taken, feeding rhythm and areas, and factors influencing feeding rate and prey selection. Some information was collected on share of the sexes in feeding chicks, and kleptoparasitism. Study was made of chick growth, fledging condition and postfledging survival. Population dynamics. About 3300 birds were ringed, providing basis for determining age at maturity, adult survival, and mortality. Pre-breeding survival was calculated using data from ringing schemes. A population model was constructed for the population, showing magnitude of immigration. Ringing provided data on dispersal.</p

The breeding biology of the Manx shearwater

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.</p

Interspecific competition between Blue and Great tits

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.</p

Sparrowhawk (Accipiter nisus) predation on tits (Parus spp.)

The present research was conducted to assess the effect of Sparrowhawk predation on the tit population of Wytham Woods, Oxford. Chapter 1 describes the history of the Wytham hawk population until this study began and the nesting biology of the hawks during the study. Six to nine pairs of hawks settled in the wood each year, but reproductive success was low due to pesticide contamination. Chapter 2 shows that tit nesting success was reduced when they nested near hawk nests. Circumstantial evidence is presented for reduced tit nesting success throughout the wood since the return of the hawks. Chapter 3 examines rates of hawk predation during the nesting period. Findings indicate that hunting rates and the percentage of the diet formed by tits are regulated by prey availability or vulnerability, with highest predation rates occurring at the time that tits fledge. In Chapter 4 the selection of tits by hawks is analysed. Results indicate that on the basis of brood and physical characteristics adult tits were selected on the basis of availability and juvenile tits were selected primarily by date of fledging. The ratio of adult to juveniles taken differed between years and was thought to be related to the number of tits available per hawk each year. Chapter 5 presents estimates showing that 22-42% of each segment of the tit population was killed by hawks each year. The effects of these losses are discussed, concentrating on the shift in the structure of the tit breeding population which has occurred since the hawks returned to the wood. In Chapter 6 the findings of the study are compared to the findings of previous predator-prey research and the attributes of Wytham as an area for studying predation discussed.</p

Factors affecting the status of the chough in Britain, with observations on its behaviour

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.</p

The habitats and feeding stations of birds in Tsavo National Park, Kenya

The land-bird community of Tsavo East National Park was studied for two years. There are two wet seasons each year: November-December and April- May, but the latter were very poor in both study years. Six habitat types were designated on the basis of canopy cover of woody vegetation. Riverine vegetation was also studied. Most ofthe open habitats extant are derived from woodland by destruction of trees, a process largely caused by elephants. Food supplies are seasonally variable. Insects and grass seeds are most abundant in December and January. In the open habitats the few fruits also peak then, but Commiphora spp., which is only in woodland and is the most abundant fruit in Tsavo, peaks July to September. The ecology of each species of land-bird is described and most are found to be ecologically separate from each other. The annual cycle of numbers of birds in each habitat is closely correlated with the food supplies. In the open habitats birds are almost all insectivorous, granivorous or both, and peak numbers (about five times the dry season density) occur in December and January. In woodland, numbers remain fairly constant through the year and frugivores are more prominent, especially around August. The wet season peaks in all habitats are caused by immigrations of different species, both from elsewhere in Africa most of which come in to breed, and species which breed in the Palaearctic. The August peak in Woodland consists mainly of species which have resident populations. The results are particularly related to the recent habitat changes in the Park. They are compared with other bird studies in African savannas and other components of the Tsavo ecosystem. Finally, the means by which birds are able to survive in the harsh and unpredictable conditions of Tsavo are discussed.</p