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.

Advisor: Perrins, C