Factors affecting survival of fledgling Manx Shearwaters Puffinus puffinus

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During an 11-year period starting in the mid-1960s, large numbers of fledgling Manx Shearwaters Puffinus puffinus were ringed and recaptured on Skokholm Island, Pembrokeshire, Wales. Since it is unlikely that any more will ever be found, this paper summarises the factors that affected the chances of a fledgling surviving long enough to be recaptured on the island. Both the date on which they were ringed and their weight at that time influenced the probability that they would be recaptured. Some implications for the annual cycle are discussed.


Skokholm Island, Pembrokeshire, Wales, is well-known for its large colony of Manx Shearwaters Puffinus puffinus (Smith et al. 2001; Newton et al. 2004). The colony is perhaps the third largest in the world, with only the adjacent island of Skomer (Smith et al. 2001; Newton et al. 2004; Perrins et al. 2012) and Rum, Scotland (Murray & Shewry 2002) having larger colonies. The pioneer studies of the breeding biology of this species were carried out on Skokholm by Ronald Lockley (Lockley 1942). On Skokholm the shearwaters nest in burrows in soft soil. These burrows are concentrated around the perimeter of the island, often in land that slopes towards the sea. Nesting in such places gives the birds easier egress to the sea than from the flatter, central areas of the island from where take-off is more difficult.

As with other procellariiforms, the Manx Shearwater has a long breeding season. The egg is laid in early May, incubation takes about 51 days and fledging about 70 days so the chicks do not leave until late August or early September. The whole breeding season is some 15 days longer than this because after pairing and before laying the female leaves the colony for two weeks to forage for the food for egg-formation (Brooke 1990; Warham 1990).

During the first 50-60 days of the nestling period as the chicks grow they also lay down large reserves of fat, reaching a peak weight which may be as high as 800 g or more, about double the adult weight. During the last fortnight or so in the nest the amount of food brought to the chick drops markedly and many chicks do not receive any food; they lose about 15 g per day. Often the parents leave the colony well before their chick. For the last few nights prior to departure the chicks come up onto the surface and exercise their wings (Perrins et al. 1973). Usually they only do this on dark nights since they are vulnerable to predation by gulls if it is light enough for the gulls to be able to hunt them.

During the years 1964-74, as part of studies of the pattern of return to the island by the pre-breeders, some 70,000 fledglings were ringed and much effort was put into recapturing them when they returned (Perrins et al. 1973; Brooke 1978a, 1990). Large numbers of non-breeding birds return to land on dark nights around the new moon periods in June and July. One-year old birds rarely come to land, but from the age of two, the cohorts become progressively more common until age 5-6 after which they become progressively scarcer (Perrins et al. 1973). This is thought to be largely because once they are paired or start breeding, usually about age 6-7 (Brooke 1990), they are too occupied with breeding to sit around on the surface.

Bird ringing (and recapturing) was stopped by the owner of the island after 1976 and so for the years 1972-74 progressively fewer recaptures were made. Since that date only a few birds found dead have been recorded. The island has recently become available for seabird studies again. Sadly, the shearwaters' shuffling gait rubs the rings against the ground and they abrade badly; the rings used at that time had a limited life-span and needed to be replaced from time to time. Since this could not be done, it is unlikely that any birds ringed prior to 1976 and still alive are carrying rings, even though a few of them are probably still alive; Manx Shearwaters have been recorded as living to the age of 50 (McKee 2003; Brown 2009). Extensive capturing in recent years has not yielded any birds ringed in 1964- 74. This paper re-examines the recapture rates of the fledglings ringed during this period. At the time of the study, there were thought to be around 35,000 breeding pairs of shearwaters on the island (Harris 1966; Perrins 1968).


This was a team effort. Thanks are due to the Wildlife Trust for South & West Wales (its current name) who administered the Bird Observatory and to the many visitors who ringed and recaptured the birds throughout this period. Mary Perrins weighed the shearwater chicks. Dr. C. Cornwallis helped with the analyses presented in Tables 3 & 4. Dr Mike Brooke, the editor and referees commented on earlier drafts of the manuscript.


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