The young of wading birds that breed in some parts of the arctic hatch when their insect food supply is most abundant. This appears to be the ultimate factor determining the date of egg‐laying. In various other arctic birds the date of breeding may be influenced, directly or indirectly, by the time at which snow or ice melts. The Joint Biological Expedition to N.E. Greenland 1974 studied Ringed plovers Charadrius hiaticula, Dunlins Calidris aplina, Sanderlings Calidris alba, Knots Calidris canutus, and Turnstones Arenaria interpres breeding in various areas within 80 km of each other. In 1974 the thaw was two or three weeks later than normal in the region. The time at which snow cleared and the dates of breeding of the waders differed considerably between valleys: breeding was delayed in valleys that cleared late. In all areas egg‐laying ceased early in July, probably because chicks hatching from eggs laid later would not be ready to migrate by the end of the summer. Hence the mean date and length of the egg‐laying period were determined by the date of snow clearance from the breeding areas. Studies of the potential food resources indicate that the waders in N.E. Greenland do not appear to time their breeding so that the young hatch at some peak of food abundance. Sufficient food resources are probably available over a long period of time.
Published observations on the breeding biology of the Razorbill A k a torda either have been limited to those obtained during a single breeding season (e.g., Paludan 1947, Plumb 1965, Bedard 1969, or have formed part of a more general study of seabirds (e.g., Belopolski 1957). My study was carried out during 1971-73 on a population of 470-480 pairs nesting on the island of Skokholm, Pembrokeshire. Many of the birds on the island were already ringed when the study began. Recoveries of dead ringed birds have provided information on migration patterns (Lloyd 1974). Analysis of information from ringed birds recaptured both before and during the present study has shown that young birds breed for the first time at four or more usually five years of age, the annual survival of breeding birds is 90-92% and that only about 13 of the young produced each year by 100 breeding pairs survive to breeding age (Lloyd & Perrins 1977). It was also demonstrated experimentally in 1973 that some Razorbills on the island could rear twin chicks to fledging (Lloyd 1977).Most of the Razorbills on Skokholm nest in clearly defined groups, mainly on the north and west cliffs; separate groups, usually situated on a point or in a bay, are referred to below as 'colonies'. Data were collected by daily visits to selected colonies in different parts of the island, referred to here as 'study colonies', in which about 56% of the total population nested. Other colonies were visited infrequently and were therefore less disturbed than the study colonies. Study colonies situated on cliffs, where the Razorbills nested on inaccessible open ledges, permitted visual records only, whilst those in which the birds' nest-sites were mostly accessible (i.e., under rocks or in crevices) provided egg measurements and chick weights, and allowed the capture of adult birds.The timing and success of breeding were recorded in all study colonies. As nest sites were visited at most once a day, records of laying and hatching dates (taken to be the day upon which the egg or chick was first discovered) were accurate to within 24 hours. The cause of loss of eggs and chicks often remained speculative.The length and breadth of eggs were measured with vernier calipers to the nearest whole millimetre, and the egg volume was calculated (after Barth 1953) using an 'egg constant' of 0.54, obtained from a sample of 64 freshly laid eggs both weighed and measured in 1971. Most Razorbills laid within a 4-5 week period each year and for the purpose of this analysis the laying season was arbitrarily divided into three 'laying periods' so that the middle period covered the time during which about half of all eggs were produced, and the first and last laying periods each covered a quarter of layings. Further division into 'laying intervals' was also made for detailed analysis (see Table 1).Samples of Razorbill chicks were weighed at the same time each day (or on alternate days), using a Pesola spring balance, to the nearest whole gramme. The calculated slope of the regression line through ...
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