The Breeding biology and survival of the Razorbill Alca torda L

The purpose of this study was to collect information on the breeding biology and life history of the Razorbill for comparison with similar studies in other species. An estimated 205,000 pairs of Razorbills breed in colonies either side of the northern Atlantic; about 70% of the birds are found in British and Irish colonies (Chapter 1).
The study was conducted on the island of Skokholm, Pembrokeshire (Dyfed), South Wales between 1971 and 1973, with additional observations in 1970 and 1974. Adult Razorbills and varying numbers of chicks have been ringed on the island nearly annually since 1934 and since 1963 the chicks have been given, colour rings to denote age classes. Adults trapped in the present study were colour ringed so that they could be identified without repeated retrapping. The sexes proved impossible to distinguish on body measurements but head size provided an alternative method of sexing adults (Chapter 2).
Colony attendance was recorded (Chapter 3), both during the pre-laying period and the breeding season. Breeding and non-breeding birds were separated whenever possible and diurnal and seasonal attendance at two types of colony compared. The effect of prevailing weather on attendance was evaluated.
Courtship behaviour and mating frequency were recorded (Chapter 4A). Most birds returned to their natal colony to breed and remained there, usually retaining the same nest site. Birds appeared to keep the same mate so far as the annual mortality allowed (Chapter 4Bi). Laying (in May) occurred on average 10-13 days later in 1972 than in 1971 or 1973 (Chapter 4Bii). Birds advanced their laying date with age so that those breeding late in the season were mainly young birds. The little information available on the seasonal abundance of the razorbills' chief food species Ammodytes spp., suggested that the onset of breeding coincided with an increase in available food which appeared to be the main factor determining the timing of the breeding season. The development of synchronised colony attendance during the pre-laying period, was also thought to play an important part in timing breeding.
Length, breadth, weight, shell thickness and pigmentation pattern were measured on samples of eggs in the 3 years. Shell thickness in 'fresh' eggs from Skokholm appeared similar to that of museum eggs collected before 1940. Shell pigmentation patterns were consistent from one year to the next in eggs laid by the same female. A seasonal decline in egg volume was recorded, apparently related to the age structure of the population. Young birds laid small eggs, but egg volume increased with age. On average 30% of all eggs laid failed to hatch and more than 70% of this loss was attributable to gulls and particularly Jackdaws Corvus monedula; the latter first bred on the island in 1965 and are increasing annually. About a quarter of the eggs lost, mainly those laid and lost early in the season, were replaced. At least some of the two egg clutches found were produced when a replacement egg was laid for a first egg which had become chilled during incubation.
Incubation was on average 35 days and the nestling period averaged 17 days. About 7% of all chicks hatched failed to fledge (Chapter 4Biii). Most losses occurred in the first five days of life whilst chick weight was still related to the size of the egg from which it hatched. Behaviour at fledging (the female of the pair usually took the chick to the sea) is discussed in relation to the brevity of the nestling period and similar behaviour in other species.
The frequency and composition of feeds to the chicks were examined and growth recorded by regular measurements of weight, feather length and tarsus length. Growth rates were fastest in 1973. Growth was independent of hatching date although chicks? from early eggs were the heaviest at fledging. As this was not associated with the seasonal trend in egg size,