The influence of eruptive movements, age, population size and other factors on the survival of the Shag (Phalacrocorax aristotelis (L.))
Little is known of the causes of natural mortality in birds, mainly because it is difficult to find an adequate and unbiased sample of the dead. This is particularly true of the mortality in sea-birds which occurs away from the breeding grounds outside the breeding season or in the pre-breeding years. Despite these difficulties, and in the absence of formal proof, density-governing adult mortality is presumed by Lack (1954, 1966) and Wynne-Edwards (1962) to be widespread in bird species.
In recent years the gross annual adult mortality rate has been determined for more than a dozen species of sea-bird, but there has been no investigation of the relative importance of the various mortality, factors in relation to age, sex or breeding experience. This is surprising from a logical point of view since many of the theories which account for deferred maturity (Wynne-Edwards 1955, 1962; Lack 1954; Ashmole 1963; Amadon 1964) can only be resolved by a fuller knowledge of the causes of adult mortality. For example, the evolution of deferred maturity was explained in terms of group selection (Wynne-Edwards 1962) before any species had been examined thoroughly enough to exclude a more conventional explanation in terms of natural selection (Lack 1966). This is analogous to studying the evolution of intraspecific variation in clutch size without investigating the genetics of clutch size determination (see Birch & Ehrlich 1967).
Some investigations of the dispersal or mortality of species in the genus Phalacrocorax have already been made. Kortlandt (1942) gives an account of the population dynamics of the Dutch population of the cormorant P. carbo (L.) and includes estimates of the mortality in relation to sex and age, though he considered that the mortality rate would be doubled in a severe winter. The average annual adult mortality rate of the breeding adults was 10%, but difficulties were encountered in corrections for the loss of rings and through ring wear. Only a few of the breeding adults were individually colour-ringed, so that the recognition of individuals was very restricted. Coulson & White (1957) estimated the annual adult mortality of the shag (P. aristotelis) as 13±9% using Jackson's negative method as interpreted in the formulae of Bailey (1951, 1952). This method assumes an absence of emigration and is unsuitable for examining annual variation or for dealing with the individuals which could not be caught. The shags used in this pilot study were not of known age or sex, nor was their breeding success known. Snow (1960) gives an annual mortality estimate of 7% for adult shags on Lundy, Bristol Channel, but it was based on a sample of only 28 risk-years.