Patterson I.J., Makepeace M. & Williams M. (1983) Limitation of local population size in the Shelduck. ARDEA 71 (1): 105-116

This study describes changes in the size of a local population of Shelducks, measures population processes which contribute to these changes and discusses the limitation of population size. The Shelduck population of the Ythan estuary, Aberdeenshire, Scotland fluctuated over 17 years, with peaks of about 230 in 1962 and 1977 and a low of 141 in 1970. Territorial and non-territorial (flock) birds fluctuated similarly but territorial numbers were more consistent (although their dispersion within the estuary changed). Both territorial and flock birds included a wide range of age groups but yearlings made up almost half of the flock, the proportion being significantly higher after years of greater production of fledglings. There were more yearling females than males in the flock and three-quarters of Ythan-reared ducklings which survived and returned to their natal area were female. Most Shelducks became territorial by 3 years of age. Mean annual survival of adults was higher in territorial birds (88.6 per cent) than in non-territorial ones and was lowest in newly-ringed birds. Only 18.7% of ringed fledglings returned. Transient or immigrant birds were detected in the population, especially in winter but no territorial or breeding adults seemed to emigrate. Some Ythan-reared ducklings settled to breed elsewhere. Nesting success of whole clutches (mean 40.5%), fledging success of ducklings hatched (34.6%) and the number of fledglings per territorial pair (0.99) all decreased significantly with increasing number of territorial birds. About 10% of the eggs in successful nests failed to produce day-old ducklings. Changes in flock size were not significantly related to the previous year's production of fledglings or their survival but flock size was increased by experimental provision of food, suggesting that immigration was influenced by food supply. Changes in the number of territorial birds were not correlated with flock size in the previous one or two years. The Ythan Shelduck population was probably not producing enough locally-bred recruits to replace annual losses, and in most years the population was maintained by immigration. This study suggests that limitation of population size may occur at two levels, recruitment to the flock, affected by food supply and recruitment from flock to territory, affected by territorial behaviour.