Performance of the roseate tern population breeding in north-west Europe - Ireland, Britain and France, 1960-94

The breeding population of the roseate tern in north-west Europe - Ireland, Britain and France - suffered an almost continuous decline from a peak of 3812 nesting pairs in 1968 to 561 pairs in 1987, a reduction of 85%. The population then stabilised at 580-590 pairs, increasing to 686 pairs in 1994. Data are presented to suggest that a major contributory factor responsible for the period of steepest decline in the Irish and British population from 1968 to 1972 was increased mortality on the west African wintering grounds. Significantly higher recovery rates of ringed birds from west Africa occurred during the main period of decline. The population decrease affected colonies throughout the breeding range, suggesting a common factor at work. Productivity appears to have been normal during the population decline, and disturbances by human interference, predators, habitat loss and climatic effects do not appear to have been responsible for the period of steepest decline, but may have been contributory factors later as the population declined further. Levels of organochlorine residues in eggs and fish prey were low and unlikely to have had any effects on mortality or productivity rates. There is no evidence to suggest that the birds 'lost' during the population decline had moved away to new or unknown breeding grounds.