Breeding ecology of the Merlin Falco columbarius in Shetland

The breeding Merlin population in Shetland declined by about 50% between 1981 and 1983, and in 1982 breeding success was low at only 50%. Since then the population has substantially recovered and on average 68% of nesting pairs have been successful. However, 97% of nest failures occurred at the egg stage, with 59% apparently due to egg breakage by the Merlins themselves. The causes of the reduction in population and breeding success are not known; adverse weather conditions and nest predation were unlikely to have been significant, and levels of organochlorine residues and PCBs in addled eggs were low in comparison with Britain as a whole. However, mercury levels were the highest recorded in Britain and may have been implicated in nest failures.