Use of ring recoveries to monitor long-term changes in the survival rates of British and Irish Cormorants Phalacrocorax carbo

Ring recovery data were used to assess changes in the survival rates of Cormorants in Britain and Ireland between 1965 and 1994. It was found that 99% were ringed as nestlings, thus necessitating an assumption of constant reporting rate across age-classes. The wide geographical spread of ringing effort since 1965 produced sufficient recoveries to enable analyses of trends in survival and reporting rates, but regional subdivisions of the data resulted in high sampling correlations between survival estimates. The assumption of a constant reporting rate across age-classes may be violated for British and Irish Cormorants, because first-year birds are more likely to be shot or caught in fishing nets than older birds and they have a differing seasonal pattern of recoveries. A simple simulation showed that first-year, but not adult, survival estimates were particularly sensitive to age-specific variation in reporting rates. However, the observed trend in first-year survival appeared robust to the assumption of constant reporting rates across age-classes. Declining reporting rates in Britain and Ireland imply that larger numbers of Cormorants will need to be ringed in order to monitor future changes in survival. Regional colour marking schemes for Cormorants have the potential to provide high quality survival information, to supplement that provided by dead recoveries and to offset the observed decline in reporting rates.