Using ringing data to study the effect of hunting on bird populations.
The scale of mortality due to hunting of quarry birds in Europe raises conservation questions concerning the effect of hunting on their populations. Typical questions relate to the effect on survival, and to geographical differences and temporal trends in hunting pressure. Geographical differences and temporal trends in hunting pressure can be examined using ringing data by constructing indices based either solely on recoveries of dead birds, or combining recoveries and ringing totals. Both types of index may vary through factors unrelated to hunting. Annual survival rates may be calculated from ringing recoveries, either using models based solely on recoveries of dead birds, or combining recoveries and ringing totals. Measuring an effect of hunting on survival rates is best achieved by comparing rates between hunted and non-hunted populations. Capture-recapture data may serve the same purpose, but are appropriate only for intensive studies. Deciding whether mortality due to hunting is additive or compensatory is difficult. Standard methods involve either indirect assessment (testing predictions based on additivity or compensation) or direct analysis of ringing-recovery data using appropriate statistical models of survival. In practice, neither method seems satisfactory. Concentrating on the issue of additivity/compensation ignores the wider play of density-dependent responses within a population and may not be relevant to understanding the effect of hunting among all possible causes of population change. Ringing and recovery data can certainly contribute to answering questions concerning the effects of hunting, but they need to be validated using other sources of information. An Integrated approach, achieved by monitoring annual gains and losses, should be an essential part of research to understand the role of hunting in population change.