Summary. Key ecological studies involve the regular censusing of populations of wild animals, resulting in individual case history data which record when marked individuals are seen alive and/or found dead. We show how current conditional methods of analysing case history data may be biased. We then show how a correction can be applied, making use of results from a mark–recovery–recapture analysis. This allows a simple investigation of the effect of time-varying individual covariates such as weight that often contain missing values. The work is motivated and illustrated by the study of Soay sheep in the St Kilda archipelago.