Exploring the consequences of reducing survey effort for detecting individual and temporal variability in survival

1. Long-term monitoring programmes often involve substantial input of skilled staff time. In mark–recapture studies, considerable effort is usually devoted to both marking and recapturing/resighting individuals. Given increasing budgetary constraints, it is essential to streamline field protocols to minimize data redundancy while still achieving targets such as detecting trends or ecological effects. 2. We evaluated different levels of field effort investment in marking and resighting individuals by resampling existing mark–recapture–recovery data to construct plausible scenarios of changes in field protocols. We demonstrate the method with 26 years data from a common guillemot Uria aalge monitoring programme at a major North Sea colony. We also assess the impact of stopping the ringing of chicks on our ability to study population demography using integrated population models (IPM) fitted to data including information on breeding adults. Different data sets were removed artificially to explore the ability to compensate for missing data. 3. Current ringing effort at this colony appears adequate but resighting effort could be halved while still maintaining the capacity to monitor first-year survival and detect the effect of hatch date on survival prospects. 4. The IPM appears robust for estimating survival, productivity or abundance of the breeding population, but has limited capacity to recover year-specific first-year survival when chick data are omitted. If productivity were not monitored, the inclusion of chick data would be essential to estimate it, albeit imprecisely. 5. Synthesis and applications: Post-study evaluation can help streamline existing long-term environmental monitoring programmes. To our knowledge, this study is the first use of data thinning of existing mark–recapture–recovery data to identify potential field effort reductions. We also highlight how alternative monitoring scenarios can be evaluated with integrated population models when data are collected on different aspects of demography and abundance. When effort reduction is necessary, both approaches provide decision-support tools for adjusting field protocols to collect demographic data. The framework has broad applicability to other taxa and demographic parameters, provided suitable long-term data are available, and we discuss its use in different contexts