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Determining the effects of oil spills on wildlife can be difficult. The collection and publication of data on wildlife impacts and mortalities from oil spills are vital steps to help understand overall effects. However, the way wildlife are collected and processed affects the results and how information can be used. Information for threatened and endangered species is particularly important, as effects on small population size or range-restricted species need to be evaluated as accurately as possible. This paper outlines the procedures for the collection of wildlife, both alive and dead, during the first 6 wk following the MV 'Rena' incident in New Zealand and discusses these processes in terms of assessing mortality. The container ship MV 'Rena' struck the Astrolabe Reef, 22 km off Tauranga, New Zealand, spilling an estimated 350 t of heavy fuel oil on 5 October 2011. An oiled-wildlife response team undertook search and collection of all live oiled and all dead birds in the affected area. In the first 6 wk of the response, 428 live birds were taken into the rehabilitation facility, and 1376 oiled and 687 unoiled bird carcasses (representing 49 species) were assessed. To maximise information gathering from oil spill mortalities, we recommend the development of clear collection procedures and documentation, experts for species identification and, where possible, necropsies of carcasses. Direct counts of mortality and post-release (for rehabilitated wildlife) monitoring studies of oiled wildlife are still rare; however, this is critical research that should be undertaken during and after oil-spill events.