Medium to small petrels that mostly nest in burrows or crevices represent a large fraction of the world’s seabirds, yet their population trends are largely unknown. This lack of knowledge, which has implications for conservation planning, results mostly from methodological difficulties and from the approaches that have been used for their monitoring. Here, we present the surveying scheme created to monitor Cory’s Shearwater Calonectris borealis breeding numbers at their largest known colony, Selvagem Grande (Portugal). We defined 60 circular plots at fixed locations on this 2.45 km2 island and counted nests with eggs annually at the end of laying. Results show that the population increased at 1.45% (95% confidence interval [CI]: 0.72–2.01%) per year between 2009 and 2023. We estimate that the current population size is 38,830 pairs (95% quantile CIs = 34,373–43,713). To the best of our knowledge, this study provides the first systematic information (using fully repeatable methods and providing CIs for the estimates) on population trends of Cory’s Shearwaters, one of the most abundant seabirds in the warm temperate and subtropical North Atlantic and one of the most studied petrels globally. Monitoring using the approach detailed here requires two days of work (by 2–3 persons) per year. Our results and simulations indicate that this is a powerful methodology, with narrow confidence limits for estimated trends and an ability to detect small population changes over short time spans. We suggest that more monitoring protocols similar to this one (with necessary site-specific adaptations, particularly for potential colony expansion where suitable habitat exists) should be developed and implemented in a range of colonies with crevice and burrowing nesting petrels to improve our knowledge on the population status of a broad fraction of the world’s seabirds.