The effect of environmental conditions on captures, survival and breeding success of a winter-breeding seabird

Few studies have explored local and global environmental effects on the demographic rates of small seabird species such as storm-petrels. We analysed ringing data (from 2011-2022) to investigate the demographic parameters of a breeding population of Band-rumped Storm-petrels Hydrobates castro, which nest on Farilhao Grande Islet, Berlengas archipelago, Portugal. We used capture-mark-recapture analyses to estimate annual capture probabilities, apparent survival and abundance. The effects of environmental and capture effort-related variables on demographic parameters were then evaluated. The mean annual survival estimate was low (0.68 +/- 0.02 SE) in comparison to other storm-petrel species, but this estimate substantially increased to 0.79 +/- 0.02 after removing transient individuals. During the study period we determined breeding success in 10 years. In four of these years we also monitored breeding attempts with automatic cameras (2014-2017). Breeding success was low (0.56 +/- 0.12 fledglings per active nest) with some observed cases of predation by Yellow-legged Gull Larus michahellis of both adults and chicks. The size of the Band-rumped Storm-petrel population was estimated at 1511.0 +/- 266.3 SE individuals in 2017 when the mist netting effort was highest. There is no evidence that this population experienced a large decrease over the last 27 years and the trend from the previous 11 years seems to indicate a fluctuation rather than a clear decrease. The North Atlantic Oscillation Index showed a positive effect on the number of captured birds, while there was a negative effect from moonlight and local winds. We conclude that mist netting is a valuable method for long-term demographic studies on ground-nesting seabirds in which nests are difficult to access, but estimates of demographic parameters are influenced by environmental and capture effort-related variables.