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 Farilhão 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.