Seabirds are in decline globally and climate change is likely to increase the pressure on already struggling species. The indirect effects of climate change are widely studied, they have been shown to have a significant effect on both seabird survival and reproductive success, but the direct effects are less well understood. Climate predictions suggest that one of the direct effects, extreme weather, is predicted to increase in both frequency and intensity. Skomer Island is the largest Manx

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... rwater colony in the world and the population has been increasing over recent decades, but the specific effects of extreme weather on reproductive success are unknown. This study compared the effects of average and extreme weather conditions on Manx Shearwater reproductive success, taking into account the effect of known breeding pairs and the potential effects of individual experience. It also considered the effect of inter-specific competition between shearwaters and Atlantic puffins on shearwater reproductive success. This study found that colony-level reproductive success showed no significant trend over the study period of 1995-2019, however fledging success showed a significant decline. When individual-level analysis was carried out no such trend was found: experienced breeders may be more likely to successfully raise a chick. Extreme weather was shown to have significant effects on reproductive success at the individual-level, particularly on fledging success, but this did not cause a significant decrease in fledging success over the study period. Population estimates show that shearwaters are increasing on Skomer and it is clear that weather, extreme or not, is not currently the most significant factor in determining reproductive success of Manx Shearwaters. This study also found no evidence that puffins are influencing the reproductive success of Manx Shearwaters on Skomer Island. The effects of climate change, indirect and direct, will interact and have many complex effects, especially if predictions regarding future climate change are met. Extreme weather and the effects of demography can only be studied where long- term datasets exist, therefore projects such as this are vital for ongoing seabird research and conservation.