The breeding demography and egg size of North Norwegian Atlantic Puffins Fratercula arctica and Razorbills Alca torda during 20 years of climatic variability

The mean egg-laying dates of Atlantic Puffins Fratercula arctica and Razorbills Alca torda in north-eastern Norway varied between 13-30 May and 21 May-2 June respectively in the years 1980-1982 and 1988-2000. Atlantic Puffins laid on average 7 days before Razorbills, and the mean spread of laying dates each year was larger (22 days) for the former than the latter (19 days). There were significant correlations between egg-laying dates and air and sea temperatures in April and May, with lower temperatures resulting in delays in laying at a rate of 1-4 days °C-1. Late access to nesting sites due to snow and ice in burrows might explain some of the delay in egg-laying during cold springs. Such variation should be considered when planning field trips to monitor breeding populations. There was no inter-annual variation in the mean volumes of Atlantic Puffin eggs. Although Razorbill egg volume did vary between years, it was not influenced by either sea or air temperatures. This suggests that neither species adjusted egg size in order to advance laying dates.