Increased parental effort fails to buffer the cascading effects of warmer seas on common guillemot demographic rates.

Climate warming can reduce food resources for animal populations. In species exhibiting parental care, parental effort is a 'barometer' of changes in environmental conditions. A key issue is the extent to which variation in parental effort can buffer demographic rates against environmental change. Seabirds breed in large, dense colonies and globally are major predators of small fish that are often sensitive to ocean warming. We explored the causes and consequences of annual variation in parental effort as indicated by standardised checks of the proportions of chicks attended by both, one or neither parent, in a population of common guillemots Uria aalge over four decades during which there was marked variation in marine climate and chick diet. We predicted that, for parental effort to be an effective buffer, there would be a link between environmental conditions and parental effort, but not between parental effort and demographic rates. Environmental conditions influenced multiple aspects of the prey delivered by parents to their chicks with prey species, length and energy density all influenced by spring sea surface temperature (sSST) in the current and/or previous year. Overall, the mean annual daily energy intake of chicks declined significantly when sSST in the current year was higher. In accordance with our first prediction, we found that parental effort increased with sSST in the current and previous year. However, the increase was insufficient to maintain chick daily energy intake. In contrast to our second prediction, we found that increased parental effort had major demographic consequences such that growth rate and fledging success of chicks, and body mass and overwinter survival of breeding adults all decreased significantly. Common guillemot parents were unable to compensate effectively for temperature-mediated variation in feeding conditions through behavioural flexibility, resulting in immediate consequences for breeding population size because of lower adult survival and potentially longer-term impacts on recruitment because of lower productivity. These findings highlight that a critical issue for species' responses to future climate change will be the extent to which behavioural buffering can offer resilience to deteriorating environmental conditions.