When climate is changing: effects of phenology and local climate on individual fitness

The fitness impacts of climate change on seasonal breeding organisms is, in part, linked to local climatic effects on phenology (i.e. the timing of major life events, such as breeding and migration) and how these relate to temporal changes in resource distribution. Thus, in this thesis we examine relationships between phenology, measures of fitness (reproduction and survival) and local climate for a long-distance migratory bird species, the northern wheatear (Oenanthe oenanthe). There was a seasonal fitness decline in this population, but the relative contribution of different reproductive components to this decline varied. Because declines in these fitness components were driven by different factors this suggests climate change is unlikely to affect fitness components equally. This unequal susceptibility of different fitness components to climate change was further demonstrated by different responses in reproduction and survival to rainfall, and that these effects were age-, sex- and time-dependent. During the past 20 years, wheatears have advanced their migration to arrive and begin breeding ~8 days earlier than in the early 1990s. Regardless, this change in migration and breeding phenology may be lagging behind current climate change because birds are breeding increasingly later relative to the progression of spring. One factor potentially contributing to this phenological mismatch is limited plasticity in the arrival-breeding interval, arising from constraints on how much the interval can be reduced after migration. Another constraint on the ability for birds to advance their breeding time is the cost of reproduction carried over from one breeding season to the next. This impacts on individual fitness through delays in breeding phenology and the fitness loss associated with seasonal fitness declines. Overall, this thesis shows that to avoid underestimation of potential effects of climate change on population demography, all fitness components and their drivers need to be considered. In addition, it highlights potential constraints preventing individuals from optimally adjusting their breeding time in order to track environmental change. Such knowledge is crucial if we are to make informed decisions regarding the management of populations in the future.