Conclusions
This study highlights the importance of integrating different approaches used to study life-history variation in order to gain a better understanding of the ecological causes and ultimately the evolutionary consequences of variation in the tempo of life-history strategies (i.e. pace-of-life). By combining an analysis of individual variation in life-histories based upon individual projection matrices, along with age- and density-dependent survival and reproduction models, we were able to understand the genetic and demographic sources of individual variation in generation time within a metapopulation of house sparrows. Furthermore, we show how competitive regimes can affect the mean age at reproduction, supporting predictions from classic life-history theory predicting that density dependence is a key determinant in shaping the pace-of-life. Because generation time determines the speed of evolutionary responses to selection, detailed understanding of how ecological dynamics affect the mean age at reproduction is therefore a key to predicting evolutionary adaptations to environmental change.