We focus on the causes and consequences of individual variation in life histories and the underlying mechanisms as schematically summarized in the figure above, with an emphasis on senescence. Study organisms are birds, mostly captive zebra finches and free-living jackdaws, that we both study in the context of long-term experiments that modulate ageing and lifespan. The ‘state’ concept is central in our work, and in theory comprises every possible trait, but we concentrate on traits that are of functional importance in the sense that they predict variation in life history decisions or (components of) fitness. In this context we have focused on telomeres in birds and humans, and are now shifting attention to DNA methylation.