In this dissertation, I use phenotypic data on age and life-history trait expression from long-lived Nazca boobies (Sula granti, a seabird) breeding in Galápagos to understand cause and effect relationships underlying variation in reproduction, survival, and ageing patterns. In the first chapter, age-specific demographic data spanning the loss of a preferred prey item were used to forecast the response of the Nazca booby population to future exclusion of the key prey species (sardines, Sardinops sagax), by ocean warming, from the foraging envelope. Breeding success was dramatically reduced when sardines were not available. Adult female survival increased, but not enough to keep the population growth rate from falling below 1, indicating a shrinking population. Population growth may not recover because predicted ocean warming around Galápagos should displace sardines permanently from the boobies’ island-constrained foraging range. In the second chapter, I test the expectation, made by the life history theories of ageing, that increased senescence should follow relatively high reproductive effort. In Nazca boobies, females recruit earlier and breed more often, predicting more intense actuarial and reproductive senescence in that sex. Females did show stronger senescence for survival and offspring production, but a causal link between sex differences in early-reproduction and sex-differences in ageing was not supported because, within each sex, individual reproductive effort positively predicted late-life performance. Absent evidence for early reproductive effort trading off with late-life performance, patterns of sex differences in senescence may result from age- and condition-dependent mate-choice interacting with this population’s male-biased sex ratio and mate rotation. Finally, in the third chapter, I show that age and current environment may interact to determine breeding responses to environmental change. Young and old Nazca booby females had lower performance than middle-age birds for all reproductive traits, and these age effects sometimes increased under a challenging environment. Environmental quality was assessed with respect to the El Niño-Southern Oscillation (ENSO). Surprisingly, Nazca booby responses to the ENSO were not in step with ENSO-forced changes in primary productivity within the boobies’ foraging envelope. Instead, trophic interactions with subsurface predators may influence Nazca booby responses to oceanographic variables. As a whole, this dissertation shows the ability of long-term, individual-based, ecological studies to address important topics from ecology and evolutionary biology, including ageing, trophic interactions, and the responses of individuals and populations to environmental change.