Populations of many species are decreasing, but the underlying causes are often poorly understood, impeding effective conservation action. One useful approach is to determine which demographic rates drive variation in population growth rate () and identify ecological and environmental causes. I identified key demographic rates driving the decrease in Ring Ouzel (Turdus torquatus) population size, a species of high conservation concern in the UK, and explored underlying ecological and environmental causes. The number of breeding pairs decreased by 44-100% during 1979-2009 across 13 study areas throughout the UK. The population in my study area in Glen Clunie, Aberdeenshire, Scotland, decreased by 67%, from 39 to 13 breeding pairs, during 1998-2009. Mean  calculated from annual censuses was 0.91. I recorded reproductive success, and used re-sightings of marked individuals to estimate survival rates in each year, thus measuring the mean, variance and covariance among key demographic rates. Prospective elasticity analysis indicated that  was most sensitive to adult survival. However, integrated elasticity analysis, accounting for estimated demographic covariance, indicated that  was most sensitive to first-year survival. Retrospective decomposition of variance indicated that first-year survival contributed most to observed variation in . However, adult survival was low compared with species with similar life histories. Juveniles fledged from early-season broods had higher survival probability during each four-day period post fledging (0.952 ± 0.011) than juveniles fledged from late-season broods (0.837 ± 0.021). Predation was the main apparent cause of mortality. Juveniles foraged on invertebrates in grass-rich areas during June to mid-July, but then switched to feed mainly on moorland berries in higher-altitude, heather-rich, areas during mid-July to early-September. Thus, a variety of habitats providing different food types is required during the late summer. This thesis illustrates the value of integrating demographic and ecological studies to understand population change in species of conservation concern.