Franklin D. Roosevelt famously said “the only thing we have to fear is fear itself”. I will take you through a series of field experiments showing that this rhetorical flourish accurately reflects a fundamental biological fact. Predators kill prey thereby reducing the survival rates and sizes of populations. We show that the ‘fear’ of falling victim to a predator is powerful enough to chronically alter the physiological profile and behaviour of songbird prey to such an extent that it reduces both fecundity and survival even when no prey are actually killed. Thus, killing and the fear of being killed must both be considered when evaluating the total impact that predators have on wildlife. I will also outline how our results may have implications for gene expression in the brain and post-traumatic stress disorder in humans.