Whether predators always attack the most vulnerable prey or simply attack prey that exceeds a minimum vulnerability level is an important question to answer in furthering our understanding of predator and antipredation behaviour. Predators may attack any reasonably vulnerable prey rather than waste time identifying the most vulnerable prey, particularly when prey can respond quickly to alter their vulnerability in response to a predator. We tested whether sparrowhawks always choose to attack the group of prey that maximises their capture probability, or whether they simply attack any group above a minimum vulnerability. We modelled sparrowhawk attack success when hunting redshanks using data from three winters and found that probability of capture increased when group size or distance to predator-concealing cover decreased. We then used this model to predict the relative vulnerability to capture of redshank groups occurring in pairs in a fourth winter and found that sparrowhawks attacked the most vulnerable prey group twice as often as not (66% n=59 pairs). When sparrowhawks attacked the less vulnerable group, there was no tendency for both groups to be particularly vulnerable or for the difference in the vulnerability between the two groups to be relatively small. This suggests that, while sparrowhawks do on average attack the most vulnerable group available, they consider other factors that affect vulnerability or that additional factors lead them to also attack opportunistically. This suggests that there will be selection for the predator to monitor a large number of prey individuals and groups and for prey to have the ability to monitor the behaviour of conspecifics in the same and different groups so that they can assess relative vulnerability.