To test the hypothesis that variation in aerial flocking behaviour is adaptively related to predation risk, we described and quantified the flocking patterns of starlings, approaching two urban roosts, which differed in predation pressure (by peregrine falcons, Falco peregrinus). We predicted that the higher predation pressure in one of the roosts would be reflected in larger and more compact flocks, thought to be less vulnerable to predation than small flocks. Incoming flocks, not under direct attack, were observed during winter for 53 days. We identified 12 flocking shapes. Significantly higher frequencies of compact and large flocks were observed in the roost with high predation pressure, while small flocks and singletons were more frequent at the roost with low predation pressure. Similar patterns were observed in both roosts when other flocks displayed antipredator behaviour, even when far away and in the absence of the predator at the focal roost. This may indicate that social information passed between flocks affects flocking decisions. Predation success was higher at the roost with low predation. These results suggest that aerial flocking patterns are affected by predation risk and possibly by the behaviour of other flocks in response to direct attacks.