The behavioural rhythms of organisms are thought to be under strong selection, influenced by the rhythmicity of the environment. Such behavioural rhythms are well studied in isolated individuals under laboratory conditions, but free-living individuals have to temporally synchronize their activities with those of others, including potential mates, competitors, prey and predators. Individuals can temporally segregate their daily activities (for example, prey avoiding predators, subordinates avoiding dominants) or synchronize their activities (for example, group foraging, communal defence, pairs reproducing or caring for offspring). The behavioural rhythms that emerge from such social synchronization and the underlying evolutionary and ecological drivers that shape them remain poorly understood. Here these rhythms are investigated in the context of biparental care, a particularly sensitive phase of social synchronization where pair members potentially compromise their individual rhythms. Using data from 729 nests of 91populations of 32 biparentally incubating shorebird species, we report remarkable within- and between-species diversity in incubation rhythms. Between species, the length of incubation bouts was unrelated to variables reflecting energetic demands, but species relying on crypsis  had longer incubation bouts than those that are readily visible or who actively protect their nest against predators. Results indicate that even under similar environmental conditions, social synchronization can generate far more diverse behavioural rhythms than expected from studies of individuals in captivity. The risk of predation, not the risk of starvation, may be a key factor underlying the diversity in these rhythms. informacion[at]ebd.csic.es: Bulla et al (2016) Unexpected diversity in socially synchronized rhythms of shorebirds. Nature. 2016 Nov 23. doi: 10.1038/nature20563