Vigilance studies have mostly focused on adaptive changes in antipredator behavior during the day. Adjustments in vigilance patterns in species that also forage at night are poorly known. Birds may forage at night to evade predation or other sources of disturbance, in which case levels of vigilance are expected to decrease at night. However, low visibility at night, especially on moonless nights, may compromise visual detection of predators, in which case levels of vigilance are expected to increase at night. We examined the effect of time of day on vigilance in non-breeding Greater Flamingos foraging in a tropical lagoon complex in Venezuela. Vigilance increased on darker nights but overall levels of vigilance at night did not differ from daytime values. Vigilance decreased with group size in a similar fashion both during day and night. Flamingos foraged in smaller flocks at night than during the day. The occurrence of small flocks, in which vigilance was quite high, and the increase in vigilance on dark nights imply that the amount of time allocated to foraging must be generally lower at night in flamingos. The results suggest that the night was not perceived as safer by foraging flamingos, and that visual detection of predators was probably compromised by low light levels.