For many animals, strategies for optimally timing reproduction involve monitoring not only the physical environment, but also the social context. To explore the potential for social factors to modulate reproductive seasonality, the influence of social and environmental cues on birth timing was examined in greater spear-nosed bats Phyllostomus hastatus. Births were observed or dated from pup growth curves in three caves on Trinidad, West Indies, over 4 years. Nearly 40% of the variability in birth dates could be explained by environmental factors because birth dates differed significantly between years and showed consistent differences between locations that receive differing rainfall amounts. Nevertheless, the timing of births within caves and in captivity indicated that social cues also affect the timing and synchrony of births within female social groups. Within each cave, social groups differed significantly in mean birth dates. Two groups of greater spear-nosed bats brought into captivity and maintained without seasonal cues initially exhibited less birth synchrony than wild groups, but birth synchrony did not decline over 3 subsequent years. Further evidence for the influence of social cues on reproductive timing came from four females that were transferred between the captive groups and then gave birth in synchrony with their new group and out of schedule with their original group. Social cues influencing reproductive timing are unlikely to be volatile chemicals or other passively transferred cues, given that frequent prolonged physical contact did not increase birth synchrony between adjacent wild groups. Cues are more probably transferred actively between female group members, perhaps by grooming, or through mating with one attendant male.