We analyzed birth dates recorded during an 18-year period in a group of Japanese macaques housed in the Rome zoo to assess the influence of environmental, physiological, and social factors on birth seasonality. Birth timing differed significantly among years. Birth timing was affected by reproductive condition of females—nes that had given birth in the previous year delivered significantly later than those that had not—but not by their age or dominance rank. We conducted further analyses separately on females that had or had not given birth in the previous year. In both subgroups of females mean birth date was not influenced either by environmental temperature and rainfall during the previous mating season or by group size. On the contrary, among females that had not given birth in the previous year, socionomic sex ratio—ratio of sexually mature males to sexually mature females—is positively correlated with both mean birth date and date of the first birth, but not with date of the last birth. Contrarily, among females that had given birth in the previous year, there is no significant relationship between these variables. We hypothesize that the effects of socionomic sex ratio on birth timing might depend on competition among males for access to fertile females. When the number of males per female was higher, mutual disruption of consort pairs may have led to a delay in the onset of mating.