A female's decision to join an aggregation or to calve solitarily depends on her parturition date relative to other individuals and on the number of other females in the birth area. The results are robust to changes in other parameter values. The Evolutionarily Stable Strategy is to join an aggregation and stay for the full period of neonatal vulnerability if calving early in the birth period, to join the aggregation but to leave it early if calving later in the birth period, and to calve solitarily if calving at the end of the birth period. The possible effects of human disturbance on female behaviour are investigated, and testable hypotheses are presented about the behaviour of females in the birth period. The game-theoretic approach to the decision to join an aggregation developed here is broadly applicable to many problems in the ecology of grouping behaviour.