Individual differences in body mass exert a major influence on several life-history traits of mammals. We investigated the factors influencing variation in body mass of calves of migratory caribou (Rangifer tarandus) at birth (June, 19 years of data) and in autumn (October, 15 years of data) in the Rivière-aux-Feuilles (Feuilles, 1991–2003) herd and the Rivière-George (George, 1978–2003) herd in Québec and Labrador, Canada. Mass at birth (hereafter, birth mass) did not differ between herds, possibly because part of their winter ranges overlapped. However, Feuilles calves were smaller in autumn than George calves, possibly reflecting differences in summer ranges. The birth mass of calves also varied with year, likely as the outcome of both intrinsic and extrinsic factors. Birth and autumn body mass were influenced positively by habitat quality in June, estimated by the normalized difference vegetation index. The North Atlantic Oscillation of the previous winter was positively correlated with autumn mass of the George calves. Previous winter snowfall was negatively related to the mass of George calves, and daily movement rates in summer were negatively correlated with the mass of calves of both herds in autumn. Birth mass was positively related with productivity in October in the George herd and also with productivity 3 and 4 years later, which corresponds to the beginning of reproduction of females. We suggest that a mechanism of delayed quality effect of the calves could have been involved in the decrease of fall productivity and population size of the George herd.