The empirical literature on the effects of migration on receiving populations has largely ignored fertility outcomes of natives. Statistical models of population projections estimating the impact of migration inflows on population growth often assumed that natives do not change their fertility behaviour as a response to immigration. We challenge this assumption and address the gap in the literature by showing that fertility outcomes of natives are affected by migration. Using exogenous geographical variation provided by the massive forced migration from the Northern Syria to specific Turkish provinces shortly after 2011, we show that natives in the affected provinces increase their fertility considerably compared to the provinces that are less affected by mass migration. We provide additional analyses to explore mechanisms at play and to show heterogeneity in the fertility response by population subgroups, using three independent datasets: 1) Administrative Data on Vital Statistics, 2) Turkish Labor Force Surveys 3) Gallup Surveys. Our preliminary results show that fertility increase in natives is primarily driven by the behaviour of prime age, low-skilled natives, and by homemakers. We rule out mechanisms related to the cost of childcare, house prices and we suggest that mechanisms related to social interactions (e.g. competitive breeding or cultural diffusion) are plausible.