A partially migratory population consists of non-migrant and migrant individuals that share a common site during one period of the annual cycle. In this paper, we derive the expected equilibrium population sizes of migrants and non-migrants and show how the abundance of one type is dependent on the other because their dynamics are coupled through density-dependent effects. We present an approach for developing hypotheses about how changes in the environment will influence partially migratory populations and for formulating testable predictions about the effects of future changes on the proportions of migrants and non-migrants. We apply this approach to a hypothesis put forward by Berthold that improved environmental conditions at the shared site will generally increase the number of non-migrants and decrease the number of migrants, and to a study by Nilsson et al. which observed an increase in the number of migrants in a partially migratory blue tit Cyanistes caeruleus population in Sweden, but an overall maintenance of the proportion of migrants.