To further an explanation of the post-1970 United States demographic phenomenon of increased population for non-metropolitan areas, the relationship of 3 ecological phenomena to non-metropolitan net migration rates between the 1960's and the 1970's is examined for a random stratified sample of 380 United States non-metropolitan counties (primarily using U.S. Department of Agriculture data). The ecological phenomena studied as possible explanations of the change are: sustenance organization, including overall functional specialization and employment change; change in the age structure, especially the population in the highly mobile "young adult" years; and "spillover", the expansion of metropolitan population and activities into non-metropolitan areas. Findings support existing literature and indicate that changes in the overall number of employment opportunities, changes in specialization of sustenance organization, and sustenance activity in all categories but mining are not significantly related to net migration change. There is, however, a significant link between net non-metropolitan migration change and change in age structure of non-metropolitan populations as well as metropolitan "spillover". Therefore, gains in net migration rates of non-metropolitan counties in the 1970's do not signal the development of new population redistribution mechanisms; rather, they can be viewed as a result of the continued operation of traditional processes. (Author/SB)