The higher rural fertility contributes to urban growth by adding to the large volume of potential rural migrants in this populous, and still largely rural, country. The demographic dynamics underlying urban growth are extremely complex, and analyses based on census data often miss such important factors as circular migration, and less direct or long-term effects of migration. These temporary migrants, who are more commonly men in Africa and women in many countries of Asia and Latin America, often fill niches in particular occupations in the manufacturing and service sectors. They often work in construction in southern China, for example, and domestic service in countries of Latin America. 26 Circular migration is often tied to seasonal patterns or agricultural cycles.
Temporary migrants can cause large swings in population size.
In Thailand, thousands of people move to Bangkok from the agricultural north and northeast areas during the dry season when there are fewer jobs in these regions, and return during the wet season when work is more plentiful. In the early 1990s, Bangkok's population of roughly 8 million was about 10 percent larger during the months of the dry season in north and northeastern Thailand than during the wet season. In many countries, such temporary migrants are counted as rural residents if they spend most of the year in rural areas, if they have not lived in an urban area long enough to qualify as urban residents, or for other reasons. Official statistics thus tend to underestimate the actual contribution of migration to urban growth. Rural migrants also contribute to urban growth when they have children and add to natural increase.