They can and do but it's much less reliable. Births and deaths are systematically recorded, and fertility rates are shaped by comparatively stable factors like

They can and do but it's much less reliable. Births and deaths are systematically recorded, and fertility rates are shaped by comparatively stable factors like education levels, access to family planning and gradual cultural shifts. Mortality rates follow similarly predictable trends. Whereas migration flows are heavily influenced by sudden events that are difficult to anticipate, like economic crises, political instability, wars, natural disasters and so on. The data is less complete too, for example unauthorized border crossings will not be recorded officially in cases where these are undetected.