MAIN FACTORS AFFECTING POPULATION SIZE are mortality and natality and immigration and emigration (hard to demonstrate). Mortality is studied by means of life tables. Birds are rather unique since the tables show that mortality is not correlated with age -- in other words, no matter what the age of a bird is, it is about equally liable to die.
Controversy exists about how populations are regulated. Is the regulation density dependent or independent? Density dependent factors, especially food, seems to be most important (game management would come in here if one was to cover it).