We have seen that different groups of birds and even different populations have characteristic clutch sizes. WHY? How did these evolve? High mortality requires fairly large clutches (to offset mortality). Birds with low clutches presumably suffer less mortality and higher clutches would lead to overpopulation.
Why do birds have low clutch sizes then? Why not just go for the maximum number? Low clutch size is not a mechanism to avoid overpopulation, however, not group selection. Few eggs must be the best way to achieve OPTIMUM SURVIVAL. Advantages of small clutch size include that it does not overly tax food supply and it results in healthier nestlings that develop quicker (each get more food).