Why do birds only lay a few eggs?

The number of eggs laid by a bird varies widely between species, and can range from one to more than twenty. However, in general, the smaller the bird the more eggs will be laid at once.

Most species are designed to invest as few resources per nestling as possible and hatch many, and replace the ones eaten by predators by renesting. Others invest heavily in parental care for their limited number of broods so invest more resources in each single one and are consequently able to protect those few to adulthood to avoid losing their resources.