A common question that many birders have often is asked is why one species of birds may only lay a few eggs while another species may lay many eggs.
The common term for how many eggs are laid by each bird species is the clutch size. A group of scientists has completed a study of over 5,000 species of birds examining the reasons for different size clutches. What they found is that clutch size is influenced by factors other than the species alone. These include environment and nest type as well as behavior and evolutionary variables.
For example, in studying woodpeckers which are cavity nesters they are found to have larger clutch sizes than say, that of open nesting species. A cavity nest is a nest that is build inside something, more than likely a tree. As it is inside, it is harder to get to and therefore offers more protection than an open nest. An open nest is that common nest which is formed by birds in a branch using grass, dirt and other materials. Also, tropical birds have much smaller clutches than those living in northern areas.
The simple answer though is that while the other variables do play into the mix of clutch size, there is one major underlying factor that drives the clutch size. And that is the propensity or likelihood of survival. So, for example, those birds that more likely to survive only lay a few eggs. But those species that are more likely to be killed lay many eggs in hopes that a couple will survive.
When you think of survival you may think this is due to predators alone. Predators do a play a role especially in open-nests vs. cavity nests. However, another factor that can influence the survival rate is climate.
Some birds that live in the tropics have smaller clutch sizes because the weather is more favorable, however, their survival is dependent on the climate conditions that their species has adapted to over years. This makes them extremely vulnerable to any climate changes which could drastically effect their ability to reproduce.