A clutch is the total number of eggs, or egg, laid by a bird during a single nesting period. Clutch size will differ greatly between species, sometimes even within the same genus. It may also differ within the same species due to many factors including habitat, health, nutrition, predation pressures, and time of year. Clutch size variation can also reflect variation in optimal reproduction effort. Long lived species tend to have smaller clutch sizes than short lived species.