Egg Viability as a Determinant of Clutch Size in Birds: A Basic Analysis

Many hypotheses address the determinants of clutch size in birds. One of the more recent is the egg viability hypothesis, which holds that a drop in egg viability with days left unincubated acts to limit clutch size, especially in tropical birds. We provide a basic theoretical analysis of this hypothesis in the form of a simulation model that considers different types of reproductive mortality and the role of multiple breeding attempts per season. We consider both biologically plausible and implausible scenarios to illustrate key points. In its simplest form, assuming a limit to seasonal egg production and the absence of predation on parent or nest, a drop in egg viability favours the allocation of eggs into the smallest clutches that allow eggs to be laid over multiple nesting attempts. This allocation maximises overall egg viability (or minimises egg mortality). Drops in egg viability also lead to smaller clutches when no egg limitation is assumed, as long as multiple nesting attempts are possible. A drop in egg viability alone does not favour small clutches when only a single nesting attempt is possible. The possibility of nest predation renders clutch sizes sensitive to egg viability decreases over time, and generally acts to limit clutch sizes still further. However, steep drops in the viability of unincubated eggs can render clutch size insensitive to changes in the risk of predation to parent or nest. Similarly, a high risk of nest predation renders clutch size insensitive to changes in egg viability.