Effect of egg size on size of subsequent eggs: experimental evidence for performance‐based feedback within the clutch of a wild bird

An egg's size should reflect the net benefit of its production, which itself should vary with the environment. Although the environment is often complex, performance‐based feedback from the size of an egg could signal the cumulative effects of the environment and enable females to optimize size of subsequent eggs, as a recent model and experiment support. That experiment did not rule out the signaling role of size‐correlates, however. Therefore, using free‐ranging Lincoln's sparrows Melospiza lincolnii , we conducted an experiment in which we substituted the first‐laid egg – on the day it was laid – with an artificial egg from one of two groups that were identical except in size. We discovered that, compared to a small substitute, a large substitute yielded a larger increase in size from a previous egg to a subsequent egg (egg 4 in the clutch). This was consistent with the model and with the hypothesis that the size specifically of an initial egg can affect the size of subsequently laid eggs. Under unpredictably variable environmental conditions, such performance‐based feedback may contribute to the optimization of not only egg size but a diversity of organismal processes.