Cognitive Decision Rules for Egg Rejection
Egg rejection is the best studied behavioral adaptation by hosts to avian brood parasitism. Investigations of the mechanism(s) by which a host accomplishes the task of perceiving and deciding to reject a foreign egg have been a hotbed of debate and discovery for decades. The two most often tested cognitive explanations for this behavior are: (1) the host rejects the egg most dissimilar from the other eggs in the nest (discordancy mechanism) and (2) the host compares each egg to an internal template of the appearance of its own eggs (template recognition mechanism). While many years of published work have purported sole support for the template recognition hypothesis (for instance, hosts can experimentally reject foreign eggs which do not represent a quantitative minority in the clutch), in recent years an increasingly prevalent argument that the two mechanisms are working in tandem has come to light. Furthermore, there is also a steadily building body of work indicating that hosts have plastic discrimination thresholds, such that the extent to which parasitic eggs must be different from a host’s own egg before rejection occurs appears to be both socio-ecological context dependent and shaped by earlier experiences through a learning component. Overall, the cognitive architecture of egg rejection decisions appears to be complex and shaped by the particular coevolutionary histories of hosts and parasites.
We thank M. Soler for his invitation to contribute this chapter. We also thank Eivin Røskaft, Francisco Ruiz-Raya, and Manolo Soler for their comments on the manuscript. For financial support, we thank the Human Frontier Science Program and the National Science Foundation (to MEH). This study was also supported by the National Research, Development and Innovation Office (NKFIH, NN118194 to CM).