Ecophysiology of egg rejection in hosts of avian brood parasites: new insights and perspectives

Abstract

Egg rejection is the most effective and widespread defense used by host species to counteract the extreme fitness costs frequently imposed by obligate avian brood parasites. Yet, the proximate mechanisms underlying between- and within-individual variation in host responses remain poorly explored. Emerging evidence suggests that egg rejection is dependent on individual physiological states, and draws attention to the role of hormones as mediators of flexible antiparasitic responses. In this perspective article, I outline recent advances in our understanding of the proximate factors that mediate egg rejection. I also point out some areas where knowledge remains still lacking, especially those related to the development and maintenance of effective cognitive functions, the potential role of oxidative stress, immunological state, and developmental stressors. I propose new hypotheses that stimulate future research on behavioral host responses toward brood parasitism.

Keywords: cognitive performance; condition-dependence; corticosterone; flexible responses; prolactin.

Figure 1.

Potential physiological mechanisms mediating egg rejection. Egg rejection is a multi-stage process that requires effective perceptual and cognitive functions (cognitive performance), as well as the accurate integration of external cues of parasitism and the assessment of potential costs associated with the response (decision). Besides the genetic background, cognitive performance is determined by individual physiological states (cognitive flexibility; e.g., hormone levels, immunological state, and oxidative status) and developmental conditions (developmental cognitive plasticity). Main hormones involved in the regulation of parental care, such as prolactin, CORT, and testosterone, could also mediate changes in incubation and associative maternal behaviors in the nest, thereby impacting the probabilities of egg rejection.