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

Abstract

Egg rejection is the most effective and widespread defence 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 state, 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 behavioural host responses towards brood parasitism.

Egg burial in the ringneck dove (Streptopelia risoria): A potential laboratory model system for egg-rejection research?

“…The current research already demonstrated the feasibility of not only experimental egg rejection (yes/no) research on captive subjects with known familial histories and breeding experience but also a future ability to manipulate both experiential (e.g., prior breeding and egg-rejection experience; Amundsen et al, 2002; Moskát et al, 2014) and physiological (e.g., endocrine factors: Abolins-Abols & Hauber, 2018; Ruiz-Raya, 2021) contexts in which egg-rejection behaviors can be studied. For example, in our Experiment 2, we varied the day of incubation at which the foreign egg type was introduced into the nests, although in this species this did not impact the pattern of egg rejection statistically (but see in other species: Liu et al, 2021; Moskát & Hauber, 2007).…”

“…Furthermore, doves already serve as a neuroethological model species for the analyses of pair-bonding (Burns-Cusato & Cusato, 2013) and parental care (Burns-Cusato et al, 2021) in captivity, allowing a suite of neuro/physiological manipulations to be also applied and queried in future studies in the context of parental care versus egg-rejection behaviors (Abolins-Abols & Hauber, 2018). For example, previous work established that both corticosterone and prolactin can serve alone to up- or downregulate, respectively, the propensity of egg-rejection behaviors in Turdus thrushes in response to model parasite eggs; however, both of these studies were conducted on females only because in Turdus thrushes the female is typically the incubating (and egg rejecting) sex (reviewed in Ruiz-Raya, 2021).…”

“…Studying antiparasitic egg-rejection behavior in well-controlled, laboratory-breeding milieus has been, to date, limited to a handful of captive populations and studies because most egg-rejecter host species are not routinely kept as pets or other captive stocks and are difficult to breed in cages or aviaries in sufficient replicate sample sizes (Manna et al, 2017; but see Collias, 1993), especially when many of the host species are cooperatively breeding (Feeney et al, 2013). This methodological constraint of not being able to study the ontogenetic, cognitive, and physiological, including the endocrine (Abolins-Abols & Hauber, 2020), basis of egg-rejection behaviors in a controlled setting (i.e., in captivity) has posed some limitations on the study of the proximate bases of avian host–parasite interactions (Abolins-Abols & Hauber, 2018; Ruiz-Raya, 2021); specifically, the roles of both individual experience in and the heritability of egg-rejection responses to real or model parasitic eggs have been difficult to estimate in the field due to generally low return rates of wild host species’ progeny, including variably egg-rejecter host species of diverse brood parasitic lineages (Hauber et al, 2012; Moskát et al, 2014). Note that studying fully acceptor or fully rejecter host species in captivity still does not advance the analysis of heritability, plasticity, and individual experience in egg-rejection behaviors.…”

Physiological stress responses to non-mimetic model brood parasite eggs: leukocyte profiles and heat-shock protein Hsp70 levels

“…Over the past decades, much research has focused on the ecological and behavioural aspects of avian brood parasite-host interactions (Soler, 2017). However, the physiological mechanisms underlying host responses to brood parasitism have received comparatively little attention despite the fact that brood parasitism may potentially trigger significate adjustments in host physiology, which can have important consequences for the expression and evolution of key anti-parasitic defences such as egg rejection (Abolins-Abols and Hauber, 2018; Avilés, 2018; Ruiz-Raya, 2021).…”

Physiological stress responses to nonmimetic model brood parasite eggs: Leukocyte profiles and heat‐shock protein Hsp70 levels

“…Over the past decades, much research has focused on the ecological and behavioral aspects of avian brood parasite–host interactions (Soler, 2017). However, the physiological mechanisms underlying host responses to brood parasitism have received comparatively little attention despite the fact that brood parasitism may potentially trigger significate adjustments in host physiology, which can have important consequences for the expression and evolution of key antiparasitic defences such as egg rejection (Abolins‐Abols & Hauber, 2018; Ruiz‐Raya, 2021).…”