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

Patterns of stress response to foreign eggs by a rejecter host of an obligate avian brood parasite

One of the most effective defenses of avian hosts against obligate brood parasites is the ejection of parasitic eggs from the nests. Despite the clear fitness benefits of this behavior, individuals within so-called "egg-rejecter" host species still show substantial variation in their propensity to eliminate foreign eggs from the nest. We argue that this variation can be further understood by studying the physiological mechanisms of host responses to brood parasitic egg stimuli: independent lines of research increasingly support the hypothesis that stress-related physiological response to parasitic eggs may trigger egg rejection. The "stress-mediated egg rejection" hypothesis requires that hosts activate the stress-response when responding to parasitic egg stimuli. We tested this prediction by asking whether hosts showed differential stress response when exposed to host-like (mimetic) or parasite-like (non-mimetic) eggs. We experimentally parasitized incubating American robins Turdus migratorius, a robust egg-rejecter host to obligate brood parasitic brown-headed cowbirds Molothrus ater, with mimetic or non-mimetic model eggs. To assess the stress response, we measured the heart rate in incubating females immediately after experimental parasitism. We also measured plasma corticosterone and, in a subset of birds, used RNA-sequencing to analyze the expression of proopiomelanocortin (POMC), a precursor of adrenocorticotropic hormone, 2 h after experimental parasitism. We found that egg type had no effect on heart rate. Two hours following experimental parasitism, plasma corticosterone did not differ between the differently-colored model egg treatments or between rejecter and accepter females within the non-mimetic treatment. However, females exposed to non-mimetic eggs showed an upregulation of POMC gene expression (before FDR correction) in the pituitary compared with females treated with mimetic eggs. Our findings suggest that in an egg-rejecter host species, non-mimetic parasitic eggs may increase the activity of the stress-related hypothalamic-pituitary-adrenal axis compared with mimetic eggs, although the temporal dynamics of this response are not yet understood.

Non-invasive elevation of circulating corticosterone increases the rejection of foreign eggs in female American robins (Turdus migratorius)

Avian obligate brood parasites rely on other species to raise their offspring. In turn, many brood parasite hosts have evolved defensive behaviors to reduce the costs of brood parasitism, yet the proximate bases underlying these defenses remain poorly understood. Recent studies regarding the potential endocrine mechanisms of foreign-egg rejection have implicated corticosterone as a physiological mediator of anti-parasitic defenses. For example, corticosterone is elevated in response to non-mimetic eggs in an egg rejecter thrush, the Eurasian blackbird (Turdus merula) and this hormone's suppression reduces egg rejection rates in the congeneric American robin (T. migratorius). American robins are also among the few host species of obligate brood parasitic brown-headed cowbirds (Molothrus ater) that readily reject foreign eggs from their nests. We non-invasively elevated corticosterone levels in incubating female robins by dissolving it in DMSO gel which was then applied onto eggs already in the clutch. Relative to controls treated with pure DMSO gel, corticosterone-treated female robins were more likely to reject a non-mimetic, cowbird-sized foreign egg (72 %) than control females (50 %) when accounting for the known effect of lower clutch sizes on greater egg rejection. Future studies are needed to assess the sensory and cognitive impact(s) of corticosterone, as well as other hormones essential for parental care, in this and other hosts' defense behaviors against avian brood parasitism.

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

Obligate avian brood parasites lay their eggs in the nest of other bird species, known as hosts. Brood parasitism often imposes severe fitness costs on hosts, selecting for the evolution of effective antiparasitic defences, such as recognition and rejection of brood parasite eggs. Glucocorticoids have been recently found to mediate host physiological and behavioral adjustments in response to brood parasite eggs; however, it remains unclear whether brood parasitism triggers a general response involving multiple physiological elements. In this study, we experimentally investigated whether a salient brood parasitic stimulus (the presence of a nonmimetic model egg in the nest) causes physiological adjustments in adult Eurasian blackbirds (Turdus merula) at immune (leukocyte profiles) and cellular (heat-shock protein Hsp70 synthesis) level. Also, we explored whether these physiological changes are mediated by variations in corticosterone (CORT) levels. We found that experimental brood parasitism caused an increase in heterophils and a decrease in lymphocytes, leading to higher heterophils and lymphocytes ratios in parasitized birds. Nevertheless, we did not find tradeoffs between immune function and CORT levels. Hsp70 synthesis was not affected by our experimental manipulation. Our findings provide evidence that brood parasite eggs trigger a general stress response in egg-rejecter hosts, including changes in cellular immune profiles.