Patterns of stress response to foreign eggs by a rejecter host of an
obligate avian brood parasite
Abstract
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
eggs. We tested this prediction by experimentally parasitizing
incubating American robins Turdus migratorius, an 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
(ACTH), two hours 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 in the
pituitary compared to females treated with mimetic eggs. Our findings
suggest that parasitic eggs may activate the stress-related
hypothalamic-pituitary-adrenal axis in an egg-rejecter host species,
although the dynamics of this response are not yet understood.