Early acoustic experience alters genome-wide methylation in the auditory forebrain of songbird embryos

Early exposure to salient cues can critically shape the development of social behaviors. For example, both oscine birds and humans can hear and learn to recognize familiar sounds in ovo and in utero and recognize them following hatching and birth, respectively. Here we demonstrate that different chronic acoustic playbacks alter genome-wide methylation of the auditory forebrain in late-stage zebra finch (Taeniopygia guttata) embryos. Within the same subjects, immediate early gene activation in response to acute con- or heterospecific song exposure is negatively correlated with methylation extent in response to repeated daily prior exposure to the same type of stimuli. Specifically, we report less relative global methylation following playbacks of conspecific songs and more methylation following playbacks of distantly-related heterospecific songs. These findings offer a neuroepigenomic mechanism for the ontogenetic impacts of early acoustic experiences in songbirds.

Inter-Individual Variation in Anti-Parasitic Egg Rejection Behavior: A Test of the Maternal Investment Hypothesis

Hosts of avian brood parasites may reduce or forego the costs of caring for foreign young by rejecting parasitic eggs from the nest. Yet, many host species accept parasitic eggs and, even among rejecter species, some individuals go on to incubate and hatch them. The factors explaining the variation in egg rejection between species have received much theoretical and empirical attention, but the causes of intraspecific variation in different individuals’ propensity for accepting parasitic eggs are less well understood. Here we tested the maternal investment hypothesis, which predicts that hosts with costlier clutches will be more likely to reject parasitic eggs from their nest. We studied variation in the egg rejection responses of American robins (Turdus migratorius), a robust egg-rejecter host of the brood parasitic brown-headed cowbird (Molothrus ater), to 3D-printed cowbird-sized eggs which were painted dark blue, a color known to induce variable and repeatable egg rejection responses in individual robins. Costlier clutch investment was estimated by earlier laying date, larger clutch size, heavier unincubated yolk mass, and variable yolk steroid hormone concentrations. There was no statistical support for most of our predictions. However, we detected more concentrated and greater overall amount of deoxycorticosterone deposited in egg yolks of rejecters relative to acceptors, although this accounted for no more than 14% of variance in the data. Future work should test experimentally the potential physiological linkage between maternal egg yolk steroid investment and egg rejection propensity in this and other host species of avian brood parasites.

Exposure to a mimetic or non-mimetic model avian brood parasite egg does not produce differential glucocorticoid responses in an egg-accepter host species

Avian obligate brood parasitism, a reproductive strategy where a parasite lays its egg into the nest of another species, imposes significant fitness costs upon host parents and their offspring. To combat brood parasitism, many host species recognize and reject foreign eggs (rejecters), but others are accepters that raise the parasitic progeny. Some accepter hosts may be unable to grasp or pierce parasitic eggs even if they recognize them as foreign eggs in the clutch, whereas other accepters may not have evolved the cognitive skillsets to recognize dissimilar eggs in the nest. Here we assessed the endocrine responses of an accepter host species to model parasitic eggs to address these two alternatives. We experimentally parasitized nests of a locally common host of the brood-parasitic brown-headed cowbird (Molothrus ater), the prothonotary warbler (Protonotaria citrea; a cowbird-egg accepter), with a mimetic or non-mimetic model cowbird-sized egg. Our goal was to determine whether they perceived the non-mimetic egg as a greater stressor by measuring circulating corticosterone levels. We added eggs to nests during the incubation stage and obtained blood plasma samples from females on the nest 2 h later, using females with unmanipulated clutches as controls. Incubating females showed no differences in baseline plasma corticosterone levels between our different treatments. We conclude that exposure to foreign eggs does not activate the hypothalamic-pituitary-adrenal axis of prothonotary warbler hosts in this experimental paradigm.