A seasonal comparison of the mesotocin system in a brood parasite, the brown headed cowbird (Molothrus ater), with its nonparasitic relative the red-winged blackbird (Agelaius phoeniceus)

Nest building and incubation is a complex maternal behaviour which is characterised by regression of the ovary, cessation of egg laying and increased maternal defence of the offspring and nest and the neuroendocrine system plays a crucial role in its regulation. The nonapeptide mesotocin plays an important role in promoting reproductive behaviour as we recently we discovered a role for mesotocin in nest building in zebra finches (Hall et al 2015). Such discoveries raise the question of the role of the mesotocin system and social behaviour neural network in a species that doesn’t build a nest or display parental care. Oxytocin, the avian homologue of mesotocin and their receptors have been shown in mammals to be critical for controlling social behaviours including maternal behaviour. In this study, we used immunocytochemistry to determine numbers of mesotocin labelled neurones in the paraventricular nucleus (PVN) and the bed nucleus of the stria terminalus (BNST) of the parasitic brown-headed cowbird (Molothrus ater) and the non-parasitic red-winged blackbird (Agelaius phoeniceus) across the reproductive cycle. Brown-headed cowbirds are obligate brood parasites and do not build nests or express parental care. Females search for and find host nests one or more days before egg laying. In contrast its non-parasitic relative, the red-winged blackbird, builds a nest, lays eggs, incubates and feeds the chicks. Free-living wild birds were captured in mist nets in Western Ontario during pre-breeding, breeding and post-breeding stages, euthanized with isoflurane perfused with formaldehyde and brains cut on a freezing microtome. In females, the number of immunoreactive mesotocin neurones were lower in the PVN of brown-headed cowbirds, correlating to the lack of maternal behaviour in this species. There was no differences in mesotocin immunoreactivity in the BNST between species nor were there any seasonal changes.

Research supported by NSERC, BBSRC and the Society for Reproduction and Fertility.