To flock or fight: Neurochemical signatures of divergent life histories in sparrows

Abstract

Many bird species exhibit dramatic seasonal switches between territoriality and flocking, but whereas neuroendocrine mechanisms of territorial aggression have been extensively studied, those of seasonal flocking are unknown. We collected brains in spring and winter from male field sparrows ( Spizella pusilla ), which seasonally flock, and male song sparrows ( Melospiza melodia ), which are territorial year-round in much of their range. Spring collections were preceded by field-based assessments of aggression. Tissue series were immunofluorescently multilabeled for vasotocin, mesotocin (MT), corticotropin-releasing hormone (CRH), vasoactive intestinal polypeptide, tyrosine hydroxylase, and aromatase, and labeling densities were measured in many socially relevant brain areas. Extensive seasonal differences are shared by both species. Many measures correlate significantly with both individual and species differences in aggression, likely reflecting evolved mechanisms that differentiate the less aggressive field sparrow from the more aggressive song sparrow. Winter-specific species differences include a substantial increase of MT and CRH immunoreactivity in the dorsal lateral septum (LS) and medial amygdala of field sparrows but not song sparrows. These species differences likely relate to flocking rather than the suppression of winter aggression in field sparrows, because similar winter differences were found for two other emberizids that are not territorial in winter—dark-eyed juncos ( Junco hyemalis ), which seasonally flock, and eastern towhees ( Pipilo erythropthalmus ), which do not flock. MT signaling in the dorsal LS is also associated with year-round species differences in grouping in estrildid finches, suggesting that common mechanisms are targeted during the evolution of different life histories.

Hypothalamic oxytocin and vasopressin neurons exert sex-specific effects on pair bonding, gregariousness, and aggression in finches

“…Although collected only in males, other correlational data from sparrows and mice are likewise consistent with the present findings. Sparrows that flock during winter exhibit significantly more VP neurons in the PVN compared with species that do not flock (this is not observed in the spring), and also exhibit a winter increase in the direct OT innervation of the dorsal LS (33). This innervation likely originates in the PVN, as suggested by findings in rats (32).…”

“…Numerous observations suggest that BSTm VP neurons exert similar effects in mammals. For instance, in both birds and mice, Fos expression in BSTm VP neurons is induced by positive, affiliative interactions but not by fighting or the presentation of negative social stimuli (38)(39)(40), and VP immunoreactivity in the BSTm is negatively correlated with aggression (33,41).…”

An aggression-specific cell type in the anterior hypothalamus of finches

“…The present findings also suggest a functional heterogeneity of hypothalamic VIP populations, with VIP neurons of the AH and tuberal hypothalamus exhibiting opposing relationships to aggression, although effects in the tuberal hypothalamus are not strong. Similarly, recent studies in sparrows (Emberizidae) show that, whereas VIP immunolabeling in the AH and caudal septum is positively related to aggression, immunolabeling in the mediobasal hypothalamus is negatively related aggression (36).…”

Vision in avian emberizid foragers: maximizing both binocular vision and fronto-lateral visual acuity

“…Species that have the highest visual acuity (relative to body mass) most commonly prey on flying insects (Eastern towhee, white-throated sparrow and American tree sparrow; supplementary material Table S1). Additionally, species with relatively higher visual acuity (towhees and song sparrows) tend to be more territorial compared with species with relatively lower acuity (hence, with lower probabilities of detecting predators from far away; Tisdale and Fernández-Juricic, 2009), which tend to flock more (field sparrows, dark-eyed juncos; Goodson et al, 2012). The implication is that the benefits of flocking (dilution and collective detection effects; Krause and Ruxton, 2002) might compensate for some sensory constraints.…”