Why Do Birds Flock? A Role for Opioids in the Reinforcement of Gregarious Social Interactions

Abstract

The formation of social groups provides safety and opportunities for individuals to develop and practice important social skills. However, joining a social group does not result in any form of obvious, immediate reinforcement (e.g., it does not result in immediate copulation or a food reward), and individuals often remain in social groups despite agonistic responses from conspecifics. Much is known about neural and endocrine mechanisms underlying the motivation to perform mate- or offspring-directed behaviors. In contrast, relatively little is known about mechanisms underlying affiliative behaviors outside of these primary reproductive contexts. Studies on flocking behavior in songbirds are beginning to fill this knowledge gap. Here we review behavioral evidence that supports the hypothesis that non-sexual affiliative, flocking behaviors are both (1) rewarded by positive social interactions with conspecifics, and (2) reinforced because affiliative contact reduces a negative affective state caused by social isolation. We provide evidence from studies in European starlings, Sturnus vulgaris , that mu opioid receptors in the medial preoptic nucleus (mPOA) play a central role in both reward and the reduction of a negative affective state induced by social interactions in flocks, and discuss potential roles for nonapeptide/opioid interactions and steroid hormones. Finally, we develop the case that non-sexual affiliative social behaviors may be modified by two complementary output pathways from mPOA, with a projection from mPOA to the periaqueductal gray integrating information during social interactions that reduces negative affect and a projection from mPOA to the ventral tegmental area integrating information leading to social approach and reward.

Mu opioid receptors in the medial preoptic area govern social play behavior in adolescent male rats

“…Opioids in the MPOA of songbirds act to stimulate and maintain affiliative, nonsexual vocal-social interactions. 1,53,54 The present study extended research findings on this type of affiliative, rewarding social behavior from songbirds to rats. Using an IEG expression approach, we found that social play behaviors were associated with a robust increase in expression of Egr1 in the MPOA.…”

Birdsong and the Neural Regulation of Positive Emotion

“…Song in this context is facilitated by the presence of flock mates ( Gochfeld, 1977 ; Petrinovich, 1988 ; Jesse and Riebel, 2012 ), but unlike sexually motivated song it is commonly produced by birds facing away from other birds and it appears to be ignored ( Feare, 1984 ; Zann, 1996 ; Riters et al, 2000 ). The immediate function of song in these flocks is not clear, but it is important for song learning and practice and proposed to play a role in flock cohesion ( Riters et al, 2019 ). This is a type of song commonly referred to as undirected song in zebra finches ( Dunn and Zann, 1996 ).…”

μ-Opioid Receptor Stimulation in the Nucleus Accumbens Increases Vocal–Social Interactions in Flocking European Starlings,Sturnus Vulgaris

“…Many bird species form remarkable flocks, often outside the context of breeding, that offer protection from predators and increase foraging efficiency ( Powell, 1974 ; Sullivan, 1984 ). It has been proposed that such large social groups may be maintained by rewarding positive interactions with conspecifics and by reducing negative states caused by social separation ( Emlen, 1952 ; Riters et al, 2019a ). Moreover, within a flock optimal social spacing between conspecifics is important, indicating that individuals may optimize spacing to be close but not too close to conspecifics.…”