At least three lines of research support the proposal that flocking is strengthened by mutual positive associations with flock mates. First, in choice tests zebra finches, Taeniopygia guttata prefer to spend more time near larger flocks of conspecifics (consisting of 10 same-sex individuals) compared to smaller flocks (consisting of two same-sex individuals) (Kelly et al., 2011). This finding indicates that in a species that is highly gregarious in nature, the presence of large numbers of conspecifics may be rewarding. Second, isolated European starlings, Sturnus vulgaris are willing to work (i.e., trigger sensors) to view images of conspecifics in the absence of any other reward. Furthermore, they respond more to pictures of starlings compared to pictures of landscapes or monkeys (Perret et al., 2015). This demonstrates that conspecific, social stimuli are primary reinforcers in this social species. Third, studies using conditioned place preference (CPP) tests [a common method to assay reward (Carr et al., 1989; Tzschentke, 2007; Trezza et al., 2009; Riters et al., 2013)] show that vocal-social interactions in non-breeding flocks of starlings and zebra finches are associated with a positive affective state (Riters and Stevenson, 2012; Riters et al., 2014; Hahn et al., 2017). In these studies, flocks of male starlings and zebra finches were observed singing in aviaries for 30 min. Each bird was then immediately placed individually into one of two distinctly decorated sides of a conditioning cage for 30 min and afterwards returned to its home aviary. The next day each bird was placed back into the conditioning cage and allowed to move freely between the previously song-paired and the non-song-paired sides of the cage, and the amount of time each individual spent on each side was recorded. The prediction was that if vocal-social interactions in non-breeding flocks are associated with a positive affective state, then birds would learn to associate the positive affective state with the distinctly decorated side of the cage and when given a choice spend most of their time on that side (i.e., they would develop a CPP). Results demonstrated that both male zebra finches and starlings developed a CPP for the chamber that had been paired with production of song in gregarious flocks (Riters and Stevenson, 2012; Riters et al., 2014; Hahn et al., 2017) (Figure 1). However, birds did not develop a CPP associated with song produced in mating or agonistic contexts, indicating that the factors that reward goal-directed, immediately reinforced behaviors differ from those underlying affiliative behaviors produced in non-sexual, gregarious contexts.

Although CPP tests are commonly used to examine rewarding properties of drug use, feeding and sexual behaviors, the function of CPPs in wild animals in natural contexts is seldom considered. The finding that vocal-social interactions in gregarious flocks can lead to the development of a CPP suggests that a natural function of this type of conditioning may be to strengthen group cohesion through a conditioned preference for a particular flock. Overall, these findings offer support for the hypothesis that social interactions in gregarious contexts can be positively reinforced.

It is common to observe birds that are separated from flocks appearing to rush to reunite with conspecifics. Early ethologists compared the motivation to flock in gregarious animals to a “hunger, a craving or sensation of discomfort” or a “state of agitation” in the absence of a physical requirement that can only be relieved by reunion with a flock (Trotter, 1916; Craig, 1918; Emlen, 1952a). This suggests that flocking may be strengthened not only because interactions with flock mates induce a positive affective state but also because they reduce a negative affective state. That is, flocking may be negatively reinforced. In gregarious vertebrates, social separation is painful (Macdonald and Leary, 2005; Eisenberger, 2012). For example, separation of young rats from mothers or the removal of guinea pigs, domestic chicks or zebra finches from conspecifics leads to the production of “distress vocalizations” or contact calls that are eliminated by reunion with group mates (Zann, 1985; Hofer et al., 1993; Kyuhou and Gemba, 1998; Warnick et al., 2005). Several studies demonstrate that the same neural systems and modulators that process physical pain also regulate the pain of social separation, and this is proposed to be adaptive given that social disconnection in gregarious species threatens survival (Macdonald and Leary, 2005; Eisenberger, 2012). For example, treatments that reduce physical pain (i.e., induce analgesia) are also found to reduce signs of social pain in animals as well as activity in brain regions underlying physical pain. This includes treatment with opioids such as morphine and even treatment with the common pain reliever acetaminophen (Herman and Panksepp, 1978; Panksepp et al., 1978; Dewall et al., 2010).

The data showing that social and physical pain share underlying mechanisms lead to the prediction that social interactions in flocks may reduce social pain. Some support for this idea comes from a CPP study in male zebra finches. In zebra finches, separation from a flock increased production of distance contact calls, which are triggered by separation from partners or flock mates and reduced when individuals are reunited (Guttinger and Nicolai, 1973; Simpson and Vicario, 1990; Zann, 1996). In contrast to the CPP identified for song in flocks described above, zebra finch contact calls correlated negatively with an individual’s affective state measured using CPP (i.e., males that produced high numbers of contact calls avoided the side of the CPP chamber associated with these calls) (Riters and Stevenson, 2012). This indicates that the affective state associated with separation-induced calls is negative.

The idea that social interactions in flocks relieve social pain via mechanisms that also regulate physical pain is supported by a song-associated analgesia study in male starlings (Kelm-Nelson et al., 2012). Singing was recorded in gregarious non-breeding flocks or in a mate-directed context. Each bird was then immediately captured and one foot was submerged in a hot water bath (hot enough to be mildly aversive; some birds retracted the foot immediately, but others did not respond). The amount of time it took for each bird to remove its foot from hot water was noted as a measure of analgesia. A linear positive correlation was found between this measure of analgesia and the production of song in flocks, indicating that vocal-social interactions in flocks are associated with pain reduction (Figure 2). A similar relationship was not found between sexually motivated female-directed song and analgesia, which again suggests that mechanisms mediating mate-directed, immediately reinforced behaviors differ from those underlying affiliative behaviors produced in gregarious contexts. This form of analgesia may function to strengthen group cohesion through the reduction of an aversive state. Thus, these findings offer support for the hypothesis that social interactions in non-sexual, gregarious contexts can be negatively reinforced.

Based on the associations revealed by these behavioral studies between singing in flocks and both positive affect and analgesia, we propose that flocking likely involves a combination of incentives and reinforcement mechanisms. This includes positive reinforcement (i.e., a behavior is strengthened because it leads to a positive outcome) and negative reinforcement (i.e., a behavior is strengthened because it leads to reduction of an aversive state). For example, it may be that an aversion to being alone pushes gregarious individuals to join flocks. Joining a flock may thus be negatively reinforced because it reduces the aversive state associated with isolation. Then, once a bird has joined a flock, behaviors produced within the flock, such as singing, may induce a positive affective state that then functions to positively reinforce associations with the flock.

Opioid neuropeptides are the neuromodulators that to date have been best studied for their roles in both physical and social pain (Macdonald and Leary, 2005; Eisenberger, 2012). Opioids that bind to mu opioid receptors are also well-known for their rewarding and analgesic properties (Matthes et al., 1996; Trescot et al., 2008; Jhou et al., 2012; Fields and Margolis, 2015). Animals will readily self-administer mu opioid receptor agonists, such as morphine and develop strong CPPs for places associated with mu receptor agonist treatment (Wise, 1989; McBride et al., 1999). Mu receptor agonists also decrease the amount of time rats spend near conspecifics, which has been interpreted to suggest that reward induced by the agonist replaces the need for reward (or relief) that is normally induced by social contact (Herman and Panksepp, 1978; Panksepp et al., 1979). A role for opioids in pain relief induced by social interactions is also supported by past research. For example, reunion of male mice with siblings has been found to induce opioid-dependent analgesia (D’Amato and Pavone, 1993; D’Amato, 1998). Furthermore, the hot water foot dip test of analgesia used in the starling study described above is opioid sensitive (Evrard and Balthazart, 2002; Kelm-Nelson et al., 2012), suggesting that for starlings social contact within flocks (as reflected by flock singing) may release opioids to relieve the pain of isolation.

Data from other species offer more direct support for opioid-mediated social reward and/or reduction of social pain. Mu receptor agonists reduce separation distress vocalizations in rodents and primates; whereas the opioid receptor antagonist naloxone increases these calls (Herman and Panksepp, 1978; Panksepp et al., 1978; Kalin et al., 1988). Distress vocalizations in domestic chicks separated from flock mates were also reduced by systemic treatment with a mu opioid receptor agonist (Warnick et al., 2005). Manipulations of delta and kappa opioid receptors were ineffective, indicating that reduction of social distress induced by social reunion in chicks is mediated selectively by mu receptors. In contrast, peripheral injections of the opioid receptor antagonist naloxone in male zebra finches suppressed “undirected” singing behavior (Dunn and Zann, 1996; Zann, 1996; Khurshid et al., 2010). This is a type of song that is predominantly produced in gregarious flock settings; however, a caveat is that in this study undirected song was produced by birds in isolation rather than in a flock setting. Although differences may be found in birds producing this type of song as part of a gregarious flock, unpublished data in starlings singing in flocks also indicate that mu opioid receptor agonism facilitates singing in gregarious flocks (Riters et al., unpublished data). These findings appear to contradict the studies in rats that show mu agonism decreases time with conspecifics (reviewed above). However, one interpretation is that an optimal level of mu opioid receptor stimulation is needed to facilitate gregariousness, with low doses of MOR agonist stimulating behavior to a point, after which increasing doses suppress behavior (resulting in an inverted-U shaped curve). This idea is supported multiple studies that show inverted-U shaped relationships between mu opioid stimulation and behavior [e.g., for sucrose consumption (Zhang and Kelley, 1997), for self-injection of MOR agonist (Simmons and Self, 2009)]. Thus when taken together, studies to date indicate that opioids that act at mu receptors both reduce vocal behavior indicative of distress and stimulate vocal behavior indicative of a positive (or less negative) affective state, consistent with a role for opioids in the regulation of flocking through both pain reduction (i.e., negative reinforcement) and pleasure (i.e., positive reinforcement).

There are several brain regions in which the activation of mu opioid receptors can induce reward and analgesia. One such region is the medial preoptic nucleus (mPOA; often referred to as POM in birds) (Tseng et al., 1980; Tseng and Wang, 1992), which has been implicated strongly in the regulation of the affective state associated with birdsong (Riters, 2012; Riters et al., 2017). It is well known across vertebrates that the mPOA stimulates goal-directed, sexually motivated behaviors, including sexually motivated birdsong (Riters and Ball, 1999; Alger and Riters, 2006; Alger et al., 2009). However, this region also appears to play a role in non-sexual vocal-social interactions in flocks of male starlings. In contrast to the inhibition of sexually motivated song by mPOA, lesions of the mPOA tend to promote song in flocks in non-breeding male starlings (Alger and Riters, 2006). In mPOA, opioids that bind to mu receptors inhibit neuronal firing in birds and mammals (Diez-Guerra et al., 1987; Furukawa et al., 1995), suggesting that opioid release associated with vocal-social interactions in flocks may inhibit activity in mPOA to facilitate song in non-breeding flocks.

Opioid measures in mPOA correlate with vocal-social interactions in gregarious flocks. Measures of mu opioid receptor immunolabeling in mPOA correlate positively with vocal-social interactions in gregarious flocks, but only to a point, after which higher rates of song are associated with lower densities of receptor labeling, resulting in inverted-U shaped relationships (Kelm-Nelson and Riters, 2013) (Figure 3). While it could be that opioid signaling decreases in birds singing high rates of non-breeding flock song, met-enkephalin labeling in mPOA relates positively to singing behavior even in the birds singing at the highest rates (Riters et al., 2005). Mu opioid receptors down-regulate in response to sustained occupation by enkephalin (Chang et al., 1982; Harrison et al., 1998), suggesting that high levels of opioid release in mPOA associated with the highest rates of vocal-social interaction may cause mu-opioid receptors to down-regulate (explaining the low mu densities in the second half of the curve; Figure 3).

Opioids (i.e., enkephalins and other mu receptor agonists) in the mPOA, have been shown in rats to induce analgesia as well as reward (analgesia: Tseng et al., 1980; Tseng and Wang, 1992); reward: Agmo and Gomez, 1991, 1993; Le Merrer et al., 2009). These previous data lead to the hypothesis that opioid release in mPOA caused by joining a flock and interacting socially with flock mates underlies both the reduction in pain (i.e., analgesia) and reward (i.e., song-associated CPP) observed in starling flocks. Consistent with this hypothesis, in male starlings, the affective state associated with vocal-social interactions in flocks (measured using CPP) correlated positively with both preproenkephalin (the precursor of the opioid met-enkephalin) and mu opioid receptor mRNA expression levels in mPOA (Riters et al., 2014). Moreover, preliminary data from a study on the mPOA show that selective downregulation of MOR in this region (induced by siRNA infusion) in male starlings suppresses affiliative song and disrupts song-associated reward, (Riters et al., unpublished results).

Dense dopaminergic projections from the VTA to the NAc are among the best studied components of the canonical mesolimbic reward pathway [for recent review see (Baik, 2013)]. Multiple studies demonstrate these dopaminergic projections to be crucial for motivated, approach responses to rewarding stimuli (Berridge and Robinson, 1998; Ikemoto and Panksepp, 1999; Ikemoto et al., 2015; Volkow et al., 2017). These projections are highly evolutionarily conserved and underlie multiple motivated, reward directed behaviors, with a few studies suggesting this role may extend to flocking behavior. For example, gregarious species of estrildid finch have more neurons labeled for tyrosine hydroxylase (a rate-limiting enzyme in catecholamine synthesis) in VTA than non-gregarious, territorial species (Goodson et al., 2009a). Tyrosine hydroxylase mRNA in VTA also correlates positively with vocal-social interactions in flocks of male starlings (Merullo et al., 2016) (Figure 5). These studies are consistent with the possibility that dopaminergic VTA projections modulate the motivation to interact with flock mates. Dopamine is also released in the VTA projection region Area X in male zebra finches singing undirected song (a type of song produced commonly in flocks) (Sasaki et al., 2006). Studies also implicate other neuromodulators, including neurotensin and endocannabinoids, in the VTA in non-sexual, vocal-social interactions in starling flocks. Neurotensin strongly modulates activity of dopamine neurons in the VTA (Steinberg et al., 1995; Kortleven et al., 2012; Stuhrman and Roseberry, 2015) and neurotensin mRNA in VTA correlates positively with singing behavior in starling flocks (Merullo et al., 2016) (Figure 5). Endocannabinoid CB₁ receptors also modulate the firing of dopamine neurons in VTA (Merullo et al., 2016), and CB₁ receptor mRNA expression in VTA correlates positively with CPP measures of flock song-associated reward (Merullo et al., 2016; Hahn et al., 2017) (Figure 5). In mammals, opioids in the VTA have also been found to indirectly stimulate dopaminergic neurons and motivated approach behaviors, with a similar mechanism identified in songbirds (Gale and Perkel, 2006). However, correlational studies so far do not (Riters et al., 2014), or only weakly (Riters et al., 2005), implicate opioids in the VTA in vocal-social interactions in flocks.

In contrast to the well-studied role of VTA dopamine projections in motivated responses to rewarding stimuli, opioids binding to mu receptors in the NAc are implicated in reward (i.e., the hedonic pleasure induced when a reward is received) (Smith and Berridge, 2007; Berridge, 2009). Although a putative location for the NAc has been identified in birds based on neurochemical and hodological similarity to mammalian NAc (Balint and Csillag, 2007; Balint et al., 2011; Husband and Shimizu, 2011), to date no studies have experimentally examined the role of mu opioid receptors in the NAc in social reward in birds. An important next step in this line of research will be to examine the role of mu receptors in NAc in flocking behavior.

The PAG has been well studied as a site in which mu opioid receptors act to induce analgesia (Bodnar et al., 1988; Spinella et al., 1999; Wiedenmayer and Barr, 2000; Morgan et al., 2014). The stimulation of mu opioid receptors in PAG also induces CPP (Olmstead and Franklin, 1997). Studies in birds and mammals suggest that the PAG gathers and integrates information about affective state from other brain regions, including mPOA, which it then relays to vocal production areas so that an animal emits a vocal signal reflective of its emotional state (Jurgens and Pratt, 1979; Absil et al., 2001; Gruber-Dujardin, 2010). In mammals, electrical input from mPOA to PAG stimulates vocalizations produced in positive contexts (i.e., calls produced during sexual behavior in guinea pigs or clucking in monkeys) but not distress vocalizations (i.e., isolation distress calls in guinea pigs or shrieks in monkeys) (Kyuhou and Gemba, 1998; Dujardin and Jurgens, 2006). Data also show that enkephalin opioids and stimulation of mu receptors in the PAG suppress negative vocalizations (i.e., hissing in cats) (Shaikh et al., 1991a,b). In starlings, similar to what was found for mPOA, inverted-U shaped relationships were detected between vocal-social interactions in flocks and measures of mu opioid receptor immunolabeling PAG (Kelm-Nelson and Riters, 2013) (Figure 6). Numbers of neurons labeled for the immediate early gene ZENK (also referred to as Egr-1) and numbers of ZENK labeled neurons double labeled for tyrosine hydroxylase were also higher in PAG in male zebra finches producing undirected song in flocks compared to silent males (Lynch et al., 2008). A similar result was observed in Bengalese finches, Lonchura domestica producing undirected song when isolated (Matheson and Sakata, 2015). Studies in male starlings also show that the dopamine metabolite DOPAC in PAG correlates positively with singing behavior in gregarious starling flocks (Heimovics et al., 2011) (Figure 6). Markers for cannabinoids (i.e., cannabinoid receptors and the cannabinoid transporter FABP7) also correlate negatively with song-associated CPP (Hahn et al., 2017) (Figure 6). The function of these relationships must now be tested experimentally using site-specific pharmacological or gene manipulations; however, together with past studies, these correlational data suggest potential roles for the PAG as well as the VTA in vocal-social interactions in flocks and affective state.

The PAG and VTA may modulate motivated behavior by different mechanisms. For example, it has been speculated that infusion of the mu opioid receptor agonist morphine into the PAG induces CPP (Olmstead and Franklin, 1997) by reducing negative affect (rather than by inducing a positive affective state), based on studies showing that a main role for the PAG is to modulate responses to aversive stimuli (Lieberman and Eisenberger, 2009; Wright and Panksepp, 2011). In contrast, morphine in VTA may induce CPP by increasing positive affect, based on studies showing the VTA to be important for reward and approach (Bozarth and Wise, 1981; Wise and Bozarth, 1987; Berridge and Robinson, 1998; Ikemoto and Panksepp, 1999; Ikemoto et al., 2015; Volkow et al., 2017). This suggests that the pleasure and pain that facilitate flocking behavior may be modified by these two complementary output pathways from mPOA, with PAG integrating information during social interactions that leads to pain reduction and the VTA integrating information leading to social approach and reward.

As introduced earlier, many songbirds shift seasonally from pair or solitary living during the breeding season to the formation of flocks after the breeding season. In an early review of flocking behavior, Emlen (1952a) suggested that seasonal increases in steroid hormone concentrations may disrupt a default state of gregariousness by facilitating behaviors disruptive to social cohesion (i.e., agonistic and sexual behaviors) (Emlen, 1952a). Consistent with this idea, in many temperate zone breeding songbirds, gonadal steroid concentrations are low outside the breeding season when birds often congregate in flocks (e.g., Wingfield and Farner, 1978; Dawson, 1983; Van Duyse et al., 2003). Low sex steroid hormones outside the breeding season in male songbirds are associated with the production of relatively shorter and less stereotyped songs (Smith et al., 1995, 1997; Riters et al., 2000; Alger et al., 2016). These songs are less attractive to females and less threatening to male competitors compared to longer, more stereotyped songs that are produced in primary reproductive contexts (Searcy and Yasukawa, 1996; Woolley and Doupe, 2008). Thus, social tolerance may be promoted in flocks through the de-emphasis of song features that induce sexual and agonistic responses.

As concentrations of testosterone in males and estradiol in females rise at the onset of the breeding season, flocks disperse and former flock mates begin to aggressively defend breeding territories and to compete for mates, singing long, stereotyped songs that repel conspecifics and attract mates (Wingfield and Farner, 1978; Dawson, 1983; Smith et al., 1995, 1997; Searcy and Yasukawa, 1996; Riters et al., 2000; Van Duyse et al., 2003; Alger et al., 2016). In some species, treating wintering birds with testosterone increases the frequency of aggressive interactions in flocks and can lead to increased spacing among flock individuals (Emlen and Lorenz, 1942; Baptista et al., 1987; Archawaranon et al., 1991). Furthermore, the mPOA has been identified as a central site in birds and mammals in which testosterone acts to promote agonistic and sexual motivation (Schlinger and Callard, 1989; Watson and Adkins-Regan, 1989a,b; Balthazart et al., 1990; Riters et al., 1998; Coolen and Wood, 1999; Hull et al., 1999; Ball and Balthazart, 2004; Balthazart and Ball, 2007), as well as the production of sexually motivated songs in canaries, Serinus canaria (Alward et al., 2013, 2018). Thus testosterone may act in mPOA to dissociate flocks by promoting aggressive and sexual behaviors among flock members.

In many vertebrates, opioids in the mPOA and/or their receptors change seasonally and are regulated by steroid hormones (Watson et al., 1986; Hughes et al., 1990; Mateo et al., 1992; Hammer et al., 1994; Eckersell et al., 1998; Holland et al., 1998; Scott et al., 2008; Woods et al., 2010; Spool et al., 2016). [Nonapeptides and/or their receptors also change seasonally and are regulated by steroid hormones, but we limit our discussion here to opioids because they are the main focus of this review and there are several papers that detail region- and species-differences in steroid effects on nonapeptides (e.g., DeVries et al., 1985; Goodson and Bass, 2001; Panzica et al., 2002; Plumari et al., 2004; Goodson and Thompson, 2010)]. One possibility is that steroid-dependent changes in opioids (and nonapeptides) modify flocking seasonally. Results of studies in starlings are consistent with this hypothesis. Treatment of castrated male starlings with testosterone alters opioid markers in a complex fashion, resulting in increased mu-opioid receptor and preproenkephalin mRNA and steroid-related mRNA (i.e., androgen receptors and aromatase) in mPOA relative to controls, but also reducing numbers of immunolabeled mu opioid receptor cell bodies in the rostral portion of mPOA (Figure 7) (Spool et al., 2016, 2018b), a subregion thought to modulate appetitive sexual behavior (Riters and Ball, 1999; Balthazart and Ball, 2007). In female starlings, individuals with elevated concentrations of estradiol tend to have higher preproenkephalin mRNA in mPOA relative to females with low concentrations of estradiol (Spool et al., 2018a). Additionally, in free-living songbirds mu opioid receptor mRNA in the mPOA changes seasonally, peaking during the breeding season (Woods et al., 2010). The results of these studies suggest that seasonal, steroid-dependent changes in opioid activity in mPOA may modulate seasonal changes in gregariousness such as those observed in European starlings.

A study in house sparrows, Passer domesticus, another seasonally gregarious songbird offers additional indirect evidence that changes in seasonal flocking behavior are linked to opioid release. In this study testosterone treatment was found to induce analgesia as measured using the opioid-sensitive hot water foot withdrawal test of analgesia described above in starlings (Hau et al., 2004). This suggests that seasonal changes in testosterone may alter opioid release, or tissue sensitivity to opioids, to reduce aversive states induced by solitude at times when it is adaptive for individuals to remove themselves from social flocks. Given the overlap in the role of opioids in both physical and social pain, it may be that a testosterone-induced increase in opioid release or receptor numbers that occurs during the breeding season replaces the need for pain relief that is induced by social interactions in flocks in the non-breeding season.

Although there is much evidence to support a role for steroid hormones in shifting animals from a prosocial, tolerant state to an agonistic, intolerant state, the relationship between steroid hormones and flocking behavior in birds is complex. For example, in many species some level of aggression persists in non-breeding flocks despite low concentrations of circulating steroids, usually as the result of competition over food and roosting resources (Sabine, 1949; Lockie, 1956; Pinxten et al., 2000; Smith et al., 2005), and the flock remains together despite these conflicts. This type of aggression has been found to be regulated by brain site-specific de novo steroid synthesis (Heimovics et al., 2015a,b), yet the extent to which local neurosteroid synthesis modulates flocking behavior has not been studied. Furthermore, some seasonally breeding birds mate in colonial settings and exhibit social tolerance even while sex steroid hormones are elevated. Animals in these breeding groups typically defend small spaces around nest sites [e.g., zebra finches (Zann, 1996), magellanic penguins, Spheniscus magellanicus (Stokes and Boersma, 2000), black skimmers, Rynchops niger (Burger, 1981), cliff swallows, Petrochelidon pyrrhonota (Emlen, 1952b)]. Larger colony sizes (i.e., greater numbers of potential competitors) in cliff swallows are associated with greater circulating concentrations of testosterone in both males and females (Smith et al., 2005). Furthermore, in male starlings testosterone was higher in males nesting in a dense colony compared to males nesting at more dispersed sites (Ball and Wingfield, 1987). Thus in many species increases in sex steroid hormones are associated with flock dissociation at the beginning of the breeding season, yet in other species they promote small-scale territoriality and resource guarding within colonial groups without causing flocks to dissociate. Season and species-specific differences in steroid effects on modulators, such as opioids or nonapeptides, in brain regions involved in flocking may reconcile these variable findings across species and seasons.