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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.

At the termination of the breeding season, many bird species leave their exclusive territories and join flocks that range from small parties to thousands of individuals. This dramatic seasonal shift in behavioral phenotype undoubtedly has profound fitness implications, but to our knowledge, no studies have addressed the neural or endocrine mechanisms that promote seasonal flocking. In contrast, mechanistic studies of avian territorial aggression are relatively extensive and have inarguably revolutionized the field of behavioral endocrinology (Wingfield, 2005; Soma, 2006). However, few of these studies explore the brain mechanisms of territoriality (Soma, 2006; Maney and Goodson, 2011). Using four emberizid songbird species that have evolved divergent life-history strategies, we here examine seasonal variation and evolutionary diversity in six neurochemical systems and demonstrate links of those systems to both winter flocking and territorial aggression.

On the basis of the immediate early gene responses of (i) male rodents to resident–intruder encounters, and (ii) male song sparrows (Melospiza melodia) to simulated territorial intrusion (playback of song and presentation of a caged male decoy), it seems that the neural substrates of territorial aggression are extensively comparable in birds and mammals. Thus, in both taxa significant activation is observed in the medial bed nucleus of the stria terminalis (BSTm), lateral septum (LS), paraventricular nucleus of the hypothalamus (PVN), anterior hypothalamus (AH), lateral portion of the ventromedial hypothalamus (VMH), and midbrain central gray [Kollack-Walker et al. (1997), Maney and Ball (2003), Goodson and Evans (2004), Goodson et al. (2005); also see Kingsbury et al. (2011)]. For the year-round territorial song sparrow, immediate early gene results are largely comparable in winter and summer (Goodson and Evans, 2004; Goodson et al., 2005), although microarray data suggest that hypothalamic responses to simulated intrusion are very different in winter and summer, perhaps reflecting the fact that luteinizing hormone is released during territorial challenges only in the breeding season (Mukai et al., 2009). Conversely, neurons that produce steroidogenic enzymes such as aromatase (ARO) may show greater activity in winter, given that territoriality in song sparrows shifts from reliance on gonadal steroids during the breeding season to nongonadal hormone production during the fall and winter (Wingfield, 2005; Soma, 2006).

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