Discussion
Our current understanding of social behavior in wild turkeys is based on visual observations of interactions during the breeding season (Watts and Stokes 1971, Healy 1992). We used high resolution movement data to assess social behaviors coupled with timing of nest initiation to infer social rank amongst females, and provide an alternative approach to infer behaviors and evaluate reproductive synchrony in wild turkeys (Bakner et al. 2019, Lohr et al. 2020). We found that female wild turkeys had social organization within flocks, likely maintained by social hierarchies that influence behaviors of individuals throughout their lives (Watts and Stoke 1971, Healy 1992). Our findings offer evidence that social dominance may influence timing of reproduction in wild turkeys, and subsequently influence individual reproductive success.
We found that female wild turkeys rarely left social groups prior to initiation of their first nest attempts. Similarly, Badyaev et al. (1996a ) observed that female wild turkeys in Arkansas typically left their winter flocks at the same time regardless of physiological factors or age. We also found that prior to the onset of nest initiation, >80% of females within social groups overlapped ranges, so clearly females within social groups spatially constituted a single social unit (Brown 1975). Stable social groups have been observed in multiple avian species, including female black grouse (Lyrurus tetrix tetrix ) who frequently occupied the same territory while foraging (Kruijt and Hogan 1967). Similarly, groups of female greater prairie chickens (Tympanuchus cupido pinnatus ) exhibited social interactions during visits to leks (Robel and Ballard 1974) and female sage grouse (Centrocercus urophasianus ) maintained social hierarchies that influenced timing of reproduction (Scott 1942).
We observed that timing of the onset of first nest initiation at the population level (e.g., across our study site) was similar across years. Researchers have noted similar observations previously, and attributed synchronous nesting behaviors at the population level to the potential that photoperiod most influences timing of reproduction (Healy 1992, Migaud et al. 2006, Walton et al. 2011). We also noted that average dates of first nest initiation for female wild turkeys on our site were comparable to dates reported across populations throughout the southeastern United States (Thogmartin and Johnson 1999, Palmer et al. 2013, Crawford et al. 2021). Conversely, within social groups we found substantive temporal variation in timing of nest initiation within years. Because social rank can dictate access to mates in species with pronounced dominance hierarchies, the fact that asynchronous nesting occurred within social groups on our study site was not surprising (Robel and Ballard 1974, Foster 1981, Webster 1994). We also observed that ~30% of successful initial first nests were produced by ~4% of females in our population, females we presumed to be dominant within their respective social groups. We note that these initial first nests by default were successful early in the nesting season, and previous works have noted the importance of successful early nesting attempts in sustaining wild turkey populations (Porter et al. 1983, Crawford et al. 2021). Contemporary literature has detailed the ecological significance of having females be successful at hatching clutches early in the nesting season for various species, including willow ptarmigan (Lagopus lagopus ; Wilson et al. 2007) and greater sandhill cranes (Antigone canadensis ; Ivey and Dugger 2008).
Within social groups, we expected 1–2 days to occur between each female initiating their first nest attempt, based on observations of nesting behaviors in captive wild turkeys detailed in Healy (1992). However, we found that on average, 3–7 days elapsed between successive nest attempts by individual females within a social group. We acknowledge that we have no way of confirming that we captured the dominant female within each social group, and recognize that we didn’t capture all individuals in every social group we studied. However, we offer that it’s reasonable to assume that dominance hierarchies existed within the social groups we monitored, and that these hierarchies influenced timing of nest initiation across females. Robel and Ballard (1974) noted that disruption of subordinate female greater prairie chickens by dominant females caused 2-3 day delays in copulation. Logically, disruption could increase time between successive nest attempts across subordinate females within a group, which may ultimately benefit dominant females if copulation was delayed in subordinate females or if they were forced to mate with inferior males (Foster 1981, Bro-Jørgensen 2002). However, female disruption only offers a partial explanation for the delay between successive nest attempts we observed. In lekking species, females are constrained in mate choice, and disruption of copulations between females and dominant males can occur via harassment from low-ranking males, which also could delay nesting (Foster 1983, Trail 1985). Likewise, if females are forced to travel farther to find suitable dominant males or return to breeding areas repeatedly to copulate with those same males, onset of nesting could be delayed (Alatalo et al. 1988).
Our findings suggest that larger social groups tended to exhibit more synchronized nest initiation, that number of days nest attempts across females declined as group size increased, and that more synchronized nesting attempts within a social group (as determined by days between nest attempts across females) resulted in greater probability of nest success. Previous research has demonstrated that larger male coalitions of wild turkeys attract more breeding opportunities through improved mate attraction relative to smaller coalitions (Krakauer 2008), which also has been observed in other species that use forms of leks (Jiguet and Bretagnolle 2006, Ryder et al. 2009). Likewise, in various species including wild turkeys, larger male coalitions are typically associated with larger social groups of females, are socially dominant over smaller coalitions, and would contain the highest ranked males within the breeding population (Watts and Stokes 1971, Krakauer 2005, Bygott et al. 1979). Conversely, smaller groups of female wild turkeys are often associated with pairs of subordinate males or singletons (Watts and Stokes 1971). In species with pronounced social hierarchies, breeding with dominant males confers females with fitness benefits such as greater reproductive success (Wong and Candolin 2005, Majolo et al. 2012), offering a partial explanation for our observation that nest success was greater in larger groups with improved nesting synchrony. Lastly, in birds where females aggregate into social groups, reproductive success in general may be positively associated with group size (Burger 1979, Canestrari et al. 2008).
Previous works noted that female wild turkeys typically nested within ~4 km of their winter range (Vander Haegen 1988, Badyaev and Faust 1996). Wild turkeys often shift ranges and exhibit temporal variation in resource selection as winter flocks dissolve and females move to breeding areas (Badyaev et al. 1996b , Miller and Conner 2007, Little et al. 2016). We found that females located first nest attempts an average of 2,107 m from the centroid of their 21-day range prior to nest initiation, a considerably shorter distance than reported by Badyaev et al. (1996a ) specific to distances from nest sites to center of winter ranges. We predicted that dominant females would move shorter distances than subordinate females between the center of their 21-day range and their initial nest location, but our findings did not support this prediction. Likewise, we found that distances between nest sites and the centroid of the 21-day range before nest initiation failed to influence nest fate. This finding contradicts Badyaev et al. (1996b ) who reported that females that traveled greater distances between pre-nesting ranges and their eventual nest site had greater nest success.
Our findings suggest that dominance hierarchies within social groups may influence timing of nest initiation in female wild turkeys. Established social bonds among individuals within social groups may be maintained throughout multiple seasons (Brandl et al. 2019). Thus, individuals in the same winter flocks are more likely to become members of social groups prior to breeding (Riehl 2011, Watkins et al. 2022), and subsequently reproduce in areas near other group members (Liu et al. 2013, Firth and Sheldon 2016). The genetic structure of social groups can have important implications for fitness, as emerging evidence suggests that females identify a cost to association with kin during the early reproductive season (Watkins et al. 2022). We suggest future research evaluating potential influences of sociality and dominance on nest success also incorporate genetic data on individuals within social groups, as sociality may also influence genetic variation, and therefore may play an important role in reproductive success (Sugg et al. 1996).