Discussion
Previous literature detailing how weather influences gobbling activity has reported contradictory results (Scott and Boeker 1972, Bevill 1973, Kienzler et al. 1996, Miller 1997, Palumbo et al. 2019), leading to uncertainty in the relative contribution of weather variables to daily fluctuations in gobbling activity. We used the most comprehensive dataset currently available on wild turkey gobbling activity, coupled with local weather metrics, to evaluate relationships between gobbling activity and weather. Collectively, our findings suggest weather variables can influence daily gobbling activity and are at least partially responsible for oscillations in gobbling activity throughout the spring reproductive season.
Gobbling is a behavior males use to attract females and ensure reproductive opportunities (Buchholz 1997). However, gobbling increases predation risk as predators are attracted to calls, so males must balance increasing predation risk with attracting mates (Tuttle and Ryan 1981, Burk 1982, Jennions et al. 1997). In birds, weather conditions can also increase predation risk, therefore males may adopt varying calling strategies in response to weather conditions (Carr and Lima 2010, Digby et al. 2014). We found rain had the greatest influence on gobbling activity, as has been shown in earlier works (Bevil 1973, Kienzeler et al. 1996). During rain events, calling males may be more vulnerable to predation as their hearing and vision, which they rely on for detecting predators, are compromised (Healy 1992, Candolin and Voigt 1998, Hedrick 2000). Furthermore, during rain events sound attenuation is increased, making it harder for the gobble to be heard by other individuals (Lengagne and Slater 2002). Alternatively, rain may simply reduce the ability of the ARU to detect gobbles, although we detected 21,180 gobbles during rain events and literature on other bird species reported that rain negatively influenced calling (Staicer et al. 1996, Bruni et al. 2014, Digby et al. 2014).We posit that the influence of rain on gobbling activity recorded by ARUs is likely a combination of detection and ecology, but when reporting gobbling chronology should be considered.
Increases in animal activity and calling have previously been associated with increases in barometric pressure across multiple species (Oseen and Wassersug 2002, Wellendorf et al. 2004, Pellegrino et al. 2013, Zagvazdina et al. 2015). Changing barometric pressure is a well-known predictor of storm fronts, with barometric pressure falling as inclement weather approaches and rising as the storm system dissipates (Saucier 2003, Breuner et al. 2013). Miller et al. (1997) found no relationship between gobbling and barometric pressure, but we observed an increase in barometric pressure from one day to the next resulted in increased gobbling activity. Given changes in barometric pressure and its relationship to inclement weather such as rain, we conclude that this relationship is best explained by turkeys gobbling more in weather conditions not associated with storm systems.
Extant literature has noted a significant relationship between decreased calling and higher temperature in various birds that use auditory courtship behaviors (Hansen and Guthery 2001, Gudka et al. 2019). Vocalization and thermal relationships are likely related to overheating and higher metabolic rates that can occur with increased ambient temperatures, especially for endotherms who use energetically costly courtship behaviors (Dillon et al. 2010, Silva et al. 2015). We observed a similar relationship between higher temperatures and gobbling activity but note that previous studies at southern latitudes reported no relationship between temperature and gobbling activity (Miller et al. 1997, Palumbo et al. 2019), whereas at more northern latitudes studies have reported positive relationships between gobbling and temperature (Kienzler et al. 1996). One could speculate that this relationship could be related to the removal of males causing drops in gobbling later in the sampling period when temperatures are warmer. However, given that we had 5 years of data on an un-hunted site where gobbling continued until the end of the sampling period (Figure 2), we suspect that this is not the case. Wild turkeys at southern latitudes may reduce gobbling at higher temperatures, but we offer that the pattern may not be similar at northern latitudes.
Sound attenuation increases at greater wind speeds, and previous studies have demonstrated wind can negatively influence calling frequency and the ability to hear calls in multiple species (Lengagne 1999, Lengagne and Slater 2002, Yip et al. 2017). We observed that greater wind speeds had a negative effect on daily gobbling, consistent with previous studies (Bevill 1973, Kienzler et al. 1996, Miller et al. 1997). The ability for either human observers or ARUs to detect gobbling as wind speeds increase may be diminished (Kienzler et al. 1996). Alternatively, during high wind speeds birds may change behaviors as perceived risk increases, as individuals have increased difficulty detecting predators due to confusion with moving vegetation (Boyko et al. 2004, Carr and Lima 2010). We suspect males may be less inclined to gobble as wind speeds increase because the desired outcome from calling may be limited by the ability of receptive females to hear the call, and because predation risk may increase.
Wakefield et al. (2020) used the same modeling approach that we used, focused on describing the influences of female reproduction (laying or incubating) and cumulative removal of males on daily gobbling activity. Wakefield et al. (2020) found the proportion of females in reproduction positively influenced gobbling activity, but the impact of male removal at the same time had a greater negative impact on gobbling activity. We also recognize other variables not measured in our or previous studies may be contributing to variation in daily gobbling, such as varying levels of testosterone in males, interactions/encounters with females, and population vital rates such as male age structure (Miller et al. 1997, Chamberlain et al. 2018, Wakefield et al. 2020).
As gobbling activity is positively correlated with hunter satisfaction and linked to reproduction, it is often a key determinant used by state agencies when considering regulatory frameworks (Bevill 1975, Hoffman 1990, Little et al. 2001, Casalena et al. 2011, Isabelle et al. 2015). Given our results, we suggest that when describing gobbling activity, managers should account for how weather patterns may influence gobbling chronology. Weather variables should be coupled with site specific reproductive timing and harvest data to fully understand gobbling chronology on a given site. We recommend future studies investigate the relationship between daily gobbling activity, weather, and reproductive phenology of females in an un-hunted population and populations subjected to varying hunting seasons and harvest rates.