Influence of weather on gobbling activity of male wild turkeys

Abstract Gobbling activity of Eastern wild turkeys (Meleagris gallopavo silvestris; hereafter, turkeys) has been widely studied, focusing on drivers of daily variation. Weather variables are widely believed to influence gobbling activity, but results across studies are contradictory and often equivocal, leading to uncertainty in the relative contribution of weather variables to daily fluctuations in gobbling activity. Previous works relied on road‐based auditory surveys to collect gobbling data, which limits data consistency, duration, and quantity due to logistical difficulties associated with human observers and restricted sampling frames. Development of new methods using autonomous recording units (ARUs) allows researchers to collect continuous data in more locations for longer periods of time, providing the opportunity to delve into factors influencing daily gobbling activity. We used ARUs from 1 March to 31 May to detail gobbling activity across multiple study sites in the southeastern United States during 2014–2018. We used state‐space modeling to investigate the effects of weather variables on daily gobbling activity. Our findings suggest rainfall, greater wind speeds, and greater temperatures negatively affected gobbling activity, whereas increasing barometric pressure positively affected gobbling activity. Therefore, when using daily gobbling activity to make inferences relative to gobbling chronology, reproductive phenology, and hunting season frameworks, stakeholders should recognize and consider the potential influences of extended periods of inclement weather.

theories such as the adaptive acoustic hypothesis and risk-reward theory suggest birds adopt different vocalization strategies depending on environmental conditions to maximize the effectiveness and costs associated with calling (Lima, 2009;Luther, 2009;Orians, 1969;Zanette et al., 2006).
The wild turkey (Meleagris gallopavo) is a non-migratory upland game bird indigenous to North America whose mating strategy is a form of polygamy similar to exploded lekking (Krakauer, 2008).
Gobbling activity is believed to be influenced by a variety of factors, such as time of day, timing of female reproductive activities, conspecifics, hunting, weather, predation risk, age structure, and testosterone levels (Chamberlain et al., 2018;Kienzler et al., 1996;Miller et al., 1997a,b Wakefield et al., 2020Wightman et al., 2019).
Wildlife managers and agencies are interested in understanding factors influencing gobbling activity, as it is the primary determinant of hunter satisfaction and is likely linked to reproductive success (Casalena et al., 2011;Chamberlain et al., 2018;Isabelle & Reitz, 2015;Schroeder, 2014).
Historical research relied on roadside surveys to describe gobbling activity, but results from previous studies contained notable discrepancies in regard to drivers of variation in gobbling activity.
For example, early studies reported both positive and negative effects of nesting phenology, weather, and hunting pressure on gobbling activity (Bevill, 1975;Kienzler et al., 1996;Lehman et al., 2005;Miller et al., 1997a,b;Palumbo et al., 2019). However, no definitive relationship between any of the aforementioned variables and gobbling activity was established, likely due to a lack of uniformity in data collection, coupled with logistical difficulties in obtaining high-quality, detailed, spatially explicit gobbling data. Furthermore, roadside surveys were generally not conducted during inclement weather and can be additionally biased by observer error, sample design, and manpower limitations (Lehman et al., 2005;Miller et al., 1997a,b;Palumbo et al., 2019).
Development and use of autonomous recording units (ARU; Colbert et al., 2015;Mennill et al., 2012;Rempel et al., 2005) offer researchers the ability to thoroughly detail gobbling activity. With advancement of ARU technology, recent studies have elucidated how factors such as time of day, nesting phenology/female receptivity, and hunting influence gobbling activity using spatially and temporally robust datasets (Chamberlain et al., 2018;Wakefield et al., 2020;Wightman et al., 2019). In general, gobbling activity was highest 30 min prior to sunrise until 150-min post-sunrise (hereafter; daily gobbling activity) and fluctuated considerably from one morning to the next (Wightman et al., 2019). Additional work has indicated that female nesting phenology was positively related to gobbling activity, with the onset of reproductive activities resulting in an initial peak of gobbling (Chamberlain et al., 2018;Wakefield et al., 2020). Furthermore, contemporary literature has noted hunting activity may have a greater negative influence on gobbling activity than the positive effect of nesting phenology (Wakefield et al., 2020;Wightman et al., 2019). However, there is no existing literature using ARUs to investigate the relative influences of weather variation on gobbling activity.
Based on previous literature investigating calling activity of wild turkeys and other avian species, we hypothesized that increased temperature, humidity, wind, and occurrence of rain would negatively impact gobbling activity (Digby et al., 2014;Gudka et al., 2019;Lengagne & Slater, 2002;Miller et al., 1997a). Likewise, we hypothesized that increases in barometric pressure would likely positively influence gobbling activity (Pellegrino et al., 2013;Wellendorf et al., 2004). Therefore, our objectives were to evaluate potential relationships between gobbling activity of male Eastern wild turkeys (Meleagris gallopavo silvestris) collected using ARUs and the aforementioned weather variables across multiple study sites in the southeastern United States.

| Study area
We conducted research on 5 study sites in Georgia and South   We used a convolutional neural network (CNN) developed to autonomously search for turkey gobbles . Wightman et al., 2019), such a study design was not logistically feasible. We offer that using weather data collected on the same study site or within the distances detailed above is sufficient for detailing how daily changes in local weather conditions influence gobbling activity. We calculated mean daily values from 15-min weather recordings from 30 min prior to 150 min after sunrise for temperature (C°), relative humidity percentage, and wind speed (kph). For barometric pressure (mb), we calculated the mean for each morning and then subtracted it from the prior morning to get a change in barometric pressure. For precipitation, we classified whether rain occurred (Yes = 1, No = 0) from 30 min before to 150 min after sunrise.

| Data analysis
Our final dataset included time series data for all weather variables (scaled by subtracting variable means from observed values and dividing by the standard deviation) and daily gobbling counts. With the spatially and temporally coupled data, we used state-space modeling to evaluate the effects of weather variables on daily gobbling activity. The state-space model accounted for correlated observations and included observation error while modeling the influences of weather variables on gobbling activity. We used a hierarchical statespace model that allowed us to decompose temporally correlated weather data and gobbling counts into a process variation and observation error (Kery & Schaub, 2012). With the weather variables being the parameters of interest, the state-space model allowed us to investigate the process variation in gobbling counts relative to stochasticity in the weather variables. We calculated Pearson's correlation coefficients to test for collinearity between each of our covariates and excluded covariates with a r > .60. We fit the statespace model within the jagsUI package (Kellner, 2018) in program R (R Core Team, 2020) to estimate the effects of weather on daily gobbling activity.
We fit the Bayesian state-space model to counts of daily gobbles (N) at each site (K) during each year (i). We treated daily gobbling counts like counts in a population model but we modeled the abundance of gobbles instead of animals. The process model was: Where r expected(t) was the expected change in daily gobbling activity, Site was the fixed effect for each of the 5 sites, β temperature was the coefficient for the effect of temperature in matrix X temperature , β wind was the coefficient for the effect of wind in matrix X wind , β bp was the coefficient for the effect of the change in barometric pressure in matrix X bp , β humidity was the coefficient for the effect of humidity in matrix X humidity , β precipitation was the coefficient for the effect of precipitation in matrix X precipitation , Year was modeled as a random effect, and Units was an offset term used to account for the number of ARUs recording. We modeled the observation process as follows:
The state-space model accurately predicted gobbling activity compared with our observed gobbling activity (Figure 2), and Rvalues indicated model convergence (Table 2). Results from the state-space model indicated the occurrence of rain most impacted (negatively) gobbling activity (Table 2). Where the mean expected number of daily gobbles would be 21 (CrI = 15, 30) without rain, compared to 12 (CrI = 7, 22) if rain occurred. Conversely, an increase in barometric pressure from one day to the next was positively associated with gobbling activity (Figure 3, Table 2). We found gobbling activity was negatively influenced by increased temperatures (Figure 4, Table 2) and by greater wind speed with the largest effect occurring when wind speeds exceeded 10 kilometers per hour ( Figure 5, Table 2). Humidity had no effect on the average predicted rate of change in gobbles across all study sites and years (Table 2).

| DISCUSS ION
Previous literature detailing how weather influences gobbling activity has reported contradictory results (Bevill, 1973;Kienzler et al., 1996;Miller et al., 1997a;Palumbo et al., 2019;Scott & Boeker, 1972), 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 (Burk, 1982;Jennions & Petrie, 1997;Tuttle & Ryan, 1981). In birds, weather conditions can also increase predation risk; therefore, males may adopt varying calling strategies in response to weather conditions (Carr & Lima, 2010;Digby et al., 2014). We found rain had the greatest influence on gobbling activity, as has been shown in earlier works (Bevill, 1973;Kienzler 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 (Candolin & Voigt, 1998;Healy, 1992;Hedrick, 2000).
Furthermore, during rain events sound attenuation is increased, making it harder for the gobble to be heard by other individuals (Lengagne & 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 (Bruni et al., 2014;Digby et al., 2014;Staicer et al., 1996). 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 & Wassersug, 2002;Pellegrino et al., 2013;Wellendorf et al., 2004;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 (Breuner et al., 2013;Saucier, 2003). Miller et al. (1997a 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. Furthermore, if storms represent a period of reduced signal efficacy, it is plausible that turkeys are increasing gobbling activity as storm fronts pass to compensate for lost signaling time.
Extant literature has noted a significant relationship between decreased calling and higher temperature in various birds that use auditory courtship behaviors (Gudka et al., 2019;Hansen & Guthery, 2001 previous studies at southern latitudes reported no relationship between temperature and gobbling activity (Miller et al., 1997a;Palumbo et al., 2019), whereas at more northern latitudes studies have reported positive relationships between gobbling and temperature (Kienzler et al., 1996). Male wild turkeys are primarily hunted during the spring reproductive season, and spring harvest is the primary cause of mortality for males (Chamberlain et al., 2012). One could speculate that reduced gobbling activity during warmer temperatures 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 et al., 1999;Lengagne & 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., 1997a). 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 & 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 (Chamberlain et al., 2018;Miller et al., 1997a;Wakefield et al., 2020).

ACK N OWLED G M ENTS
We gratefully acknowledge the numerous technicians, undergraduate students, and graduate students at Louisiana State University and the University of Georgia for their efforts in collecting data and veri-

CO N FLI C T O F I NTE R E S T
None.

DATA AVA I L A B I L I T Y S TAT E M E N T
All raw gobbling and weather data from this study can be accessed on Dryad (DOI https://doi.org/10.5061/dryad.573n5 tb9k).