Modern spring turkey seasons are deliberately structured around male harvest when gobblers are most vulnerable to calling. Surprisingly, little attention has been paid to the potential impacts of this approach from an evolutionary perspective. Recent evidence suggests that sufficiently intense hunting can select for toms who, for behavioral reasons, are less susceptible to harvest and that there could be derivative impacts on hens, which include the production of broods with uneven, female-biased sex ratios.
Likely of no surprise to turkey hunters, observational studies and telemetry confirm that the frequency, duration, and spatial predictability of gobbling and displays decline over the course of the hunting season. Toms learn to avoid locations heavily used by hunters, including habitat edges, fields, and trails. As a result:
• Dominant males become less detectable
• Daily activity is greatest at dawn and dusk.
• Courtship shifts to proximity (i.e., chance encounters)
• Dominance hierarchies are disrupted
In terms of reproductive impacts, subordinate males gain more breeding opportunities, and mating encounters can be reduced or delayed, with effects on clutch size, nesting success, and the need to renest.
Because renesting is a substantial (and extra) energetic investment, it influences the physiological conditions during egg formation and often influences the sex ratios within broods for a variety of reasons, including:
• Differential hormone allocation to eggs.
• Changes in the timing of ovulation relative to copulation.
• Selective resorption of failure of embryos under stress.
As background, in turkeys and other galliform birds, juvenile sex ratios tend to be near 50:50 in undisturbed populations. However, when environmental conditions are poor or when stressors like hunting are added, male offspring show greater mortality than females. Moreover, and possibly more significantly, a multi-state study of eastern wild turkeys found that at sites with active spring hunting, as few as 38% of male poults hatched.
There are two possible explanations for this troubling effect. The first is adaptive sex allocation. Female turkeys (like many other species) can adjust the sex of offspring based on cues about male survival chances. In other words, if males are more likely to be killed, then females could (can) produce more daughters who have a higher expected survival. The other explanation is physiological stress effects. Hunting (as well as excessive predation risk) during the laying period can increase stress in breeding females, and that stress can affect maternal hormones that influence sex determination in birds.
Logically, female-biased broods could have long-term population impacts. Apart from affecting hunter success, fewer males on the landscape might alter mating dynamics, thereby reducing effective breeding opportunities.
The important takeaway is that when it comes to turkey management, there’s more to it than the number of birds harvested or the number of broods produced. The available evidence suggests that hunting, when sufficiently intense, can act as a selective evolutionary pressure that, over time, could produce durable changes not only in the number of toms but also in behaviors that affect their susceptibility to harvest.
