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Why Doesn’t the Spring Season Open Earlier?

Mary Jo Casalena & Ian Gregg, Bureau of Wildlife Mgmt, March 2013

Pennsylvania's 2006-2015 Wild Turkey Management Plan (WTMP) specifies a statewide 4-week (including 5 Saturdays) regular spring gobbler season opening the Saturday closest to May 1, plus a 1-day statewide youth-only hunt the Saturday prior to the regular season opener. In recent years the season has included a partial 5th week (Memorial Day only from 2008-10, and a fixed closing date of May 31 beginning in 2011). During some years when the Saturday closest to May 1 fell on May 2 or 3, the opening date for the regular season has been moved 1 week earlier (April 25 or 26) (Table 1). Spring season structure (opening date placement and overall length) was addressed in the WTMP because it has the potential to significantly impact wild turkey populations via changes in hen mortality / disturbance, and gobbler harvest rates. These biological issues, in relation to overall turkey population status and harvest pressure, should be taken into account when setting spring gobbler seasons.

Hen Mortality and Disturbance: Timing of the spring gobbler season is frequently viewed in relation to gobbling activity patterns or vegetation phenology ("green-up"). These phenomena may vary from one part of Pennsylvania to another, and may be occurring earlier today than in previous decades (although it should be noted that gobbling decreases with the onset of hunting activity, whenever that onset occurs). However, a more important determinant of proper season timing is the date of the average peak of nest incubation (as with most game birds, turkeys lay individual eggs over a period of about 2 weeks, but do not begin incubating until the full clutch is laid, which results in synchronous hatching and all young being ready to leave the nest soon after hatching). Why is this date important? During the egg-laying period, hens are more active, move more extensively, and are more sensitive to disturbance than during the incubation period. Consequently, they are much more susceptible to illegal harvest and nest abandonment, both of which have direct negative effects on reproduction (and thus, future populations). Though spring hen disturbance and mortality can never be completely eliminated with 230,000 hunters afield, initiating this level of hunting activity prior to the peak of incubation greatly increases the risk.

May 1 has proven to be a good approximation for the peak of nest incubation in Pennsylvania. There is no evidence that this peak has changed over time, or differs appreciably between northern and southern portions of the state. From 1953-63, the average statewide incubation initiation date was April 28. On the Michaux State Forest (WMU 5A) from 1999-2001, incubation initiation by radiotransmittered hens averaged May 8 for adults, and May 13 for juveniles. In the current hen harvest and survival rate study, median dates of incubation initiation for satellite-transmittered hens were April 24 (2010), May 3 (2011), and May 3 (2012 – a year when spring weather and vegetation green-up were exceptionally early) (Figure 1). Also, the median incubation dates for the southern (WMUs 2C, 2E, 4A, 4B, and 4D) and northern (WMUs 2F and 2G) Study Areas were the same in 2010, and differed by only one day in 2011 and 2012. These findings indicate that incubation behavior is controlled primarily by photoperiod (day length) rather than variations in weather or vegetation, and that from a management standpoint, the existing guideline of using May 1 as a target date to open Pennsylvania's spring season, and the statewide rather than geographically staggered season structure, remain appropriate.

Gobbler Harvest Rates: A secondary benefit of opening the spring season in conjunction with the average peak of incubation is that with the majority of breeding complete, a relatively high proportion of males can be removed from the population without impacting reproductive output. However, research indicates that if adult gobbler harvest rates exceed 50%, future age structure of the population is affected, with a reduction in adult gobblers. A younger age structure is not detrimental from a reproductive standpoint, but may be a social issue in that many surveys show hearing, calling, and harvesting adult gobblers to be important factors in hunter satisfaction. From 2006-2009, adult gobbler harvest rates in Pennsylvania averaged about 40%.

Data from the first 5 years (2008-12) of a partial 5th week at the end of Pennsylvania's season show no clear patterns in overall harvest compared to a 4-week season, although on a percentage basis the effect appears to be dispersing a similar late-season harvest over more days, moreso than an increase in harvest. We have not detected an effect of minor variations in opening date on harvest rates, and cannot reliably predict the effects (if any) of adding a 6th Saturday to the season. These uncertainties, and our rather limited margin for increasing harvest rates without reducing future availability of adult gobblers, should be given consideration when evaluating potential liberalizations in season structure.

Turkey Population Status: Past departures from WTMP criteria for spring gobbler season length and opening date have not resulted in demonstrable negative impacts on turkey populations. However, such changes have usually coincided with multiple-year periods of good brood-rearing weather and overall increasing populations, which likely buffered any increase in hen mortality / disturbance or gobbler harvest rates. By contrast, we currently are in a period of several consecutive years of declining / below-average populations, as indexed by summer sightings and spring harvest density. This makes any potential impacts on reproduction or gobbler age structure of added concern, especially given that the spring season is established a year in advance, prior to knowing the nesting season success and fall harvest that will occur during the intervening year. Hen mortality in our fall either-sex season is generally additive (i.e., these birds would otherwise exhibit high survival to the next spring), and adult females comprise the majority of the fall harvest during years of low recruitment.

Pennsylvania Spring Seasons Compared to Surrounding States: Several of our surrounding states open their spring seasons 1-2 weeks earlier than Pennsylvania, but it is important to note that our hunter numbers, gobbler harvest rates, and total spring harvests – and thus the potential negative effects of opening too early - are far higher than those for most of our neighbors. New York comes the closest to matching our hunter numbers and harvests; there, the spring gobbler season is a fixed May 1-31 timeframe (31 days of hunting annually). Although the absence of Sunday hunting in Pennsylvania results in 1-6 fewer days to hunt than New York overall each year, our opening date under the WTMP guidelines averages out to be the same – a few days earlier in some years, a few days later in others. (Because a constant May 1-31 structure would only provide 4 Saturdays some years, varying the opening date around May 1 maintains 5 Saturdays in the season). Pennsylvania's fall harvests (and thus the level of additive hen mortality we must account for) are also much higher than surrounding states, which increases the importance of setting our spring season appropriately.

Conclusion: Pennsylvania's wild turkey resource provides countless benefits to consumptive and non-consumptive users alike. Particularly during periods of decline in the turkey population, as are now being experienced in much of the Commonwealth, ensuring the long-term sustainability of these benefits requires careful consideration of the biological risks in any departures from the WTMP harvest management guidelines.

Table 1. Structure of Pennsylvania spring gobbler seasons, 1994-2013 and (preliminary) 2014.

 

Figure 1. Incubation Timelines for Satellite-Transmittered Hens in Pennsylvania, 2010-2012.

 

Timeline of Wild Turkey Nest Incubation, 2013, from Transmittered Hens in South-Central and Southwestern PA, and Spring Gobbler Season Date.

Management goal - open season approximately during the mean incubation date. Graph shows gobbler season should not open earlier.

 

Timeline of incubation for nesting hens in Pennsylvania Study Area 1 (WMUs 2C, 2E, 4A, 4B, and 4D) for first nests in 2013, based on transmitter activity. Median date of incubation initiation (red line) was 4 May. Shaded area indicates the interquartile range. Opening date of regular spring turkey hunting season (blue line) was 27 April.


Timeline of Wild Turkey Nest Incubation, 2013, from Transmittered Hens in North-central and Northwestern PA, and Spring Gobbler Season Date.

Management goal - open season approximately during the mean incubation date. Graph shows gobbler season should not open earlier.

 

Timeline of incubation for nesting hens in Pennsylvania Study Area 2 (WMUs 2F, 2G, and 2H) for first nests in 2013, based on transmitter activity. Median date of incubation initiation (red line) was 2 May. Shaded area indicates the interquartile range. Opening date of regular spring turkey hunting season (blue line) was 27 April.


Incubation Start Dates of Wild Turkey Nests in PA & Spring Gobbler Season Start Dates

- Gobbler season opener is biologically set to open approximately when most turkey hens are incubating nests, to minimize nest disturbance.

 

Box and whisker plot showing nest incubation start dates of wild turkey hens in Pennsylvania, from satellite transmitter data, 2010-2013, and the start date of the regular spring turkey season for each year (dots). Turkeys begin their 28-day incubation period after they lay an entire clutch of 10-12 eggs.

Box represents the range from the lower and upper halves of the median nest incubation date.
Line on left side of box is the median of the lower half of incubation dates.
Line on right side of box is the median of the upper half of incubation dates.
Line inside the box marks the median incubation start date.
'Whiskers' represent the earliest and latest incubation date.

Incubation dates did not differ between northern and southern PA.