Effects of Winter Conditions on Reproduction in a Northern Wild Turkey Population Author(s): William F. Porter, Gary C. Nelson and Kim Mattson Source: The Journal of Wildlife Management , Apr., 1983 , Vol. 47, No. 2 (Apr., 1983), pp. 281-290 Published by: Wiley on behalf of the Wildlife Society Stable URL: https://www.jstor.org/stable/3808500 JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org. Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at https://about.jstor.org/terms Wiley and Wildlife Society are collaborating with JSTOR to digitize, preserve and extend access to The Journal of Wildlife Management This content downloaded from 128.227.251.1 on Fri, 28 Feb 2025 14:14:17 UTC All use subject to https://about.jstor.org/terms EFFECTS OF WINTER CONDITIONS ON REPRODUCTION IN A NORTHERN WILD TURKEY POPULATION1 WILLIAM F. PORTER, College of Environmental Science and Forestry, State University of New York, Syracuse, NY 13210 GARY C. NELSON, Minnesota Department of Natural Resources, Altura, MN 55910 KIM MATTSON, Department of Biology, University of Minnesota, Minneapolis, MN 554552 Abstract: Nine aspects of the reproductive performance of female wild turkeys (Meleagris gallopavo) we measured in Minnesota during 1975, 1977, and 1978. Sixty-eight of 75 (90%) radio-tagged females attempte to nest, renesting was common, and eggs in nests of 67% of the females ultimately hatched. Clutch siz averaged 11.7 eggs, hatching success was 82%, and 37% of the young produced survived to late summe Winter severity, measured in terms of impacts of site-specific conditions on late-winter body weights females and mortality rates, varied within 14 different winter flocks. Females weighing less than 4.3 kg we less likely to survive to breed, less likely to nest, and had a natality rate of 2.6 females/female in the breedi population. Heavier females had higher survival and nesting rates and natality was 4.3. Strong correlatio were observed between survivorship within winter flocks and reproductive performance of females th survived to breed. Severe winter conditions appeared to reduce egg hatching success among yearling female and recruitment of young among adults. Population modeling demonstrated that winter mortality an impaired reproduction performance can result in a significant decline in the population. At least 2 breedin seasons are necessary for population recovery. Impacts on hunting harvest quotas are discussed. J. WILDL. MANAGE. 47(2):281-290 It is a basic ecological tenet that on the fringe of its geographic range the abun- dance of a species tends to be influenced primarily by physical aspects of the en- vironment. This generality seems to hold true for species of the Phasianidae (pheas- ants, quail, and turkeys). It is commonly agreed that 1 of the most important physical factors influencing phasianid populations in the upper Midwest is win- ter severity. However, the mechanism by which winter severity acts to cause vari- ation in population levels has been un- clear. Leopold (1931) and Schorger (1966) noted that historical populations of wild turkeys in the upper Midwest "ebbed and flowed" depending on the severity of win- ter. Implicit was the hypothesis that the primary impact of severe winters was ex- tensive mortality as a result of prolonged negative energy balances. Austin and DeGraff (1975) in New York, Wunz and Hayden (1975) in Pennsylvania, and Por- ter et al. (1980) in Minnesota reported data strongly supporting this hypothesis. The ultimate effect of severe winter on the population depends not only on the overwinter mortality incurred but also on the reproductive performance of those in- dividuals that do survive. Under ideal conditions, the reproductive potential of the turkey could offset substantial winter losses. However, work with many species of birds has shown that nutritional varia- tion in the wild can significantly impact breeding success (e.g., Lack 1966, 1968; Ryder 1970; Moss et al. 1974; Newton 1979). Prolonged periods of negative en- ergy budget during the winter can result in poor nutritional condition of females in the spring. Work with ring-necked pheas- 'Research supported by National Institutes of Health Training Grant 5 T01 GM 01779, the Uni- versity of Minnesota, the Minnesota Department of Natural Resources, the Minnesota State Archery As- sociation, the Special Projects Committee of the Min- nesota Big Game Club, and the State University of New York, College of Environmental Science and Forestry. 2 Present address: Institute of Ecology, University of Georgia, Athens, GA 30602. J. Wildl. Manage. 47(2):1983 281 This content downloaded from 128.227.251.1 on Fri, 28 Feb 2025 14:14:17 UTC All use subject to https://about.jstor.org/terms 282 WINTER CONDITIONS AND REPRODUCTION IN TURKEYS * Porter et al. ants (Phasianus colchicus) and northern bobwhite (Colinus virginianus) has led to the general hypothesis that the reproduc- tive performance of these species can be impaired by severe winter conditions (Breitenbach et al. 1963, Edwards et al. 1964, Gates and Woehler 1968, Labisky and Jackson 1969, Roseberry and Klimstra 1971, Gates and Hale 1975). If such a cause-and-effect relationship exists within turkeys as well, proper management should consider a time lag in the recovery of a population following a severe winter in northern regions. The reproductive ecology of the wild turkey in the upper Mississippi Valley has not been examined previously. As part of a larger investiga- tion (Porter 1978), data were acquired to document reproductive performance. The objectives of this paper are to report on the reproductive performance of wild tur- keys in southeastern Minnesota and to evaluate the effects of severe winter on reproduction and population dynamics. We acknowledge D. B. Siniff of the University of Minnesota and H. Shepperd, N. Gulden, R. Johnson, and G. Meyer of the Minnesota Department of Natural Re- sources for support of this project. D. Hamilton, S. Erickson, B. Neil, and R. Tangen provided extensive field assis- tance. D. DeMaster, J. Ludwig, J. Tester, H. Tordoff, and an anonymous reviewer provided especially helpful comments during preparation of this paper. STUDY AREA The study area included 600 km2 in the southeastern Minnesota counties of Hous- ton, Wabasha, and Winona. The area is characterized by a rolling upland, deeply and extensively dissected by the Mississip- pi River and small tributaries. Soils are calcareous and well drained. Vegetation is composed of an interspersion of mature hardwood forest and agricultural land in a 60:40 distribution. Oaks, especially northern red (Quercus rubra) and white (Q. alba), dominate the forested sites. Canopy closure in forested areas averages 80% and understories are composed pri- marily of shrubs such as gooseberry (Ribes missouriense), common pricklyash (Zan- thoylum americanum), and American fil- bert (Corylus americana). Most agricul- tural fields are planted to corn and alfalfa (61% of the agricultural land) or are in pasture (19%). In many areas, patches of corn (0.5-5.0 ha) remain standing through the winter months and are used by tur- keys as a winter food source (Porter et al. 1980). More complete descriptions of areas used for wintering and summer brood- raising activities are presented by Porter (1980) and Porter et al. (1980). Turkey populations in this region are derived from wild stock transplanted from Arkansas, Missouri, Nebraska, and South Dakota during 1964-73. The populations were undergoing rapid growth (X = 2.15) during the study and densities ranged from 6 to 17 birds/km2 in areas occupied by turkeys for at least 2 years (Porter and Ludwig 1980). METHODS Assessments of reproductive perfor- mance of turkeys were based on a sample of females captured during the winter and fitted with radio transmitters (Porter et al. 1979). All radio-tagged birds were moni- tored daily throughout the winter, spring, and summer. Reproductive patterns were character- ized through the estimation of 9 parame- ters. Nesting frequency was measured as the proportion of females in a population at breeding that showed indications of in- cubation behavior. Incubation behavior was ascertained via telemetric monitoring of activity patterns beginning in April (Gilmer et al. 1971). In most cases, activ- J. Wildl. Manage. 47(2):1983 This content downloaded from 128.227.251.1 on Fri, 28 Feb 2025 14:14:17 UTC All use subject to https://about.jstor.org/terms WINTER CONDITIONS AND REPRODUCTION IN TURKEYS * Porter et al. 283 ity was monitored for each female twice daily. Date of nest initiation was deter- mined for the first nesting attempt based on activity records during egg-laying and incubation. Clutch size was determined after hatching occurred by examination of egg shell fragments. Mean egg size was determined for each clutch in which some eggs remained unhatched (assuming that unhatched eggs were not different in di- mension from eggs that hatched). Length and width of each egg were measured and their mathematical product used in sub- sequent analyses. Nest success was the proportion of females attempting to nest that eventually hatched some eggs. Egg fertility was based on all eggs that actually hatched and on those unhatched eggs that showed some sign of embryonic develop- ment within each clutch. Eggs in ad- vanced decay were classified as infertile. Hatching success was defined as the pro- portion of eggs in each clutch that ac- tually hatched. Poult survival through the summer was based on periodic visual counts of brood flocks. Telemetry data showed that females unsuccessful in nest- ing joined brood flocks in late fall, limiting estimates for survivorship (L,) to the off- spring surviving to late summer (1 Aug- 1 Sep). Survivorship values were estimat- ed as the number of young observed per female in the brood flock in late summer relative to the number of young hatched by a given female. These reproductive parameters were summarized as age-specific female natal- ity rates (Mj), calculated as: MX = (nfx)(cx)(nsx)(hs) /2 where: nfx = nesting frequency, cx = clutch size, ns, = nesting success, and hs, = hatching success. An equal sex ratio was assumed at hatch- ing. Recruitment of young females (R) into the fall population was estimated as young surviving to late summer per female alive to breed the previous spring: R = (M.)(L,) Nesting and post-nesting survival of fe- males was determined by activity patterns obtained by telemetry and eventual visual observation. These survivorship estimates were calculated for each age-class as the proportion of radio-tagged females alive at the beginning of the interval that sur- vived through the interval (Porter et al. 1979). Nesting survival was defined as sur- vival during the period 1 April to the date on which the female no longer showed signs of incubation. Post-nesting was de- fined as the period from the end of incu- bation to 31 August. Birds that did not show signs of incubation were excluded from the estimation of nesting and post- nesting survivorship. Examination of the relationship be- tween winter severity and reproductive performance involved the use of 2 indices of winter severity based on measures of the response of turkeys to the winter con- ditions. The 1st measure was body weight, which was measured when birds were captured and radio-tagged and whenever recaptures occurred. A 2nd index was sur- vivorship within flocks; this was estimated as the proportion of radio-tagged birds (males and females) that survived to 1 April among all the birds radio-tagged in the flock. These survivorship estimates were verified by periodic visual counts of flocks. Only those flocks in which at least 4 birds were tagged were included in the analyses. RESULTS Data were available from 3 breeding seasons: 1975, 1977, and 1978. Two of these seasons (1975, 1978) followed win- ters when prolonged periods of deep snow J. Wildl. Manage. 47(2):1983 This content downloaded from 128.227.251.1 on Fri, 28 Feb 2025 14:14:17 UTC All use subject to https://about.jstor.org/terms 284 WINTER CONDITIONS AND REPRODUCTION IN TURKEYS * Porter et al. caused extensive overwinter mortality of 30-40% of the population. Spring and summer weather during all years was nor- mal to dry. Estimates of flock survivorship through the winter months were based on sample populations of radio-tagged male and fe- male turkeys. An average of 8 turkeys was tagged in each of 14 different wintering flocks containing females. General rates of survival through the winter (both sexes, all age-classes, and flocks combined) were 0.56 (N = 23), 0.85 (N = 49), and 0.65 (N = 82) for winter 1975, 1977, and 1978, respectively. However, survivorship with- in winter flocks ranged from 20 to 100%. At least I radio-tagged female survived to the nesting season from each of 14 winter flocks. Winter weights were available for 72 of 75 females that survived into the breed- ing season. Weights for adults averaged 5.15 ? 0.12 kg (9 - SE, N = 27) vs. 4.49 ? 0.07 kg (N = 45) for yearlings. Weights were variable from year to year and month to month within a given win- ter. Females were generally heaviest in December and lightest in March, losing an average of 0.5 kg (Porter et al. 1980). However, weight losses were far greater in some flocks, especially during winters 1975 and 1978. Birds tagged in late Feb- ruary or March that weighed less than 3.5 kg did not survive to the breeding season. Reproductive data were obtained from telemetry and visual observation of 27 adult and 48 yearling radio-tagged fe- males. Sixty-eight of 75 females (90.1%) attempted to nest and 42 (61.8%) were successful in hatching some eggs from their 1st nesting attempt. The 1st nesting attempt was usually initiated during the last 2 weeks in April. The average date of initiation of egg-laying was 25 April and ranged from 10 April to 24 June. Clutch sizes averaged approximately 12 eggs. Fertility exceeded 90% and egg hatching success was approximately 80%. This re- sulted in an average of 9.55 poults hatched per successfully nesting female or 5.29 poults/female present in the population at the beginning of the breeding season. Renesting was observed in 11 of 17 cases in which the female survived at least a month after the 1st nest was disrupted. Four of 7 adults and 7 of 10 yearlings attempted to renest; 3 and 5, respectively, were successful. Most females moved less than 1.6 km to renest and established the 2nd nest in 9 ? 2.8 days (f SE). Clutch size and hatching success in the 2nd nest did not differ from those of the 1st nest (P > 0.50). Renesting accounted for ap- proximately 22% of the total poults hatched and thus was important to repro- duction. Survivorship of poults through the sum- mer months was estimated for broods of 25 radio-tagged females. Survival rates were lowest during the first several weeks after hatching. July brood counts showed survival of approximately 47% of the poults hatched. August brood counts showed additional losses in 1978 (11%), but none in other years. Brood counts showed an average of 3.1 young present in the fall population/suc- cessfully nesting adult female in the spring population and 3.6 young/successful yearling female. Fall recruitment was 0.97 young females/female in the breeding population (Table 1). Survival among adult and yearling fe- males during the nesting and post-nesting period averaged 83% (N = 89) for the 3 years. Most mortality occurred during the early nesting phase and again during the first weeks after hatching. There were no differences in nesting survivorship among females between the 2 age-classes or in post-nesting survivorship among years. Combined data for 1975, 1977, and 1978 J. Wildl. Manage. 47(2):1983 This content downloaded from 128.227.251.1 on Fri, 28 Feb 2025 14:14:17 UTC All use subject to https://about.jstor.org/terms WINTER CONDITIONS AND REPRODUCTION IN TURKEYS * Porter et al. 285 0 0 ?- 0 cu .6 Cu c, Cu Cu 0) "0 CD 0 cO Cu to E 0 cn cu ,. c, (D co Cu 0 co (D 0 E C (D M E Cu wu t- 0 Cu C Cu Cu V 0) C- o Cu E c_ Cu a Cu to C) E tC ._> w Cu Cu H 0D to-C oo _--qcci q c ?' 0 ?0 co 00 CIO CO COO CI z q q C11 O o o0 , c0c LO 't m. t-- Cq CI 3 m o ~C]C 't cjC ID ce LO cc ? O I'll LDOO ci cq c q 00l- m10 m: cc Go O c CZC A) - M)O G zs COL( C] C]I C] CC~~ CC) CO C d5 0 d L Cu ed u 0~ ~lc 0 (U bC 0 0 Cu 0 cd C, co cu E - Q) Co C' - P; a 'CC' U-.-0 o .~ 'CC'C' o''C showed a 91% survival rate 31 August (N = 87). Diffe years were evident in surv ing the nesting season, ran 100%. The lowest survival rate was ob- served in 1977 following a mild winter. Analysis of the relationship between re- productive performance and body weight was limited to 13 yearling and 7 adult fe- males captured during late winter (after 21 Feb). Late winter weights did not dif- fer between these 2 age-classes (P > 0.50) and averaged 4.3 ? 0.1 kg (i SE). Eight of 13 yearling and 2 of 5 adult females weighed less than 4.3 kg at the time of capture. Females weighing less than 4.3 kg were less likely to survive to breed than heavier females (survival 16 Mar-i Jun = 70 vs. 100%) and were less likely to at- tempt to nest (71 vs. 100%). No differ- ences were evident between weight classes in clutch size (12.0 + 1.4, ? SE, N = 8), nesting success (12 of 15 nests hatched some young), hatching success (s = 84.7 ? 9%, N = 8 nests), or survival through the nesting and post-nesting sea- sons (13 of 15 females with functional transmitters survived the summer). Anal- ysis of renesting within this group was limited to 2 yearling females. One female that weighed 4.2 kg in March did not re- nest after the loss of her 1st nest. A female weighing 4.9 kg in March did renest, but was unsuccessful in hatching young. Na- tality averaged 2.6 females hatched/fe- male among light females and 4.3 among heavy females. Used as an index, flock survivorship provided a measure of the cumulative ef- fects of winter on females. Flock size changed most during the last 6 weeks of the winter as a result of mortality caused by starvation and predation. Predation was important only in flocks where observa- tions indicated that birds were under se- vere stress (Porter et al. 1980). Thus, the J. Wildl. Manage. 47(2):1983 This content downloaded from 128.227.251.1 on Fri, 28 Feb 2025 14:14:17 UTC All use subject to https://about.jstor.org/terms 286 WINTER CONDITIONS AND REPRODUCTION IN TURKEYS * Porter et al. Table 2. Coefficients of determination (r2) for impact of winter conditions on wild turkeys as measured by flock survivorship in southeastern Minnesota. Age-class Adult Yearling Category r2 N r2 N Date of nest initiation 0.013 27 0.029 48 Nesting success 0.013 27 0.017 48 Clutch size 0.027 27 0.295 48 Mean egg size 0.002 8a 0.021 9a Hatching success 0.002 27 0.876 48 Recruitment 0.718 10 0.014 15 Female survival through spring and summer 0.389 27 0.208 48 a Number of clutches in which at least 1 egg was measured. index provided a measure of stress an allowed incorporation of a larger sampl of females (those weighed prior to lat winter) into the analyses. Analyses using flock survivorship as an index to site-specific winter condition gave results similar to those given by the weight analyses. Reduced flock survivor ship was correlated with reduced repro ductive performance among those fe males that did survive to breed. The strongest relationships were between flock survivorship and hatching success among yearling females and recruitment of young into the fall population among adult fe- males. A weak relationship was evident for clutch size among yearlings and for survival through the breeding season. No relationships were evident for nest phe- nology or egg size (Table 2). Analyses of renesting data showed no renesting at- tempts among 3 females from flocks with poor overwinter survivorship (<75% sur- vival) that survived long enough to have renested. In contrast, 9 of 15 females from flocks where overwinter survival was good (>75% survival) attempted to renest and 8 were successful. Natality was 2.8 fe- males hatched/female (in the breeding population) and recruitment was 0.65 fe- males among those females from flocks in which overwinter survivorship was poor. Females from flocks in which survivorship was good raised an average of 3.6 females and recruited 1.65 females/female. DISCUSSION Analyses of the relationship between se- vere winter conditions and subsequent re- production are often conducted over a rel atively large area. Such an approach requires the assumption that the degree of winter severity is homogeneous across the area. Our studies of winter ecology of wild turkeys show that substantial variation in winter conditions may exist within an area of 600 km2 (Porter et al. 1980). The de- gree of winter severity is site-specific, de- pending on the combination of food, cov- er, and weather conditions in a particular location. By examining the responses of turkeys to site-specific winter conditions our analyses integrated environmental factors influencing winter conditions and took advantage of the variation in winter conditions on the study area. These anal yses were limited to those years prior to 1979 in which density-dependent inter- actions appeared insignificant to popula- tion growth (Porter 1978). We believe the results of the study are best interpreted from an energetic per- J. Wildl. Manage. 47(2):1983 This content downloaded from 128.227.251.1 on Fri, 28 Feb 2025 14:14:17 UTC All use subject to https://about.jstor.org/terms WINTER CONDITIONS AND REPRODUCTION IN TURKEYS * Porter et al. 287 spective. Many studies have shown that egg-laying, incubation, and early brood- ing are energetically expensive. These ac- tivities frequently result in substantial depletion of energy reserves (e.g., Brei- tenbach et al. 1963, Ryder 1970, Drent 1975). In the turkey, all of these costs are borne by the female. Unlike many species of the Phasianidae, this species accumu- lates substantial fat deposits. Endogenous reserves in female turkeys at breeding are dependent on the energy budget during the weeks immediately prior to breeding. Winter conditions are most severe from mid-February through mid-March and, in Minnesota, turkeys begin nesting during the 2nd week of April (Porter 1978). Thus, there is little time for females to replenish severely depleted energy reserves prior to breeding. Intuitively, one might suspect that poor winter conditions might result in a delay in nest establishment. Attempts to test this hypothesis using pen-raised birds have demonstrated that the physiological con- dition of the female does influence the ini- tiation of egg-laying (Hayden and Nelson 1963, Gates and Woehler 1968). Our tech- nique was not adequate to document the initiation of egg-laying. We can only infer that the initiation of nesting was not sig- nificantly affected by winter conditions. The only comparable analysis conducted under field conditions of which we are aware resulted in similar conclusions (Gates and Hale 1975). It may be that nesting begins sufficiently late in the spring for females to recover enough to lay a clutch on schedule. A 2nd facet of reproduction that one might expect to be affected by winter conditions is clutch size. Such an effect was not observed in pheasants in Wiscon- sin by Gates and Hale (1975). After re- viewing this question among nidifugous birds, Lack (1968) reached the conclusion that food limitations do not influence clutch size in this group. Drent's (1975) review suggests that other factors (such as the thermodynamic properties of a clutch) exert stronger selection pressures on clutch size and that if energy reserves are inad- equate to lay a full clutch, nesting will not occur. Our data tend to support these con- clusions. A possible relationship exists for clutch size among yearling females. How- ever, winter conditions explain substan- tially less than 50% of the variation, sug- gesting other unmeasured factors are more important. Renesting appears to be another com- ponent of reproduction likely to be af- fected by severe winter conditions. Fol- lowing the investment of energy and time in laying and beginning to incubate a 1st clutch, the reserves of these females may be so low that the time required to recover precludes a 2nd nesting attempt. Pattee and Beasom (1979) have shown that nu- tritional characteristics of habitat can in- fluence the probability of renesting among turkeys. Our data are too limited to test this hypothesis adequately, but are sup- portive. The facets of reproductive perfor- mance that appear to be most clearly af- fected by winter conditions are hatching success and survival of young through the summer. These facets may be influenced by egg quality, which is in turn a product of physiological condition of the female as suggested by Moss et al. (1974) for red grouse (Lagopus lagopus scoticus). How- ever, specific investigations of Breiten- bach et al. (1963) and a general review of the galliform literature by Lack (1968) suggest that physiological condition of the female does not influence egg quality. Presuming egg size is related to egg qual- ity, our data support this latter conclusion. A 2nd hypothesis concerns the effect of the relative attentiveness of the females J. Wildl. Manage. 47(2):1983 This content downloaded from 128.227.251.1 on Fri, 28 Feb 2025 14:14:17 UTC All use subject to https://about.jstor.org/terms 288 WINTER CONDITIONS AND REPRODUCTION IN TURKEYS * Porter et al. during incubation and early brooding on the survival of offspring. Work summa- rized by Drent (1975) shows that variation in temperature of the egg during incu- bation can influence hatching success. Fe- males entering incubation with minimal endogenous energy reserves may be forced to spend more time away from the nest to satisfy their own energy requirements. Egg temperature may vary more widely, re- sulting in a decline in hatching success. Because yearling females generally have lower reserves than adults, one would pre- dict the effect to be most pronounced in this age-class. Our analyses show that more than 80% of the variation in hatching rates of yearling females can be explained by winter conditions. Survival of poults may be influenced by a similar relationship. Females that hatch young but are in poor physiological condition may be forced to spend more time actively feeding instead of brooding during the first days after hatching. As a result, mortality among their young can be expected to increase. Our analyses sug- gest that adult females have the energy reserves necessary to attend to incubation but not to provide for care of young. The lack of a similar relationship for survivorship of poults of yearling females may be attributable to the fact that eggs of few yearlings from flocks in which winter survival was low hatched. Similar- ly, the higher average rate of survival of poults of yearling females relative to adults may reflect the fact that eggs of only those yearlings in apparently good physiological condition hatched. Thus, although winter conditions show a similar overall impact on reproductive performance of adults and yearlings, the facet of the reproduction process impacted appears to differ be- tween the age-classes. We suggest that the effects of severe winter conditions on reproductive output do not operate primarily through impacts on the processes associated with laying a clutch. Rather, the primary effect appears to be related to the ability of the female to hatch eggs and rear the young. The impacts of winter conditions on the population dynamics of wild turkeys in northern regions can be substantial. A model was developed and used to com- pare growth under good and poor winter conditions. These conditions were defined as those in which overwinter survivorship values of different flocks were greater than and less than 75%, respectively. Average values of survivorship and concomitant reproductive values were used for females in these 2 classes. The model used was: Nt+l = N,(Pw)(P)() + Nt(Pw)(Pb)(P,) where: Nt = the number of females in the fall population, Pw = the probability of survival to 31 March, Pb = the probability of survival from 1 April to 31 May, R = the number of females re- cruited into the fall popu- lation, and P, = the probability of survival from 1 June to 31 August. Projections made with this model show that winter mortality alone can signifi- cantly reduce the growth rate of the pop- ulation. Assuming good reproduction (re- cruitment rate of 1.65 young females into the fall population), average reduction in overwinter survival from 92 to 61% results in continued growth but a depression in the finite rate of growth (X) from 2.0 to 1.4 (Table 3). If the reproductive perfor- mance of surviving females is also im- paired, overwinter losses are not offset in the subsequent breeding season. Projec- tions show that a 30% reduction in winter J. Wildl. Manage. 47(2):1983 This content downloaded from 128.227.251.1 on Fri, 28 Feb 2025 14:14:17 UTC All use subject to https://about.jstor.org/terms WINTER CONDITIONS AND REPRODUCTION IN TURKEYS * Porter et al. 289 Table 3. Population projections for wild turkeys in Minnesota assuming different impacts of severe winters on survival reproduction. All values refer to females only. Population projections Survivorship and No fall reproductive variables harvest 20% fall harvesta Winter impact Pw Pb, R Ps Nt Nt+1 Nt+2 Nt+l Nt+2 Nt+3 Not significant to survival or reproduction 0.92 0.85 1.65 0.91 100 200 401 160 256 411 Significant to survival only 0.61 0.88 1.65 0.86 100 135 181 107 116 125 Significant to reproduction only 0.92 0.85 0.65 0.91 100 122 149 98 95 93 Significant to survival and reproduction 0.61 0.88 0.65 0.86 100 81 65 64 42 27 a Represents fall hunting for either sex resulting in harvest of 20% of the females in the popula survival with its concomitant reproduc- tive performance (recruitment rate of 0.65 young females into the fall population) can result in a 20% decline in the population in the subsequent fall (X = 0.80). MANAGEMENT IMPLICATIONS The ultimate impact of severe winters on the population dynamics of turkeys in this region will depend primarily on the geographic extent of the conditions. As observed in this study, severe winter con- ditions are often localized. The reductions in overwinter survivorship and in recruit- ment in some areas will be offset by the high reproduction in others, even if severe conditions occur frequently. However, several winters of severe conditions can result in localized extinction of turkey populations. The implications of this phenomenon on harvest management are unclear. Our model assumes that winter impacts will occur regardless of any harvest of the pop- ulation during the preceding fall. If the relationship between fall harvest and win- ter stress-reproductive performance is of a compensatory form, population esti- mates of our model are conservative. There may be little impact of winter con- ditions on harvest management. Where actual population responses ap- pear to follow those predicted by an ad- ditive model, the impacts on harvest man- agement will depend on the type of hunting season. Harvests of males in the 2nd spring following a severe winter can be expected to decline as a result of a smaller cohort of maturing 2-year-old males. Either-sex harvests can be expected to decline in the 1st fall following the se- vere winter due to reduced recruitment. Recovery of a population following a se- vere winter will not be influenced by male- only seasons but may be by fall either-sex seasons. Modeling suggests that hunting pressure may lengthen recovery times. Where a conservative approach to popu- lation management is desirable, consid- eration of adjustments in fall harvest quo- tas may be appropriate. LITERATURE CITED AUSTIN, D. E., AND L. W. DEGRAFF. 1975. Winter survival of wild turkeys in southern Adiron- dacks. Natl. Wild Turkey Symp. 3:55-60. BREITENBACH, R. P., C. L. NAGRA, AND R. K. MEYER. 1963. Effect of limited food-intake on cyclic annual changes in ring-necked pheasant hens. J. Wildl. Manage. 27:24-36. DRENT, R. 1975. Incubation. Pages 333-420 in D. S. Farner, J. R. King, and K. C. Parks, eds. Avian biology. Vol. 5. Academic Press, New York, N.Y. EDWARDS, W. R., P. J. MIKOLAJ, AND E. A. LEITE. J. Wildl. Manage. 47(2):1983 This content downloaded from 128.227.251.1 on Fri, 28 Feb 2025 14:14:17 UTC All use subject to https://about.jstor.org/terms 290 WINTER CONDITIONS AND REPRODUCTION IN TURKEYS * Porter et al. 1964. Implications from winter-spring weights of pheasants. J. Wildl. Manage. 28:270-279. GATES, J. M., AND J. B. HALE. 1975. Reproduction in an east central Wisconsin pheasant popula- tion. Wis. Dep. Nat. Resour. Tech. Bull. 85. 70 pp. , AND E. E. WOEHLER. 1968. Winter weight loss related to subsequent weights and reproduc- tion in penned pheasant hens. J. Wildl. Manage. 32:234-247. GILMER, D. S., W. B. KUECHLE, AND I. J. BALL. 1971. A device for monitoring radio-marked animals. J. Wildl. Manage. 35:829-832. HAYDEN, A. H., AND E. NELSON. 1963. 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