Impacts of Acorns and Oaks

Because acorns are a preferred and highly utilized food source, their availability might be an important factor in turkey population processes. Mast availability and use may ultimately affect the condition, behavior, survival, and productivity of wild turkey populations.

Movements and Behavior

The home range size, movements, and habitat use of wild turkeys are apparently influenced by food availability, and winter foods exert the greatest effect (Lewis and Kurzejeski 1984, Kurzejeski and Lewis 1990, Lewis 1992). Wunz and Pack (1992) observed smaller annual home ranges in superior habitats, and winter home ranges appear to be smaller during years with abundant acorn crops (Lewis and Kurzejeski 1984, Kelley et al. 1988).

Because turkeys favor mast crops (especially acorns) as winter foods, the locations of wintering areas are determined by acorn availability (Ellis and Lewis 1967, Hurst 1992). In movements that coincide with mast availability and acorn drop, turkeys typically shift their ranges from fields and pastures to forests in fall and winter (Eaton et al. 1970, Barwick and Speake 1973, Speake et al. 1975, Eaton et al. 1976, Everett et al. 1985, Porter 1992). In a study of wild turkeys in managed pine forests, the turkeys responded to mast availability by using bottomland hardwoods more in fall and winter (Exum et al. 1987). Gould's turkeys have moved as much as 12.7 km between summer and fall ranges, probably to take advantage of mast crop availability (Schemnitz et al. 1990). When mast crops are good, these shifts in habitat can be very distinct and abrupt (Eaton et al. 1970, Barwick and Speake 1973, Healy 1992a).

Seasonal shifts in home range are reduced when food resources are abundant and distributed throughout the range (Korschgen 1967, Kurzejeski and Lewis 1990). When mast crops are scarce, flocks move to areas where other food sources (especially row crops) are abundant (Dalke et al. 1942, Ellis and Lewis 1967, Kurzejeski and Lewis 1990, Healy 1992a). As turkeys search for food resources, shifts from summer range to fall and winter range tend to be greater during years with poor mast production (Kurzejeski and Lewis 1990, Healy 1992a). In northern Missouri, turkeys traveled up to 4.8 km to use row crops in winter when acorn production was poor (Kurzejeski and Lewis 1990). When acorns are still available in the spring, after good mast crops, spring movements remain small (Godwin et al. 1994).

Winter flocking behavior seems to be influenced by the availability of acorns and other foods. When natural foods are scarce or unavailable, winter flocks usually are composed of fewer than 10 birds (Wunz and Pack 1992). As winter progresses and acorn supplies become depleted, flocks reduce their size, to facilitate food acquisition (Wheeler 1948). Larger flocks occur with abundant mast conditions (Wheeler 1948) or when winter foods are abundant and localized (e.g., crops) (Wunz and Pack 1992).

Condition of Turkeys

Acorns, with their high crude fat content and high metabolizable energy (Billingsley and Arner 1970, Decker et al. 1991), become available in the fall when energy demands are increasing. Despite their energy richness, their low protein and phosphorous contents suggest that acorns are largely inadequate to supply the nutritional needs of breeding or growing turkeys (Beck and Beck 1955, Short and Epps 1976, Pattee and Beasom 1979).

Dietary energy requirements increase during the colder fall and winter months throughout much of the wild turkey range. Standardized for a 4.23-kg turkey hen, two separate studies estimated that 247 kcal/day (New Hampshire) and 347 kcal/day (Minnesota) were required to maintain body weight at an ambient temperature of 0°C (Decker et al. 1991, Haroldson 1996). If only acorns were consumed to meet these winter energy demands at temperatures of 0°C, 4.23-kg hens from these respective studies would require 77.9 g/day (about 26 acorns) and 109.5 g/day (about 37 acorns). An extra 19 grams of acorns (about 6 acorns) would be required per hen for every 10° decrease in ambient temperature (Haroldson 1996). An 8-kg turkey gobbler would need 512 kcal/ day (239.9 g/day or about 80 acorns/day) for a maintenance diet (Decker et al. 1991). A full turkey crop, which can easily contain > 100 acorns (Mosby and Handley 1943, Schemnitz 1956), would satisfy the daily energy requirement. Because winter diets of turkeys typically are not 100% acorns (Table 16.1), fewer acorns would actually be required to maintain thermoregulation in most areas. Although individual acorns vary widely in size among and within species, we have assumed that an average acorn weighs approximately 3 grams (Christisen and Korschgen 1955, Decker 1988).

Availability of oak mast seems to influence turkey condition, but direct measurements of food availability and turkey condition have been difficult to obtain. Low body weights and weight loss were assumed to be the result of an absent acorn crop in a study in Mississippi (Seiss 1989) and of deep snows in a study in Minnesota (Porter et al. 1980). Merriam's turkeys appeared to enter the winter in better condition after good acorn crops (Korschgen 1967).

Mortality

Because turkeys have a flexible diet and the ability to fast for long periods without starving, it is assumed that mast failures are of little significance to turkey survival. Other useful foods are usually available and good populations exist where few oaks occur (Bailey et al. 1951, Korschgen 1967, Markley 1967, Ignatoski 1973). Although body weights may be reduced during years with poor mast, adaptability to food shortages gen erally prevents starvation (Uhlig and Bailey 1952, Schorger 1966, Powell 1967). Substitutes for hard mast include ferns, bulbs, tubers, spore heads of club moss (Lycopodium sp.), seeds of forbs and grasses, and crops (Wunz and Pack 1992). In northern areas, where all foods may become unavailable because of deep and continuous snow cover, turkeys are capable of surviving without food for two weeks while losing > 40% of their body weight (Hayden and Nelson 1963, Markley 1967).

During unfavorable winter weather conditions, however, fall and/or winter survival of turkeys can be related to the availability of food resources, especially acorns (Porter et al. 1980, Porter et al. 1983, Vander Haegen et al. 1989, Vangilder 1995). In areas with limited fall hunting mortality, higher fall survival rates of hens generally were associated with better mast production (Vangilder 1995). No association was apparent between mast abundance and winter mortality rates. Even so, Vangilder (1995) suggested that acorn availability might be important for winter hen survival during adverse weather conditions.

Unfavorable ambient conditions (e.g., severe winters at northern latitudes, persistent deep snows), coupled with poor acorn production, can result in negative energy balances and starvation. Access to agricultural landscapes and food plots has been shown to mitigate the combined effects of mast failures and severe winters on mortality rates (Vander Haegen et al. 1989, Roberts and Porter 1995, Vangilder 1995).

Turkey populations also may experience greater fall hunting mortality during years when mast is scarce than when it is abundant (Menzel 1975, Wunz 1979). Hunter success may improve in the fall during mast shortages because turkeys concentrate activity at localized food sources (e.g., row crops, fields), making them more visible and vulnerable (Healy 1992a, Wunz and Pack 1992). Additional analyses of a five-year turkey population dynamics study in Virginia and West Virginia (Pollock et al. 1997, Pack et al. 1999) suggested that hunting mortality rates of adult hens during the fall tended to be higher under poor mast conditions (P = 0.01) (Table 16.2). Unlike for adult hens, fall hunting mortality rates for juvenile hens did not vary (P = 0.80) as a function of mast production.

Fall hunting mortality may be disproportionately magnified when heavy hunting pressure is coupled with mast failure. When acorn crops were abundant, regions with heavy (Virginia) and light (West Virginia) fall hunting pressure had similarly low adult hen hunting mortality rates (0.05 and 0.04, respectively); but when acorn availability was scarce, hunting mortality rates of adult hens increased much more (P = 0.05)

Fall hunting mortality rates of wild turkey hens based on mast conditions and hunting pressure 1990-1993 in Virginia and West Virginia

Age

Hunting pressure

Mast conditions Good-Excellenta Poor—Fairh

Overall mortality rate

Mean

SE

Mean

SE

Mean

SE

Adult0

Low®

0.04

0.02

0.07

0.01

0.06

0.01

Highf

0.05

0.03

0.17

0.01

0.11

0.04

Overall

0.04

0.01

0.12

0.03

0.08

0.02

Juveniled

Low

0.10

0.03

0.11

0.06

0.10

0.03

High

0.16

0.08

0.18

0.06

0.17

0.04

Overall

0.13

0.04

0.14

0.04

0.13

0.02

Sources: Pollock et al. 1997, Pack et al. 1999. Although study began in 1989, due to small sample sizes in the fall of 1989, hunting mortality rates from that year were not considered.

aBased on mast surveys, the two best mast years (1991 and 1993).

bBased on mast surveys, the two worst mast years (1990 and 1992).

eIn a 4-week fall season in West Virginia with no concurrent firearms deer hunting. in an 8- or 9-week fall season in Virginia with 1 or 2 weeks of concurrent firearms deer hunting.

Sources: Pollock et al. 1997, Pack et al. 1999. Although study began in 1989, due to small sample sizes in the fall of 1989, hunting mortality rates from that year were not considered.

aBased on mast surveys, the two best mast years (1991 and 1993).

bBased on mast surveys, the two worst mast years (1990 and 1992).

eIn a 4-week fall season in West Virginia with no concurrent firearms deer hunting. in an 8- or 9-week fall season in Virginia with 1 or 2 weeks of concurrent firearms deer hunting.

in heavily hunted areas than in the lightly hunted areas (0.17 vs. 0.07, respectively). Regarding the combined effects of hunting pressure and mast conditions, no differential change in fall hunting mortality was observed for juveniles (P = 0.93). In Pennsylvania, fall turkey harvests were noticeably greater on a heavily hunted area during years with poor acorn production than on an area with low hunting pressure (Wunz 1979).

In addition to mast abundance, fall harvest is also influenced by a variety of other factors, including the availability of alternate foods, turkey population size, juvenile recruitment during the year, and hunting pressure. In West Virginia, there was actually a positive correlation between regional indices of mast abundance and fall harvest during the years 1943 through 1950 (Uhlig and Bailey 1952, Uhlig and Wilson 1952). These studies concluded that cold weather in May resulted in both poor mast production and lower turkey productivity and that warmer May temperatures resulted in good mast years and higher turkey productivity. High productivity yielded population increases for larger fall harvests (Uhlig and Bailey 1952).

Reproduction

Many studies have indicated a strong relationship between wild turkey reproductive success and nutrition. Nutritional deficiencies and poor habitat quality may lead to fewer hens' laying (Pattee and Beasom 1979), later initiation of laying (Billingsley and Arner 1970, Pattee and Beasom 1979, Porter et al. 1983), earlier end to nesting attempts (Pattee and Beasom 1979), fewer eggs being laid (Hayden and Nelson 1963, Gardner and Arner 1968, Billingsley and Arner 1970), fewer subadult nesters (Rumble and Hodorff 1993), lower renesting rates (Vander Haegen et al. 1988, Hoffman et al. 1995), fewer poults hatching (Pattee and Beasom 1979), and lower brood survival (Porter et al. 1983).

Few studies have specifically addressed the role of acorn availability prior to breeding efforts. Palmer et al. (1993) noted later nest initiation following acorn crop failures in Mississippi. However, Wunz (1979) found no clear evidence that acorn production had any effect on the following summer's reproduction. Despite wide differences in mast production over a five-year period (Pollock et al. 1997), recruitment rates did not vary among years in Virginia and West Virginia (Norman et al. 2001). Considering the importance of acorns to winter diets and that reproductive success is dependent on the energy budget and endogenous reserves of females during the weeks prior to breeding (Porter et al. 1983), winter acorn availability is likely important to turkey reproduction.

In addition to hen productivity, nutrition also affected gobbler breeding capabilities and behavior. The nutritional quality of the habitat affected the growth and testes development of subadult gobblers (Lewis and Breitenbach 1966). Small gobblers, those weighing less than 6.35 kg, rarely exhibited advanced stages of spermatogenesis (Lewis and Breitenbach 1966, Blankenship 1992). Winter food restrictions that caused weight losses of 20-30% before breeding season delayed strutting and gobbling (Hayden and Nelson 1963). Among environmental factors, available mast might influence energy reserves and subsequent gobbler investment in gobbling and breeding activities (Lint et al. 1995, Miller et al. 1997).

Population Impact

While acorn production and availability appear to exert a great influence on wild turkey diet, movement, behavior, survival, and productivity, the ultimate role of oak mast as a population limiting factor is unclear. Because good wild turkey populations exist where oaks are rare (Markley 1967, Powell 1967, Kothmann and Litton 1975, Dickson et al. 1978), and turkeys generally do not starve during mast failures (Korschgen 1967, Markley 1967), acorns appear not to be a necessity for survival. Despite the apparent influences on population parameters, winter foods seldom act as an ultimate limiting factor for the omnivorous and opportunistic wild turkey. Less obvious population influences of winter mast probably are manifested by changes in carrying capacity and population growth rates.

Winter foods, oak mast, and other habitat factors are probably most important at the northern and southern edges of turkey range and in more heavily forested habitats (Healy 1981, Lewis 1992, Porter 1992). Populations have been observed to fluctuate with changes in food resources and mast abundance (Ligon 1946, Porter 1992, Vangilder 1995), especially when deep snows influence food availability (Healy 1981, 1992b).

Other Values of Oak Trees

In addition to their value as acorn producers, oak trees and forests have other uses for turkeys. In parts of the wild turkey range, especially in the Central Plains and the West, oak trees are more valuable as roost sites than as food sources (Crockett 1973, Healy 1992b). Because their roosting options sometimes are limited, Texas Rio Grande turkeys, New Mexico Gould's turkeys, and Oklahoma Merriam's turkeys commonly roost in live oaks (Markley 1967, Beasom and Wilson 1992), Emory oaks (Q. emoryi) (Potter et al. 1985), and bur oaks (Q. macrocarpa) (Crockett 1973), respectively. Turkeys prefer coniferous trees, which usually provide better thermal cover and roosting structure than oak trees (Hoffman 1968, Kilpatrick et al. 1988, Porter 1992, Flake et al. 1995). Depending on the associated understory characteristics, oak habitats are often selected by wild turkeys for nest sites (Hoffman 1962, Shaw and Mollohan 1992) and brood rearing (Mackey and Jonas 1982, Ross and Wunz 1990, Schemnitz et al. 1990). Rio Grande gobblers use shaded groves of live oaks to escape the summer heat in Texas (Beasom and Wilson 1992). Scrub oak (Q. ilicifolia) thickets provide escape cover for turkeys in West Virginia (Glover 1948). Bur oak habitats provide important forest corridors between roosting sites in South Dakota (Flake et al. 1995).

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