FRED S. GUTHERY, Department of Forestry, Oklahoma State University, Stillwater, OK 74078
DALE ROLLINS, Texas Agricultural Extension Service, San Angelo, TX 76901
Abstract: Brush is an essential habitat component for most species of upland game birds. In areas where brush becomes too dense, brush sculpting can be used to enhance habitability via increasing the interspersion of cover types. Mechanical methods of brush control are generally preferred because of the selectivity afforded and the soil disturbance that results from clearing. Prescribed burning, while useful, is generally overrated as a brush management tool relative to game bird habitat management, at least in the western half of Texas. The manager’s objective should be to maximize the amount of “useable space” available to the target species of game bird. In general, the manager has a good deal of latitude in selecting a brush management pattern that will maximize the average density of a particular game bird species.
Woody cover is an essential component of habitat for most species of upland game birds. Woody plants serve as nest sites for white-winged doves (Zenaida asiatica) and mourning doves (Z. macroura), although mourning doves may nest on the ground when brush is reduced (Soutiere and Bolen 1976). The lack of woody cover limits habitability for game birds on Conservation Reserve Program acreages in northwest Texas (Lutz et al. 1994). Some species, such as wild turkeys (Meleagris gallopavo) and Gambel’s quail (Callipepla gambelli), use brush and trees as night-roosting cover. Virtually all species of upland game find refuge from predators and heat in or under the canopies of woody plants; this function of woody cover may serve prairie-adapted species such as lesser prairie-chickens (Tympanuchus pallidicinctus) at some stages of their life history (Donaldson 1969).
On the other hand, woody cover may become too dense for upland game. Brush invasion of prairies may be associated with the decline of Attwater’s prairie-chickens (T. cupido attwateri) along the Gulf Coast of Texas; these birds are adapted to prairie as opposed to brushy landscapes. Likewise, an overabundance of woody cover may inhibit populations of mourning doves and quail. These birds are not adapted to areas lacking openings in woody plant cover. When brush becomes too abundant, management has an excellent opportunity to increase populations of game birds. Brush control can be an excellent tool for sculpting wildlife habitat, but as it is often practiced (i.e., all woody vegetation targeted for removal) it is disastrous to most upland game birds.
Our purpose is to provide a general overview of brush management on game bird range. We will discuss regional brush problems, compare and contrast methods of brush suppression, and provide guidelines on patterns and amounts of mature brush that should be retained for game bird species.
Regional brush problems
Rolling Plains Ecoregion. Mesquite (Prosopis glandulosa) is the predominant brush problem over all of Texas, and especially so in the Rolling Plains. Other associated woody species in this area include lotebush (Ziziphus obtusifolia), netleaf hackberry (Celtis reticulata), sandplum (Prunus angustifolia), sand shinoak (Quercus spp.), sandsage (Artemisia trifolialata), skunkbush (Rhus trilobata), chittam (Bumelia lanuginosa), catclaw mimosa (Mimosa biunciferae), elbowbush (Foresteria sp.) and cottonwood (Populus deltoides). Redberry juniper (Juniperus pinchotii) dominates on calcareous sites, breaks and badlands. Pricklypear (Opuntia spp.) is a codominant with mesquite over many areas.
Mesquite provides both food and shelter (thermal, escape and loafing) for northern bobwhite (Colinus virginianus) and scaled quail (Callipepla squamata) (Jackson 1969). Mesquite beans are usually among the “top 10” quail seeds throughout the Rolling Plains (Rollins 1981). Mesquites are also key nesting, roosting and loafing sites for mourning doves. Lotebush, sandplum and skunkbush are especially desirable for bobwhites and scaled quail for loafing coverts. Hackberry and chittam produce seeds which are staples in quail and turkey diets in this region. Cottonwoods, which are common in riparian areas, are a preferred roost site for wild turkeys. Pricklypear and tasajillo (O. leptocaulis) produce seeds that are commonly consumed by bobwhites, scaled quail and turkeys. Pricklypear also serves as nesting habitat for bobwhite and scaled quail, especially in more heavily grazed areas (Carter 1995). Those nests situated in pricklypear enjoy higher survival rates than similar nests in more traditional cover (e.g., bunchgrasses like little bluestem [Schizachyrium scoparium]) (Carter 1995).
Edwards Plateau Ecoregion. Junipers (J. pinchotii and J. ashei) are the dominant woody species across the limestone soils of the Edwards Plateau. Other common woody species include mesquite, liveoak (Q. Virginiana), shinoak (Q. mohriana), lotebush, catclaw mimosa, catclaw acacia (A. greggii), agarito (Mahonia trifoliolata), hackberry and sumacs (R. trilobata, R. microphylla and R. copalina). Pricklypear and whitebrush (Aloysia lycioides) can become especially dense on deeper soils.
Junipers provide screening and thermal cover (especially during winter) for quail and turkeys (Rollins and Armstrong 1997). Liveoaks and associated species (e.g., pecan [Carya illinoiensis]) are the major turkey roosts in this ecoregion. Oaks also produce mast which is especially valuable to wild turkeys, and to a lesser degree northern bobwhites. Quail loafing cover in this area is typified by littleaf sumac, lotebush, whitebrush and taller stands of pricklypear (especially O.discata).
Rio Grande Plains Ecoregion. The Rio Grande Plains Ecoregion is characterized by diverse brush communities. Assemblages of blackbrush acacia (Acacia rigidula), ceniza (Leucophyllum frutescens) , and guajillo (A. berlandieri) dominate shallow, rocky soils. Mixed brush, including mesquite, brazil (Condalia obovata), coyotillo (Karwinskia humboldtiana), granjeno (Celtis pallida), pricklypear, and lime pricklyash (Zanthoxylum fagara), occupy loams and sandy loams with level topography. Deep sands occur along the Gulf Coast and as outcrop-pings in shallower soils in south-central portions of the Rio Grande Plains. Live oak is the most common woody plant on sands near the coast, whereas mesquite becomes dominate further inland.
Wild turkeys tend to be associated with stands of live oak, which provide roosts and mast. Strong populations of bobwhites and wild turkeys occur together where live oak and mesquite intermix. Bobwhites are well adapted to mixed brush communities in the central portion of the Rio Grande Plains, and in some cases (e.g., LaSalle County) bobwhites and scaled quail occur together at high densities. Sypatry occurs where mesquite-dominated communities on deeper soils intermingle with blackbrush-guajillo communities on shallower soils.
Trans-Pecos Desert and Mountains. The eastern portion of the Trans-Pecos shares many woody species with the Edwards Plateau, including netleaf hackberry, mesquite, lotebush, shinoak and littleleaf sumac. Other common woody species in this region include javelinabrush (Condalia ericoides), wolfberry (Lycium spp.), allthorn (Koeberlinia spinosa), and cholla (O. imbricata).
Methods for sculpting habitats
Treating with herbicides (Koerth 1996), prescribed burning (Riggs et al. 1996), and treating with mechanical implements (Fulbright and Guthery 1996) are the primary methods of brush suppression in Texas. The use of browsers (goats) to manage brush is practiced on some ranches in the western Edwards Plateau and in experimental areas.
The application of chemicals in the management of game bird cover is limited but important in some contexts. Sandy rangelands occupied by sand shinoak in northern Texas provide an example of a useful application. These rangelands may trend to nearly pure stands of shinoak. Herbicide treatment increases the amount of herbaceous ground cover useful as nesting, foraging and roosting habitat for lesser prairie-chickens. Such treatments may also benefit bobwhites and scaled quail, but post-treatment areas are more vulnerable to overgrazing than the original stands of shinoak.
Herbicide treatments also provide a relatively inexpensive methods for controlling problem, individual brush plants. In Wisconsin, brush beginning to invade the prairie habitat of greater prairie-chickens has been controlled with individual plant treatments. Such spot treatments can be used in Texas to maintain openings created with mechanical implements. The selectivity afforded by individual plant treatments negates one of the most undesirable aspects of using broadcast herbicides, i.e., an inability to spare desirable woody plants during applications.
Prescribed burning, while useful, generally is overrated as a tool for managing game bird habitat in Texas. The lore of prescribed burning arose with the work of Herbert L. Stoddard in Florida and Georgia (Stoddard 1931). Stoddard worked in subtropical areas receiving 40-60 inches of rain annually. The primary management problem for bobwhites in such environments is the build-up of hardwood cover after some treatment. Canopy closure may be complete (100%) at heights of 6-10 feet within 3 years after burning. This closure prevents bobwhites from occupying habitat space because the birds are not adapted to such conditions. One plantation in northern Florida burns 60-70% of 10,500 acres annually, and treats another 25% with roller-choppers just before hunting season.
On Texas rangelands, however, which typically receive 15-25 inches of rainfall, the environment contrives to limit the usefulness of burning. The accumulation of sufficient ground cover to carry a fire is limited by the amount of precipitation and grazing pressure. Some habitat types, such as shinoak, are not amenable to burning, depending on the density of shinoak. Other key woody plants, e.g., lotebush, are top-killed by “hot” fires and may take 5 to 10 years to attain suitable dimensions to be attractive as coverts (Renwald et al. 1978). Reclamation burns conducted under high temperatures and designed to suppress mature brush may be useful but are dangerous to execute. Burning is perhaps best applied for management of game bird cover on Texas rangelands as a means of suppressing brush regrowth on areas treated with other methods.
Pricklypear is often controlled by conducting a burn followed by an application of 0.13 to 0.25 lb a.i./acre picloram. This combination of treatments typically removes more than 95% of the pricklypear on a site (Ueckert et al. 1988). Recent evidence (Carter 1995, Slater 1996) suggests that pricklypear may be important nesting cover for bobwhites and scaled quail,especially in more arid regions where bunchgrasses are limited. Picloram also suppresses forb populations for 6-24 months, depending on the rate applied and site characteristics.
Mechanical methods of brush management generally seem more desirable than alternative methods in the management of game bird cover. Root-plowing kills a high percentage of brush individuals but has the drawback of reducing the diversity of brush species on a treated area (Fulbright and Beasom 1987); it is possible that 150-200 years must elapse to recover the brush diversity present before root-plowing (Fulbright and Guthery 1995). Generally, more diverse plant communities are considered beneficial to game birds, but there are limits on expectations. That is to say, after a certain level of diversity is attained, further increases in diversity have null effects on game bird abundance.
Less radical treatments of brush with mechanical methods include shredding, grubbing, chaining, and methods that simultaneously reduce the height of brush and disturb the soil (discing, roller-chopping, and aerating). Shredding is applied to maintain brush at a low height and, in this application, is similar to prescribed burning. Chaining takes out mature, single-stemmed plants and leaves low plants with supple stems. Discing, roller-chopping, and aerating may be likened to shredding with soil disturbance.
The soil disturbance associated with these latter practices may be quite beneficial. Fracturing of hardpan and compacted soils promotes the infiltration of rainfall, which in turns fosters the growth of forbs and grasses. The plants that appear soon after soil-disturbance treatments include many species of desirable foods for game birds.
Brush management patterns
Heeding the wisdom of some of the pioneers of game management (Stoddard 1931, Leopold 1933, Lehmann 1984), we note that populations of game birds maximize when individuals can use any part of a pasture at any time. Although this outlook was meant for bobwhites, it undoubtedly holds well for any species that is a target of management. Lehmann (1984) believed each and every square inch should be usable each and every day of the year. This philosophy has been called maximization of space-time (Guthery 1997); the philosophy serves as the basis for the patterns applied in brush management.
The pattern resulting from brush management is the arrangement of treated and untreated stands in a pasture or on a ranch. Up to a point, as treated stands become smaller and better dispersed among untreated stands, space usability increases; i.e., as stand interspersion increases, space usability increases. Space usability does not increase (and may decrease) when interspersion gets carried too far.
Habitats have a property which we shall call “slack:” many different arrangements of habitat objects, such as treated and untreated stands of brush, are equally valuable to a particular species of game bird. Slack arises for at least 4 reasons:
(1) Game birds may switch time in activities with little effect on survival and production. Within ordinary limits, it matters little to a population of game birds how much time is spent in foraging, travel, and resting.
(2) Generally, the capacity of usable space to support game birds exceeds the number of game birds present. Game birds in the Southwest are limited primarily by weather events and time within usable space. Therefore, alterations in the arrangement of habitat objects have little effect on population abundance if habitat space may be occupied at all times.
(3) Game birds (quail, doves, turkeys) are remark-ably well adapted to a variety of conditions, as evidenced by their vast distributions in North America. Many arrangements of habitat objects fit the adaptive outlook of these species.
(4) The cover functions provided by herbaceous and woody plants are, to some degree, interchangeable. Herbaceous cover may serve as loafing cover, woody cover may serve as foraging cover. Different arrangements of woody and herbaceous cover have the same outcome for game birds.
A number of practical guidelines arise from the somewhat theoretical paragraph above:
(1) Whether a brush management pattern is based on preservation of strips, preservation of blocks, or creation of openings within stands of mature brush makes no difference to game bird populations if certain distance and dispersion relationships are maintained among treated and untreated stands.
(2) The much bally-hooed Principle of Edge (or Law of Dispersion, as Leopold  called it) has definite limits in application. Game bird abundance may maximize and remain stable with different quantities of edge on an area (Guthery and Bingham 1992). The Principle of Edge fails when populations are operating within slack zones for the reasons outlined above.
(3) Functionally perfect habitat comes with many arrangements of habitat objects such as treated and untreated stands of brush. Perfect habitat is not a singular instance, but rather a composite of habitat configurations, each of which maximizes game bird abundance. In general, the manager has a good deal of latitude in selecting a brush management pattern that will maximize the average density of a particular game bird species.
Let us provide a more concrete example, using the bobwhite as a model. We start with a pasture having 75% canopy coverage of brush that is 6-10 feet tall. We wish to impose mechanical brush management such that all points in habitat space are usable at all times. Then we may apply any pattern that meets these constraints:
(1) no point in the pasture is further than 25 yards from woody cover,
(2) no more than 90% of the pasture is treated, and
(3) no woody cover object is less than 75 square feet in area.
Actually, the above prescription probably is conservative for bobwhites. We might be able to accept points up to 75 yards from woody cover, but such a configuration would be more sensitive to grazing. Also, the prescription is quite arbitrary. Thirty-two yards from woody cover, 82% treated, and 150 square feet probably are equally useful guidelines.
There are some other guidelines in managing brush for game birds.
(1) Preserve mottes instead of singletons. Wild turkeys, quail, and deer are more likely to occur in areas with mottes.
(2) Save patches of taller, mature brush. Taller brush is important on semiarid rangelands because of the cooler temperatures it creates during hot days and seasons (Johnson and Guthery 1988).
(3) Preserve wild turkey roosts and travel corridors (strips of woody cover) radiating from the roosts (Scott and Boeker 1977).
(4) Identify and preserve the integrity of “honey-holes”, i.e., special sites like sandplum or chittam thickets.
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Updated: Mar. 18, 1997