Research ArticleSimulating Northern Bobwhite PopulationResponses to Nest Predation, Nesting Habitat,and Weather in South TexasMICHAEL J. RADER,1Caesar Kleberg Wildlife Research Institute, Texas A&M University-Kingsville, Kingsville, TX 78363, USALEONARD A. BRENNAN,2Caesar Kleberg Wildlife Research Institute, Texas A&M University-Kingsville, Kingsville, TX 78363, USAKYLE A. BRAZIL,3Caesar Kleberg Wildlife Research Institute, Texas A&M University-Kingsville, Kingsville, TX 78363, USAFIDEL HERNA´NDEZ, Caesar Kleberg Wildlife Research Institute, Texas A&M University-Kingsville, Kingsville, TX 78363, USANOVA J. SILVY, Department of Wildlife and Fisheries Sciences, Texas A&M University, College Station, TX 77843, USAABSTRACT Nest predation is thought to be one of the major factors limiting northern bobwhite (Colinus virginianus) populations. Weexamined the relative impact of altering nest-predation rate, nesting habitat, and weather (i.e., temp and precipitation) on northern bobwhitepopulation dynamics in a hypothetical 15,000-ha subtropical-rangeland ecosystem in south Texas using a simulation model. The systems modelconsisted of a 3-stage (i.e., eggs, juv, and ad) bobwhite population with dynamics influenced by variables affecting production, recruitment, nestpredation, and mortality. We based model parameters on data collected from a 3-yr nest-predator study employing infrared-camera technology,from ongoing field research using a radio-marked population of wild bobwhites, and from the literature. The baseline simulated bobwhitepopulation dynamics corresponded closely to empirical data, with no difference between medians of simulated (n ¼ 30 yr) and observed bobwhiteage ratios over a 28-yr period. Similarly, a time-series comparison of simulated and observed age ratios showed most (89%) observed valuesfell within the 5th and 95th percentiles of the simulated data over the 28-yr period. We created simulated population scenarios representing1) baseline historical conditions, 2) predator control, 3) low precipitation, 4) low precipitation with predator control, 5) high temperature,6) high temperature with predator control, 7) reduced nest-clump availability, and 8) reduced nest-clump availability with predator controlthat resulted in considerably different median bobwhite densities over 10 yr. For example, under simulated predator control, populationsincreased by about 55% from the baseline scenario, whereas under simulated reduced nest-clump availability, populations decreased by about75% from the baseline scenario. Comparisons of time-series for each scenario showed that reduced nest-clump availability, low precipitation,and high temperature reduced bobwhite densities to a larger degree compared to a natural nest predation rate. Reduced nest-clump availabilityresulted in the most substantial decline of simulated bobwhit e densities. Simulations suggested that management efforts should focus onmaintaining adequate nest-clump availability and then possibly consider nest predator control as a secondary priority. ß 2011 The WildlifeSociety.KEY WORDS bobwhite, Colinus virginianus, drought, nesting habitat, nest predation, population, predator control,simulation, south Texas, stochastic modeling.Northern bobwhite (Colinus virginianus) populations arepotentially limited by many factors, including predation,weather, and habitat (Stoddard 1931, Lehmann 1984,Roseberry and Klimstra 1984, Guther y 2002). In southTexas bobwhites exist in a unique ecological context charac-terized by a diverse predator community, recurrent drought,extensive and contiguous semiarid-subtropical rangelandsubject to livestock grazing, and fee-lease hunting(Lehmann 1984; Herna´ndez et al. 2002; Rader et al.2007b). South Texas is one of the few regions where bob-white populations have not experienced significant long-term declines in North America (Brennan 1991, Churchet al. 1993, Link et al. 2008). Recent trends indicate largecattle ranches characteristic of south Texas have increasingeconomic incentives to emphasize sale of bobwhite andwhite-tailed deer (Odocoileus virgin ianus) hunting opportu-nities over livestock production. This development hasresulted in economic incentives to maintain rangeland in acondition beneficial to bobwhites and a variety of otherwildlife (Fulbright and Bryant 2002).Numerous biotic and abiotic factors influence bobwhiteproduction in South Texas. Nest predation is the main causeof bobwhite nest failure and is a recurrent managementconcern for bobwhites and other ground-nesting galliformes(Stoddard 1931, Klimstra and Roseberry 1975, Lehmann1984, Hurst et al. 1996, Rollins and Carroll 2001). Apparentbobwhite nest losses to predation generally fall between 37%and 76 % (Lehmann 1946, Simpson 1976, Staller et al. 2005,Rader et al. 2007a, Sandercock et al. 2008). Thus, there isevidence that predation might limit potential bobwhite pro-duction, but the impact of nest predation on fall and springbobwhite densities is not clear. In fact, predator removalstudies have been shown to have minimal positive effects onsouth Texas bobwhite populations (Beasom 1974, Gutheryand Beasom 1977, Lehmann 1984). Sandercock et al. (2008)observed that nest success was fourth in overall importanceafter chick survival, adult summer, and adult winter survival,with respect to annual population growth.Nesting habitat is another variable potentially amenable tomanagement and a perennial concern for managers in southTexas (Kiel 1976, Guthery 1997, Kopp et al. 1998). Numerousstudies have documented bobwhite nest-site selection andsusceptibility to predation (Spears et al. 1993, Guthery 1997,Received: 30 April 2010; Accepted: 12 May 20101Present Address: Wisconsin Department of Natural Resources, 5301Rib Mountain Drive, Wausau, WI 54401, USA.2E-mail: [email protected] Address: P.O. Box 1370, Lampasas, TX 76550, USA.Journal of Wildlife Management 75(1):61–70; 2011; DOI: 10.1002/jwmg.4Rader et al.Bobwhite Population Model 61Herna´ndez et al. 2003, Lusk et al. 2006, Rader et al. 2007a).Additionally, other studies have highlighted potential delete-rious effects of grazing on bobwhite habitat and production(Cantu and Everett 1982, Wilkins and Swank 1992).Drought is a recurrent feature in south Texas and isthought to be the driving mechanism of the characteristicboom-bust phenomenon exhibited by bobwhit e populationsin the region (Lehmann 1984, Lusk et al. 2001, Herna´ndezet al. 2002, Herna´ndez et