Research Article Population Irruptions of Northern Bobwhite: Testing an Age-Specific Reproduction Hypothesis FIDEL HERNA ´ NDEZ, 1 Caesar Kleberg Wildlife Research Institute, Texas A&M University-Kingsville, Kingsville, TX 78363, USA KRISAN M. KELLEY, Caesar Kleberg Wildlife Research Institute, Texas A&M University-Kingsville, Kingsville, TX 78363, USA JUAN A. ARREDONDO, Caesar Kleberg Wildlife Research Institute, Texas A&M University-Kingsville, Kingsville, TX 78363, USA FROYLA ´ N HERNA ´ NDEZ, 2 Caesar Kleberg Wildlife Research Institute, Texas A&M University-Kingsville, Kingsville, TX 78363, USA DAVID G. HEWITT, Caesar Kleberg Wildlife Research Institute, Texas A&M University-Kingsville, Kingsville, TX 78363, USA FRED C. BRYANT, Caesar Kleberg Wildlife Research Institute, Texas A&M University-Kingsville, Kingsville, TX 78363, USA RALPH L. BINGHAM, Caesar Kleberg Wildlife Research Institute, Texas A&M University-Kingsville, Kingsville, TX 78363, USA ABSTRACT Age-specific reproduction has been suggested for northern bobwhite (Colinus virginianus) and has been hypothesized as a factor contributing to population irruptions. However, little research has been conducted on the subject. We conducted a laboratory and field study to determine if age-specific reproduction occurred in northern bobwhites. Our objectives were to compare 7 reproductive measures (% F nesting, date of first incubated nest, egg-laying rate, nesting rate, clutch size, egg mass, and egg hatchability) between first- and second-year breeders and determine if differential reproduction was impacted by diet quality. The laboratory study consisted of a 2 3 2 factorial experiment with age and diet quality (low protein [12%] and high protein [24%]) as the factors. Data for the field study represented a 6-year data set of bobwhite reproduction (May–Sep 2000–2005) obtained from an ongoing radiotelemetry study in southern Texas, USA. We documented similar productivity (i.e., % F laying, egg-laying rate, and egg mass) and timing of laying (i.e., date of first egg) between juvenile (n ¼ 33) and adult bobwhites (n ¼ 27) in our laboratory study. However, females on the high-protein diet exhibited a greater egg-laying rate than females on the low-protein diet. Under field conditions, we also documented no difference in productivity (% F nesting, nesting rate, clutch size, egg hatchability) and timing of nesting (date of first incubated nest) between age classes (n ¼ 59 juv and 32 ad). Our findings do not support early suppositions of age-specific reproduction in quail. Quail irruptions should not be influenced by population age structure as it relates to age- specific reproduction. ( JOURNAL OF WILDLIFE MANAGEMENT 71(3):895–901; 2007) DOI: 10.2193/2006.101 KEY WORDS age-specific reproduction, Colinus virginianus, northern bobwhite, population dynamics, reproductive ecology, Texas. Most North American quail species exhibit irruptive population behavior; populations rapidly increase during relatively wet periods and abruptly decrease during drought. Irruptive behavior has been documented in northern bobwhite (Colinus virginianus; Kiel 1976), scaled quail (Callipepla squamata; Campbell et al. 1973), California quail (C. californica; Francis 1970), Gambel’s quail (C. gambelii; Heffelfinger et al. 1999), and Montezuma quail (Cyrtonyx montezumae; Brown 1989). Quail populations fluctuate so drastically on semiarid rangelands that population changes have been referred to as boom and bust. Various hypotheses have been researched to explain this phenomenon including vitamin A deficiency (Nestler and Bailey 1943), phosphorus deficiency (Cain et al. 1982), phytoestrogens (Leopold et al. 1976), water deprivation (Koerth and Guthery 1991), and heat stress (Guthery et al. 2001). Despite years of research, however, the causal factor(s) of the boom-bust phenomenon remains unknown (Guthery 2002, Herna ´ndez et al. 2002). Quail production is influenced by 4 crucial variables: proportion of females that nest, nesting rate, probability of nest success, and length of the nesting season (Guthery and Kuvlesky 1998, Herna ´ndez et al. 2005). These variables affect total nest production which is a primary determinant of percent summer gain in bobwhite populations (Dimmick 1974, Klimstra and Roseberry 1975). As such, any factor that influences nest production can exert a profound influence on quail populations. Age-specific reproduction, the phenomenon in which experienced breeders reproduce earlier and are more productive than younger, inexperienced breeders, is a process that, if present in quail, could influence quail populations via nest production. Mere changes in population age structure through time, therefore, could affect the amplitude and frequency of quail irruptions. Moreover, quail populations also could be impacted if poor nutritional conditions further exacerbated differential pro- ductivity between age classes (Kirkpatrick 1988). Age-specific reproduction is a common phenomenon in many avian species (Curio 1983, Forslund and Pa ¨rt 1995). Evidence for differential productivity in quail, however, is tenuous ( Johnsgard 1973, Kirkpatrick 1988). Although researchers have suggested that older quail exhibit both earlier nesting (Lehmann 1953, Parmalee 1955, Rosene 1969) and increased egg production (Robinson 1963, Francis 1970, Brooks 2005), evidence for age-specific reproduction in quail is weak because low sample size limits existing data. Kirkpatrick (1988) noted that almost no data existed regarding age-specific reproduction in quail. Lacking evidence to the contrary, Johnsgard (1973) proposed that quail did not exhibit age-specific reproduction. Age-specific reproduction is an unresolved question in 1 E-mail: fidel.hernandez@tamuk.edu 2 Present address: Elephant Mountain Wildlife Management Area, Texas Parks and Wildlife Department, Alpine, TX 79830, USA Herna ´ndez et al.  Age-Specific Reproduction in Bobwhites 895