Environmental drivers of Northern bobwhite fall and winter survival and resource selection in Southwest Missouri

Abstract

Northern bobwhite (Colinus virginianus) populations have declined over the last 50 years throughout the majority of their historical range, primarily due to habitat loss and degradation caused by modern farming practices, increased urbanization, succession of native grasslands to forests, decreases in native pasture acreage and increases in grazing intensity. Recent questions regarding conservation grazing and prescribed burning management regimes and extensive grassland management suggest further information on vegetation and management effects on bobwhite demographics is necessary for robust conservation planning. Information regarding the survival and resource selection through fall and winter is necessary to create a holistic management plan which benefits bobwhite at all stages of their annual cycle. Therefore, our objective was to determine the relationship of bobwhite survival and resource selection to vegetation types and management practices to fill in knowledge gaps of their annual cycle and demography. We used known-fate, logistic exposure survival models to estimate survival of birds fitted with tracking devices. We used 2068 bobwhite locations from 1 Nov to 31 Jan, 2017-2018 and 2018-2019 to fit 7 models that represented competing hypotheses concerning the amount of native grass, intensive and extensive management, and woody cover. Variation in survival was not explained by the amount of native grass managed by prescribed burning and grazing on the landscape. However, survival was negatively related to prescribed burning. Survival was negatively related to the distance to shrubs but not related to shrub edge density or proportion of shrub cover within 50 m. Survival was also positively related to the distance from the nearest tree. We used conditional multinomial logit discrete models to evaluate the relationship of vegetation and management types to bobwhite resource selection by comparing covariates among the used point and a set of random points that defined a choice set for each bobwhite location. We used 671 choice sets from 1 Nov 2018 to 31 Jan 2019 to fit 6 candidate models in a model selection framework which represented combinations of our hypothesized drivers of bobwhite fall and winter resource selection. We found no support for an effect of native warm season grasses or the effect of prescribed burning and grazing on resource selection. Bobwhite selected locations with greater woody stem counts and greater visual obstruction, both of which are indicative of shrub cover, and there was a weak positive relationship with woody edge density However, bobwhite also selected locations that were closer to trees. We found weak support for negative effects of the percentages of forb and bare ground at a given location on resource selection, while there was a negative effect of grass on winter resource selection. Woody cover was an important component of vegetation contributing to fall and winter survival and resource selection of bobwhite in Missouri. More specifically, proximity to shrubs and distance from trees were positively related to survival. Greater quantities of woody stems, indicative of shrub thickets also increased the relative probability of selection. Interestingly, closer proximity to trees also increased the relative probability of selection, potentially indicating an ecological trap. While necessary, the utility of native grasslands for bobwhite is greatly diminished if they do not contain shrub cover. Prescribed burning is perhaps the most effective tool to manage native grasslands for bobwhite habitat as it simultaneously promotes native grass quality, removes litter, and prevents shrub cover from dominating the landscape and succeeding to trees. While prescribed fire is important for controlling woody vegetation and for the creation of breeding, nesting, and brood rearing habitat, its short-term effect was to reduce survival of bobwhite in the fall and winter. We suggest that application of prescribed fire on short intervals at large scales will likely reduce the quality of habitat available to bobwhite during the fall and winter seasons. However, the use of fire in a multiyear rotation in combination with grazing in native and restored grasslands can create the needed interspersion of shrub cover for fall and winter while meeting the other annual resource needs of bobwhite. Felling mature trees while still maintaining adequate interspersion of shrubs may also prevent an ecological trap because survival was lower nearer trees. Lastly, while our study focused on fall and winter habitat, we suggest conservation planning and management needs to consider quail needs throughout their full annual cycle.