Corticosterone concentrations of Rolling Plains Quail and potential relationships with weather variables
The Northern bobwhite (Colinus virginianus, bobwhite) has been in steep decline over its entire range for several decades, including the Rolling Plains. Researchers, hunters, and land owners have all been trying to find the cause of this decline. It is within this context that Operation Idiopathic Decline began in 2011, after a disastrous decline in 2010, a year which was predicted to exhibit a sharp population increase. Within the framework of OID, I set out to investigate whether bobwhites in the Rolling Plains have been undergoing stress in recent years, as demonstrated by corticosterone concentrations. I specifically wanted to accomplish three objectives. First, I wanted to measure corticosterone concentrations in bobwhites, using their feathers as the sampling matrix. Second, once the corticosterone data were obtained, I wanted to discern any differences in concentrations between years, sex, ecoregions, etc. Finally, I wanted to investigate potential correlations between these corticosterone concentrations and weather variables such as temperature, precipitation, or drought. I successfully extracted corticosterone from bobwhite wing feathers. Concentrations ranged from 0.12 pg/mm to 13.98 pg/mm with an average of 1.42 ± 0.6 pg/mm. These numbers are comparable to those found in the literature for other bird species. I determined that the corticosterone concentrations varied between years, sex, and ecoregions, though not necessarily significantly. In particular my results suggest that birds that underwent wing molt in 2010 and 2011 were subjected to more stress than those that underwent wing molt in 2012 and 2013, possibly due to weather conditions. My comparison of concentrations between male and females between years was inconclusive, with females having greater concentrations in some years but not in others. I believe that macronutrient availability may play a role in these results. Regarding ecoregions, I determined that yearly stress levels were consistently greater in the Central Great Plains (CGP) compared to the Southwestern Tablelands (SWT) except in 2011, for which the presence of several outliers may have affected the comparison. My results suggest that bobwhites living in CGP, where native grassland has since been overtaken by invasive species, are more susceptible to drought than bobwhites living in SWT, where native grassland and rangeland is prevalent. I developed generalized mixed linear models of corticosterone response for each of the two ecoregions for each year vs principal components of the weather data on one hand and other parameters such as the Vegetation Drought Index (VegDRI), sex, weight, or intensity of eyeworm (Oxyspirura petrowi) infestation on the other hand. Results from the models indicate that the first principal component, which was mostly reflective of drought, was the most consistent parameter found in the different best-fit models. VegDRI, a more holistic indicator of drought which I hoped would show strong correlation to the corticosterone response, was only part of 3 of the 6 best models. However, in 2 of those 3 instances, it was the parameter that had the most influence on the corticosterone response. This is an encouraging sign for future studies incorporating this indicator. I believe that the reason I did not get better results for VegDRI is due to the seasonal discrepancy between the weather data used in the models (spanning Aug-Oct of each year) and VegDRI (spanning June to Aug of each year). To my knowledge, no previous study has examined bobwhite corticosterone concentrations using feathers as the sample matrix, nor am I aware of any study of bobwhite corticosterone concentrations in the Rolling Plains. I hope that the conclusions reached from this innovative study will lend to a better understanding of bobwhite quail decline and inform management practices.