COS 43-1 - Population growth and influence of prescribed fire intervals on Henslow’s Sparrow abundance in a North Carolina grassland ecosystem

Abstract: The Henslow’s Sparrow (Centronyx henslowii) is a grassland specialist, migratory songbird that breeds throughout the US Midwest and Northeast, reaching as far south as North Carolina. The species has exhibited steady long-term population declines throughout its range largely due to fire suppression and habitat fragmentation. Currently, one breeding population of Henslow’s persists in eastern North Carolina and may represent a remnant of the eastern subspecies. A major goal of our study is to develop foundational demographic data to better understand the effect of the ongoing prescribed fire regime on population density and estimate the population growth rate between 2011 and 2022. These data are necessary to model conservation scenarios regarding the optimal way to implement prescribed fires to enhance species persistence. Our study site is 1200 hectares, divided into six management units currently burned on a 3-year rotation to promote early successional habitat. Our approach required estimating population size at two scales using distance sampling, namely, at the study site and management unit levels. We used natural log-transformed annual site-level population estimates and time (year, from 2011–2022) to estimate the average observed rate of population increase. Changes in growth rate before and after the implementation of prescribed fires (2011–2015 vs 2016–2022) were determined using piecewise regression. Henslow’s Sparrow response to fire management was modeled using unit-specific fire history and population estimates per unit collected for 7 years. We used a generalized linear mixed model to determine the relationship between sparrow abundance/unit and fire rotation. Model terms were time-since-last-burn and year, with burn unit as a random effect. Preliminary findings indicate that this population of Henslow’s Sparrows is increasing, on average, 15% annually. Growth rates were steeper earlier in the time series (25%, 2011–2015), soon after existing structures (radio antennas) were removed from the site, and declined to a slower rate beginning in 2016 (11%), when prescribed fires were initiated. As of 2022, the population stood at over 446 singing males (SE: 65.89; CV: 0.15). Henlow’s Sparrows exhibited highest population density/unit two years after burn, and lowest at three years post-burn. Our assessment describes a growing population, positively responding to the current 3-year fire rotation. Population growth could be enhanced by increasing the number of units/year at the 2-year post-burn stage available to sparrows. We follow this work by considering how climate change projections may affect fire management schedules, and possibly infringe on Henslow’s Sparrows breeding phenology.