Snag density and stand age, but not stand size, explain occupancy and reproduction of an imperiled cavity nester in early successional forest

Abstract

Many early successional and disturbance-dependent bird species have declined over the past several decades. Cavity-nesting birds in early successional forests are vulnerable because they often require specific habitat characteristics and frequent disturbance events. We examined whether stand age (a proxy for forest succession), stand size, and snag density explained occupancy, nest survival, and productivity of a threatened cavity nester, the Southeastern American Kestrel (Falco sparverius paulus), in natural cavities in early successional forest (≤ 15 years since treatment). We conducted point counts, nest searching, and monitoring during the 2018–2019 breeding seasons in sand pine (Pinus clausa) scrub habitat in Ocala National Forest, Florida. Stands were created by either a clearcut, a clearcut followed by prescribed burn, or wildfire. Occupancy probability tended to peak in stands ≤ 5 years old, but this varied with snag density: βstandage²xsnagdensity =-1.40, 95% CI (-2.57,-0.23). Even slight increases in snag density were related to large increases in occupancy across stand ages. For example, our occupancy model predicted that if there were 5 snags/ha, there was a 70% probability of occurrence (per point) in a four-year-old stand, whereas 1 snag/ha led to expected occupancy probability of 30%. Nest survival was best explained by snag density: βsnagdensity = 0.71, 95% CI (0.12, 1.46). Neither occupancy nor nest survival were explained by stand size. Productivity did not have a relationship with any measured covariate. Our data indicate that snag density is an important measure of habitat quality for cavity-nesting birds and that interpreting stand age without considering the role of snag density may lead to under-or overestimating habitat quality of stands. Occupancy probability and nest survival had a low positive correlation across clearcut stands, but not in prescribed burns or wildfires. These results emphasize caution when extrapolating predictions from clearcuts to wildfire or prescribed burn stands.