Evaluating interactions between space‐use sharing and defence under increasing density conditions for the group‐territorial Red‐cockaded Woodpecker Leuconotopicus borealis

Abstract

Information about how bird species respond to increasing density conditions through either space‐use sharing or increased territoriality, and how those changes affect fitness, is essential for effective conservation planning. We used a case study of endangered Red‐cockaded Woodpeckers Leuconotopicus borealis (RCW) to address these questions. We documented over 36 000 locations from 44 RCW groups in three density conditions on two sites in South Carolina, USA, between April 2013 and March 2015. The frequency of neighbouring group interactions differed among density conditions and was highest for high‐density groups. RCW home‐ranges and core‐areas were larger under low‐density conditions (x¯normalHome‐normalrange = 88.4 ha, x¯normalCore‐normalarea = 21.0 ha) than under medium (x¯normalHome‐normalrange = 68.29 ha, x¯normalCore‐normalarea = 16.6 ha) and high‐density (x¯normalHome‐normalrange = 76.3 ha, x¯normalCore‐normalarea = 18.6 ha) conditions. Neighbouring RCWs maintained overlapping home‐ranges with nearly exclusive core‐areas across density conditions, but overlap tended to increase as neighbouring group density increased. Under high‐density conditions, home‐range overlap correlated inversely with clutch size (β ± se = −0.19 ± 0.09), nestling production (β ± se = −0.37 ± 0.09) and fledgling production (β ± se = −0.34 ± 0.08). Our results indicate that RCWs dedicate more effort to territorial defence under high‐density conditions, potentially at the expense of greater foraging efficiency and time allocated to reproduction, as evidenced by reduced fitness. Large home‐range overlap indicated limited territoriality farther away from cavity trees, but the existence of exclusive core‐areas suggests that RCW groups defend habitat closer to cavity trees. Thiessen partitions used to allocate critical foraging habitat offered comprehensive habitat protection for RCW but appear flawed for spatially explicit habitat assessments because they do not accurately delineate space used by individual RCW groups.

Effects of group size and group density on trade‐offs in resource selection by a group‐territorial central‐place foraging woodpecker

“…standardized approach to delineate critical foraging habitat at the landscape scale without the need for detailed home-range data. Previous studies have highlighted poor performance of Thiessen partitions at high group density because they do not capture increases in home-range overlap (Garabedian et al 2018a). Our study shows that differences in group size at high group density further complicate use of Thiessen foraging partitions by influencing accessibility of distant large pines for small groups.…”

“…Thus, RCW Thiessen habitat partitions are truncated as the density of neighbouring clusters increases (Schlicht et al 2014). Further, our approach reflects the overall spatial neighbourhood that RCWs are likely to encounter when interacting with neighbouring groups and dispersing (Engstrom & Mikusinski 1998) and captures density-dependent changes in territorial behaviours (Garabedian et al 2018a). Following Garabedian et al (2018a), we used ranges of 0.25-0.50, 0.51-1.00 and ≥ 1.00 groups/50 ha to represent low, moderate and high group density conditions, respectively.…”

“…Further, our approach reflects the overall spatial neighbourhood that RCWs are likely to encounter when interacting with neighbouring groups and dispersing (Engstrom & Mikusinski 1998) and captures density-dependent changes in territorial behaviours (Garabedian et al 2018a). Following Garabedian et al (2018a), we used ranges of 0.25-0.50, 0.51-1.00 and ≥ 1.00 groups/50 ha to represent low, moderate and high group density conditions, respectively. The 50-ha denominator was selected to approximate the recommended minimum amount of good-quality foraging habitat allocated to individual RCW groups (USFWS 2003).…”

“…, Garabedian et al . ). At high group density, we predicted that larger RCW groups would select for large pines at greater distances because they are more likely to outcompete smaller groups for access to high‐quality resources in shared foraging habitat (prediction 2; Hooper et al .…”

“…Accordingly, we tested five predictions. At low and moderate group density, we predicted that all RCW groups would select for pines ≥ 35.6 cm diameter at breast height (dbh) (hereafter, large pines) at greater distances because access to foraging habitat is not constrained by neighbouring groups and competition for resources is low (prediction 1; Hooper et al 1982, Garabedian et al 2018a. At high group density, we predicted that larger RCW groups would select for large pines at greater distances because they are more likely to outcompete smaller groups for access to high-quality resources in shared foraging habitat (prediction 2;Hooper et al 1982, DeLotelle et al 1987.…”