Efficacy of intervention to relieve nest box competition for Orange‐bellied Parrot
 Neophema chrysogaster

We use an experimental approach to evaluate the effectiveness of removing nests of a dominant competitor to create vacant nest boxes for a critically endangered parrot. We compared the number of times that Tree Martin (Petrochelidon nigricans – the dominant competitor at nest boxes) perched at or entered nest boxes intended for Orange-bellied Parrot (Neophema chrysogaster – the subordinate nest competitor) over three time periods (before, immediately after and one week after experimental nest destruction). In the before period, rates of nest attendance by martins in treatment and control nests were not explained by treatment group. After experimental nest destruction, total attendance at boxes by martins rose to a mean of 6.1 visits over three five-minute surveys in the treatment group, compared with 3.3 visits at control boxes. Within individual surveys, martins visited treatment boxes 4.4 times per survey one week after nest destruction, compared with only 1.6 visits in the control group. Martins in the treatment group rapidly rebuilt their nests and laid replacement clutches, and within a week, all boxes were reoccupied. Nest destruction did not increase nesting opportunities for the parrot, and increased vigilance of the dominant competitor may in fact reduce nesting opportunities in nearby boxes. Our study suggests that removing martin nests is an ineffective management action for alleviating nest competition for this parrot. Introduction Tree cavity abundance can limit the populations of cavity-dependent fauna (Newton 1994), and in some forests, cavities suitable for wildlife are rare (Stojanovic, et al. 2012). In such cases, competition among cavity-dependent species may be intense (Pearce, et al. 2011). If subordinate hollow competitors (i.e. those that lose competitive interactions) are of conservation concern, exclusion from cavities by dominant competitors may be a threat. Nest boxes are commonly deployed to overcome cavity limitation (Lindenmayer, et al. 2016), but if these are also occupied by dominant competitors, management may actually be creating new problems (e.g. if dominant species population sizes are inflated by additional nesting opportunities). Additional interventions to control dominant competitors (Stojanovic, et al. 2018c, 2018) may be necessary to protect threatened subordinate species, but the efficacy of these techniques may vary among species and study systems. We evaluate whether removing nests of a competitor from nest boxes benefits Orange-bellied Parrot (Neophema chrysogaster, hereafter: parrot). It is arguably the rarest parrot in the world because in 2016 the wild population declined to only two females (Stojanovic, et al. 2018), and the species currently depends on nest boxes for breeding (Department of Environment Land, Water & Planning 2016). Anecdotal observations indicate parrots are subordinate to Tree Martin (Petrochelidon nigricans hereafter: martin) at nest boxes (Department of Environment Land, Water & Planning 2016). Martins are cavitynesting aerial insectivores, and although smaller (18 g vs. 45 g), they can aggressively swoop at parrots until they are driven away from boxes (Department of Environment Land, Water & Planning 2016). Martins can also usurp parrot nests by covering-over their eggs with nests of leaves or by using mud to shrink the nest box entrance hole. On an ad hoc basis, conservation managers remove martin nests by cleaning nest boxes to alleviate competition, but whether this increases the availability of vacant boxes is unknown. We test this experimentally and evaluate whether nest attendance by martins (as a proxy for the intensity of box defence) changes after nest removal.