Drone-based surveys improve estimates of tree hollow abundance and accessibility

1. Tree hollows are critical habitat for many species globally, and fauna studies often include assessments of hollow abundance. However, traditional ground-based surveys for hollows can be inaccurate, either over- or under-estimating hollow abundance and/or accessibility. In order to address this inaccuracy, ground-based hollow counts have previously been calibrated using a ‘double-sampling’ method such as felling or climbing trees. Here we test whether drone-based surveys can be used to count and assess tree hollow accessibility and discuss the considerations and limitations of using drones for hollow surveys. 

2. In this study, we describe a survey of tree hollows in 134 Eucalyptus and Corymbia trees in a tropical savanna south of Darwin, Australia. Tree hollows were first counted from the ground using binoculars, then double-sampled using drone-based surveys. 

3. Drone-based surveys detected more hollows than ground-based surveys, with the latter underestimating potential habitat hollows by at least 15%. Hollows with estimated entrance diameters of 5–10 cm and 10–20 cm were most likely to be missed by ground-based surveys. Drone-based surveys also provided more information on hollow accessibility, identifying that 38% of hollows were inaccessible to fauna due to being ‘blind’ or blocked by termite material.

4. Practical implication. Drone-based surveys potentially offer a more accurate method by which to count and assess tree hollow accessibility for fauna, as well as a less biased means of calibrating ground-based hollow counts. Important considerations for drone-based hollow surveys include weather restrictions (e.g. wind and rain), time available, vegetation density and potential impacts on wildlife. Where complete hollow surveys by drone are not possible or there is insufficient time available, we recommend that a subset of ground-surveyed trees are double-sampled using drone-based hollow surveys—particularly for studies where small- to medium-sized hollows are important. Ground-based hollow surveys alone risk underestimating the abundance of an important habitat resource and overestimating the number of currently accessible hollows. Thus, using a more accurate method such as drone-based surveys to count or calibrate hollow numbers will likely provide improved estimates of landscape-scale hollow abundance and accessibility.