Powerful Owl CSI: nest box breeding success and mystery bones

Excitedly this season we have seen a pair of Powerful Owls raise two chicks in a targeted nest box in a reserve in northern Sydney.

This is just the second known record of a Powerful Owl pair nesting in a constructed timber box, and the first known record in NSW. Earlier this season we also saw Powerful Owls nesting in a supplementary hollow in central Victoria.

It has been a long process. The box was designed, built and installed back in August 2019 by Narawan Williams from Fauna Field Ecology, in collaboration with Lane Cove Council. A pair of Powerful Owls were known to use the reserve, and courting behaviours had been observed, but there appeared to be no tree hollows in the area large enough to accommodate breeding for these owls. Despite remaining in the area through the next few years, we don’t think they bred at all or used the box until this season.

In June and July this year, suspicions were raised – our Powerful Owl Project staff observed the male roosting approximately 60-65m from the nest box, with line-of-sight to the box. The female was not seen or heard on either visit. The terrible weather didn’t help but in September when the team returned to the reserve to conduct another dusk watch, a newly fledged Powerful Owlet was observed roosting with an adult, approximately 25m from the nest box. Of course, this doesn’t mean they used the box, but with extensive whitewash directly under a branch next to the box, the team were very curious. Subsequently we confirmed not one, but two fledged chicks and it was time to take a look inside the box!

Narawan climbed the tree to the box in early October – and yes, something had definitely used the box! Even without visual evidence – the smell confirmed it. He was also greeted by a thick layer of matted and compacted fur and bones and solidified chick faeces. The walls were splattered with blood stains and there was even what looked like whitewash high on the back wall. There was also fur, oil and blood stains on the perch in front of the entry to the box. A carnivorous species had definitely been in there, and given the evidence, we are confident that it was these Powerful Owls.

Narawan collected a sample of the box contents so we could take a closer look at just what was inside the box. Professor Paul A. Haynes and Dr Mary Hartley at Macquarie University helped us out and identified a range of typical Powerful Owl prey (bandicoot, possum, small bird, rodent) – and then the story took another twist…two bones collected from the box, a femur and tibia, appeared very similar in shape and size to Spotted-tailed Quoll bones (very rare in Sydney). Dr Hartley visited The Australian Museum and compared the tibia to a Spotted-tailed Quoll skeleton. Although it was very similar in shape and size to the bone in the reference skeleton, she found small differences. She also compared the bone to bandicoot, possum, cat, fox, wallaby, rabbit, and hare, and it was not from any of those animals. After further investigations, the tibia appears to belong to a bird, and the morphology narrows it down to an Australian Brush-Turkey. However, further investigations are underway to secure this identification. These are a known, but generally infrequent prey item of Powerful Owl, and they are abundant in the Lane Cove area.

In yet another interesting twist, the pair of adults were spotted back at the nest box in the early hours of the morning in early December (see the photo from Lane Cove Council on the right).

All of these developments are really exciting and interesting, but there are a couple of points to stress. Firstly, nest boxes are not the single answer to conserving Powerful Owls, or indeed other hollow-nesting species. Hollow-bearing trees are critical to our environment and our priority should be to conserve them. A nest box can never replace their importance.

This is also a one-off and there is more work to do. Nest box use by Powerful Owls is incredibly rare. This box needs to be trialled further and in other locations to understand more about the conditions that were needed to have it successfully used. It is not just the hollow – a Powerful Owl territory needs to have abundant food, canopy cover and good roost sites for the birds. We want to stress that this is not a solution for the approval of felling/removal of Powerful Owl nest trees and should never be. If you share with others the story of the owls breeding in this nest box, please share this message also.

Finally, if you come across a Powerful Owl nest tree or in fact this nest box, please keep a respectful distance and don’t share their location. Powerful Owls are very prone to nest disturbance, and we want to ensure that they are able to raise young without interference.

If you have a site where you think this box design would be worth trialling or if you know of any other records of Powerful Owls using a nest box, please contact Narawan Williams, Ecologist, Fauna Field Ecology, at faunafieldecology@gmail.com or Jenny Zvolanek, Powerful Owl Project Officer, BirdLife Australia, at powerfulowl@birdlife.org.au.