Investigation into increased deaths in predatory birds

Dr Mark Hawes
Veterinary pathologist, Agriculture Victoria Research and Wildlife Health Australia state co-ordinator, Victoria

Between May and September 2018, there was an apparent steep increase in the numbers of birds of prey being found sick or dead in Victoria.

Wildlife carers, community members and Wildlife Health Australia surveillance partners responded to total of 537 reports of sick or dead birds of prey, compared to 175 birds of prey reports in the same period in 2017.

The predominant species were barn owls (Tyto alba) (n=289), followed by Nankeen kestrels (Falco cenchroides) (n=47) and boobook owls (Ninox boobook) (n=40). Other species included kites (whistling and black-shouldered) and tawny frogmouth owls.

Some of the dead birds were submitted to either the University of Melbourne or AgriBio Veterinary Diagnostic Services for testing. The majority of cases that were submitted for examination had been found in the Bellarine, Greater Geelong and Surf Coast locations.

A total of 48 birds were necropsied. Over half of the 27 barn owls and eight of the 10 Nankeen kestrels were found to be in an emaciated condition. Most of the southern boobooks, tawny frogmouths and the single whistling kite, were in reasonable body condition. All the barn owls were sub-adults, and most were female.

There were no gross or histopathological lesions to suggest an infectious cause of mortality in any of the species examined and specific testing (qPCR) for avian influenza and avian paramyxovirus-1 was negative.

Of the barn owls, 25 per cent were diagnosed as having died from trauma (presumptively vehicular) and 7 per cent from anticoagulant rodenticide poisoning. Trauma was a relatively common diagnosis across the species examined.

Overall, it is suspected that starvation, road trauma and anti-rodenticide poisoning are common causes of death in birds of prey in any given year. It is not known why sub-adult barn owls and Nankeen kestrels were found in much greater numbers this year and in unusual locations such as the Bellarine Peninsula. It is possible that the young birds were struggling to find sufficient prey and had moved into southern Victoria.

Suggested theories as to why this event occurred include:

• the boom and bust cycle of mice impacting on the raptors that feed on them
• the drought conditions to the north may have had an effect on population movements.

This event provided an opportunity to document the presence of anticoagulant rodenticide compounds in native raptors in Victoria and showed that in a small but significant percentage of these birds, the anticoagulant rodenticides have caused morbidity and mortality.