Wedge-tailed Eagles

Our speaker was Simon Cherriman, a long-time member of the Nats’ Club (DRB Branch) and a former Serventy Memorial Prize-winning student. Wedge-tailed Eagles have been Simon’s passion since he was a child, when he would explore the bush by bike and make detailed notes on his sightings. His skill at climbing trees—very useful for such an interest—also developed from childhood. He is now working towards a PhD in which he uses satellite tracking to plot the movements of juvenile Wedge-tailed Eagles (WTEs). He has learnt to abseil and has become skilled at photography and filming from precarious places high in trees. He also has a passion for developing children’s curiosity about nature and people’s feeling for nature, for conservation, for public education on natural history, and for acknowledging the Aboriginal people: the original custodians of the land.

Simon began with the subject of identifying the WTE (Aquila audax in Latin or Waalitj in Noongar), which (obviously) has a wedge-shaped tail. Also, along with the Little Eagle, it “wears trousers” (has fully-feathered legs) and soars high in the air, whereas the White-bellied Sea Eagle “wears shorts” (has partly feathered legs). The Square-tailed Kite, often mistaken in the field for the WTE, has much smaller legs and usually keeps very close to the canopy when in flight. Juvenile WTEs have a golden colour and are highly mobile, whereas adults are darker and usually restrict their movements to a smaller area. When they find a mate, the pair-bond is nearly always long-lasting, and if one dies, the other finds a new mate. The female is bigger than the male, and the sexes have different shaped beaks and different feet. A WTE has vision 2.5 times as acute as ours, powerful talons and a wingspan of 2.5 metres. Captive birds can live up to 50 years.

Simon then took us through the Noongar seasons, describing the behaviour of the Waalitj throughout the year. Djeran (April-May) is when courtship and mating occurs, with spectacular courtship display flights, boundary clashes, and collection of nesting material. Makuru is the cold, wet weather of June and July. Nests are constructed and eggs (usually 2) are laid. The female sits patiently on the eggs through all weathers and insulates the eggs from the cold. Djilba is the beginning of the wildflower season of August and September. That is when most eaglets hatch, and both parents feed the tiny chicks—with utmost care and delicacy—pieces of meat torn from a carcass. (We saw Simon’s video footage of this.) The adults take turns to hunt for food, and even share food between themselves with what seems to be a polite “no-no, after you” attitude. There is competition between the sibling chicks, however, and a parent may sometimes literally sit on the offending one to calm it down. Kambarang or late Spring (Oct-Nov) sees a pair of “fat chooks” in the nest, with the parents having to hunt bigger prey to feed them. Birak (Dec-Jan) is when the eaglets fledge by exercising their wing muscles and then flying to nearby trees. They still beg for food, but then start feeding on a carcass as a family. Bunuru (Feb-March) is when juveniles develop their flying ability and prepare for dispersal away from their natal home range. The young eagles start circling and then undertake the extraordinary journeys that are the subject of Simon’s current research.

Simon’s research involves banding each bird in the study with a conspicuous band and attaching a GPS satellite transmitter (very lightweight and solar-powered) so that he can trace their movements on his laptop. To do this may involve climbing a 20m Jarrah tree, grabbing, bagging and lowering the bird, then attaching the device and taking the bird up again to be released.

They do not appear to be bothered by the transmitter harnessed to their back, and it eventually falls off. Simon had a setback in 2013 when his DBCA licence was revoked because of a perception that two out of the four birds in the study died because they were wearing satellite transmitters. There was no evidence for this and plausible causes of natural mortality were determined. The other two satellite-tagged birds wearing the same harnesses are still alive today. During the same period, two other eagle nestlings died as a result of a prescribed (supposedly cool) burn by DBCA.

The results of his tracking work are amazing. After several months of flying around the home territory for up to 3 km, the juveniles suddenly fly northwards for hundreds of kilometres, at 100 to 400km a day, typically to the Pilbara or the Kimberley region.

As well as the contribution to our knowledge of WTE biology, there is a conservation benefit of the transmitters being fitted. If a bird is shot, it may be possible to trace the shooter. WTEs continue to be blamed for killing lambs and are still being shot, despite being protected by law. Simon suggested that Club members can contribute to the research if they see a banded WTE, by taking a photo of it and noting the location and the band colour and number if possible. He also suggested joining the Perth Raptor Study Group, or Australian Geographic or Birdlife Australia. Simon’s photos and video of the birds in the nest were the icing on the cake of this fascinating talk.

Mike Gregson