Saturday, 10 October 2015

'Twin' Girls!

After a short walk through the Mulga scrub today, I was super excited to spy Wallu perched at the top of a tall Cue York Gum, watching my every move. He stayed for only a few minutes, just long enough for me to snap the above photo, before launching and flapping away with large wing-beats, his PTT aerial waving in the wind. As I had suspected earlier this year, Wallu and Wurru had constructed a new nest, which I spied close to Wallu's perch tree, built 10 m up in another Cue York Gum.

Well hidden among the foliage of a Cue York Gum, Wallu and Wurru's new nest is only the fourth of over 90 wedge-tail nests built in this tree species at the Matuwa/Lorna Glen study site.

My prediction was that Wallu's regular visits to the nest site recorded by our tracking data over the past 2 months indicated that a chick had hatched and continued to be fed on a daily basis. It was most exciting to discover that this also proved to be true, and a split second after spying the nest, I noticed an eaglet standing on it, looking large and healthy. Climbing the nest, however, gave me an even better surprise: not one, but TWO large, healthy eagle chicks!

Two large female eaglets, aged about 8 weeks old.

As part of my research on the movements of juvenile eagles from Matuwa, individuals are being ringed with blue leg-rings (you can read about the first year of conducting such research in 2014 here). The 'twins' from Wallu's nest were lowered to the ground in a handling bag, then removed and held for processing. Even at this age, eaglets have powerful talons which can inflict severe wounds to a human, so care is always taken to hold the birds firmly but gently, ensuring no injuries are sustained to them or the handler.

Removing one of the juvenile eagles from the handling bag - feet first!

                                                                                                                      Taking measurements from the eaglets: the tarsal length (left), and rear talon (right).

After data on their morphometrics such as weight, wing length, tarsal (lower leg) length and size of the rear talon were taken, two leg rings (a blue, numbered colour-ring, and a stainless steel ring from the Australian Bird and Bat Banding Scheme) were fitted to the birds' tarsi (all ringing is conducted under appropriate licenses from State and Federal Government authorities). The blue colour rings are unique to this study site, so we know that any eagles bearing a ring of this colour that are subsequently sighted elsewhere in Australia originated from Matuwa/Lorna Glen. As we've seen from Kuyurnpa's amazing travels, eagles can move thousands of kilometres in their first year, so no matter where you are in Australia, keep your eyes peeled!

Holding an eaglet firmly by the tarsi and tucking up the wings prevents it from causing any injuries to itself or the researcher (left). The blue colour-rings have a unique number which identify the individual birds (click images to enlarge).

It was encouraging to record both juvenile eagles were large females, each weighing over 3 kg! And at eight-weeks of age, they still have some mass to put on, so will probably have fledging weights much greater than their father Wallu (male eagles are smaller than females, weighing about 3 kg at the most, with females occasionally exceeding 5 kg).

When the processing was complete, the 'twins' were returned to the shady canopy and replaced on their nest.
These juvenile eagles are the first record of a brood of two surviving to near-fledging age in four years of research on more than 30 resident breeding wedgie pairs at this study site. Due to an overall low food supply, most pairs rear 0 young per year, and a maximum of 5 pairs have reared 1 young each in previous years. Such findings are typical for long-lived raptors and are consistent with previous research on this Australian eagle species in arid parts of its range, where rainfall is erratic. Perhaps having satellite tracking devices on eagles increases their breeding success!?! ;-) Well done Wallu, on a fantastically successful breeding season in 2015!

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