Monday, 6 October 2014

Wallu After 16 Months

As the sun began its decent late this afternoon, I drove along the edge of a dry salt lake past 'Rabbit Ridge', and was extremely excited to see Wallu and Wurru launch from among the samphire! The pair had been on the ground only 50 m from the road and caught me completely by surprise. I suspected they'd been hunting rabbits from within the dense cover at ground level. Although unusual, I've seen this behaviour a couple of times before. Eagles are quite capable of running along the ground to catch prey if they manage to surprise it, and what better way to get a rabbit than meet it face to face?

It was brilliant to see Wallu in good health and capture a fleeting shot of him bearing his PTT, which can be seen the above photograph. Wallu only flew a hundred metres before alighting on the ground once again. Was he determined to get a rabbit?

Wallu alights in the samphire about 100m from where we flushed him.

He might've been hungry for himself, but we know he wasn't after prey to feed a chick. Unfortunately we discovered last week that Wallu and Wurru had another unsuccessful breeding season this year. When looking at the tracking data in June and July (nesting season), I started noticing a pattern in Wallu's daily movements. He would often be at Rabbit Ridge at dawn, then at mid morning we would see one or two fixes at his nest site.

Here are some maps (click to enlarge):

A day in the life - Wallu has travelled just over 25 km and roosted at two favoured hunting sites.

This map shows Wallu beginning his day at the eastern edge of a salt lake ('Rabbit Ridge' - cluster of red dots, centre right), and flying to his nest at 11am. He then soars to about 1000m above sea level and remains at this altitude for the middle of the day, drifting from the far western end of his territory (green circle) to the far east (yellow circle), and back to the middle. By sunset he has gone to roost at another favoured hunting spot, near an old farm well just north-east of Rabbit Ridge. The total distance covered was about 25km - not a bad day's work!

Another day, Wallu roosted much closer to 'home' on two consecutive nights - the green circle around the red dots reveals the spot, just 1.3km from his nest (see map below). At 6am he was at roost, but by 7am he was on the nest, at 8am he sat in a tree high on the ridge 800m to the north. At 9 and 10am Wallu was back on the nest, then as the clock struck 11am, he was airborne, cruising at 30km/h above Rabbit Ridge (as shown by the blue arrow). He spent from noon until 2pm sitting in his perch tree only 50m from the nest, headed off 'rabbiting' again from 3-5pm, and by 6pm, Wallu roosted back at the same spot again (green circle).

This data is consistent with Wallu delivering prey to his incubating female, 'guarding' her from a nearby perch, and, on occasions where he spent more time at the nest, taking on an incubation shift while Wurru has a break, perhaps to feed or stretch her wings.

In August I was thrilled to visit their nest (Nest 57) with Neil Hamilton and make an exciting discovery, confirming that Wurru had laid eggs. As we neared the nest tree  we crept through the bush, making every effort to keep a dense thicket of mulga trees between us and the nest. A stiff breeze ruffled the foliage, and through a few fine gaps I could make out the shape of Wurru sitting on her nest. She was incubating!! We retreated quickly as to not disturb her, our movements being concealed by the quivers of surrounding vegetation constantly in motion from the wind. Such conditions are good for remaining 'camouflaged' to an eagle's eye, which on a still day detects even the slightest movement from a long distance.

I arrived back at Lorna Glen last week (late September), excited to return with the strong hope that our inaugural satellite-tagged eagle would become a father for the first time. My Mum was accompanying me as a volunteer to help with my fieldwork. However, we were both disappointed to find Nest 57 empty (right). It contained fresh Eucalypt leaves and a slightly flattened nest cavity, but no eggshell, which almost certainly means the eggs had hatched but the chicks had not survived past their first week or so. Why?

Food is the ongoing factor which influences raptor breeding success, and although Wallu hunts regularly at several active rabbit warrens, and has access to other prey (kangaroos, monitor lizards) in his home range, he still has to supply plenty to a growing family, and this isn't easy in the arid zone. There are several other factors which also influence breeding success, and you can read about these here.

October now marks the 16th month we have been tracking Wallu by satellite, and even after this time we are still learning new things. It is only through satellite tracking that we are able to have such detailed insights into the lives of these magnificent eagles.

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