Wallu feeding Wurru

Today I looked at our eagle data for the last 3 days, and noticed a slight pattern in Wallu’s daily routine. Around 10 or 11am, he is recorded as being AT one of his nest sites – Nest 57 – which, as we discovered early on, was in mid-June being freshly lined in preparation for use. Wallu’s fixes show us that he begins the day near the rabbit warren (probably hunting), then delivers a kill to Wurru, who I think is incubating eggs on Nest 57, and takes over sitting duties for a short period while she feeds away from the nest. When she is full of ‘roo or rabbit and ready to sit again, Wurru returns to incubate and allows Wallu to fly. Judging by the large distance between fixes, he might well be spending the rest of his day performing territorial duties at the far boundaries of his patch.

It really looks like our Wallu has his work cut out this year!

More updates soon.

Gidjee nest update

Gidjee is still incubating!

Imagine sitting in a tree all day with this view. This is what our female eagle is doing at the moment during the incubation period.

Her movements today show, well… no real movement! This is a good sign that she has settled on her clutch of eggs and is well into the incubation phase of this year’s breeding season. Mulga is obviously doing a good job at providing her with food, allowing this dedicated incubator to focus on the task at hand.

Speed Dives

How fast does an eagle fly? It’s probably quite a common question, but not something that is easy to answer as wedgies don’t fly past multi-novas very often! Luckily, one of the many variables our sophisticated PTT devices have the ability to record is speed, measured in knots. I’ve been keeping track of the speeds recorded at each GPS fix and not noticed anything that exciting. Both Wallu and Gidjee are usually stationary (i.e. perched, or, in Gidjee’s case, incubating eggs), or when they are in flight, they cruise at relatively low speeds of 5 – 10 knots, roughly 10 – 20 km per hour.

But today something awesome came through.

Check out this fix:

It was taken on Wallu at 11 am 2 days ago as he dived from the sky. If we convert knots to km/h, it clocks him at nearly 70km/h!! For a large bird of prey this is quite impressive. I hadn’t really paid much attention to speeds prior to today, but this fix made me light up and scroll back through the last couple of weeks’ data, and notice a faster dive occurred on 5^th July at the same time of day, when Wallu travelled at 80km/h! How cool is THAT!?

Why so fast?

Currently we are in the breeding season, and it’s known that male Wedge-tailed Eagles defend their territory with aggressive aerial displays. They dive from the heavens like a bullet, showing the two white dots which are clearly visible at the point where each wing joins the body, on the dorsal surface. To another eagle these dots would appear as bold markings, and in a display dive indicate to their neighbours that means ‘I’m a big scary object at high speed, so don’t come near my patch – it’s taken’!

There is also the possibility that such a fast dive is in response to a sudden appearance of a prey animal. Or, when deciding that he needs to be closer to ground level, he might as well do it in the quickest fashion possible! If you can – why not? I know that if I was capable of stooping from the sky in a controlled fashion, I certainly would!  

Gidjee in a nest – but nest in a gidgee!

By visiting some of the favoured roost trees of our two eagles, we have been able to see on-ground what these places look like, giving us a good idea of the birds’ reasons for being there. We discovered a couple of weeks ago that Gidjee was frequently roosting quite close to a small patch of gum trees inside the fenced enclosure, and the reason for this was she and her mate Mulga had built a new nest here. I could tell by its good condition that this nest had been newly constructed and was most likely the site at which Gidjee and Mulga would breed this year.

However, today I know my predictions were wrong.

The above tracking data shows Gidjee’s behaviour over the last few days has contracted to a small spot on a ridge overlooking the fenced enclosure. On the 2^nd, 3^rd and 4^th July she spent most of each day at this location, only moving away a few times. This behaviour aligns with her making the final preparations at a nest by adding fresh leaves to the nest cavity, and spending more time there prior to laying. Considering we are expecting her to be laying eggs soon, we would assume she would be closing in on Nest 63, if this was her chosen nest in the 2013 breeding season. Apparently it isn’t!

I can now see that Gidjee and Mulga must have another nest, one we have not yet located, on the ridge, and THIS is where she is breeding! Being situated in this prominent location means the nest is almost certainly in a gidgee tree, hence the title of this blog post. Since the three days shown above, almost all the daily fixes have occurred at this same point, with Gidjee only leaving the nest briefly during late morning or mid afternoon, probably to receive a meal from Mulga (male wedge-tails usually feed their females on the nest during incubation). Each time we get a fix on her being away from the nest for some time, this almost certainly means her mate is incubating the eggs while she takes a break. Such is the cooperation present between well-bonded adult Wedge-tailed Eagles.

Altitude Findings

I am now back in Perth, 1200km away from the Lorna Glen eagle study site. But every 3 days, a nice little package of GPS coordinates streams into my computer telling me what our two adult wedgies, Wallu and Gidjee, have been up to. After just over a fortnight of tracking, we are starting to see some really amazing stuff! Here’s a little video to share with you the wonders of GPS PTT technology:

Wedge-tailed Eagle Satellite Tracking from Simon Cherriman on Vimeo.

Invaders?

This morning I collected a motion-sensing camera which I had set up on the kangaroo carcass we used to catch Wallu just over a week ago. The 'roo had now been dragged out of the trap and the trap closed, to ensure we didn't catch any other eagles. Setting the camera up had indeed paid off - it gave us the first close glimpse at Wallu with his transmitter we've had since we released him!

This video shows Wallu's mate Wurru feeding at the carcass, then looking up and calling softly as he dropped down from the perch to join her, offering the camera a good glimpse of his PTT (shown circled in the picture above). You can see he's still very wary - the last time he fed on this carcass he was caught by some researchers!

Then something really interesting happens. Wurru is disturbed by something, looking skyward and running out of shot before taking off. Wallu soon follows, and only minutes later, a THIRD eagle lands to investigate the carcass! This one is a sub-adult male. You can see this from his much paler nape (neck feathers) and more golden wing-bar that he is a younger bird, and his sex is revealed by his relative size (smaller than the female), and shorter head and bill. There is also a Willie Wagtail in shot - but somehow I don't think he is the reason the eagles flew away :-D




Eagles Caught on Camera from Simon Cherriman on Vimeo.

Is this bird an invader? Why does his presence disturb Wallu and his mate so much? Maybe they left instantly to defend their territory. And maybe Wallu's relationship is being tested by the presence of a younger, more hansom male! We will find out further into the breeding season if Wallu remains in his territory and fathers offspring. More updates soon!

Tracking them Down

The excitement out bush continued today when we spotted an eagle perched in a dead tree not far from Trap 2, the location at which Wallu was trapped last week. A closer look at the bird revealed it was a large female – Wallu’s mate Wurru! I knew he couldn’t be far away, and minutes later I saw him perched in a live gidgee tree on an adjacent hill. He flew as soon as I left the vehicle, as did the female, but we noticed both birds settled in another dead perch tree a few hundred metres off the road. Although the odds were it was Wallu, I needed to sight his PTT (transmitter) to confirm. Neil and I stalked them, keeping hidden behind a small ridge, and managed to get within about 100m.

I set up my long zoom-lens and framed both birds – Wurru facing sideways, the male facing us front on. Where was the PTT!? I couldn’t see his back! As though hearing my thoughts, he suddenly turned around and revealed his back… and I saw it! The PTT sat perfectly, aerial probing skywards. He was alive and well!

Wallu's PTT is visible as he turns to fly from the perch tree.

A sighting of Gidjee earlier that morning revealed she was also doing fine, flying low over the pen not far from her release site. We now had a couple of records of our eagles since their release, making us confident they’ve resumed their normal lives. Tracking information from the PTTs also confirmed this, and for the first time we have been able to view the birds’ movements and ‘see’ what they got up to after we released them. Here’s a map of Wallu’s story…

 

You can see from this amazing imagery (thanks to Google Earth!), Wallu roosted on a hill about 1km from the trap site (green circle). First thing the next morning, he flew straight to an area bordering the lake system and spent a few hours there (blue circle). (Later inspection revealed this site was a rabbit hotspot, littered with piles of dung, diggings and a few large warrens. It wasn’t surprising – the day after trapping, Wallu was hungry, and flew nearly 4 kilometres to exactly where he knew he could get a meal). He then returned to the ridge to the north of the trap site and spent some time at one of his nest sites, roosting that night close by. The next day was spent close to this nest site, possibly because he was helping Wurru gather lining for it in preparation for the upcoming breeding season. Yesterday at mid-morning, Wallu again made a beeline to Rabbit Ridge, probably for another feed, before flying to perch near the road where we sighted him

'Rabbit Ridge', showing one of the perch trees Wallu has been using on the right.

The map from Gidjee’s territory also reveals her movements, shown in close proximity to the fenced enclosure (blue line) containing many reintroduced native mammals… 

We know Gidjee roosted not far from the trap the night of her capture (shown by the red circle just above 'Trap 1' in the above map). The next day she spent the morning on the ridge overlooking the enclosure, before perching in a tall patch of eucalypts on the plains below (yellow circle). Roosting nearby that night, she began her Monday morning with a flight above the nearby ridge, then spent the afternoon perching in patches of tall eucalypts, both in the enclosure (near an active Boodie warren), and then another patch further south of the enclosure. This behaviour may well be associated with investigating possible prey areas: these eucalypt areas are productive, usually supporting good numbers of rabbits, kangaroos and birds.

Later on we visited the area of tall eucalypts inside the pen and discovered Gidjee and her mate Mulga had built a new nest here! This is probably in response to an increase in prey density inside the fenced enclosure, as the native marsupials thrive in the absence of feral cats and wild dogs. More light will be shed on Gidjee and Mulga's breeding in this new nest as the year progresses.