Eagle Breeding - Success and Failure

Wedge-tailed Eagle nests in the Perth Hills are normally built high up in one of the largest trees available, which gives the birds a good view over their territory. Large trees usually have at least one sturdy fork in which the birds can construct their massive stick platform, but such trees are not necessarily entirely stable. While checking on one pair of eagles recently, I parked at a familiar monitoring location - a roadside parking bay - and peered up to the nest with binoculars. Despite being familiar with the site, I was unable to see the eyrie right away, and I wondered if the canopy of trees in the foreground had grown up and obscured my view. After several minutes scanning, I still had no luck, so I decided to walk up to the ridge and have a closer look. Judging by observations of an incubating adult from earlier in the season, I anticipated the nest should contain a month-old chick.

While walking up the steep, rocky ridge towards the nest tree, I pondered the whereabouts of the nest, casting my mind back to last breeding season, when it was clearly visible. Why couldn't I see it? I then remembered the entire nest tree was dead, and suddenly realised what must have happened. I had a vision of what lay in front of me, and sure enough, as I neared the site, I noticed my instinct was right.

The entire nest tree, a very large, straight but entirely dead Marri, lay on the forest floor. Its rotted root-system had prized up a large chunk of earth, and as I inspected the ground, I realised the relatively wet weather this year must have made the ground soft, and strong winds (not to mention a very heavy eagle nest!) caused it to topple over. As you can see in the above image, the nest-tree still has bark, indicating it has probably only died in the last 2 years. I've noticed that mature eucalypts like this seem to be 'dropping off the perch' at an increasingly frequent rate, a result of increased stress during the hotter, drier summers associated with a changing climate.

The grim news was that I confirmed the nest had indeed contained an eaglet aged about 5 weeks, evident from its remains found amongst the mass of collapsed branches and nest lining.

Examining the remains of a 5 week-old eaglet which died when the nest came crashing down.

The eaglet's leg can just be made out in the remains of the giant, collapsed eyrie.

This was an unfortunate discovery as the eaglet was born in a productive territory with ample food, so would have had every chance of fledging. Nevertheless, such events emphasise the delicate balance between life and death, and prove that not every bird which breeds fledges chicks in the 'happily ever after' scenario that humans often perceive. They are an interesting and important part of ongoing research.

Unlike the above site, however, most other eagle nests in the hills have been successful this year, and it has been exciting to commence ringing/banding the first chicks of the season. This is now the second year of the colour-marking study which is part of a long-term research project investing the movements and survival of juvenile Wedge-tailed Eagles, which commenced last year. I have worked towards setting up such a study ever since I found the first Perth Hills wedgie nest over 15 years ago, and put many hours into obtaining the appropriate licenses from State and Federal Government departments, so it is a fantastic feeling to know the study is off the ground at last!!

I climb to each nest using rope-access techniques, and the chick is carefully secured by the talons and placed into a handling bag, then lowered to the ground. This allows us to perform all the necessary tasks beneath the tree and on a flat place which doesn't sway around in the breeze! Once inside the bag, the eaglets normally sit on their haunches until they are taken out and held firmly by an assistant.

Mick holds the eaglet while the footspan is measured.

It was brilliant to have my good friends Mick and Rianna, and their boys Jarrahn and Bhodi, helping with the first eaglets of the year. Mick has been reporting eagle sightings and assisting with monitoring of nests for many years now, so it was a pleasure to head bush with him and his family, and this also created a great opportunity for an educational activity for the next generation of ornithologists!

The first nest we visited had a single chick aged about 8 weeks. It was in very healthy condition and had been recently feeding on a freshly killed Australian Raven (Corvus coronoides), whose carcass lay on the nest cavity.

At the second site east of the Mundaring Shire, we were delighted to discover a brood of 'twins', aged between five and six weeks. These birds were in a nest built high in a dead Marri tree and had an amazing view over the surrounding Wandoo woodland. Wedgies normally lay two eggs but only rear one chick, as the second either doesn't hatch or dies in the first few weeks of nest life. After the nesting failure described above, it was wonderful to have a 'replacement' turn up so quickly!

7 year-old Jarrahn was delighted to see this female eaglet up close.

The first part of processing eaglets involves placing two metal rings/bands around the their legs: a stainless-steel ring from the Australian Bird and Bat Banding Scheme, and a specially designed aluminium colour-ring fitted with pop-rivets, coloured yellow which is unique to my Perth region study area. I have learned the method of applying such bands from Golden Eagle researchers in Scotland, where I spent time volunteering and gaining experience earlier this year, and it has been fantastic to carry on with the same type of work back home.

After ringing/banding, several measurements are taken to record the birds' morphometrics and help us determine their sex. Female eagles have a larger head and a much broader footspan than males, and this difference is apparent after the age of about one month.

Measuring the head-bill length. The long cere indicates this bird is a female.

Measuring the rear talon (halux). Juvenile talons are pin-sharp as they haven't yet been worn.

The ringing and measuring usually takes about 15 minutes, and we carefully monitor the birds to ensure they remain calm throughout. A key difference I've noticed between wedgie and goldie chicks is their personality, with the latter being much calmer overall, sitting quietly and allowing their measurements to be taken without moving much. Wedgies on the other hand sit up in a threat display to warn of any would-be predator (or scientist!) and often strike with their feet at anything that moves beneath them. For this reason, it is safer for the birds and us handlers if they are held firmly but gently during the process.

When we have recorded all the necessary information, the birds are again placed in the handling bag and carefully hoisted back to be returned to the nest cavity. The usual reaction is for chicks to go back on the nest and immediately adopt a defence posture, spreading their wings to make themselves look bigger. This is a good sign that normal behaviour has resumed, and after taking a few photos, I descend the ropes and leave them in peace.

Eagles really are remarkable creatures, and such important parts of our natural heritage. They captivate people with their majestic, soaring flight and inspire us with their almighty powers of vision and capabilities of predation. But they are often simply just there, and we don't necessarily know exactly what they (in particular, young, non-breeding birds) need to ensure their survival. It is for this reason I am driven to continuing research to find out as much as possible about their ecology, and fill knowledge gaps regarding juvenile dispersal. Where will these yellow-ringed birds end up? Keep your eyes peeled, and you might one day be able to tell me!

If you do see a Wedge-tailed Eagle bearing metal leg rings/bands, please photograph it, record the time, date, GPS location, and if possible, the 3-digit colour ring number, and contact me to report this information.