Shandon's Crowned Eagles:  Blog 2

12 December 2019

Dr Garth Batchelor, Peter Retief and Petri Viljoen

Blog 1 summary:

•  Raspberry Pi computer installation February 2019
• 2 eggs noted on nest 2 September
• Incubation 53/54 days
• Chicks hatched on Saturday 12 October.

Two chicks were recorded on the nest initially but a week later only one was present. Eagles have a peculiar behaviour in that in many species where two eggs are laid, the strongest chick will usually kill or injure the second chick to the extent that it dies. This is known as sibling aggression or infanticide.  The Cain and Abel behaviour is known to occur in Crowned Eagles and it was hoped to document this with the remote camera. The images recorded from the camera were unable to detect this aggressive behaviour due to the deepness of the nest cup. However, this practice of sibling aggression and infanticide has been documented for other eagle species including Verreauxs', Wahlberg's and Martial Eagles. The first chick to hatch is usually the chick that survives. It is presumed to be an insurance policy should one of the eggs be infertile or a chick being deformed.

The Crowned Eagle chicks, as with most other eagle chicks, are also covered in white down on hatching.  They remained covered in snow white down until their wing feathers start to erupt 40 days. These feathers are grey or black in sharp contrast to the white. The quills for the secondary and primary feathers on the wings were first noticed on 21 November 2019. Soon after, the dark feathers on the back started appearing.  

It was noted that while the chick was very small, the female Crowned Eagle used to collect a spray of green leaves, often from a round-leafed kiaat, (Pterocarpus rotundifolius), or a mingerhout, (Breonadia salicifolia), and place it on the nest. Later as the chick got larger she would bring in a spray of leaves at any time during the day but less frequently. The purpose of bringing in fresh foliage is not known but it is thought to be related to nest hygiene. The leaves appear to cover over old prey. On other nests, leaves from a number of different species have been recorded. These are often aromatic for example pine needles and Eucaluptus spp. The leaves of white syringa, Kirkia wilmsii and kiepersol , Cussonia spicata have also been recorded.

When the chick was small and defenseless the female Crowned Eagle remained on the nest or on a branch in the nest tree for most of the time.  She spent the night on the nest with the chick. As the chick became stronger and able to move around the nest, the female spent more and more time away. She no longer slept on the nest. On very hot days, even when the chick was over a month old, she would return to the nest and mantle over it to help keep it from overheating. She herself became extremely hot and could be seen to be gular panting, with her beak wide open. Birds, being covered in feathers, which are extremely efficient insulars against heat loss, cool down through gular panting. This is even more important in eagles whose entire legs are feathered unlike other birds. The female eagle also returned to the nest during storms and heavy rain to protect the chick from becoming chilled. She appeared almost saturated after heavy rain events. The chick still being covered mostly by down would have been very vulnerable to chilling should the female not have protected it, feathers being more waterproof than down.

The images from the Raspberry Pi camera showed that the male eagle continued to deliver prey to the nest after the eggs hatched even when the chicks were very small. On arrival to the nest with prey, the female eagle would rush up to him, collect the prey from his talons in her beak and then mantle over it. He would then depart.  Sometimes he would sit on a nearby branch in the nest tree or else fly directly off. Most of the prey was delivered in the morning usually after 10h00 although this was not always the case. On the 16 October 2019 the male delivered prey at 06h51 while on 30 November 2019 he delivered prey at 16h14.

Initially during incubation the female eagle would sometimes leave the nest for short periods. The male eagle delivered a number of items of prey during this time. In all only 8 deliveries were recorded. Most of the prey was small mammals. Amongst these, Greater Bushbabies were recorded and an antelope leg. On a number of occasions she would leave the nest for short intervals and it is assumed that she went off to feed on prey which had been delivered by the male.  An assortment of prey items have been recorded on the nest to date, these include Water Mongoose, Greater -spotted Genet, Grey Duiker, Red Duiker, Impala fawns and Vervet Monkeys.

It is not only the Crowned Eagles who have been seen at the nest. Cape Glossy Starlings have been recorded at the nest a few times. Initially it was thought that they were coming to collect scraps of meat from the nest.  Although this is possible, a starling was seen collecting a piece of eagle down from the nest for its own nesting material. I have also recorded Grey-headed Sparrows collecting down off another Crowned Eagle nest.

Some amazing images have been recorded by the Raspberry Pi camera of the female Crowned Eagle and the chick during extreme weather events. A severe storm came in from the south on the evening of the 23 November. At 18h11 the storm hit the valley accompanied by strong winds and rain. It is amazing the nest did not blow out of the tree!    

On 30 November, the temperature soared to 39C . Images show both the chick and the female eagle gular panting  and the chick being sheltered from the heat by the female. Her dark charcoal plumage on her back may camouflage her in dappled shade but would not bring relief from the sun! From the 9 to 12 December the Lowveld experienced a period of cold weather with almost continuous rain. Both the chick exposed on the nest and the female eagle sitting on a nearby branch became saturated. The chicks down clung to its body and it became stained brown from the nest and nest contents. While watching, it from the hide on 10 December its continual shivering was clearly visible.

 Two week old white downy chick

 Female Crowned eagle gular panting during exceptionally hot weather

 Female Crowned Eagle shading chick in hot weather 

 Starling collecting eagle down (Image  G.R. Batchelor)

Six week old chick showing emerging primary and secondary feathers on wings (Image G.R. Batchelor)

Crowned Eagle being blown about on nest during a storm