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

2019.05.17 Pre-laying  courtship behaviour of Crowned Eagles on nest. (Female on left).

191015 11h52 Female Crowned Eagle delivering fresh leaves to nest with two,  1 day old eaglets.

20190507 10h59 . Pre-laying  courtship behaviour of Crowned Eagles on nest.  (Female on left).

20190508 10h50 Pre-laying  courtship behaviour of Crowned Eagles on nest. (Female on left). 

190726 14h14 Courtship feeding, Male Crowned Eagle delivering prey to female Crowned Eagle . Prey unidentified.

190708 13h34 Male, wings raised, female courtsey display behaviour.

190826 10h09 Female Crowned Eagle inspecting newly laid egg.

191008 10h50 Female Crowned eagle collecting head of Red Duiker from male Eagle. The image was lightened to show the head of the duiker.

20130911 An immature Crowned Eagle flying fledged from the Municipal Nature Reserve nest flying over Shandon Estate.

192029 06h51 Crowned Eagle female with 15 day old chick

Background

For the past twelve years the Crowned Eagles around Nelspruit/Mbombela have been studied. The focus of the study has been on determining the population density and whether the population is a source or a sink. In other words, is their population a stable one producing a surplus of chicks or is the population shrinking. To date the information obtained from monitoring the nests tends to indicate that the population is healthy, producing  a surplus of chicks. The known nests are used regularly, usually every alternative year, or annually if they have a breeding failure. 

The pair of Crowned Eagles that now nest on Shandon Estate used a nest in the municipal nature reserve for a number of years. Their nest was first discovered in 2007 and was already a massive structure so had obviously been used  for many years previously.  This nest was abandoned in 2011 and they moved onto the Shandon Estate in 2012. This pair of eagles is unique in that it is the only pair being monitored that has attempted to breed every year since their nest was discovered. Most pairs breed every second year. The reason for breeding annually is not known  as they have had chicks annually and at least two have successfully fledged.  The presence of a fledged chick  is thought to inhibit the urge to breed during the year after fledging as it still needs to be fed for as much as ten months after fledging. This is similar to the Martial Eagle which also breeds every alternative year. A possible reason why this pair breeds every year is that the chick is not surviving.  This was confirmed during  the 2018 breeding season when  in December, the three quarter grown chick was suspected to have been killed by a Baboon on the nest.  Quilled feathers from the chick's wings were found below the nest. A male Baboon had been observed sitting near to the nest just two days earlier.  It is possible that baboons have also been responsible for other chicks not surviving and this being the reason why this pair of eagles breed every year. 

In order to test this theory, a proposal was put to the Shandon Estate management to install a remote recording camera at the nest.  With the guidance of Peter Retief, a local computer specialist, a sophisticated remote miniature computer called a Raspberry Pi and attached small camera, has been placed near the nest to try to record the breeding activities of the eagles on a continual basis for an entire breeding season.  The camera and its accessories were funded by the owners of Shandon Estate.  The camera was installed in March, long before the commencement of any nesting activities,  so not to disturb the eagles. The camera was placed in the tree over twenty metres up by John Davies from the Endangered Wildlife Trust. It  has already taken some amazing images of the pair of Crowned Eagles on their nest. The advantage of this camera is that it can be programmed to take images as programmed by the computer from Peter's desk! He can also observe the images being taken from his office without disturbing the eagles. This is the first time this technology has been used in South Africa to document bird nesting behaviour.

Discussion

The Raspberry Pi camera has been recording images daily now for seven months without a hitch. A number of very interesting images have been recorded during this period documenting behaviour  patterns not previously recorded such as  pair bonding and also the breeding behaviour of the eagles on the nest.  The camera allowed us to determine when the eggs were laid even though we could not actually see them. It also allowed us to determine the exact day of hatching from recording the behaviour of the eagles.  The eggs took 51 days to hatch. The "Cain and Abel"  battle where one chick succumbs to the pecking by its sibling, was unfortunately not documented clearly due to the depth of the cup in the nest. The surviving chick is now two weeks old. Where possible, prey items are being identified. So far greater bushbabies, red duiker, rusty spotted genet and dwarf mongoose have been brought to the nest.  If all goes well, the chick will start leaving the nest in about 90 days time! 

Acknowledgements

Peter Retief for developing and programming the Raspberry Pi  computer and camera and his endless enthusiasm of the project, Petri Viljoen for initiating the project, Jaco Badenhorst, the General Manager of the Shandon Estates for his continued enthusiasm for the project and all property owners who have contributed towards this unique project in various ways.

Garth Batchelor