Sharing a table

Egyptian Goose below the Crowned Eagle nest

I knew very early on in the season that Egyptian Geese had an interest in the Zimbali crowned eagle nest.  On the 25th of September, even while the eagles were incubating, I observed a pair of geese flying up to branches on the underside of the nest, at first I thought they were trying to steal nesting material, but they were probably trying to steal the entire nest.  Fortunately the female eagle sat strong and steady and the geese backed off. I have no record of the geese interactions during the incubation period and the first 20 days of chick rearing, I only suppose that the eagles were very forceful or the geese wisely cautious.  And during the months after the camera was installed, while the young eagle was flightless and sedentary on the nest, there were no incidents observed. On the 18th of December the eagle chick was ringed with a yellow N2 band, she was measured as a large female and returned to the nest. N2 fledged at about 115 days of age around the 26th of January.

After fledging, young crowned eagles are still completely reliant on the adults for delivering food, and unlike many other raptors which will chase and steal prey from the incoming adults crowned eagle fledglings wait courteously on the nest for the food to be served.  This can go on for five months post-fledging and so my diet studies using the cameras continue for months after.

I timed my next camera service once the young eagle had made its first flight, and didn't notice anything unusual - unknown to me at the time was that the first few eggs of the clutch were deceptively hidden, covered by the detritus of nesting material .  On the next camera maintenance visit three weeks later the goose nest was obvious, lined with goose down, and again the eggs were deceptively covered.  I got a hell of a fright when one of the geese swooped past me and landed on the nest hissing with fury  -  I thought it was one of the eagles!  I laughed as the adrenaline rush of fear gave way to amused chagrin at the silly goose.

Silly goose on the left !

In any case – the camera was kept operating to see what would happen at this peculiar situation, and Zimbali has been a thrilling data-set to analyse both because of an exceptional diversity of prey remains, and this peculiar interaction between geese and eagles.

One would think that the eagles would simple dominate here, overpowering, even killing the geese right there on the nest site.  Recent Black Sparrowhawk research revealed that geese overpowered and usurped sparrowhawks, but the interaction between this particular eagle and goose duo has been somewhat of an example of commensalism.  In ecology, commensalism is a class of relationship between two organisms where one organism benefits without affecting the other- derived from the latin commensalis, meaning 'sharing a table'.

A summary of the situation – which I hope to describe in more detail on a journal note (so take heed of the copyright and intellectual property statement at the footer to the blog), is:

On the 28th of January, just a few days after the young N2 fledged, the goose first visits the nest and digs a goose-sized divot in the rotting nest lining. The first egg is laid on the 2nd of Feb and is subsequently kicked out - a full clutch of 7 eggs is laid by the 8th.

During the early stages of incubation the eggs are left covered and concealed for long periods while the eagles continued to deliver food, and the juvenile continued to eat casually on the nest. I am utterly shocked at how long these eggs can go without being brooded – and during data analysis held little hope for the success of this clutch of geese. Even up until the 10th of Feb N2 slept the night on the nest.  For the next few weeks the nest is shared between eagle feeding and geese brooding.

By the latter weeks in February the antagonism becomes clearer, maybe the geese eggs require far more attentiveness in order to ensure the survival of the duck embryos. So while the adult eagles still drop prey at the nest, it appears that the geese throw them out.  Zimbali visitors have had wonderful experiences with the trusting N2 while she had to eat her meals on the forest floor.

Floor food, a Blue Duiker leg. Photo by Benine Du Toit

The clutch of eggs were last seen on the evening 12th of March, and even on this day N2 had a nice feast of cane rat on the nest. By the morning of the 13th, the brooding behavior of the goose had noticeably changed – she now held her wings dropped – the eggs had probably hatched during the night, and the first glimpse of ducklings are seen shortly before 10am.  She keeps a very close watch over her brood and only leaves the nest once, very briefly, that evening.  The next morning, just 24 hours after hatching, its time to scarper before the little ducklings become eagle snacks.  At 6.42am the mother goose guides her little brood off the nest.  They jump and parachute 25 meters to the ground!

N2 received another meal from her parents at midday, and after eating her fill, she curiously crushed the hollow half-shells left in the nest.  Another month of camera monitoring until the 8th of April revealed no further trace of a goosey interest. N2 continued to receive meals, and consume them in peace on the nest.  The most recent photos were taken of a happy and healthy 'Miss Zimbali' on the 26th of May, and so it seems, life goes on!

Miss Zimbali as she is affectionately known by Adele Kieser (photographer)

Photo by Adele Kieser