Avian Demography Unit
Department of Statistical Sciences
University of Cape Town
|BIRD NUMBERS||Volume 10 Number 1, July 2001|
05. Four White Storks vanish, but one keeps going
Les G. Underhill
White Storks Ciconia ciconia were first recorded breeding in South Africa in the 1930s, and there has probably been at least one pair breeding at one of three localities in the Western Cape since the early 1960s. In spring 2000, there were five breeding pairs, one between Cape Agulhas and Bredasdorp, and four at the Tygerberg Zoo near Cape Town.
In comparison with the intensity of effort that is invested in White Storks in Europe, our monitoring of this fascinating population has been pitifully weak! In the past year or two, ornithologists from the Vogelwarte Radolfzell of the Max Planck Institute in Germany, who study storks intensively under the leadership of Professor Peter Berthold, have started taking an interest in these birds. The big questions about the offspring of South African-breeding White Storks is: ‘Where do they go after they fledge, and where do they breed?’
A team of ornithologists from Radolfzell and the Belgische Natuur- en Vogelreservaten travelled to Cape Town in November–December 2000 in search of an answer to the first question. In a collaborative project with the ADU, they placed satellite transmitters on five fledglings. Four of these satellite tags were attached to nestlings at the Tygerberg Zoo near Cape Town.
The first satellite tag was put on a nestling we called ‘Tiger’ on Friday 1 December 2000; this was the largest nestling in any of the nests at the Tygerberg Zoo on this date. The following Wednesday, three tags were placed on siblings ‘Misty’ and ‘Leo’, whose nest was on top of the panther’s cage, and on ‘Saturn’, who grew up on the nest platform in the giant tortoise enclosure. The fifth tag had meanwhile been placed on the largest nestling in the last remaining nest in the Bredasdorp district; this bird we named ‘Rembrandt’, the artist of the Nachtwacht, the name of a famous stork farm in this area.
As was the case with the three satellite-tagged penguins, Peter, Pamela and Percy, who swam from Port Elizabeth to Cape Town in July 2000, the ADU’s website reported the progress of the Famous Five. The positions of the storks were updated regularly. There was a crucial difference between the devices used for the penguins and those used for the storks: the penguin devices were powered by batteries with a maximum life of a couple of months, whereas the stork devices were solar-powered, and theoretically could transmit for years.
At the time that the birds were tagged, we wrote on the website: ‘The harsh reality is that the first few weeks and months out of the nest are extremely hazardous for a young bird. It is likely that several (or even all) of the five young birds will die before they move any great distance.’
We were lucky – all five birds fledged successfully. During late December and early January, the signals indicated that they had left their nests, and were making short-distance trips into the neighbouring areas. By 21 January 2001, all were outside the borders of the Western Cape. By the end of the month they were outside South Africa. They had survived the critical first few weeks, and we breathed a bit easier. But a bit too soon. We had not expected the rains over the Zambezi River valley, which brought devastating floods to Mozambique.
Tiger set out from Tygerberg Zoo on 8 January, and by 27 January was in the Luangwa valley in northeastern Zambia, some 4000 km away. It spent nearly two weeks there, the daily satellite positions coming from different places within the valley. Pete Leonard, one of Zambia’s leading birders, reported: ‘The area is not densely populated with people. The valley has rolling hills covered in scrub and poor miombo woodland until you hit the escarpments on either side and get into decent woodland. The storks make use of the oxbows on the main river but are probably more often on the smallish floodplains along the tributaries.’
We do not know if Tiger made this journey in the company of adult White Storks on their way back to Europe, but this seems likely. If the storks had continued travelling northwards at the speed they had been travelling, they would have arrived in Europe while it was still winter there. The Luangwa valley is probably one of the feeding areas which they use along the route, and where they remain for a few weeks before moving on.
By early February, siblings Misty and Leo were both in the vicinity of the Kariba Dam, Rembrandt was in Botswana and Saturn was in the Save River floodplain in Mozambique. Then the rains came, and regular signals were received only from Misty and Saturn. This was not alarming in itself, because sunny conditions are needed to power the transmitters, and also because the signals transmitted up to the satellites do not easily penetrate thick cloud or rain. Suddenly, on 28 February, the ‘activity counter’ in Misty’s satellite tag got stuck on 166, indicating that this bird was definitely dead. At the time of going to press, Saturn had crossed from Mozambique into the eastern highlands of Zimbabwe. Its progress will continue to be reported on the ADU website.
Days are growing to weeks, and the weeks to over a month, and there have been no signals from any of the remaining three birds, Tiger, Leo and Rembrandt. It is probably safe to assume that they are dead, but there is still a small chance that they will make a reappearance. Having got over the hurdle of the first few weeks out of the nest, it is disappointing that four out of five birds were to be lost in such a short period of time. But these are the realities of life for White Storks, and also for most wild animals.
We are still digesting what we have learnt. The biggest surprise was that the birds left their nest areas so early. We had expected them to head northwards at the time we believe the adults migrate, late in February and March. We were surprised at the long distances flown each day. We were surprised that the birds followed such direct routes; all except Saturn followed the ‘stork highway’ to Europe, determined by a decade of satellite tracking of northern hemisphere birds. This is what makes us believe they were in the company of adults on their way back to Europe. This year, this main route took the birds into a hostile weather system, and only the bird that survived took the unorthodox route through the Kruger National Park to Mozambique and eastern Zimbabwe. The Vogelwarte Radolfzell in Germany has been so intrigued at the results, that they intend to repeat the exercise in the coming breeding season.
For more information, and up-to-date positions, see the ADU's White Stork pages
Office Avian Demography Unit
Enquiries/More Information: firstname.lastname@example.org
Document posted: 24-Aug-2001
Office Avian Demography Unit