Photo Gordon Scholtz Adult and juvenile Caspian Terns

Seabirds

Caspian Tern Hydroprogne caspia

Please keep watch for colour-ringed Caspian Terns, and report all interesting sightings

Keith Spencer, Tony Tree & Les Underhill

The Caspian Tern is classified as "Near-threatened" in the 2000 Red Data Book

In South Africa, Caspian Terns Hydroprogne caspia occur both along the coastline and in the interior. Along the coastline, this tern is found mostly at estuaries; records along the open shore are mostly of birds that are on the move. In the interior, most records are from the highveld, along the major rivers, at dams and in the panveld. Occurrence in the interior is a relatively new phenomenon, with breeding first reported in 1968. Now, some of the largest concentrations of Caspian Terns in southern Africa are here: recently there were 335 at Gariep Dam, along the Orange River in the Northern Cape, and 100 at Barberspan, in the North-West Province. In spite of the apparent increase in range and in numbers, the Caspian Tern was retained in the 2000 Red Data Book, where it was classified as "Near-threatened". It is considered to remain a sufficiently rare species that it needs to be carefully monitored.

This book contains masses of fascinating information about waterbird movements

Rather little is known about the movements of Caspian Terns. The Review of Ring Recoveries of Waterbirds in Southern Africa, published in 1999, summarizes the available information. There have been only four really interesting recoveries. These are all of birds ringed as nestlings. There have been movements from Port Elizabeth to St Lucia, KwaZulu-Natal (906 km), and to Maputo Bay, Mozambique (1130 km), and two from St Lucia to southern Mozambique (228 km and 384 km). All four of these recoveries were made within nine months of ringing. This tiny sample of records suggests that young birds from colonies in the Eastern Cape and KwaZulu-Natal move eastwards to "nursery areas" in southern Mozambique (where Caspian Terns have only been recorded breeding once, and that was along the lower Zambezi River, in the 1860s!). We know absolutely nothing about the movements of birds that breed in the interior of southern Africa or those of birds that breed along the southern and western coasts of South Africa or along the Namibian coast.

Photo Gordon Scholtz

A newly hatched Caspian Tern chick, at the De Mond Nature Reserve. The Heuningnes estuary within this reserve is one of South Africa's Ramsar sites, proclaimed in 1986

On 12 December 2000, 19 Caspian Tern fledglings were colour-ringed with leg flags at the De Mond Nature Reserve, just east of Cape Agulhas, in the Western Cape. The ringing was undertaken as part of a Western Cape Nature Conservation Board initiative at the De Mond Nature Reserve. The objective was to see where these young moved to once they had fledged, and how long they remained with their parents.

Resightings of juveniles with leg-flags (Updated by Tony Tree on 23 February 2002)

The following resightings of these birds have been reported:

• Sightings from the time of ringing up to August 2001
• On 28 December, the first of these were seen at the estuary of the Heuningnes River, within the reserve.
• On 25 January 2001, one of these fledglings was observed, together with an adult bird, at Cape Recife, Port Elizabeth, by Tony Tree. The birds arrived from the west, rested for a brief time at Cape Recife, and then moved on eastwards. The distance from De Mond is about 500 km, implying an average eastward drift of around 18 km per day. Between 12-15 February he again saw a leg-flagged chick with parent at Cape Recife and this pair was present on most visits up to 10 March but not on many subsequent visits. This was quite possibly the same twosome that were there on 25 January. On 5 May these or another twosome appeared at Cape Recife and remained until 4 July but was not seen subsequently. An adult plus flagged young seen nearby at King's Beach, Port Elizabeth, on 18 May may have been the Cape Recife birds.
• Tony Tree found another leg-flagged bird, together with parent, at the mouth of the Kabeljous River, Jeffrey's Bay, west of Port Elizabeth on 15/16 February. This twosome remained in the area until 9 August and were seen on virtually any visit between those dates by several observers.
• A flagged bird plus parent were found by Tony at the Kromme River mouth on 20 February and these birds were seen regularly by a variety of observers up until 30 July.
• On 20 February Gregg Darling reported an adult, with a faded orange-yellow colour ring, accompanied by a leg-flagged juvenile at Cape St Francis Point. (We have not yet tracked down who put the colour ring on the adult but there is a strong possibility that it may have originally been bred at St Lucia, KwaZulu-Natal, where many birds were colour ringed in the past.)

Overview in August 2001 Thus it would appear that the period of dependence of the fledged young bird on the adult may be as much as seven to eight months, an extremely long period for a tern. In European populations a maximum figure of about eight months is also given. Does such a lengthy period of dependence affect the annual breeding cycle of the adults? Do the adults accompany the juveniles further eastwards or desert the young and return to the natal areas again?

• Sightings since September 2001
• On 12 December 2001 there were two adults and two juveniles, one with leg flags, at Kings Beach, Port Elizabeth (R. Paton).
• On 24 December 2001, there were two Caspian Terns, one with flags, at Cape Recife (A. Taylor).
• On 13 and 14 February 2002, Tony Tree saw a juvenile with flags at the Kabeljous River mouth. This bird was observed begging from an adult, which chased it.

  Tony Tree's overview in February 2002 The impression that I am getting is that dependence may be more than the seven or eight months suspected earlier and may be up to 14 months. I suspect that the Kabeljous River bird was being chased by its parent who probably thought it about time that its offspring became independent. This then raises the question, do successful breeders breed in the following season? Only seven pairs attempted to breed in late 2001 at de Mond as against the 38 pairs (19 successful) in 2000. Of course not too much can be read into that because it is possible that birds shift colonies between years. The Caspian Tern breeding colony in Port Elizabeth deserted completely last year.

  Photo Gordon Scholtz Staff of the Western Cape Nature Conservation Board ringing Caspian Tern chicks

  These resightings are all east of the point of ringing. They suggest that the juveniles are moving in the direction suggested by the four recoveries - eastwards towards northern KwaZulu-Natal and southern Mozambique.

  Please RINGWATCH

  So, whether you are in this predicted area or not, please check every Caspian Tern you see for leg flags. The picture below shows what you should be looking for. Because of the short projections, leg flags are more conspicuous than ordinary colour rings. Under good conditions, they are also visible during flight. There is a colour for each breeding season, and a colour for each area. Make a note of the colours of the leg flags, and their positions relative to the metal ring. Report this combination, plus the time, date, place and any other relevant information, to Dieter Oschadleus at the South African Bird Ringing Unit (SAFRING).

  Photo Gordon Scholtz

  This is what the colour leg flags on the Caspian Terns look like. They are more conspicuous than colour rings

  Although population sizes of Caspian Terns are monitored by the Avian Demography Unit's Coordinated Waterbird Counts (CWAC) project, this provides us with information for only one day in midsummer and one day in midwinter each year. To enable us to get a clearer insight into the patterns of movement of this rare species, and the full set of localities that it utilizes, we need to be gathering more information than this. Any sightings of Caspian Terns at any time of the year should therefore be reported on the Avian Demography Unit's system for archiving interesting records (jump to Bird Sightings.)