Page 549 - Atlas Sea Birds Ver1

3.15 Numbers, trends, status, and conservation of Roseate Tern
(Sterna dougallii) in South Africa

Authors: RJM Crawford, PA Whittington, BM Dyer, and AB Makhado

Abstract:
The South African population of Roseate Terns (Sterna dougallii) is relatively discrete from other populations of the species but there
may be some interchange with birds in the southwest Indian Ocean. In South Africa, Roseate Terns have bred at six sites, including one
mainland colony, and may have bred at a seventh, but since 1972 have only nested at three islands. Numbers estimated to have bred
from 1977–2022 varied between 93 pairs and 305 pairs (N = 27), with no obvious trend and a mean and standard deviation of 178 ± 56
pairs. The most recent count was of 213 pairs in 2022. The mature component of the South African population is < 1,000 individuals so
that it should be classified as regionally Vulnerable (VU). There may be grounds for maintaining its present classification as regionally
Endangered. Both Dyer Island Nature Reserve and the Algoa Bay Islands section of Addo Elephant National Park qualify as Important
Bird and Biodiversity Areas for the species in South Africa. Threats to the South African population include reduced reproductive success
as a result of human disturbance or unfavourable weather and possible outbreaks of infectious avian diseases at colonies or tern roosts.

Introduction:
Roseate Terns (Sterna dougallii) have bred at six sites in South Africa (Figure 1, Randall and Randall 1980) but not in Angola or Namibia.
Elsewhere they have bred in the north Atlantic, North Sea, Indian and west Pacific oceans (del Hoyo et al. 1998). In the southwest Indian
Ocean, they have bred at Mafia Island in Tanzania, at Aride Island in the Seychelles, at the Cargados Carajos archipelago (north of
Mauritius), at Barren Islands, Nosy Manitra, and an island south of Tamatave off Madagascar. They were observed in nuptial plumage
elsewhere around Madagascar and displaying on a sand-spit south of Inhassoro, opposite Bazaruto Island in south Mozambique (Tree
2005a). In South Africa, they bred at Dyer Island, Western Cape, on the headland at Cape Recife and at St Croix, Seal, Stag, and Bird
islands in Algoa Bay, Eastern Cape (Randall et al. 1991), at the eastern boundary of the Benguela upwelling ecosystem (Makhado et al.
2021). They were suspected to have bred at Jahleel Island in Algoa Bay in 1983 (Randall et al. 1991). If Cape Agulhas is taken to be the
junction between the Atlantic and Indian oceans, Dyer Island would fall in the Atlantic Ocean. The Algoa Bay sites are in the Indian Ocean
inshore of the warm Agulhas Current that flows to the southwest. Resightings and recoveries of Roseate Terns banded in South Africa
have shown that they range from 850 km west of Bird Island (Algoa Bay) to 785 km northeast of it (Tree et al. 2019).
Based on an ability of Roseate Terns to adapt to altered conditions at colonies by shifting breeding sites, on substantial fluctuations in
numbers breeding in Algoa Bay, and on a low return rate of marked birds to Algoa Bay, it was postulated that some Roseate Terns may
move between breeding sites in South Africa and elsewhere in the southwest Indian Ocean (Tree 2005a). However, the South African
population is unique in its genetic characteristics (Lashko 2004) and moult data from 404 individuals in South Africa failed to identify
subpopulations (Tree et al. 2019). Stable isotope information suggested that 152 of 156 Roseate Terns likely moulted their scapulars in
South African waters and that the other four birds moulted theirs elsewhere (Tree et al. 2019).
This chapter collates estimates of numbers of Roseate Terns breeding in South Africa. It makes a preliminary assessment of the Red
List status of, identifies Important Bird and Biodiversity Areas (IBAs) for, and summarises the main threats to the South African population.

Numbers and population trend
Records of breeding at South African sites from 1909–2022
were collated in Table 1. For counts of nests or pairs, the
mid-point of ranges given was used. When numbers of
eggs were counted, they were divided by 1.3 to estimate
the number of pairs because at St Croix Island, in 1977,
30% of nests had two eggs (Randall and Randall 1980).
Sources of information were: Ross (1975), Every (1975),
Randall and Randall (1980), Randall et al. (1991), Tree and
Klages (2003), Tree (2005a), Crawford et al. (2007, 2009),
Hagen (2015), and Tree et al. (2019). These were updated
with unpublished estimates made by CapeNature, South
African National Parks, and South Africa’s Department of
Forestry, Fisheries and the Environment (DFFE). Roseate
Terns bred at Dyer Island in unknown numbers in several A Roseate Tern with a fish (photo BM Dyer)
years between 1909 and 1971, but the species was not
thereafter recorded at the island until 1982 (Tree 2005a). were recorded. About 70 pairs bred at Stag Island in 1971.
Disturbance caused by guano collection, which stopped in Roseate Terns bred at Bird Island, also at the east of
1985, may have caused the cessation of breeding there Algoa Bay, in many years between 1948 and 2022, with a
(Crawford 1997). One pair bred again at Dyer Island maximum of 300 pairs in 2010. They bred at Cape Recife,
in 1991 and they bred there annually from 1996–2022, at the west of Algoa Bay from 1963–1968. Based on counts
the maximum number observed being 54 pairs in 2021. of eggs about 55 pairs bred there in 1968. Roseate Terns
Breeding was reported for Seal or Stag islands at the east bred at St Croix Island, also at the west of Algoa Bay, from
of Algoa Bay between 1936 and 1947 but no numbers 1969–2008, with a maximum of 134 pairs in 1986.

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