Page 551 - Atlas Sea Birds Ver1

As noted by Tree (2005a), numbers of Roseate Terns sites in South Africa: Dyer, St Croix, and Bird (Algoa Bay)
estimated to breed in South Africa have shown substantial islands. Breeding birds mainly forage within 2 km of their
oscillations. They rose from 128 pairs in 1977 to 264 pairs islands (Tree 2005b), or across an area of c. 16 km .
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in 1986, fell to 121 pairs in 1992, increased to 261 pairs in Hence their area of occupancy (AOO) during the breeding
2001, dropped to 93 pairs in 2004, climbed to 305 pairs in season would be ≤ 50 km , but non-breeding birds would
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2010, decreased to 100 pairs in 2011, went up to 211 pairs range over a larger area as would post-breeding adults. As
in 2016, down to 119 pairs in 2018, and up to 206 pairs there have been wide fluctuations in estimates of numbers
in 2019. In 2022, 213 pairs bred. However, there was no breeding (Figure 2), the South Africa population may
apparent trend in the overall numbers nesting in this period qualify for classification as Endangered in terms of criterion
(Figure 2). The mean and standard deviation of annual B2 (AOO < 500 km ) together with (a) (≤ 5 locations) and
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numbers recorded breeding from 1977–2022 was 178 ± 56 (c) extreme fluctuations in (iv) number of mature individuals
pairs (N = 27). Tree (2005a) thought the oscillations might (IUCN 2022).
be attributable to some of the terns breeding elsewhere in Important Bird and Biodiversity Areas (IBAs) are
the southwest Indian Ocean in years when low numbers identified through the use of specific criteria. Criterion
bred in South Africa. Other possibilities are altered A1 is that ‘the site is known or thought regularly to hold
participation in breeding, including deferring or advancing significant numbers of a Globally Threatened species’
the age at first breeding, and changes in recruitment into (BirdLife International 2020). Guidelines for the application
the breeding population. Both of these probably influenced of IBA criteria recommend that to meet A1 ‘a site must
numbers of Greater Crested Terns (Thalassarche bergii) support: at least 30 individuals (the equivalent of 10 Pairs/
breeding in South Africa (Crawford 2009). Roseate Terns Reproductive Units) of a species classified as Vulnerable’
may visit colonies at an age of 1 year, rarely breed at (BirdLife International 2020). Then, based on the most
2 years, mostly breed at 3 years, but may delay breeding recent counts at sites, both Dyer Island and Bird Island
until 4 years or later (Tree 2005b). Complete breeding (Algoa Bay) would qualify as IBAs for South Africa’s
failure of Roseate Terns in four of the five years that they Roseate Tern population. The former falls within Dyer
bred at St Croix Island between 1979 and 1986 (Randall Island Nature Reserve and the latter within the Algoa Bay
et al. 1991), at Bird Island in 2002 (Tree 2005a), and at Islands section of Addo Elephant National Park.
Dyer Island in 2011 (Hagen 2015) would have influenced In South Africa, when disturbed, breeding Roseate Terns
later recruitment to breeding colonies. Fledging success in may leave their nests and generally they do not return
Algoa Bay varied from 0.09–0.4 chicks/nest (Tree 2005b). immediately. This leaves their eggs and chicks vulnerable
Changes in adult survival would also influence numbers to predation by Kelp Gulls (Larus dominicanus) (Randall
breeding. No large mortality events of Roseate Terns have and Randall 1981) or extreme temperatures. Wet weather
been noted in South Africa but four partly-eaten adults and caused breeding failure of Roseate Terns at both St Croix
the remains of seven adults were observed at Cape Recife and Bird (Algoa Bay) islands (Randall et al. 1991, Tree
in September 1967 (Every 1975). If 22 pairs nested at 2005a). In 2007, high seas swamped the breeding area
Cape Recife that year (Table 1), these losses amounted to of Roseate Terns at Bird Island and caused abandonment
25% of birds breeding at the site. Deaths of birds away from of their breeding attempt (BMD, PAW unpubl.). In August
colonies are harder to monitor. Annual survival of Roseate 2009, an African Grass Owl (Tyto capensis) caused the
Terns has been estimated at 0.74–0.84 (Tree 2005b). colony at Bird Island to abandon breeding (Tree 2010).
Of interest were lesser fluctuations and a long-term Highly pathogenic avian influenza (HPAI) caused deaths
increase in numbers of Roseate Terns breeding at Dyer of four more abundant species of tern in Algoa Bay in
Island (Table 1). More difficult and less frequent access 2018 (Khomenko et al. 2018). Roseate Terns were present
to breeding sites in Algoa Bay may have resulted in some in mixed-species tern roosts in all 28 visits to the Cape
colonies there being overlooked or visited outside the Recife area made in 1986 (Randall et al. 1991). Such
breeding peak in years of low counts. A virtual absence of associations make them susceptible to infectious diseases
records of the presence of Roseate Terns at Stag and Seal and consequent mortality.
islands from 1978–1986 was thought to reflect low search 350
effort at those islands (Randall et al. 1991).
300
Conservation status, Important Bird and Biodiversity
Areas, and threats 250
In South Africa, Roseate Terns were classified as 200
Endangered in 2015 (Hagen 2015). Assuming the South Pairs
African population of Roseate Terns is relatively discrete 150
from other populations of the species, the mean annual
count between 1977 and 2022 (178 pairs), the maximum 100
annual count in this period (305 pairs) and the sum of most
recent counts at colonies (213 pairs) suggest a mature 50
population of 356–610 mature individuals. The population
should then be classified as Vulnerable (VU) in terms 1977 1980 1983 1986 1989 1992 1998 2019 2022
criterion D1 (number of mature individuals < 1,000) of 1995 2001 2004 2007 2010 2013 2016
the International Union for Conservation of Nature (IUCN Figure 2. Estimates of overall numbers (pairs) of Roseate Terns
2022). Since 1972, Roseate Terns have only bred at three breeding in South Africa, 1977–2022

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