Page 506 - Atlas Sea Birds Ver1

A roost of Hartlaub’s Gulls (photo L Upfold)

number that bred was < 10 pairs; five of these had a solitary and for birds older than 2 years (φ ) 0.87 (Underhill and
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pair (Table 1). Underhill 1986). Based on these parameters, and assuming
In 1990, it was estimated that c. 12,000 pairs of an equilibrium situation, 59 of 159 post-fledged birds,
Hartlaub’s Gull bred at 48 sites (Williams et al. 1990). or c. 37%, would be pre-breeders (13 of 100 birds aged
Maxima recorded by Williams et al. (1990) at different sites three years or older would die each year, necessitating
are shown in Table 2 and totalled 13,711 pairs, of which 15 individuals aged two years that would replace them, and
11% were in Namibia. In 2007, the overall population was similarly 19 aged one year, and 25 fledged young). Then
thought to number 6,865 pairs, again with 11% being in 2,520 of the 4,000 birds estimated to be in Namibia would
Namibia (Kemper et al. 2007). Maximum numbers observed have been breeders, equivalent to 1,260 pairs. This value is
at sites totalled 10,577 pairs in the 1970s, 16,059 pairs in not overly disparate from the 935 pairs estimated to breed
the 1980s, 15,007 pairs in the 1990s, 14,186 pairs in the in Namibia from the most recent counts at colonies there
2000s, and 8,902 pairs in the 2010s, indicating an ongoing (Table 2), especially when it is borne in mind that sizeable
decrease since the 1980s (Table 2). However, because proportions of the mature population may not breed in
Hartlaub’s Gulls can alter the location of their breeding, some years (Crawford and Underhill 2003).
summing maxima at sites across a period of time may Also based on the above parameters, generation length
overestimate their population. On the other hand, if shorter (G) for Hartlaub’s Gulls may be calculated as G = A +
periods result in exclusion of colonies, the population may (1 / (1 − ϕ ))(Birdlife International 2000) = 10.7 years. Then
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be underestimated. Numbers of Hartlaub’s Gulls breeding 3G is 32 years and the decrease of 50% over 30 years
in the Western Cape likely decreased after the 1980s between 1990 and 2020 was equivalent to c. 53% in 3G.
(Crawford and Underhill 2003). Though inexact, this estimated rate of decrease in numbers
The sum of maximum numbers at colonies in 2018–2022 breeding meets criterion A1 of the International Union
was 6,279 pairs and the sum of the most recent count at for Conservation of Nature (IUCN 2022) for classification
each colony was 5,817 pairs, of which 16% was in Namibia of Hartlaub’s Gull as Vulnerable (VU). The species was
and 84% in South Africa (Table 2). These two values previously classified as VU in Namibia (Simmons 2015) but
were similar and had a mean of c. 6,000 pairs, which was not in South Africa (Taylor et al. 2015). Further monitoring
approximately half of the 12,000 pairs that were estimated to confirm an overall VU status should be undertaken.
to breed in 1990 by Williams et al. (1990).
In 2015, counts of birds suggested that the population of Important Bird and Biodiversity Areas
Hartlaub’s Gulls in Namibia was c. 4,000 birds (Simmons
2015). Age at first breeding (A) for the species was three Important Bird and Biodiversity Areas (IBAs) are identified
years (Crawford and Underhill 2003). Annual survivorship through the use of specific criteria. Criterion A1 is that
from fledging to 1 year was 0.76, from 1 to 2 years 0.79, ‘the site is known or thought regularly to hold significant

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