Egg morphology of Swift Terns in South Africa

Morphology of Swift Tern Thalasseus bergii bergii eggs was examined on Robben Island, South Africa. A recently formed colony was found abandoned en masse, probably following human disturbance, and 146 freshly laid eggs were collected. The mean ± SD nest density was 7 ± 2.5 nests m-2 and 3% of nests contained two eggs. Eggs ranged in shape from oval to pyriform and displayed black markings (blotched, streaked, scrawled or speckled) overlaying the eggs’ colour. The mean length and width of a sample of 105 eggs was 62.2 mm (56.3–66.9 mm) and 42.3 mm (39.8–45.3 mm), respectively. All collected eggs were weighed and the mean mass was 57.9 ± 3.72 g. Estimated volume of eggs was calculated to be 56.3 ± 3.74 cm3. This is the first report of mass measurements obtained from freshly laid eggs Swift Tern eggs and provides insight on egg morphology, for which knowledge is limited for this species.Keywords: egg morphology, Hoyt’s equation, Robben Island, Thalasseus bergii bergi