The colours and patterns of the eggs and chicks of ground-nesting terns are generally considered to have evolved to provide camouflage against the background of the nesting areas. This works to a certain extent: Sooty Tern eggs are difficult to see on a sandy substrate and dark brown downy chicks are barely visible among driftwood or dead vegetation. However, the generally pale background of their eggs renders Sooty Tern eggs readily visible on dark driftwood and chicks stand out like sore thumbs on white sand! Sooty Tern chicks vary slightly in the colour and pattern of their down but it is always some shade of brown with or without paler markings but this does little to offer protection from predators in several of Bird Island’s habitats within the nesting colony.
Brown Noddies also nest in their thousands on Bird Island. Most build bulky nests in trees, especially in the crowns of coconut trees, but since the eradication of rats from the island in 1996 many have taken to nesting on the ground, particularly around the hotel. Brown Noddy chicks are also downy when they hatch and most are dark brown. A small proportion, however, are white, and there are even some that are intermediate between the two extremes.
This phenomenon is not restricted to Bird Island, for white chicks have also been recorded on Cousine Island, also in Seychelles. In none of the Brown Noddy nesting habitats would white chicks offer camouflage. The downy chicks have dark eyes, dark bills and dark feet, and so are not albinos. Furthermore, when true feathers moult through these chicks develop into normally-coloured Brown Noddies.
There appears to be some geographical variation in the proportion of white downy chicks as most downy chicks on the Kermadec Islands, in the Pacific Ocean, are white, and on Ascension Island in the Atlantic are white.
While the variations in colour of the downy chicks does not seem to offer protection from predators, there have been no other satisfactory explanations of how the colour forms exist alongside each other. A suggestion that different colours might confer different advantages in regulation of internal body temperature seems unlikely as downy chicks are normally protected from direct heat from the sun by a parent. The mystery remains.