Abstract
Of the five species of black cockatoo in the genus Calyptorhynchus, those species with red tail bands (Red-tailed Black Cockatoo and Glossy Black Cockatoo) lay clutches of only one egg and those with white or yellow tail bands (Carnaby's Cockatoo, Baudin's Cockatoo and Yellow-tailed Black Cockatoo) usually lay clutches of two. The breeding of the endangered Carnaby's Cockatoo has been studied from 1969 to 2012 at a number of localities throughout its range in south-western Australia within a region largely cleared for agriculture. When raising nestlings the species feeds on seeds of native vegetation, and there was a strong but not significant negative relationship between nesting success and percentage loss of native vegetation within 6 and 12 km of nest hollows. There was a significant negative relationship between the health of nestlings and percentage loss of native vegetation around nest hollows. While the usual clutch size is two, average clutch size tended to be lower in areas where much native vegetation has been cleared. While both eggs hatch in 77% of two-egg clutches, the species normally fledges only one young. However, the species is capable of fledging both nestlings from a breeding attempt. Sets of siblings are usually the product of older, more experienced females nesting in areas where more native vegetation has been retained. The conservation implications of these findings are discussed in the light of predicted changes to the climate of south-western Australia.
Highlights
In the Psittaciformes, despite the variation in the number of eggs in the clutches of some species, there is an inverse relationship between average clutch size and body weight (Saunders et al, 1984)
Red-tailed Black Cockatoo (Saunders, 1977; body weight 530–870 g) and Glossy Black Cockatoo (Garnett et al, 1999; 400–460 g) lay single-egg clutches, while clutch size is variable in Yellow-tailed Black Cockatoo (Saunders, 1979a; 505–900 g) and Carnaby’s Cockatoo (Saunders, 1982; 480–790 g; weights and clutch sizes from Saunders, 2009), but clutches of two predominate
Why produce two eggs in a breeding attempt when only one nestling is usually raised successfully? It has been postulated that in birds, clutch size is an adaptation to the largest number of young for which parents can provide enough food (Lack, 1954)
Summary
In the Psittaciformes, despite the variation in the number of eggs in the clutches of some species, there is an inverse relationship between average clutch size and body weight (Saunders et al, 1984). The five recognized species of black cockatoo in the genus Calyptorhynchus [viz. Red-tailed Black Cockatoo Glossy Black Cockatoo (C. lathami), Yellow-tailed Black Cockatoo (C. funereus), Baudin’s Cockatoo (C. baudinii) and Carnaby’s Cockatoo (C. latirostris)] do not conform to this pattern. Red-tailed Black Cockatoo (Saunders, 1977; body weight 530–870 g) and Glossy Black Cockatoo (Garnett et al, 1999; 400–460 g) lay single-egg clutches, while clutch size is variable in Yellow-tailed Black Cockatoo (Saunders, 1979a; 505–900 g) and Carnaby’s Cockatoo (Saunders, 1982; 480–790 g; weights and clutch sizes from Saunders, 2009), but clutches of two predominate. In Yellow-tailed Black Cockatoo and Carnaby’s Cockatoo, when both eggs hatch, the younger nestling usually comes from a smaller egg and dies within 1–2 days of hatching (Saunders, 1982; Higgins, 1999; Forshaw, 2002)
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