The comparative ecology of Australian corvids. III. Breeding seasons

Abstract

This paper compares the breeding seasons of the five species of Corvus inhabiting Australia. The more sedentary species (C. coronoides, C. orru, and C. tasmanicus) tend to have regular annual breeding seasons. The more nomadic species (C. bennetti and C. mellori) tend to breed in late winter and early spring but if this time is unfavourable they may respond to suitable environmental stimuli (usually rainfall) whenever they occur. All species have an immature period of 2 or 3 years. Adult males actively produce sperm over a 10-week period each year. In females the ovaries rapidly regress after egg-laying but retain the capacity to redevelop. The species of Corvus studied were typically single-brooded and in the event of failure a replacement clutch rarely occurred later than 5 weeks after the first eggs had been laid. Only the female incubates and her brood patch develops when the first egg is laid. In Australian Corvus the colour of the eye changes from brown to white with age, and in each species only birds with white eyes were found breeding. From this observation and records of aviary birds of known age it was concluded that C. orru, C. bennetti, and C. mellori do not breed until they are more than 2 yr old, and C. coronoides and C. tasmanicus more than 3 yr of age. Observations in the field on known-age birds banded as nestlings showed that breeding rarely occurred before a bird of any species was 3-4 yr of age.