Breeding biology of the Grey Shrike-thrush (Colluricincla harmonica)

Grey Shrike-thrushes (Colluricincla harmonica) occur in a wide range of habitats over most of Australia but their life history and breeding behaviour have not been fully documented. Over eight seasons, from 1996 to 2004, we studied the reproductive efforts of Grey Shrike-thrushes using primarily property homesteads as nest sites in the Warrumbungle Mountains near Coonabarabran, New South Wales, Australia. Twenty-four nests were observed at three sites, involving six male and six female birds. Eggs were laid at intervals of similar to 24 h and the clutch size was usually three (range 2 - 4). Parents contributed almost equally to incubation, brooding and feeding nestlings and removing faecal sacs during the day, whereas the female alone incubated and brooded at night. After the laying period, the birds combined to incubate the eggs for 93 - 97% of time. Incubation bouts averaged 41.5 min during laying, 50.7 min during early incubation, and 42.6 min (males) and 50.2 min (females) during late incubation. Mean length of bouts decreased to 10.5 min during the hatchling period and 13.9 min during the early nestling period. The mean incubation period was 17.4 days, the nestling period 15.8 days and the breeding cycle (incubation plus nestling) 32.5 days. Successful fledging of one brood was often followed by re-nesting, with up to three attempts in a single breeding season. The median period to re-nesting (last young fledged to first egg of new clutch) was 16 days, and median length of the breeding season was 18.3 weeks (4.2 months). There was a significant decrease in the length of the nestling period and the breeding cycle for later (second or third) attempts compared with first attempts. Habituated Shrike-thrushes (those nesting near houses; n = 22 attempts) fledged on average 2.7 young per nest (89% success rate). Experienced pairs of habituated Shrike-thrushes fledged an average of 3.1 young per nest compared with inexperienced pairs, which averaged 1.3 young per nest.