Nesting biology of the spotted catbird, Ailuroedus melanotis, a monogamous bowerbird (Ptilonorhynchidae), in Australian Wet Tropics upland rainforests

Abstract

Spotted catbird, Ailuroedus melanotis, nests (104 active and 59 old) were studied on the Paluma Range, north Queensland during 1978–89. Nesting began in September: 97% of clutches were incubated during October–December, 83% of nests had nestlings during November–December, and 70% of nestlings fledged in December. Nests were 3.1 1.3 m (n = 119) above ground. Nest dispersion was clumped, reflecting preference for particular topography and traditional use of a site over years. Four individuals used the same nest location for at least four consecutive seasons. Mean nearest-neighbour distance between 30 nests, built at 11 traditional locations over different seasons, averaged 42.0 31.4 m. At least 32 pairs nested within 50 ha in season 1980–81, at an average of one pair per 1.6 ha (home range) and at a mean nearest-neighbour nest distance of 96.9 24.8 m. Parents foraged at a mean of 68 36 m from their nests. Two replacement nests were built and laid in within 16 days of the loss of a brood. Eggs were laid 9–29 days after nest completion. Mean clutch size was 2.0 and eggs were laid on alternate days. At laying, eggs averaged 11% of mean adult female weight. Only females built nests, incubated, and brooded. Males fed females and nestlings. Females sometimes incubated before clutch completion. Median and mean incubation periods were 22–23 days. Eggs were incubated for 62% of total diurnal time, bouts averaging 19 min at a frequency of 2.0 per hour. Median nestling period was 19–20 days, the mean 20 days, when parents spent an average of 48% of diurnal time at their nest. Visits to nestlings averaged 4.9 min at a frequency of 5.9 per hour. During the first 15 days of nestling life, females spent 12% more time brooding a single nestling than a larger brood. A single nestling was provided with an average of 3.9 meals per hour and a larger brood 5.7 meals per hour. On average, females delivered 63% of all meals (n = 602) to broods at a rate of 3.3 per hour, males doing so at an average of 2.0 meals per hour. Of 323 identified nestling meals, 80% were of fruit (at least 33 plant species, of which 29 were identifiable) and 20% animal (of 182 meals 54% were insects, 21% pieces of birds). Proportionately more fruit was fed to older nestlings. Of 137 eggs laid in 72 nests, 88% hatched and 60% of nestlings left the nest; 46% of all eggs produced fledglings, averaging 1.0 per nest. Overall nest success rate was 51%, but of only those nests for which the fate was known it was 57%. The success rate of nests for which the fate was known during an exceptionally dry breeding season was 8%, with only 5% of eggs laid producing fledglings, at a rate of 0.1 per nest

The nesting biology of the spotted catbird is compared with that of the green catbird, A. crassirostris, and found to be similar except in mean nest height and in proportions of fruit/animal foods fed to nestlings. The nesting biology of these two monogamous species is compared with that of the polygynous bowerbirds, and the significance of Ficus figs to the evolution of catbird monogamy discussed. The limited extent of nest attendance by the male catbird is discussed in the context of the evolution of contrasting bowerbird mating systems.