Breeding behaviour of a pair of Spotted Harriers Circus assimilis near Tamworth, New South Wales

Abstract

A pair of breeding Spotted Harriers Circus assimilis was observed in an agricultural environment near Tamworth, New South Wales, in winter–spring 2020 for 290 hours over 68 days from pre-laying and incubation to the post-fledging period. Laying occurred at the end of June, hatching at the beginning of August and fledging in mid September. Only the female incubated and brooded, and the male supplied all prey from the incubation period to late in Week 3 of the nestling period, after which the female contributed. Two chicks fledged of three that hatched; there was some sibling competition for food. The incubation period lasted 34 ± 1 days, and the nestling period 43 days for the older (female) chick and c. 40 days for the younger (male) chick. Prey consisted of ground-dwelling small mammals, birds and reptiles that are typical of agricultural land. The male’s prey-delivery rate to the incubating female was 0.22 item/h. Overall, the male delivered 0.31 item/h and the female 0.09 item/h to the nestlings, for a combined average of 0.4 item/h, peaking in the pre-fledging fortnight. The combined rate in the post-fledging period was 0.39 item/h (Week 1), declining to 0.16 item/h to the fledglings in Week 2 when only the male delivered prey and the adult female consumed some of his catch. Estimated biomass delivered ranged between ~20 and ~100 g/item (often 20–50 g/item). Breeding behaviour and nestling/fledgling growth and development are described. The juveniles appeared to be dependent or partly so for at least 3 weeks, and apparently roosted in the nest area in Week 4, after which they could not be located.