Partial loss of migratory behaviour by Torresian Imperial-Pigeons Ducula spilorrhoa in Darwin, Northern Territory

The Northern Territory population of the Torresian Imperial-Pigeon Ducula spilorrhoa was once considered to be completely migratory, departing for New Guinea in February and returning to breed in August. However, reporting rates from Darwin suggest that since at least the early 1990s a substantial proportion of birds 'over-winter' in the region. We monitored an evening roost-site each month from May 2011 to August 2014, counting birds in 5-minute intervals as they arrived. Mean numbers were highest during the late wet-early dry season transition months (March through May) and lowest during the late dry-early wet season transition period (September through November), although the maximum number (1325 birds) was counted in June 2011, following the wettest wet season on record. These observations confirm that many Torresian Imperial-Pigeons do not migrate to New Guinea after the breeding season, but it is not yet known if any individual birds are long-term residents. It is possible that residency in the early dry season has evolved because of year-round availability of food resources in the urban environment but we were unable to test this hypothesis.