Occurrence and diet of a Barn Owl (Tyto alba) in the sub‐Antarctic

Abstract

A Barn Owl Tyto alba was recorded at Grytviken, South Georgia (54°15′S, 36°45′W), in the South Atlantic during 1997 and 1998. Although Barn Owls are known to breed on the Falkland Islands (52°S, 58°W), this is the first known record of this species in the sub-Antarctic. In the 1997 austral winter a bird was found roosting in an old freezer plant in the abandoned whaling station at Grytviken. There were no sightings during the summer but a bird was again seen regularly from 13 July 1998 until 6 October 1998. Pellets were collected from the roost site from 30 August to 6 October 1998. After this date there were no fresh pellets and no subsequent sightings. A total of 19 intact pellets was examined and found to contain the skulls, bones and fur of the Common Rat Rattus norvegicus. Nine of these pellets contained single intact skulls, along with skeletal remains. Skulls were absent from the remaining 10 but most contained a set of rat limb bones. A further group of pellets that had become partially decomposed and therefore could not be separated into individual pellets contained a total of seven intact skulls. The body mass of rats taken was estimated from the length of the left jaw (Morris 1979). The mean jaw length was 22.3 mm (se = ±0.26 range 20.3–24.7) which gave an estimated body mass of 119 g (se = ±6 range 75–189 g). The origin of the Barn Owl was unknown. However, the nearest breeding site of the subspecies T. a. tuidara is in the Falkland Islands, 1400 km away (Woods & Woods 1997). Barn Owls have occasionally been recorded travelling large distances. A bird ringed as a nestling in New Jersey in 1981 was recovered in Bermuda 20 months later, a cross water journey of 1250 km (Soucy 1985). It is possible that migrations of this distance are aided by ship travel. Mueller and Berger (1959) reported a Barn Owl that had been ringed as an adult in Wisconsin and captured on a ship 360 km east of Savanna, Georgia, USA. Previous reports show that birds have travelled to South Georgia onboard ships. Examples of these include a Peregrine Falcon Falco peregrinus that came aboard a ship and flew ashore on South Georgia in 1986 and 26 Cattle Egrets Bubulcus ibis in 1993 that joined the RV Nathanial Palmer half way between the Falkland Islands and South Georgia and flew ashore at Stromness (Prince & Croxall 1996). It therefore seems likely that a species such as a Barn Owl travelled in a similar way and it may be no coincidence that Grytviken is the most frequently visited location by tour ships and fishing vessels on South Georgia. The survival of the Barn Owl in sub-Antarctic conditions was possible because of suitable roost sites provided by the disused whaling station and an abundant food supply of rats that were introduced by sealers and whalers in the 1800s (Pye & Bonner 1980). The only other introduced rodent present on South Georgia is the House Mouse Mus musculus that was found to occur in an isolated population at Queen Maud Bay more than 60 km from Grytviken (Berry et al. 1979). Rats are found in the diet of Barn Owls throughout the world (Taylor 1994). Because of their size rats provide a considerable energy source and it would appear that this allowed either one individual to survive for more than a year or two birds for several months. However, Barn Owls appear to be poorly adapted to low temperatures, high wind speeds and prolonged snow cover (Taylor 1994). The Barn Owl may have the ability to colonize new areas by long-distance displacements of far-ranging birds. However, the disappearance of this individual(s) suggested that the present climatic conditions of the sub-Antarctic were too extreme for long-term survival.