Kleptoparasitism by Brown Skuas on albatrosses and giant-petrels in the Indian Ocean

Auk, The, Apr 1999 by Spear, Larry B, Howell, Steve N G, Oedekoven, Cornelia S, Legay, Delphine, Bried, Joel

The behavior of skuas and jaegers (subfamily Stercorarinae) when kleptoparasitizing other seabirds has been studied in detail at breeding colonies (Furness 1987a, b; Maxson and Bernstein 1982; Le Corre and Jouventin 1997), at a coastal estuary along a migration route in Britain (Taylor 1979), in boundary currents off Peru and south Africa (Duffy 1982), and in pelagic waters of the Eastern Tropical Pacific (Spear and Ainley 1993). The size of the host is usually smaller than that of the kleptoparasite. Indeed, only 9 of the 40 host species recorded had a mean body mass greater than that of the parasite (mean body mass of the host was 1.1 to 2.4 times greater than that of the kleptoparasite; Table 1).

Attacks by skuas on giant-petrels and albatrosses have not been described. Therefore, we were surprised to see attempted kleptoparasitism by Brown Skuas (form from the Crozet and Kerguelen archipelagoes = Catharacta antarctica lonnhergi sensu Devillers 1978, Mailing-Olsen and Larsson 1997; but see Peter et al.1994) on a Wandering Albatross (Diomedea exulans) and a Northern Giant-Petrel (Macronectes halli), because these hosts are 5.9 and 2.6 times larger, respectively; than the skuas (Table 1). We also report an attempted kleptoparasitism by a Brown Skua on a Light-mantled Albatross (Phoebetria palpebrata) in the Kerguelen Archipelago, and four attempts by Brown Skuas (the Amsterdam Island [Indian Ocean] form, which is similar in size to the Tristan Brown Skua [C. a. harniltoni]) on Yellow-nosed Albatrosses (Thalassarche chlororhyochos bassi) adjacent to the primary breeding colony of the latter at Amsterdam Island.

The kleptoparasitism attempts on the Wandering Albatross and Northern Giant-Petrel were observed while we surveyed seabirds from the R/V Marion Dufrense in the southwestern Indian Ocean. The incident involving the albatross happened on 9 November 1997 at 44 deg 09'S, 51 deg 26'E (238 km from the Crozet Islands), and that involving the giant-petrel occurred on 14 November 1997 at 49 deg 22'S, 66 deg 43'E (147 km from the Kerguelen Islands). LBS, CSO, and SNGH saw both attempts from the flying bridge; DL saw the first attempt from the bridge.

The Brown Skua that attacked the albatross was in its first year, based on uniform dull-brown feather color, no dorsal flecking or hackle sheen on the neck feathers, and a bicolored bill. The skua making the attempt on the giant-petrel was a subadult (i.e. older than one year but not an adult) based on presence of light flecking on the dorsal surface but absence of prominent hackles. Based on plumage, both the albatross and the giant-petrel were subadults or adults.

The attack on the Wandering Albatross was initiated from 20 m above sea level (asl). Using continuous flapping flight, the skua descended at an angle of about 350 directly toward and from behind the albatross, which was gliding 2 to 3 m asl. The skua did not strike the albatross, although the latter began continuous flapping flight, increased its speed, and initiated evasive maneuvers using an irregular flight path, including a 1600 change in course. The skua usually remained 0.5 to 2 m behind the albatross, but several times it grabbed the rectrices or inner secondaries of the albatross. The most aggressive contacts occurred on the two occasions that the albatross tried to increase its altitude. The chase ended unsuccessfully (i.e. the skua obtained no food) after a chase of 1 to 1.5 min, just after the skua had pulled vigorously on the tail of the albatross for 2 to 3 s.

The attack on the Northern Giant-Petrel also was unsuccessful and was initiated from 30 m asl at an angle of descent of about 45 deg. The giant-petrel was gliding with periodic wing beats at 2 to 3 m asl. The skua did not strike the petrel but came within 0.5 m as it swooped over and past it. The skua then increased its altitude steeply to 10 m above and beyond the petrel, reversed direction, increased its speed with strong flapping, and descended again. Upon reaching the petrel, the skua grabbed its scapulars or inner secondaries as the petrel tried to increase its altitude. Upon being grabbed, the petrel dropped to the water from 3 m asl. The skua hovered for several seconds 2 m above the petrel and then departed.

On 29 December 1993, JB observed a probable attempted kleptoparasitism by an adult Brown Skua on a Light-mantled Albatross at Mayes Island (49 deg 28'S, 69 deg 57'E) in the Kerguelen Islands. The skua stooped on the albatross from 60 m asl at an angle of about 40 deg as the albatross was flying 25 to 30 m asl along an island ledge 300 m inland from the sea. When the skua tried to grab the nape of the albatross, the albatross began evasive flight and uttered a shrill call but did not regurgitate any food. Upon completing the attack, the skua returned to its territory on an adjacent slope. The attack may have been an act of territory defense, but this was unlikely because skuas at Mayes Island were not seen attacking albatrosses during frequent flights of the latter over the skuas' territories.