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

On 23 and 24 November 1997, SNGH saw four kleptoparasitism attempts by Brown Skuas on Yellow-nosed Albatrosses. Three of the attempts occurred while the ship was anchored 1 km from Amsterdam Island (37 deg 50'S, 77 deg 33'E); fishing activity by the ship's crew had attracted about 50 Yellow-nosed Albatrosses, 2 Shy Albatrosses (Thalassarche cauta), about 20 White-chinned Petrels (Procellaria aequinoctialis), and 3 Brown Skuas (C. a. hamiltoni). A successful attempt occurred when a subadult Brown Skua descended at an angle of 30 from 30 m above and behind an adult Yellow-nosed Albatross that was gliding (with occasional flapping) 1 m asl and carrying a piece of offal in its bill. The skua did not strike the albatross but caused it to adopt an erratic course and increase its speed using continuous flapping. The skua pulled up to 10 m and made a second stoop, causing the albatross to drop the offal, which the skua caught and swallowed in flight. The second and third attempts occurred 20 min later when an adult Brown Skua briefly chased two different adult Yellow-nosed Albatrosses that were carrying offal. In both cases, up to eight other Yellow-nosed Albatrosses followed behind the skua and the birds with the offal. When the latter landed on the water, surrounded by squabbling albatrosses, the skua was reluctant to approach and discontinued the chase. The fourth attempt occurred at St. Paul Island (38 deg 43'S, 77 deg 32'E), 95 km from Amsterdam Island, on 24 November 1997. SNGH and JB saw a Brown Skua stoop on an adult Yellow-nosed Albatross from about 350 m asl at an angle of 30 as the albatross was gliding along the island's crater rim. The albatross, which was not carrying prey, took evasive action (increased flapping with erratic course) as the skua pulled up to make a second stoop before departing.

The size ratios of the Brown Skuas and their "small" albatross hosts that we observed at Amsterdam, St. Paul, and Mayes islands (host:skua = 1.4:1 and 1.9:1; Table 1) were similar to those of prior observations where the hosts were larger than the parasites (range 1.1:1 to 2.4:1). Our observations of kleptoparasitism by Brown Skuas on small albatrosses were also similar to those of skuas chasing Northern Gannets (Morus bassanus), Masked Boobies (Sula dactylatra), Imperial Shags (Phalacrocorax atriceps), Common Murres (Urin nalge), and Razorbills (Alca torda) because in each case (i.e. 6 of the 13 parasite-host relationships; Table 1), the skuas operated near breeding colonies of their hosts. This facilitated frequent attempts, thus increasing the odds for success with minimal energy expenditure when searching for potential hosts.

Although three of the five attempts by Brown Skuas on albatrosses were in an unnatural context (human fishing activities), the occasional use of this behavior in a natural context suggests that attempted kleptoparasitism on species of smaller albatrosses is not unusual. This conclusion is consistent with observations elsewhere (Table 1), in which host species weighing up to 2.4 times more than skuas were kleptoparasitized occasionally (but see below). However, even when considering the aggressiveness, strength, and tenacity of skuas, the attacks on the Wandering Albatross and the Northern Giant-Petrel were unexpected because the Crozet-Kerguelen form of Brown Skua averages six times smaller than the albatross, and the formidability of giant-petrels is well known. For example, during land-based confrontations over carrion, giant-petrels dominate all other avian species, including skuas (L. Spear, S. Howell, and J. Bried pers. obs.). Similarly, during confrontations over dead prey at sea, the giant-petrel was the only species (others involved were Black-browed [Thalassarche melanophris], Gray-headed [T. chrysostomal, and Sooty [Phoebetria fusca] albatross) able to exclude the Wandering Albatross (Weimerskirch et al. 1986), which averages 2.3 times larger than the petrel (Table 1).