NEST MAINTENANCE AND STONE THEFT IN (PYGOSCELIS-ANTARCTICA) .2. EFFECTS OF BREEDING GROUP-SIZE

The intensity of stone collection and stone theft by breeding Chinstra p penguins was measured, and estimations made of the number of stones per nest in large (> 400 nests) and small subcolonies (< 50 nests) in the large Vapour Col colony on Deception Island, South Shetland Island s. Stone availability was significantly higher both inside and outside small subcolonies. Penguins carried stones to the nest at the same ra te in large and small subcolonies, but stole more intensively in large subcolonies. Stones obtained by theft were significantly larger than those collected elsewhere. When stone availability was increased exper imentally, individuals of large subcolonies collected more intensively than control individuals in large and small subcolonies, and stole si gnificantly less than control individuals in large subcolonies, and as much as individuals in small subcolonies. The greater theft pressure in large subcolonies was accompanied by more aggressive defence by nes t owners and by reduced success in stealing. However, the reduced avai lability of stones on the ground near large subcolonies led to a signi ficantly lower number of stones per nest than in small subcolonies. Th ese results are interpreted in the light of the geometric effects of b reeding group size (perimeter to surface ratio) on stone accessibility .