The skinks Mabuya sechellensis and M. wrightii scavenged among the seabird colonies of Cousin Island. particularly under the colony of Lesser noddies Anous tenuirostris which, in 1979, contained about 60.000 pairs. The density of skinks in different areas of the island did not vary greatly through the year. The mean density of M. sechellensis was 1393 individuals/ha equivalent to a biomass of 21.7 kg/ha and of M. wrightii 320 individuals/ha equivalent to 17.7 kg/ha. These biomass figures are the highest recorded for any terrestrial lizard population. Skink and Lesser noddy density were closely correlated. The skinks deposited fat equivalent to up to about 50% of their lean dry weight during the noddy nesting season but were leaner and lighter at other times. Our calculations of the amount of skink food emanating from the noddy colonies, in the form of faeces, dropped fish and eggs suggest that there is sufficient to maintain a skink population of the density observed.
The skinks bred throughout the year, the larger M. wrightii more frequently than the smaller M. sechellensis, and the young/adult ratio was higher in M. wrigh:htii. The possible relevance of these differences to the two species' coexistence is discussed. The young/adult ratio varied according to skink density; in both species few young were found at high density.
Mean home range sizes of M. sechellensis and M. wrightii were 26·6 and 78· m2, respectively.