--African Penguins (Spheniscus demersus), which hrecd more successfully in burrows than on the surface (Frost ctal. 1976a), arc subject to heat stress, which can cause them to desert their nests (Randall 1983, Williams and Cooper 1984). As burrow-nesting reduces the effects of ambient temperature extremes experienced by surface-nesting birds (Frost et al. 1976b), the incidence of surface-nesting would be expected to be higher in the austral summer than in winter when ambient temperatures arc higher and insolation most intense. Also, as burrows in sandy soil are prone to collapse and flood during heavy rains (Frost et al. 1976b, pets. ohs.), a higher incidence of surface-nesting would be expected in sandy areas. l tested these two hypotheses during visits to Dassen Island (33ø25'S, 18ø05'E), on the west coast of South Africa, where peak hrccding occurs in the austral winter (June) and summer (November- December) (Cooper 1980). From 1983 to 1985, two 200-m 2 burrow-nesting sites were checked for occupied nests. 132 THE WILSON BULLETIN ß Vol. 100, No. 1, March 1988 Site 1 was in an area of sandy soil; Site 2 was in an area of firmly packed gravel. The numbers of surface nests and burrows in use at each site were counted during five visits in winter (June-August) and summer (February). African Penguins nested on the surface at sandy Site I more frequently in winter (19 surface nests, 111 burrows) than in summer (3 surface nests, 60 burrows) (X 2 = 4.08, df = 1, P < 0.05). Season had no effect on the number of surface nests at Site 2 (X 2 = 0.025, df = 1, P < 0.01) (winter: 3 surface nests, 97 burrows; summer 3 surface nests, 85 burrows), which is not susceptible to burrow-flooding and collapse. In winter a significantly greater proportion of penguins nested on the surface at Site 1 (the sandy site), which was susceptible to burrow-flooding and collapse than at Site 2 (X 2 = 8.81, P ( 0.01, df = 1). In summer there was no significant difference in the proportion of surface nesters between the two sites (X 2 = 0.176, P > 0.05, df = 1). The results indicate that, for African Penguins, ambient temperature and insolation affect the proportion of the population that occupies surface nests. Scolaro (1984) found that Magellanic Penguins (S. magellanicus) show a tendency to nest in soils of high clay content compared to soils of low clay content. The results of this study confirm the tendency of penguins preferentially to burrow in suitable substrata and suggest that the quality of the burrowing substratum is important for spheniscid penguins. Acknowledgments.--I thank the Sea Fisheries Research Institute for transport to and accommodation on Dassen Island, and D.C. Duffy and S. Hunter for commenting on the manuscript. This work forms part of the Benguela Ecology Programme, sponsored by the South African National Committee for Oceanographic Research. LITERATURE CITED COOPER, J. 1980. Breeding biology of the Jackass Penguin with special reference to its conservation. Pp. 227-231 in Proc. 4th Pan-African Ornithol. Congr., Mahe 1976. South African Ornithological Society, Johannesburg, South Africa. FROST, P. G. H., W. R. SIEGFRIED, AND J. COOPER. 1976a. Conservation of the Jackass Penguin (Spheniscus demersus) (L.). Biol. Conserv. 9:79-99. --,ADA.E. BuRGER. 1976b. Behavioural adaptations ofthe Jackass Penguin Sphenicus demersus to a hot, arid environment. J. Zool. Lond. 179:165-187. RANIALL, R.M. 1983. Biology of the Jackass Penguin (Spheniscus demersus) (L.) at St. Croix Island, South Africa. Ph.D. diss. Univ. Port Elizabeth, Port Elizabeth, South Africa. SCOtARO, J. A. 1984. Biologia y seleccion del habitat de reproduccion de Spheniscus magellanicus, en Patagonia, Argentina. Ph.D. diss. Univ. Cordoba, Argentina. WILLIAMS, A. J. AND J. COOPER. 1984. Aspects of the breeding biology of the Jackass Penguin Spheniscus demersus. Pp. 841-853 in Proc. 5th Pan-African Ornithol. Congr., Lilongwe 1980. South African Ornithol. Society, Johannesburg, South Africa. G. D. LA COCK, Percy FitzPatrick Institute of African Ornithology, Univ. Cape Town, Ron- debosch 7700, South Africa. Received 4 Apr. 1986, accepted 21 Aug. 1987.