NEST-SITE CHARACTERISTICS AND REPRODUCTIVE SUCCESS IN MAGELLANIC PENGUINS (SPHENISCUS-MAGELLANICUS)

We used cross-sectional, longitudinal, and experimental data to invest igate the effects of habitat at the smallest spatial scale-the nest si te-on reproductive success of Magellanic Penguins (Spheniscus magellan icus). Over an eight-year period, the amount of nest cover was positiv ely correlated with fledging success. The same pairs tended to be more successful when they had more nest cover, and experimental increases and decreases in cover significantly affected survival of nest content s. Other characteristics of nest sites, such as nest type and type of vegetation over the nest, did not affect success. The positive effect of cover resulted mainly from reduced exposure of nest contents to pre dators during incubation and to high temperatures when chicks were you ng. Roof cover was positively correlated with fledging success in nest s from all areas. Cover on the sides of the nest giving the most prote ction from the sun was positively correlated with fledging success in warmer sites and with survival of young chicks in all areas. Young chi cks at nests with less cover were more likely to move from their nests and to die on hot days. Experimental results indicated that the likel ihood of egg detection bq predators decreased with increasing nest cov er Height of nest entrance was a significant predictor of egg loss, su ggesting that accessibility of nest contents to predators was an impor tant component of predation risk. Thermal properties of nests and risk of predation were related; predation of nest contents was more likely when adults were absent, and during hot weather adults were more ofte n absent from nests with little cover. Although the effect of cover on success was small relative to the large yearly variation in success d ue to food conditions, cover is likely to influence lifetime reproduct ive success substantially. Large and long-term data sets and experimen tal approaches may be necessary to identify subtle but biologically im portant factors among long-lived organisms that inhabit variable envir onments.