Nest site selection by the red-tailed tropicbird

-Red-tailed Tropicbirds (Phaethon rubricauda) nest under shrubs in loosely associated groups on Christmas Island, Central Pacific Ocean. Aggressive interactions among adults at nest sites are high; yet, large areas of superficially similar habitat contain no tropicbird nests at all. We compared physiognomic features of nest sites with those of two sets of randomly generated "nest sites" by discriminant function analysis. Of those variables measured, the number of stems within a nest space and the amount of peripheral cover influenced the tropicbirds' choice of nest sites. Differences among localities existed. Areas where tropicbirds nested had shrubs with few stems and more peripheral cover. Preferred nest sites were rare in areas other than those in which tropicbirds were nesting already. This study demonstrates the utility of the discriminant function technique in quantifying nestsite selection and nest-site availability when the appropriate null data sets are used.