Multivariate Analysis of Early and Late Nest Sites of Abert's Towhees

Abstract

Seasonal variation in nest site selection by the Abert's towhee (Pipilo aberti) was studied in honey mesquite (Prosopis glandulosa) habitat along the lower Colorado River from March to July, 1981. Stepwise discriminant function analysis identified nest vegetation type, nest direction, and nest height as the three most important variables that characterized the difference between early and late nest sites. Early nests were typically placed low on the southeastern sides of small shrubs or mistletoe, while later nests were built higher in mesquite and mistletoe. Although mesquite was a preferred nest shrub species after it leafed out in April, flexibility in nest site choice allowed towhees to initiate reproduction in early March. By breeding earlier than migratory songbirds, multi-brooded Abert's towhees lengthen their breeding season, and thus increase their chances of nesting successfully. Abert's towhee (Pipilo aberti) is a sedentary species endemic to the Southwest and is restricted to desert riparian habitats. Along the lower Colorado River, mesquite woodlands are its preferred breeding habitat (Anderson and Ohmart, 1976a), yet only anecdotal information is available