"Bird populations and distributions are known to be influenced by environmental factors (e.g., Graber and Graber 1979; Cawthorne and Marchant 1980; Root 1988a, b, c; Arcese et al. 1992; Mehlman 1997). Nevertheless, 'whether and how most animal populations are regulated remains one of the principal unanswered questions in ecology' (Rodenhouse et al. 1997). Another major gap in our understanding concerns the scale over which species' populations fluctuate (Holmes et al. 1986)