Ecological determinants of the distribution, abundance and breeding success of phainopeplas (Phainopepla nitens, Cl. Aves) at the northern edge of their range

In the northeastern Mojave Desert, phainopeplas are apparently declining. There, phainopeplas and their principal food, desert mistletoe berries, reach the edge of their range and may experience large cold- and drought-influenced fluctuations in abundance and reproductive success. Furthermore, their habitat is undergoing rapid fragmentation. Yet the relative importance of mistletoe abundance, woodland size and other factors in determining phainopepla abundance and breeding success is unknown. Therefore, I surveyed phainopepla presence and density, mistletoe abundance and habitat structure along 70-76 line transects in woodlands of different sizes in the winter and breeding seasons of 2000-2003. I examined nest site selection and reproductive success at two spatial scales within eight woodlands. In 2003, I experimentally removed half the mistletoe berries from six phainopepla territories and compared their reproductive success to eight control territories. Phainopeplas occupied most woodlands for at least part of the year. Spring occupancy was limited to a more restricted set of woodlands. Mean density differed significantly between 2002 (1.4 phainopeplas/ha) and 2003 (0.89 phainopeplas/ha). Patch occupancy was significantly positively correlated with berry abundance in both seasons, and negatively correlated with tree height in the spring. Density in both seasons was significantly positively correlated with berry abundance and the interaction between berry abundance and woodland area. Within woodlands, breeding phainopeplas selected tall nest trees (n=147) with abundant berries and dense cover, and nest patches (n=138) containing many tall, infected trees and abundant berries. Nest survival (n=160 nests) was strongly positively correlated with nest tree height and cover, strongly negatively correlated with nest tree mistletoe volume, and weakly positively correlated with density, height and berry abundance of trees in the nest patch. Factors that predicted nest site selection and nest survival generally corresponded, likely because they reduced predation, the major cause of nest failure. Nest survival and breeding pair density also were positively correlated with patch size. Nest survival in the removal treatment was lower, due to nest predation, suggesting that food availability and predation risk may have interactive effects on reproductive success. Combined, these results indicate that conservation efforts for phainopeplas must focus on multiple factors in numerous large sites.