Mistletoe-bearing trees positively influence bird foraging patterns across urban and natural environments

Abstract Worldwide, parasitic mistletoe species have become successful inhabitants of urban forests, despite novel host trees and communities of potential avian dispersers compared to natural habitats. Though land managers often remove mistletoe because of its impact on urban trees, these hemiparasites may nonetheless provide important food resources and structural heterogeneity for local avifauna. This study compares bigleaf mistletoe’s ( Phoradendron leucarpum spp. macrophyllum ) host distribution and relationships with birds in the urban and natural landscapes of California’s Central Valley. We established vegetation plots in urban parks and riparian forests to survey mistletoe distributions and measured avian visitation and behavior within individual trees. Mistletoe was detected in 10 of 32 surveyed tree species. Mistletoe presence did not vary across land-uses or tree origins (i.e., native vs. non-native trees); however, infestation levels were highest on larger and nonnative trees. Bird species richness and abundance were higher in trees with mistletoe, a pattern driven by insectivorous gleaning species. Birds were more likely to be observed foraging in trees with mistletoe in natural areas but not urban areas. Finally, birds observed foraging or gleaning were more likely to use mistletoe compared to birds observed perching. Our study reinforces ideas surrounding mistletoe’s host adaptability and importance for native birds. While prior work has focused on the importance of its berries and nectar, we found that mistletoe may also be important for invertebrate-gleaning birds. Looking forward, land managers should look to balance the biodiversity benefits of native mistletoe against potential effects on tree health.