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

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.