The Value of Native and Invasive Fruit-Bearing Shrubs for Migrating Songbirds

Abstract

The success of annual migrations for songbirds is greatly affected by habitat quality at stopover sites, particularly in relation to food needed for rapid refueling. The abundance and nutritional quality of important food resources may be linked to the presence of deciduous shrub species that provide seasonal fruits in the fall and support insects in the spring. The objective of this study was to determine whether migrating songbirds benefit from resources provided by native or invasive fruit-bearing shrubs found at 2 bird-banding stations in Rochester, NY. We conducted nutritional analyses (energy density, fat content, total soluble solids) on the fruits of common shrub species at the study sites, monitored removal of the fall fruits of focal native and invasive shrub species in the field, and measured the abundance of midges—a common insect resource for migrating songbirds— supported by the focal shrub species in the spring. The highest fat content and energy densities were found in fruits of native shrubs, ranging from 6.57 to 48.72% fat and 18.83 to 28.68 kJ/g of energy. All invasive fruits had ≤0.99% fat and ≤17.17 kJ/g of energy. We also found a significant positive correlation between fat and energy content of the fruits. Native dogwood fruits were consumed by migrating songbirds at higher rates than invasive fruits over the fall migration period. However, there was no clear pattern of midge abundance between native and invasive shrub species during the spring migration period. Our results suggest that fruits of native shrubs are of greater nutritional value to migrating songbirds than the fruits of invasive shrubs during fall migration, which is supported by the higher removal rates by songbirds of native dogwood fruits than fruits of the 4 other invasive fruit species. This finding suggests that removal of invasive fruit-bearing shrubs or plants will not negatively impact migrating birds when high-quality native fruit-bearing shrubs are available. However, additional study on the relative value of these shrubs in the spring and over multiple seasons is needed to provide insight into their overall value for birds during annual migrations.