In many northeastern United States forests, invasive shrubs have moved into the understory displacing native trees and shrubs. The majority of these invasive species were originally brought over as horticultural specimens and have since escaped cultivation. Viburnum dilatatum and Viburnum sieboldii are two examples of horticultural plants that have become naturalized in northeastern forests. These two non-native species have expanded their range in forest understories for the past 30 years in the greater New York City region, central New Jersey, and the greater Philadelphia area. While they are not yet recognized as significant invaders, they have been noted as a local concern for invasiveness. The potential exists for these species to further expand their range through long distance seed dispersal by birds. Our two motivating questions are as follows: are the dispersal strategies employed by these closely related viburnum species comparable and if not, what are the potential implications for invasiveness? To answer our questions we conducted research at Lewis Morris County Park and Fosterfields Living Historical Farm in Morristown, NJ as both viburnum species are present. We conducted six bird point count surveys per viburnum species and operated game cameras around both viburnum communities from August-February to determine if migratory bird species (long distance dispersers) or residents (local dispersers) were consuming fruit. We quantified nutritional content (energy density and percent of crude fat) of each viburnum fruit to help explain any observed differences in bird fruit preference and consumption time.
Although both species fruit ripen at roughly the same time, we found V. sieboldii fruit is mainly consumed by Gray Catbirds (Dumetella carolinensis) during the fall migration and V. dilatatum fruit is mainly consumed by American Robins (Turdus migratorius) in the winter. Therefore we conclude that V. sieboldii has a higher probability for long distance seed dispersal because it is being dispersed by migratory species in the fall whereas V. dilatatum is being dispersed by resident birds in the winter and therefore is only spread locally. We also discovered that there is a difference in the average nutritional content between the two shrubs. V. sieboldii fruit has higher energy density, 21.78 kJ/g and fat content, 13.41% crude fat compared to that of V. dilatatum fruit that has 15.91 kJ/g of energy density and 1.98% of crude fat. This could mean that bird consumption time is largely driven by nutritional content of fruit, with higher nutrient fruit being eaten first.