Fruit availability and consumer demand within a migration bottleneck
September 27, 2016
By Bryan Watts, Center for Conservation Biology
During the fall of 2014, The Center for Conservation Biology, along with the U.S. Fish and Wildlife Service, conducted a study to examine the availability of fruit and which fruits were important to passerine migrants on the lower Delmarva Peninsula (download fruit availability report). CCB technicians Sarah Rosche and Arianne Millet performed more than 2,000 vegetation assays to evaluate the composition and density of fruiting plants and the density of fruit production within selected forest and shrub patches. They monitored nearly 500 fruiting branches of 12 species that supported more than 24,000 fruits weekly during the study period to assess patterns in fruit ripening. These branches were included in an exclusion experiment (covered versus exposed) that we used to evaluate rates of fruit loss, fruit consumption, and fruit preference.
The seasonal schedule of fruit ripening varied dramatically between plant species such that the availability of ripe fruits changed during the migration period. Some of the fruits, including American holly and hackberry, matured too late to have relevance for most migrants. Based on the exclusion experiment, an index of consumption varied significantly between fruit species. Sassafras, devil’s walkingstick, fox grapes, and autumn olive had consumption rates of more than 15% per week compared to hackberry, beautyberry, and bayberry that were less than 5% per week. Fruit species fall into three preference categories including high demand, medium demand, and low demand. Migrants stripped virtually 100% of fruits in the high demand category during the height of the migration season.
Mature forest and shrub patches differ dramatically with respect to the composition of the fruiting plant community, plant density, fruit density, and the extent to which they support preferred fruit species. Although fruit density within shrub habitat was more than ten-fold higher than forest patches, 95% of the crop is of low demand or is produced by an exotic invasive species. Shrub patches should be managed to broaden out the fruiting plant community to include preferred fruit species. Management prescriptions should be developed that drive the footprint of the less desirable plants down and expand the more desirable elements.
This field study follows a previous study by CCB that examined metabolic demand by migrants stopping over on the Lower Delmarva Peninsula and conservation limits (download conservation limits report). These projects have been completed to inform land management within this important staging site.