Fruit availability and consumer demand within a major migration stopover area: The Lower Delmarva Peninsula
The lower Delmarva Peninsula is one of the most significant migration bottlenecks in eastern North America where large numbers of birds become concentrated within a relatively small land area. Habitats on the peninsula receive extremely high use by migrant landbirds during the fall months and are considered to have some of the highest conservation values on the continent. Over the past 20 years blocks of private land have been acquired by state and federal agencies for the stated purpose of restoring habitat for migratory land birds. This activity represents a sea change in both the character and purpose of this landscape and has the potential to improve the survivorship of many species of conservation concern. How to manage these lands to achieve maximum benefit to migrants continues to be an important question. Resource agencies have identified two management endpoints including mature forest and maintained shrubland intended to improve conditions for migrants on the lower Delmarva Peninsula. An important question within this landscape is what resources these conservation endpoints will provide to migrants. Fruit is an essential component of habitat quality for fall migrants. We performed more than 2,000 vegetation assays during the fall of 2014 to evaluate the composition and density of fruiting plants and the density of fruit production within reference forest and shrub patches. We monitored nearly 500 fruiting branches (12 species) supporting more than 24,000 fruits weekly during the study period to assess patterns in fruiting phenology. These branches were included in an exclusion experiment (bagged vs unbagged) that we used to evaluate rates of fruit loss, fruit consumption, and fruit preference. The seasonal schedule of fruit ripening varied dramatically between 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 walking stick, fox grapes and autumn olive had consumption rates of more than 15%/wk compared to hackberry, beautyberry and bayberry that were less than 5%/wk. Based on the first two months of the migratory period fruit species fall into three preference categories including high demand, medium demand and low demand. The two management endpoints (mature forest vs shrub) 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. 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. The reference forest patches used in this study are of high quality but atypical habitats within the lower Delmarva landscape. Most of the forest patches within the study area are much younger, have closed canopies, support fewer fruit-producing plants and should be managed within an open-canopy system in order to mimic the reference patches and produce higher fruit densities.