Fall Migration and Vehicle Disturbance of Shorebirds at South Core Banks, North Carolina.

Abstract

Anthropogenic disturbance has been implicated as a factor related to declines in shorebird populations because they depend upon coastal stopover sites, where human recreation is concentrated, for resting and refueling between long, energetically-expensive migration flights. We examined the use of South Core Banks, a barrier island on North Carolina’s Outer Banks, by migrating shorebirds and recreationists during fall and measured the effects of vehicle disturbance on shorebird behavior and habitat use. To describe spatial, temporal, and tidal patterns in shorebird and vehicle abundance, we performed weekly surveys of birds and vehicles from all-terrain vehicles, recording the species, numbers, and microhabitat locations (i.e. surf, swash zone, dry sand, and wet sand) of all individuals within half-mile ocean beach segments. We summarized survey data by week, tide, beach section, and daylight hour in order to identify patterns in abundance. Shorebird densities on South Core Banks were similar to those reported for other sites on the Outer Banks, and their numbers decreased slightly throughout the season, but peaked several times. Gull and vehicle numbers increased throughout the fall while tern numbers decreased. As a group, shorebirds were more or less evenly distributed along the southeast facing beach, but individual shorebird species showed unique spatial patterns in abundance. Several species, including Sanderlings (Calidris alba), Black-bellied Plovers (Pluvialis squatarola), Semipalmated Plovers (Charadrius semipalmatus), Red Knots (Calidris canutus), and Ruddy Turnstones (Arenaria interpres), were more abundant on the ocean beach during high tide than during low tide. They used a sand spit and a portion of the ocean beach on the southern half of the island as roosting sites at high tide. Shorebirds were abundant in areas where vehicle abundance was also relatively high, but their distribution among microhabitats was opposite that of vehicles; vehicles were primarily located on dry sand while shorebirds were typically found in the swash zone and wet sand microhabitats. Many environmental, habitat, and biological factors influence the distributions of nonbreeding shorebird, and they are often confounded. To examine whether or not vehicle disturbance is one of these factors, we employed a before-after-control-impact (BACI) experimental study design that isolated disturbance effects from spatial or temporal differences among sites. We manipulated disturbance levels within beach closures using paired control and impact plots and measured bird abundance and Sanderling behavior during before and after periods on both control and impact plots. Control plots were closed to vehicles during both the before and after periods. Treatment plots were closed to vehicles during the before period but subjected to a fixed level of vehicle disturbance during the after period. Differences in shorebird abundance and behavior between paired control and treatment plots provided an estimate of vehicle disturbance effects. We found that disturbance has a negative effect on site use by shorebirds, all birds, and Black-bellied Plovers. The two most abundant species of shorebird at our study sites, Sanderlings and Willets (Catoptrophorus semipalmatus), did not show a significant decrease in abundance in response to disturbance, but disturbance influenced Sanderling activity by decreasing the proportion of time that they spent roosting and increasing the proportion of time that they spent active. Microhabitat use shifted towards the swash zone when disturbance was introduced. We conclude that vehicle disturbance influences shorebirds’ use of ocean beach habitat for roosting during the nonbreeding season and that experimental BACI study designs provide a practical tool for measuring the effects of disturbance on wildlife without the confounding that affects purely observational approaches.