Tom McGinnis
One of my study sites. Royal Terns at a nesting colony.
Royal and Sandwich Terns nest together in dense colonies on small isolated dredge spoil islands in North Carolina. Presumably both species benefit by the added protection from nest predators, since several terns confront intruders. If food is a limiting resource and both species compete for the same prey, competitive exclusion would be expected. I observed that the foraging habitats of these species do not substantially overlap during the nesting season. Sandwich Terns primarily forage along the ocean shore, while Royal Terns seem to prefer the estuary. Oxygen isotope ratios in eggshells support these findings.
Eggs of Sandwich Tern (left, 5cm long) and Royal Tern (right, 6.5cm long).
Seawater contains a higher concentration of 18O than fresh water due to the selective evaporation of the lighter 16O isotope in the hydrologic cycle. Carbon and Nitrogen isotope concentrations also will be analyzed to complete the signatures of foraging habitats. Eggshell analysis is non-traumatic to the birds, as only left-over fragments are collected at seabird colonies.
Shore-based foraging surveys were conducted from fixed points along the Cape Fear River and the shore of Onslow Bay near Wilmington, North Carolina. The next step will be to study these terns at sea.
Seabirds
will be surveyed from the RV Cape Fear once
per month as part of an interdisciplinary study of Onslow Bay funded by the
National Oceanographic and Atmospheric Administration (NOAA).
Cruises will conducted from the Intracoastal Waterway near Wilmington and
cross the western Gulf Stream front near Frying Pan Shoal.
The location of the Gulf Stream will be determined by Advanced Very High Resolution Radiometry (AVHRR) imagery, which detects sea surface temperatures. Chlorophyll a concentrations measured in the field will be compared to Sea-viewing Wide Field-of-view Sensor (SeaWiFS) images.
The warm Gulf Stream flows northeast by the North Carolina coast. Long Bay is the first bay near the bottom of this AVHRR image. Onslow Bay is in the middle, and the line of black to the North is the Outer Banks.
The Gulf Stream flows close to the Outer Banks colonies, where David Lee of the North Carolina State Museum of Natural Sciences regularly observed Royal Terns carrying fish toward colonies. Since the Gulf Stream is farther offshore in the southern part of the State, royals may forage primarily in the estuary and river plume, but take advantage of high production along eddies which occasionally spin off the Gulf Stream into Onslow Bay.
This will be the first in-depth ecological comparison of Royal and Sandwich Terns in North America. Foraging differences may be seen in these species since ecological segregation occurs when similar species compete for limiting resources. This study will contribute to our understanding of two seabird species with notable breeding colonies along the east coasts of North and South America.
At
my half-way point here at UNCW I
thank my thesis committee chairman/advisor, Steve Emslie and committee members
Mike Durako and Martin Posey for their advice. Thank you Lisa McGinnis,
Kelly McGinnis, Ellen Wambach, Terri Maness and John Hackney for help in the field,
David Allen of NC Wildlife for logistic support, data collection and granting
access to the northern tern colonies and Walker Golder of the Audubon Society
for allowing access to the breeding colonies he administers. Fred Bingham
helped with AVHRR imagery. NASA, UNCW's Center for Marine Science Research, and
Sea Grant generously provided financial support.