Royal Terns (Sterna maxima) on the
mid-Atlantic Coast in the Eastern U. S.

Steven D. Emslie 
Department of Biological Sciences
University of North Carolina
Wilmington, NC  28403

Introduction

The Royal Tern (Sterna maxima) is a large seabird that occurs along coastal regions in the Atlantic and eastern Pacific oceans where it forms dense breeding colonies on isolated beaches or islands.  It migrates to Central and South America in winter, traveling as far south as the coasts of Peru and Argentina, before returning to breeding grounds to the north in early spring.  In the mid-Atlantic region of the eastern U. S., Royal Terns breed primarily on dredge islands on rivers and estuaries from Maryland to Florida.  In North Carolina, Royal Terns breed near inlets at Cape Hatteras, Cape Lookout, and on the Cape Fear River where it feeds primarily on larval fish that spawn in estuaries.  At least five colonies are formed each year in these regions and number from hundreds to thousands of breeding pairs per colony.  All colonies are protected from human disturbances by the North Carolina Wildlife Resources Commission and the local chapter of the National Audubon Society.

In summer 1999, the author initiated investigations of Royal Terns with graduate and undergraduate students at UNCW.  The primary focus of these investigations has been the foraging ecology of this species, especially in relation to fishery stock abundance and annual variation in the marine environment.  Other studies have been on the impact of pollutants and toxic compounds ingested by adults and chicks near their breeding sites, annual variation in chick growth by colony, and sex ratios in adult and juvenile terns as determined by blood analyses.  

In addition, it has been possible to study age-related foraging and breeding success by known-age individuals for the first time in this species.  Since the 1960’s, Royal Tern chicks have been banded at their breeding colonies using standard USFWS metal leg bands.  Each band has a unique number stamped on it that identifies where and when the bird was banded.  Dr. John Weske has been actively banding most tern chicks at North Carolina breeding colonies for the past 15-20 years.  As a result, up to 80% of adults found breeding at these colonies today are banded, known-age individuals.  Thanks to these efforts, it is now possible to study birds of different ages, from inexperienced young breeders to old, experienced birds, to see how foraging and breeding success might vary between them and why.  Three UNCW graduate students (Ellen Wambach, Terri Maness, and Tom McGinnis) so far have completed Master's theses in this program and one (Deniz Aygen) currently is studying chick diet and growth rates at Fisherman Island, Virginia.  Undergraduate students also assist with these studies each year, either for Honors thesis research or for Directed Individual Studies.

Research Objectives

Foraging ecology of Royal Terns breeding on Ferry Slip Island, Cape Fear River, was initiated using radio telemetry of known-age individuals to determine duration of foraging trips while feeding chicks.  Telemetry equipment was purchased from Advanced Telemetry Systems, Inc., Isanti, Minnesota, to complete this portion of the study.  A 6-gram radio transmitter was attached on the back of each of ten adult terns using epoxy and plastic cable ties.

A tern with a transmitter attached with epoxy and plastic cable
ties. The device will be shed by the bird during its annual molt.

An ATS R2100 receiver and DCCII data logger with antenna were placed in a metal box near the breeding colony to automatically record presence/absence of radio signals from these birds.  The equipment was adjusted to record signals only when the bird with the transmitter was at the breeding colony.  In this manner, we were able to determine the average time for each bird to complete foraging trips while feeding its chick on the island. Transmitters were placed on birds of known-age within each of four age classes: young (3-5 years old), middle I (6-8 years), middle II (9-11 years), and old (12+ years).  In two cases, transmitters were placed on both members of a breeding pair to facilitate studies of how these pairs coordinate foraging and chick guarding at the colony.  In addition, systematic observations of chick feedings were completed at the colony by E. Wambach to identify prey brought to chicks by known-age birds.  These birds were captured and marked with picric acid on the neck and breast for easy identification.  Color bands also were placed on their legs with color codes to identify the age class of a bird using the same age categories. These observations were completed at high and low tides to determine if birds take different prey during certain tidal conditions (Wambach and Emslie 2003).

   
Transmitter equipment with antenna (left); data logger, receiver, 
   and marine battery as a  power source.

 
Royal Tern with transmitter feeding its chick a fish 
while its mate, also with transmitter, looks on.
                                                                                              

The effect of pollutants and toxic compounds on Royal Terns was investigated by T. Maness using blood samples from adults and chicks that were collected at four breeding colonies.  Adults were captured on the nest with a net trap, or in mist nets placed near the colony.  Each adult was identified by its metal band, weighed, measured, and sampled for blood.  Only 20-40 ul of blood was taken from each bird by pricking the toe and drawing the blood into a capillary tube.  The blood was transferred from the tube into a buffer solution, placed on ice, and returned to the laboratory for processing.  Genotoxicity, or damage to DNA due to toxic compounds, was determined by mounting blood cells on a slide and assessing damage based on a tail or comet assay.  These investigations allowed assessment of DNA damage not only to birds at different breeding sites and foraging locations, but also to birds of different ages.  Relative levels of pollutant exposure at each colony were ranked from cleanest to most polluted based on previous published studies on water and sediment analyses in coastal estuaries completed by the Environmental Protection Agency (EPA) and the Center for Marine Science Research, UNCW.

Results of this study indicated that adult and juvenile terns had similar exposures to pollutants at each colony.  In other words, where DNA damage was high to the adults, it also was high for the juveniles, indicating that a local pollutant most likely is causing this damage (rather than adults being exposed to pollutants at wintering grounds).  In addition, the two colonies with the highest levels of DNA damage were at Wainwright and Sand Bag Islands in the Core Sound, while the lowest levels were at Ferry Slip Island on the Cape Fear River, and Big Foot Island and Island L in the Pamlico Sound (Maness and Emslie 2001).  These results do not correspond with water and sediment analyses that indicate relatively low pollutant loads in the Core Sound compared to other locations.  Thus, it is possible that Core Sound birds are being exposed to an undetected genotoxic contaminant that requires additional investigation.  Moreover, a repeat analysis of blood from terns collected at all colonies in 2000, the year following heavy flooding and increased sediment and pollutant loads in North Carolina estuaries from Hurricane Floyd, inidicated an increase in genotoxic damage at all locations.  These increases matched the results in 1999, with the two Core Sound sites showing significantly higher genotoxic exposure than the other colonies.

Graduate students Terri Maness (left) and Ellen Wambach (right)
collect a sample of blood from a Royal Tern.

Variation in the foraging ecology of Royal Terns and another species, the Sandwich Tern (Sterna sandvicensis) at four breeding sites was investigated by T. McGinnis.  These two species frequently breed in intermixed colonies, possibly as a means to reduce the effect of predation by gulls.  Colonies located in the Outer Banks in northern North Carolina are relatively closer to the Gulf Stream and Royal Terns have been observed foraging 20-40 km offshore in this region.  In the Cape Fear River, Royal Terns are most frequently observed foraging in the estuary while Sandwich Terns seem to prefer the inshore coastal zone.  How these species differ in their foraging ecology can be determined by analyzing carbon and oxygen isotopes in their eggshells.  Isotope ratios stored in these shells reflect the relative amount of foraging by birds in inshore versus offshore zones, and whether the primary prey is low or high on the food chain (e.g., squid, crabs, and shrimp versus fish).  Ten eggshells of each species were collected at each colony to test for differences in foraging strategies in northern and southern colonies in North Carolina.  In addition, feces samples of each species were collected to extract and identify food remains including invertebrate parts, fish bones and otoliths. 

Satellite image of the Eastern U. S. showing the Gulf 
Stream (in dark red) as it moves northward along the coast.

Results of this study indicated that Royal and Sandwich Terns partition food resources by habitat and prey choice in North Carolina.  Royals were observed feeding more in estuaries while Sandwich Terns preferred  inshore or coastal foraging sites.  Overlap in foraging areas occurred at inlets and river mouths.  In addition, Sandwich Terns had significantly higher carbon, nitrogen, and oxygen stable isotopes recorded in their eggshell further indicating differences in foraging habits.  These results are consistent with expected isotope values for birds feeding primarily in estuaries versus ocean waters (McGinnis and Emslie 2001).

UNCW students Leah Harman and Manuela Campo 
record observations on foraging terns at the colony.

Results of dietary studies on Royal Terns, 1999-2000, at Ferry Slip
Island, North Carolina (see Wambach and Emslie 2003).

Studies on Royal Tern chick growth and diet have continued at five colonies in North Carolina and one in Virginia each year.  At each colony, 100 chicks are weighed and measured two to four weeks after hatch as a part of this program.  This research is conducted in late June through July during annual chick bandings by Dr. John Weske, when all chicks at each colony are herded into an enclosure and banded with stainless steel USFWS bands.  Data gathered in this manner over the past three years (2001 - 2003) so far indicate significant variation in growth rates and weight at all colonies, after calibrating the data against a known growth curve.  These results demonstrate that annual variation in the marine/estuarine environments impacts the growth and survival of Royal Terns along the mid-Atlantic seaboard.  

 
Banding Royal Tern chicks at Ferry Slip Island (left) and data generated from chick weight and wing chord meaurements for New Dump/Sand Bag Island, 2000 - 2003 (right), showing significant variation in annual growth rates.

In 2004, the Royal Tern Working Group was formed by a consortium of Federal, State, and University researchers from Maryland, Virginia, and North and South Carolina, to better understand population trends in this species in the mid-Atlantic region over the past 20 to 30 years.  Royal Terns breeding in these states have been recognized as a metapopulation based on the relative isolation of these colonies from those farther to the south in Georgia and Florida.  Royal Tern populations tend to fluctuate from year to year, but census data gathered by State and Federal researchers in the mid-Atlantic region indicate a general decline has occurred since a population boom in the 1980s.  Explanations for the boom and subsequent decline currently are being investigated by this group.

Trends in Royal Tern populations in Virginia and North Carolina collectively based 
on numbers of chicks banded each year compared to more limited nest census data.

The studies on tern population trends and annual variation in chick growth are being continued each year with UNCW graduate and undergraduate students as part of a long-term investigation of terns as indicator species for the health of marine and estuarine environments in North Carolina.

This research was funded by grants from National Aeronautics and Space Administration (NAG5-7629), the North Carolina Sea Grant Program (99-0103), and the UNCW Charles L. Cahill Award and the Center for Marine Science Research. Ongoing monitoring research each summer is supported by the UNCW Ornithology Trust Fund.

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