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F.J. Sanders and S.J. Ray
22001155 SOUTHEASTERN NATURALIST Vol.1 144(,1 N):1o–. 81
Encounters of Royal Terns (Thalasseus maximus) Banded in
South Carolina
Felicia J. Sanders1,* and Stacy J. Ray2
Abstract - Encounters of Thalasseus maximus (Royal Terns) (n = 1387) banded as chicks
in South Carolina were used to expand our knowledge regarding winter and migration sites
of immature and mature birds, to identify causes of mortality, and to examine natal-site fidelity.
Ages of encountered terns ranged from zero (less than one year old) to 27 years old.
Mortality was highest in the first year of life. The majority of the encounters (64%, n = 879)
were in Florida. Banded birds were also observed outside the US as far west as the Gulf
coast of Mexico, east to Guyana, and south to Peru. Royal Terns appear to delay their first
migration from winter grounds to breeding sites until the fourth summer. The vast majority
of encounters (92%, n = 1275) were due to mortalities; the remaining 8% (n = 112) were
of live terns encountered by resighting or capturing. In total, 35% (n = 485) of live or dead
encounters were directly attributed to human activities, including entanglement in fishing
gear (recorded as late as 1991) and shooting (recorded as late as 1980).
Introduction
Thalasseus maximus Boddaert (Royal Tern) breeds along the Atlantic Coast of
the US from Maryland to Texas (Buckley and Buckley 2002). Nesting numbers
of Royal Terns declined from Maryland to South Carolina from the mid-1980s to
late 1990s. Although the breeding population is presently stable to increasing from
Maryland to North Carolina, nest numbers in South Carolina have not recovered to
1970s and 1980s levels (Emslie et al. 2009, Jodice et al. 2007).
Royal Terns are migratory; thus, identifying causes of population decline are
difficult if they are only studied on their breeding grounds. To identify areas
used throughout the year, Royal Terns have been banded in South Carolina since
the early 1930s (Danny Bystrak, US Geological Survey Bird Banding Laboratory,
Laurel, MD, pers. comm.), but band-encounter data have not been thoroughly
analyzed. Only 21 encounters of Royal Terns banded in South Carolina were
summarized in 1971, with the majority of the band recoveries in Florida (Van
Velzen 1971). An analysis of 46 band encounters of Royal Tern chicks banded in
Virginia suggest that Royal Terns that breed in the southeastern US also winter
as far south as Central and South America (Van Velzen 1968). We used over 1300
encounters of Royal Terns banded as chicks in South Carolina to expand our
knowledge of winter and migration sites of mature and immature birds, to identify
causes of mortality, and to examine natal-site fidelity.
1South Carolina Department of Natural Resources, 220 Santee Gun Club Road, McClellanville,
SC 29458. 2College of Charleston Master’s of Environmental Studies Program, 66
George Street, Charleston, SC 29424. *Corresponding author - sandersf@dnr.sc.gov.
Manuscript Editor: Paul Leberg
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2015 Vol. 14, No. 1
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Methods
The US Geological Survey Bird Banding Laboratory provided us with 1327 bandencounter
records of Royal Tern chicks banded between 1930 and 2009 in South
Carolina. We removed band recoveries with inadequate information for each analysis
as appropriate, and thus sample size of encounters varied by analysis. For example,
we removed band-recovery locations without data on latitude and longitude from our
analysis of distance from banding to encounter site. For our analysis of season of encounter
and age of tern, we removed encounters represented by a band found on a tern
leg bone, or a tern carcass, or if date of recovery was unclear. We calculated distance
between banding and encounter site from latitude and longitude of each, respectively.
Encounter locations were mapped using a GIS (ESRI 2009).
In March or early April, adult Royal Terns begin migrating from overwintering
sites to breeding sites. Most individuals are present at their breeding sites by late
April and remain there until August or September (Buckley and Buckley 2002).
Thus, we defined breeding season encounters as those in May–July. We removed
birds banded and recovered in the breeding season of the same year from distance
analysis because they probably did not migrate from their natal sites. Because most
individuals reach their overwintering destination by December and remain there
until February (Buckley and Buckley 2002), we defined winter recoveries as those
in December–February. Royal Terns remain on wintering grounds for at least 2
breeding seasons; the first breeding attempt is thought to occur after the third year
(Buckley and Buckley 2002). Thus, we divided terns into mature and immature age
groups (≥4 years old and less than 4 years old, respectively). Terns were aged using 1 June
as the average hatch date; therefore, we categorized all Royal Terns as a year older
after 1 June each year following hatching or banding. Data were not normally
distributed; thus, we used the Kruskal-Wallis test to analyze differences in median
distance between different age groups and seasons. We employed a post hoc Dunn’s
test to compare the groups using Bonferroni adjustments for multiple comparisons.
Confidence intervals were controlled at a family error rate of 0.2. All statistical
analyses were performed using Minitab 15 Statistical Software (2010).
Results
Royal Terns were banded during May–August at 7 sites in South Carolina from
1930 to 2006 (Fig. 1). Due to lack of electronic records for birds banded before
1960, the total number of Royal Terns banded was not available. Of the total banded,
1387 were later encountered. Electronic banding records indicate that 29,250
Royal Terns were banded in South Carolina after 1960, with most of the banding
(28,544 birds) done between 1963 and 1975 (Fig. 2). Of the Royal Terns banded
after 1960, ~3% (n = 954) were encountered. Encounter numbers peaked in the mid-
1960s (Fig. 2). The age range for encountered terns was 0–27 years old (Fig. 3).
Encounter locations were dispersed throughout the Atlantic Coast of the US from
New Jersey to Florida and on the Gulf coast from Florida to Texas. A majority of
the total band recoveries were in Florida (63%, n = 879); only 3 of these were in
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2015 Vol. 14, No. 1
the Florida Panhandle (the northwest region of Florida). Encounters also occurred
outside the US west to the Gulf coast of Mexico, east to Guyana, and south to Peru
(Fig. 1). Recoveries from Royal Terns less than one year old (n = 831) were from
Figure 1. Encounter locations for 1364 Royal Terns banded as chicks in South Carolina.
Encounters occurred between 1930 and 2009.
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New Jersey (1), Maryland (1), Virginia (7), North Carolina (14), South Carolina
(23), Georgia (9), Mississippi (1), Florida (604), and outside the US (171) west to
Mexico and south to southern Peru. Encounters of Royal Terns more than one year
old north of South Carolina (n = 97) were primarily from August through November
(n = 50). Although the majority (60%, n = 30) of breeding season recoveries
of mature Royal Terns was in South Carolina, 40% were found in other states and
countries: Florida 16% (8), Georgia 12% (6), North Carolina 4% (2), Colombia 4%
(2), and Cuba 4% (2). Three mature (age 13–15 years) Royal Tern breeding-season
recoveries were made from 1988 to 1990; they were all in South Carolina. There
were no encounters of mature birds during the breeding seasons after 1991.
Figure 2. Number of Royal Tern chicks banded in South Carolina from 1961 to 2009 (number
of terns banded before 1961 is unknown). Number of band encounters from 1930 to
2009 of Royal Terns banded in South Carolina.
Figure 3. Age
at encounter
for 1318 Royal
Terns banded
in South Carolina
as chicks.
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2015 Vol. 14, No. 1
Median distances between banding and encounter sites were significantly different
between mature and immature Royal Terns during winter and breeding seasons
(H = 98.21, df = 3, P < 0.001). Median distances from mature Royal Tern banding
sites to encounter sites during the breeding season were significantly different
from the median distances recorded for the following groups: immature wintering
Royal Terns (Z = 9.4443, P < 0.001), immature terns during the breeding season
(Z = 8.0799, P < 0.0001), and mature wintering terns (Z = 4.7791, P < 0.0001)
(Table 1). Additionally, median distances from mature wintering tern banding sites
to encounter sites were significantly different from the median distances recorded
for immature wintering terns (Z = 3.5182, P = 0.0004) and immature terns during
the breeding season (Z = 2.9079, P = 0.0036). Recoveries of dead or injured Royal
Terns less than one year old were highest from December to February with a peak
in January (Fig. 4). The vast majority (1275; 92%) of band recoveries for all ages
were due to mortalities; 112 (8%) were of live terns encountered by resighting or
capturing (Table 2). In total, we attributed 485 (35%) live or dead encounters directly
to human activities, including entanglement in fishing gear (recorded as late
as 1991) and shooting (recorded as late as 1980). Natural causes of encounters were
predation by raptors and fish, and weather-related occurrences. We lumped together
all recoveries due to roadside collisions into one encounter class: struck vehicles or
Table 1. Mean ± standard error (sample size) and median distances from banding sites to encounter
sites of Royal Terns banded as chicks in South Carolina. Mature Royal Terns were ≥4 years old; immature
Royal Terns were less than 4 years old. Breeding season = May–July; winter = December–February.
Differences = results of analysis using the Kruskal-Wallis test followed by Dunn’s test using Bonferroni
adjustments for multiple comparisons; different letters denote that each group had significantly
different median distances from banding to encounter site (P ≤ 0.05).
Age group Season Mean distance (km) + SE (n) Median distance (km) Differences
Immature Breeding 948 + 74 (107) 704 a
Immature Winter 840 + 22 (576) 709 a
Mature Breeding 263 + 73 (50) 66 b
Mature Winter 697 + 73 (60) 593 c
Table 2. Causes of encounters of Royal Terns banded as nestlings in South Carolina encountered between
1930 and 2009. Natural causes of encounters included predation by raptors or fish and weather
events.
Cause Number (%)
Unknown/misc 665 (47.9)
Entangled in fishing gear 304 (21.9)
Shot 146 (10.5)
Trapped or caught 91 (6.6)
Band on bone/band only 69 (5.0)
Disease 36 (2.6)
Struck object or vehicle 29 (2.1)
Natural causes 26 (1.9)
Resighted 21 (1.5)
Total 1387 (100.0)
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objects, which included birds that hit vehicles, wires or bridges, and other highwayrelated
mortalities or injuries.
Discussion
This study represents the most comprehensive analysis of Royal Tern band
encounters to date. The 27-year-old Royal Tern recorded in our study is the oldest
age record for this species; the bird was banded in 1975 and resighted at
Sanibel Island, FL, in November 2002. The majority of band recoveries were in
Florida, particularly in the southern part of the state (Fig. 1). Band returns suggest
that the main non-breeding range of Royal Terns banded in South Carolina
extends from Florida through the Greater Antilles (Cuba, Jamaica, Hispañola).
The encounter rate for this study (3%) is the same as the return rate of Royal
Terns banded in Virginia in 1964 and recovered from 1964 to 1969 (Van Velzen
1968). Similarly, a large percentage, 34%, (n = 17) of Royal Terns banded in
Virginia were also encountered in peninsular Florida but none were recovered
from the Florida Panhandle (Van Velzen 1968). Royal Terns banded in Maryland,
North Carolina, Texas, Louisiana, and Mississippi were also recovered in Florida
(Stevenson and Anderson 1994). Together with encounters of Royal Terns banded
in Virginia, these results suggest the wintering range of Royal Terns from the
Atlantic coast of the Southeast US extends as far south as the coast of Peru (Van
Velzen 1968). Although most of the birds detected outside of the breeding season
were located to the south of the banding sites, there was also evidence of some
northward movement in the fall and early winter.
Many colonial breeding seabirds exhibit natal philopatry (Furness and Monaghan
1987, Greenwood and Harvey 1982, Schreiber and Burger 2002); however, it is
Figure 4. Number of encounters by month of 807 Royal Terns banded as chicks in South
Carolina and encountered dead or injured when less than one yea r old.
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unknown if Royal Terns return to their natal site to breed. In this study, encounters
of mature Royal Terns during the breeding season (40% outside of South Carolina)
suggest the birds may disperse from natal sites in South Carolina to breed and do
not always exhibit natal philopatry. Nesting sites are ephemeral sandy islands, so
dispersal within South Carolina is expected. Because only 50 mature Royal Terns
were recovered during the breeding season, additional banding and resighting efforts
are needed to examine natal philopatry and to determine if recent local breeding
season declines are due to emigration or population level decreases.
Years with a high number of encounters generally followed a year in which
many birds were banded (Fig. 2), indicating that mortality of Royal Terns is greatest
during the first year of life. This finding is consistent with those for other avian species
that have high juvenile mortality rates (Burger and Gochfeld 1990). Because
a majority of the encounters in our data were from the 1960s, additional banding
could update our knowledge and understanding of the major current causes of
Royal Tern death. Encounter distances for immature terns were not significantly
different between the breeding season and winter. These data support other studies
that suggest Royal Terns remain on wintering sites for 3 years until their first
breeding season (Buckley and Buckley 2002). This life-history pattern is consistent
with behaviors observed for other seabirds and shorebirds that delay migration for
several years; postponing migration may increase survival (Gabrey 1996, Hockey
et al. 1998). Although Royal Terns winter in large numbers on the Gulf coast of
Texas and Mexico as well as on the Pacific coast of Central America (Buckley and
Buckley 2002), few Royal Terns banded in South Carolina were recovered in these
areas. Royal Terns encountered on the Pacific Coast south of Panama may cross
Central America at the Isthmus of Panama (Buckley and Buckley 2002) but do not
appear to cross north of Panama.
Band-encounter studies can lead to erroneous conclusions on habitat selection
and migration destinations because band-recovery reporting may not be uniform
throughout the species range or life cycle and is often based on encounters with
dead birds. Additionally, encounters may increase in areas that do not necessarily
have the highest density of birds but have a high density of people that report bands;
for example, the sites with the most recoveries in this study are near large cities
in Florida. Thus it is difficult to determine if more Royal Terns winter in Florida
than other areas, especially south of the US, or if the high number of recoveries in
Florida is just a result of a dense coastal human population that reports encounters.
However, encounter data are often the only available and most widely used tool to
identify areas used by migratory birds. Currently, encounter data is the only source
of information available to increase our understanding of Royal Tern movement.
Acknowledgments
We would like to acknowledge the many individuals who banded Royal Terns, especially
E. Milby Burton, Edward Burnham Chamberlain, Ted Beckett, and staff at Cape
Romain National Wildlife Refuge. Without their efforts, this analysis would not have been
possible. We thank those who reported recoveries to the Bird Banding Laboratory of the US
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2015 Vol. 14, No. 1
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Geological Survey and to the staff of the laboratory. The College of Charleston provided
GIS and computer support. Audubon South Carolina and South Carolina Department of
Natural Resources provided partial funding. We would like to thank B. Doyle, M. Hughes,
P. Jodice, L. Ferguson, J. Weske, Paul Leberg, and anonymous reviewers for contributing to
earlier versions of this manuscript. Thanks to K. Sloan for help with statistics and T. Murphy
for encouraging this project.
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