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Encounters of Royal Terns (Thalasseus maximus) Banded in South Carolina
Felicia J. Sanders and Stacy J. Ray

Southeastern Naturalist, Volume 14, Issue 1 (2015): 1–8

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Southeastern Naturalist 1 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 Southeastern Naturalist F.J. Sanders and S.J. Ray 2015 Vol. 14, No. 1 2 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 Southeastern Naturalist 3 F.J. Sanders and S.J. Ray 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. Southeastern Naturalist F.J. Sanders and S.J. Ray 2015 Vol. 14, No. 1 4 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. Southeastern Naturalist 5 F.J. Sanders and S.J. Ray 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) Southeastern Naturalist F.J. Sanders and S.J. Ray 2015 Vol. 14, No. 1 6 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. Southeastern Naturalist 7 F.J. Sanders and S.J. Ray 2015 Vol. 14, No. 1 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 Southeastern Naturalist F.J. Sanders and S.J. Ray 2015 Vol. 14, No. 1 8 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. Literature Cited Buckley, P.A., and F.G. Buckley. 2002. Royal Tern (Sterna maxima). Number 609, In A. Poole and F. Gills (Eds.). The Birds of North America. The Birds of North America, Inc. Philadelphia, PA. Burger, J., and M. Gochfeld. 1990. The Black Skimmer: Social dynamics of a colonial species. Columbia University Press, New York, NY. 355 pp. Environmental Systems Research Institute, Inc. (ESRI). 2009. ArcMap Version 9.3., Redlands, CA. Emslie, S.D., J.S. Weske, M.M. Browne, S. Cameron, R. Boettcher, D.F. Brinker, and W. Golder. 2009. Population trends in Royal and Sandwich Terns along the mid-Atlantic seaboard, USA, 1975–2005. Waterbirds 32:54–63. Furness, R.W., and P. Monaghan. 1987. Seabird Ecology. Chapman and Hall, New York, NY. 164 pp. Gabrey, S.W. 1996. Migration and dispersal in Great Lakes Ring-billed and Herring Gulls. Journal of Field Ornithology 67:32–339. Greenwood, P.J., and P.H. Harvey. 1982. The natal and breeding dispersal of birds. Annual Reviews of Ecology and Systematics 13:1–21. Hockey, P.A.R., J.K. Turpie, and C.R. Velasquez. 1998. What selective pressures have driven the evolution of deferred northward migration by juvenile waders? Journal of Avian Biology 29:325–330. Jodice, P.G.R., T.M. Murphy, F.J. Sanders, and L.M. Ferguson. 2007. Long-term trends in nest counts of colonial seabirds in South Carolina, USA. Waterbirds 30:40–51. Minitab 15 Statistical Software (2010). Minitab, Inc., State College, PA. Schreiber, E.A., and J. Burger. 2002. Seabirds in the marine environment. Pp. 1–15, In E.A. Schreiber and J. Burger (Eds.). Biology of Marine Birds. CRC Press, Boca Raton, FL. Stevenson, M.H., and B.H. Anderson. 1994. The Birdlife of Florida. University Press of Florida, Gainesville, FL. 892 pp. Van Velzen, W.T. 1968. The status and dispersal of Virginia Royal Terns. The Raven 39:55–60. Van Velzen, W.T. 1971. Recoveries of Royal Terns banded in the Carolinas. Chat 35:64–66.