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Effects of Management Strategies on the Reproductive Success of Least Terns on Dredge Spoil in Georgia
Kathryn A. Spear, Sara H. Schweitzer, Robin Goodloe, and Deborah C. Harris

Southeastern Naturalist, Volume 6, Number 1 (2007): 27–34

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2007 SOUTHEASTERN NATURALIST 6(1):27–34 Effects of Management Strategies on the Reproductive Success of Least Terns on Dredge Spoil in Georgia Kathryn A. Spear1, Sara H. Schweitzer1,*, Robin Goodloe2, and Deborah C. Harris2 Abstract - Sterna antillarum antillarum (Eastern Least Tern) historically nested on Atlantic Coast beaches and barrier island shores, but has moved inland to artificial habitats, such as dredge-spoil sites, as available natural habitat has been lost to development and increased human recreational activities. Least Terns readily nest on artificial sites, but the effects of different habitat characteristics and depredation conditions on reproductive success are unclear. We examined the effects of management strategies, disking and electric fencing, on daily survival rate (DSR) and 21-day survival rate (DSR21) of clutches, and on apparent nesting success on a dredge-spoil site in Georgia from 1993 through 1998. All 3 estimates of reproductive success increased as management intensity increased. Significantly (􀁲2 2 = 185.8, P < 0.001), DSR increased from 0.88 (1993, no management) to 0.97 (1998, disking in March to remove vegetation and enclosure with an electric fence). Corresponding DSR21 were 0.06 and 0.59, respectively. Artificial nesting sites can be improved by management actions, and such work may be increasingly important as natural habitat for beachnesting birds continues to decline in availability and quality. Introduction There are 3 recognized subspecies of Sterna antillarum Lesson (Least Tern) in North America: S. a. browni (California), S. a. athalassos (Interior), and S. a. antillarum (Eastern) (American Ornithologists’ Union 1957, Draheim and Haig 2005; but see Whittier et al. 2006). The Eastern subspecies is distributed from southwestern Maine to the Florida Keys and west along the Gulf Coast to Texas (US Department of Interior 1983). Although the Eastern subspecies is not listed by the US Fish and Wildlife Service under the Endangered Species Act, it is listed as “rare” by the Georgia Department of Natural Resources (2004). Least Terns nest in colonies from April to mid-June along the Atlantic Coast. Their nests are a shallow scrape made in dry sand, rarely lined with shell fragments. They lay 2 or 3 eggs; incubation begins with the first egg and lasts 20–25 days, or an average of 21 days (Hays 1980, Massey 1974). Typically, Least Terns only rear 1 brood per year (Burleigh 1958). Historically, Least Terns nested on beaches, sand spits, and barrier island shores, but they have moved inland to sites such as roof-tops (Fisk 1978, Cimbaro 1993, Cooper 1994, Gore and Kinnison 1991, Krogh and 1D.B. Warnell School of Forestry and Natural Resources, University of Georgia, Athens, GA 30602. 2US Fish and Wildlife Service, Ecological Services, 105 Westpark Drive, Athens, GA 30606. *Corresponding author - 28 Southeastern Naturalist Vol. 6, No. 1 Schweitzer 1999) and dredge-spoil sites (Krogh and Schweitzer 1999, Kushlan and White 1985, McNair 2000, Miller 1994) as available shore habitat has declined. The quality of these inland sites for nesting is questionable. Estimated apparent nesting success on Georgia barrier islands (Ossabaw, Sapelo, and Little St. Simons) ranged from 0–40% with 10% nest abandonment (Corbat 1990). Krogh and Schweitzer (1999) calculated apparent nesting success for different habitat types in Georgia. Apparent nesting success on beach habitat (Ossabaw and Sea Islands) ranged from 0–29%. On dredge-spoil habitat (Crab Island, Andrews Island, and Mainside Spoil), apparent nesting success ranged from 0–32%, and on flat gravel roofs, it ranged from 22.7–53%. Mallach and Leberg (1999) found that the fine texture of dredged spoil was not as favorable to nesting success as fragmented shell substrates. Kotliar and Burger (1986) found that dredge-spoil sites had a greater colony turnover rate and smaller colonies than beach sites. Causes of nest failure include predation on eggs and chicks by mammals (Burger and Gochfeld 1990, Rimmer and Deblinger 1992), birds (Jenks-Jay 1982, O’Connell and Beck 2002, Rimmer and Deblinger 1992), and Solenopsis xyloni McCook (southern fire ants; Hooper-Bui et al. 2004); extreme weather events (e.g., hail, winds, thunderstorms); tidal flooding (Cowgill 1989, O’Connell and Beck 2002); and human disturbance (Burger 1984, Burger and Gochfeld 1990). Kotliar and Burger (1986) concluded that depredation events were more likely at inland dredge-spoil sites. Fencing has been used successfully to discourage predation and increase nesting success of Least Tern and other beach-nesting bird colonies (Goodrich 1982, Minsky 1980, Rimmer and Deblinger 1992). We conducted this project to determine the nesting success of Least Terns on an inland dredge-spoil site relative to management activities. We hypothesized that nesting success would increase as the intensity of management practices increased. Our findings will be useful for the conservation of other beach-nesting, migratory bird species as natural beach habitat is progressively diminished, and more birds are forced to move inland to similar dredge-spoil sites. Study Area We conducted this study on Andrews Island (31º07'N, 81º30'W), Brunswick, Glynn County, GA. Andrews Island is a 312-ha, dredge-spoil site located in the Turtle River, a tributary to the Brunswick River, St. Simons Sound, and the Atlantic Ocean. An earthen causeway connected the island to the mainland. Access to the island was limited by a locked gate and fence at the causeway. Vegetation within the spoil areas included Eupatorium capillifolium (Lam.) Small (dogfennel), Andropogon virginicus L. (broomsedge), Tamarix gallica L. (tamarisk), Myrica cerifera (L.) Small (wax myrtle), Baccharis halimifolia L. (eastern baccharis), and other lowgrowing vegetation. Silt and clay dredged from the Brunswick River covered 2007 K.A. Spear, S.H. Schweitzer, R. Goodloe, and D.C. Harris 29 most of the island, but a small number of areas were covered in sand and shell from newly dredged portions of the river. Least Terns nested within an 8-ha site in the southwestern corner of the island each year. Methods We monitored Least Tern nests between 0700 and 1000 AM, twice a week in May and once a week in June and July from 1993–1998, except in 1994, when all monitoring was done once a week. We developed a grid system over the 8-ha site, within which we walked parallel transects 6 m apart, from which we located nests. We placed a 15-cm long, uniquely numbered wooden marker approximately 0.5 m east of each active nest, and recorded the number of eggs present in each nest. In subsequent visits, if a nest was empty, we recorded the presence of chicks, signs of depredation, or signs of washing-out from storms to determine nest fate. During each March 1994–1998, the 8-ha nesting site was disked to remove vegetation. A mesh (30.5-cm width x 8.25-cm height [12-in. width x 3.25-in. height]), battery-charged electric fence, was placed around the site in 1998. During the nesting season, signs were posted to deter human interference. Hence, our hypothesis that nesting success would increase with increasing management intensity, was tested relative to 3 management actions— no management, disking in March, and electric fence installation to exclude mammalian predators as well as disking in March. For each year of the study, we used the program MAYFIELD (Bart and Robson 1982, Hines 1996a, Mayfield 1961) to estimate the daily survival rate (DSR) and 21-day survivorship rates (DSR21) of clutches as measures of reproductive success. We calculated the annual apparent nesting success (number of successful nests/total number of nests) for compatibility with previous studies that only used this estimate. We defined a successful nest as one where at least 1 egg hatched. Program CONTRAST (Hines 1996b, Hines and Sauer 1989, Sauer and Williams 1989) was used to make multiple comparisons of rate data (P < 0.05 for all tests), incorporating associated variance and covariance estimates. Data were sorted by management practice: no management (1993), disking in March (1994–1997), and electric fencing as well as disking in March (1998). For comparisons of 3 rates, CONTRAST used an asymptotically chi-square quadratic model, a matrix within which chi-square tests determined differences among rates. Results Least Terns began nesting on Andrews Island during early April each year. We monitored from 216 to 459 nests annually (Table 1). The estimated 21-day survivorship rate ranged from 0.06 during 1993, when there was no habitat enhancement or protection from mammalian predators, to 0.59 during 1998, when management included disking in March to reduce vegetation and electric fencing to exclude mammals. The cause of nest failure in 1993 30 Southeastern Naturalist Vol. 6, No. 1 was unknown. From 1994–1997, we estimated that 2.0% of failed nests were depredated by southern fire ants, 4.8% were preyed on by mammals, 0.1% were lost to avian predators, and 93.1% were lost for unknown reasons. In 1998, 4.4% of failed nests were lost to southern fire ants, 1.4% were lost to avian predators, and 94.2% were lost for unknown reasons. Daily survival rates increased as the level of management activity increased (􀁲2 2 = 185.8, P < 0.001; Table 1). The daily survival rate for 1993 (no management) was 0.88, the daily survival rate for 1994–1997 (March disking) was 0.95, and the daily survival rate for 1998 (March disking and electric fence) was 0.97. Similarly, 21-day survivorship rates increased as management activity increased: 0.06, 0.41, and 0.59, for no management, March disking, and March disking and electric fence, respectively. Discussion The Eastern population of the Least Tern is not federally endangered, but its population status in some states is rare or of concern, and in Georgia, it is listed as rare. Consequently, conservation actions such as increased protection and enhancement of its nesting sites are warranted and will benefit other species with similar listing status and nesting habits, such as Charadrius wilsonia Ord. (Wilson’s Plover), C. melodus Ord. (Piping Plover), Haematopus palliadus Temminck (American Oystercatcher), and Rynchops niger Linnaeus (Black Skimmer). Because the Least Tern is an adaptable species, likely because of its habit of using ephemeral, sandy areas for nesting, it accepts artificial sites for nesting when natural beach sites are scarce due to development or unsuitable because of disturbance by human activities. When species are excluded from natural nesting habitats and must seek alternate sites, it is Table 1. Least Tern nesting data from a dredge-spoil island, Andrews Island, GA, 1993–1998. 95% Daily confidence 21-day Apparent Number survival interval survival nesting Year of nests Treatment rateA Lower Upper rateB successC 1993 251 None 0.8776 0.8617 0.8936 0.0645 0.092 1994 365 DiskingD 0.9565 0.9506 0.9625 0.3930 0.340 1995 459 Disking 0.9725 0.9690 0.9761 0.5568 0.514 1996 378 Disking 0.9168 0.9078 0.9257 0.1613 0.161 1997 216 Disking 0.9693 0.9640 0.9745 0.5195 0.394 1998 362 Disking and 0.9749 0.9714 0.9785 0.5864 0.486 electric fence ADaily survival rate of clutches was calculated using program MAYFIELD (Bart and Robson 1982, Hines 1996a, Mayfield 1961). BSurvival rate of clutches where mean length of incubation is 21 days (Bart and Robson 1982). CApparent nesting success = number of successful nests/total number of nests. Successful nests were those in which at least 1 egg hatched. DAll disking was conducted in March. 2007 K.A. Spear, S.H. Schweitzer, R. Goodloe, and D.C. Harris 31 expected that their reproductive success rate would decline. Nesting on dredge spoil may lead to lower reproductive rates than those on natural beach sites due to increased predation pressure (Kotliar and Burger 1986), exposure to contaminants (Winger et al. 2000), impermeable soils (Mallach and Leberg 1999), and increased disturbance from human activities (Kotliar and Burger 1986). Reproductive rates on natural beach habitats decline when predation rates (Sus scrofa Linnaeus on Ossabow Island) and disturbance from recreationists (Sea Island) are high (Krogh and Schweitzer 1999). We suspected that the reproductive success of Least Terns would increase if appropriate and intensive management actions were implemented, in this case, at a dredge-spoil site on Andrews Island, GA. The overall apparent nesting success from 6 years of data collection ranged from 9.2–51.4%, and was slightly greater than other measures of apparent nesting success reported in Georgia on beach (0–40%, Corbat 1990; 0–29%, Krogh and Schweitzer 1999) and dredged-spoil sites (0– 32%, Krogh and Schweitzer 1999). Estimates of apparent nesting success on roof sites (22.7–53%, Krogh and Schweitzer 1999) were slightly higher than those in this study at the dredge-spoil site. Roof sites are protected from most mammalian predators, but are susceptible to avian predation (Voigts 1999), environmental extremes, and may not have parapets that prevent eggs and chicks from falling (Krogh and Schweitzer 1999). The relatively greater apparent nesting success at the Andrews Island dredgespoil site was due to implementation of management strategies in the last 5 years of the study, specifically, March disking to eliminate vegetation at the beginning of the nesting season and electric fencing to exclude mammalian predators. Statistical analyses of daily survival rates of clutches concurred with the observation of increasing apparent nesting success and detected significant increases in daily survival rates as management intensity increased. The 21-day survivorship rates followed suit and increased with increasing management of the dredge-spoil site. There are several management practices that improve natural and artificial habitats when natural nesting conditions are unavailable or of poor quality, and selection of one or more practice(s) may significantly improve reproductive rates of vulnerable populations, as demonstrated in this and other studies. Terns and Black Skimmers readily nest on dredged spoil, and adding shell to the dredge-spoil substrate enhances the site (Mallach and Leberg 1999). Erecting an electric fence around colonies successfully excludes most mammalian predators and increases reproductive success (Minsky 1980, Rimmer and Deblinger 1992, Sargeant et al. 1974). Various “chick shelters” may provide refugia and protect chicks from avian predation (Jenks-Jay 1982). Artificial nest structures designed to elevate nests off the ground to prevent them from washing out in tidal flooding have been successful in some studies (Loftin and Thompson 1979). Disking and fencing enhanced the dredge-spoil site on Andrews Island in this study. 32 Southeastern Naturalist Vol. 6, No. 1 Beach-nesting birds along the Atlantic Coast must adapt to increased development and human recreation. One adaptive response is to seek alternative nesting sites, such as dredge-spoil islands used increasingly in New Jersey and North Carolina (Erwin et al. 2003). Coastal biologists must be aware of alternative habitats selected by these species and increase management activities that optimize nesting conditions (Erwin et al. 2003) to maintain or increase reproductive rates of species of concern. Acknowledgments We thank Drs. Robert J. Cooper and John P. Carroll for reviewing and improving earlier drafts of our paper. Mr. John B. Sabine provided assistance with the programs MAYFIELD and CONTRAST. We greatly appreciate Ms. Cheryl Snow’s exceptional assistance with field data collection. Literature Cited American Ornithologists’ Union. 1957. Check-list of North American Birds, Fifth Edition. American Ornithologists’ Union, Ithaca, NY. Bart, J., and D.S. Robson. 1982. Estimating survivorship when the subjects are visited periodically. Ecology 63:1078–1090. Burger, J. 1984. 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