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Roof-nesting Least Terns Travel to Forage in Brackish/Marine Waters
Elizabeth A. Forys, Arya Poppema-Bannon, Kristina Krajcik, and William A. Szelistowski

Southeastern Naturalist, Volume 12, Issue 1 (2013): 238–242

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238 Southeastern Naturalist Vol. 12, No. 1 238 Roof-nesting Least Terns Travel to Forage in Brackish/ Marine Waters Elizabeth A. Forys1,*, Arya Poppema-Bannon2, Kristina Krajcik1, and William A. Szelistowski2 Abstract - Sternula antillarum (Least Tern) are small, migratory seabirds that are listed as threatened or endangered in every state in which they occur. While their natural nesting habitat is open beach, Least Terns have adapted to nest on flat tar and gravel roofs, sometimes many kilometers inland. We used the “fish drop” technique to collect fish that birds dropped around a roof colony in Pinellas County, FL to assess use of fresh and saltwater fish species as prey. We collected 37 fish from 12 different species. The most common (n = 11) was Dorosoma petenense (Threadfin Shad), a species found primarily in marine/brackish waters, but often stocked in man-made freshwater ponds to supplement piscivorous fish prey. Eighteen of the remaining 26 fish, encompassing 7 of the remaining 11 species, are marine/brackish species, 5 individuals are freshwater species, and 3 are freshwater/brackish species. Roof-nesting Least Terns appeared to forage in stormwater ponds and a lake found near the colony, but they also traveled a minimum of 4.8 km to forage in marine/brackish waters. Roof nesters may face greater challenges than beach nesters, because they are potentially exposed to more pollutants in stormwater ponds and expend more energy and time to travel farther to forage at saltwater sites. Introduction Sternula antillarum Lesson (Least Tern) is a small, colonial-nesting seabird which breeds along portions of both the eastern and western coasts of North America and throughout major rivers in the interior United States (Thompson et al. 1997). Because of past over-hunting, habitat loss, and disturbance on beaches, Least Terns are listed as threatened or endangered in every state in which they occur (Thompson et al. 1997). One adaptation the birds have developed is to nest on tar and gravel roofs (Forys and Borboen-Abrams 2006). In surveys conducted from 1998–2000 throughout Florida, 83.5% of the total breeding population nested on roofs (Gore et al. 2007). As of 2011, there were 15 active roof-nesting colonies of Least Terns in Pinellas County, FL (Florida Fish and Wildlife Conservation Commission 2011). Least Terns are surface plungers that search for small fish from above the water surface (Eriksson 1985). After capture, they either consume the fish while flying or carry it to the nest to feed their mate and chicks (Atwood and Kelly 1984). Least Terns consume a wide variety of small fish, and it appears that fish size (2.5–9.0 cm) is more important to prey choice than type of species (Thompson et al. 1997). Beach-nesting Least Terns generally feed <2 km from colonies 1Environmental Studies Discipline, Eckerd College, 4200 54th Avenue South, St. Petersburg, FL 33711. 2Marine Science Discipline, Eckerd College, 4200 54th Avenue South, St. Petersburg, FL 33711. *Corresponding author - forysea@eckerd.edu. 2013 SOUTHEASTERN NATURALIST 12(1):238–242 2013 E.A. Forys, A. Poppema-Bannon, K. Krajcik, and W.A. Szelistowski 239 until chicks have fledged (Atwood and Minsky 1983). While nearly all Florida roof colonies are located in coastal counties, some roofs are >10 km from saltwater (Gore et al. 2007), indicating that roof nesters either travel farther than beach nesters or forage in other bodies of water. The purpose of our study was to determine foraging location by identifying species of fish that roof-nesting Least Terns brought back to the colony. Study Area and Methods This study was conducted at a large Least Tern colony (≈200 nesting pairs) located on the tar and gravel roofs of two adjacent warehouses in an industrial park in Pinellas County, FL, approximately 8 km from the Gulf of Mexico (Fig. 1). To determine which fish were brought to the colony, we used the “fish drop” technique (Atwood and Kelly 1984). Fish brought to feed a mate or chick are occasionally dropped by accident or because they are unsuitable for consumption Figure 1. Location of the roof-colony warehouses (W) in relation to bodies of water suitable for Least Tern foraging in Pinellas County, FL. Lake Seminole and the stormwater ponds (shaded in dark grey) are freshwater. Boca Ciega Bay ranges from brackish (<5 ppt) in its upper reaches near Lake Seminole to nearly marine (>31 ppt) in the lower portions of the Bay adjacent to the Gulf of Mexico. 240 Southeastern Naturalist Vol. 12, No. 1 because of size or presence of spines. Dropped fish were collected from the pavement surrounding the warehouses daily from 2–23 June 2010 and 1–18 June 2011 while the colony was feeding primarily unfledged chicks. The age of the chicks varied throughout the sample period. Dropped fish were fixed in 10% formalin solution and stored in 60% ethanol. Potential sources of these fish included five freshwater retention ponds (located 0.4–1.0 km from the colony), a 289-ha freshwater lake (Lake Seminole, located 0.8 km from the colony), and a nearby estuarine bay (Boca Ciega Bay, located 4.8 km from the colony). We looked for Least Terns foraging at these locations twice per week from 2–23 June 2010. Observations were made a few meters from the shore to avoid disturbance using binoculars (Swift 10 x 42), lasted 30 minutes, and were made between peak foraging hours of 0700–0900. Results and Discussion Least Terns were observed foraging at least once at all sites surveyed, although it is possible that some birds observed were from other roof or ground colonies. We collected a total of 37 fish from 12 species (Table 1). The most common (n =11) was Dorosoma petenense (Threadfin Shad), a species found primarily in marine/brackish waters but often stocked in man-made freshwater ponds to supplement piscivorous fish prey (DeVries et al. 1991). Eighteen of the remaining 26 fish, encompassing 7 of the remaining 11 species recorded, are primarily marine/brackish species, 5 individuals are freshwater species, and 3 individuals are common in both brackish and freshwater. This study is the first published record of 5 of these species being used by Least Terns (Table 1; Thompson et al. 1997, Zuria and Mellink 2005). Fish size ranged from 30–95 mm, which is generally within the size range of fish ingested by Least Terns (Table 1). Table 1. Fish dropped by Least Terns at a warehouse roof colony in Largo, FL. Fish sizes are expressed in standard length (SL). # = number collected. Size range given in mm SL. Size Scientific name Common name # range Primary habitat Dorosoma petenense (Günther) Threadfin Shad 11 35–65 Marine/brackish/freshwater Hyporhamphus unifasciatus Halfbeak* 7 74–95 Marine/brackish T.N. Gill Mugil cephalus L. Striped Mullet 3 47–60 Marine/brackish Anchoa mitchilli (L.) Bay Anchovy 3 50–54 Marine/brackish Lepomis macrochirus Rafinesque Bluegill 3 42–52 Freshwater Brevoortia sp. Menhaden 2 49–51 Marine/brackish Poecilia latipinna (Lesueur) Sailfin Molly* 2 50–52 Freshwater/brackish Micropterus salmoides (Lacépède) Largemouth Bass 2 43–64 Freshwater Lagodon rhomboides L. Pinfish* 1 42 Marine/brackish Sardinella aurita Valenciennes Spanish Sardine* 1 62 Marine/brackish Leiostomus xanthurus Lacépède Spot* 1 50 Marine/brackish Gambusia holbrooki Girard Eastern Mosquitofish 1 30 Freshwater/brackish *Indicates species not previously recorded as being used by Lea st Terns. 2013 E.A. Forys, A. Poppema-Bannon, K. Krajcik, and W.A. Szelistowski 241 Research in 2008 and 2009 at Lake Seminole found all these freshwater and freshwater/brackish species to be present, but did not find any of the marine/ brackish species (Champeau et al. 2009, Pouder et al. 2010). These saltwater species are dependent on marine waters at some point in their life cycles and therefore would not have been captured in isolated freshwater bodies such as Lake Seminole or the stormwater ponds. The closest location where Least Terns could have obtained these fish is Boca Ciega Bay, found 4.8 km from the nesting sites. Marine/brackish species were brought to the colony on at least 12 separate days and during both nesting seasons, suggesting that foraging in marine/brackish areas was a common practice for roof nesters. Compared to Least Terns examined in other studies, roof nesters traveled farther to obtain food between chick hatching and fledging and likely spent more time away from their mates and young. Atwood and Minsky (1983) observed California Least Terns feeding within a 2-km radius from the beach colony until chicks had fledged. Interior Least Terns foraged up to 12 km away from their colony during the egg-laying period and when the chicks were fledging, but stayed close to the colony while brooding and feeding young chicks (Schweitzer and Leslie 1996). Interior Least Terns nesting on spoil piles—an artificial habitat— foraged 1.5 km or more, but still remained closer than the distance birds traveled in our study (Wilson et al. 1993). Further research should investigate if the additional energy and time needed to forage farther away decreases nest productivity and/or increases mortality of adult Least Terns and should include multiple roof colonies. It appears there are advantages to foraging in Bay and Gulf waters rather than in much closer freshwater ponds and Lake Seminole. Least Terns elsewhere have shifted their foraging locations in response to prey abundance (Zuria and Mellink 2005). Additionally, if turbidity is lower in marine waters or if fish swim closer to the surface, these conditions would favor foraging by Least Terns because of their surface-plunging feeding technique (Eriksson 1985). Least Terns foraging in freshwater ponds and lakes could be ingesting fish with elevated levels of heavy metals and pesticide byproducts (Allen et al. 1998). Future research should also examine the potential impact of contaminants on roof-nesting Least Terns. Acknowledgments Many volunteers of St. Petersburg and Clearwater Audubon helped with this project, including R. Hanley, J. Hood, B. and S. Jenks, M. Korosy, and W. Meehan. M. Flock of the Pinellas County Department of Engineering and Environmental Services and B. Pouder of the Florida Fish and Wildlife Conservation Commission provided access to data on Lake Seminole fishes. This research was funded in part by grants from the Natural Science Summer Research Program of Eckerd College. Literature Cited Allen, G.T., S.H. Blackford, and D. Welsh. 1998. Mercury, selenium, and organochlorides and reproduction of Interior Least Terns in the Northern Great Plains, 1992– 1994. Colonial Waterbirds 21:356–366. 242 Southeastern Naturalist Vol. 12, No. 1 Atwood, J.L., and P.R. Kelly. 1984. Fish dropped on breeding colonies as indicators of Least Tern food habits. Wilson Bulletin 96:34–47. Atwood, J.L., and D.E. Minsky. 1983. Least Tern foraging ecology at three major California breeding colonies. Western Birds 14:57–72. Champeau, T.R., W.F. Pouder, J.S. Willitzer, M.E. Call, and P.W. Thomas. 2009. Southwest Regional Fisheries Management Project: Annual report. Florida Fish and Wildlife Conservation Commission, Tallahassee, FL. DeVries, R., R.A. Stein, J.G. Miner, and G.G. Mittelbach. 1991. Stocking Threadfin Shad: Consequences for young-of-year fishes. Transactions of the American Fisheries Society 120:368–381. Eriksson, M.O.G. 1985. Prey detectability for fish-eating birds in relation to fish density and water transparency. Ornis Scandinavica 16:1–7. Florida Fish and Wildlife Conservation Commission. 2011. Florida shorebird database. Available online at https://public.myfwc.com/crossdoi/shorebirds/. Accessed 6 June 2012. Forys, E.A., and M. Borboen-Abrams. 2006. Roof-top selection by Least Terns in Pinellas County, Florida. Waterbirds 29:501–506. Gore, J.A., J.A. Hovis, G.L. Sprandel, and N.J. Douglass. 2007. Distribution and abundance of feeding seabirds along the coast of Florida, 1998–2000. Final Performance Report, Florida Fish and Wildlife Conservation Commission, Tallahassee, FL. Pouder, W.F., N.A. Trippel, and J.R. Dotson. 2010. Comparison of mortality and diet composition of pellet-reared advanced-fingerling and early-cohort age-0 wild Largemouth Bass through 90 days poststocking at Lake Seminole, Florida. North American Journal of Fisheries Management 30:1270–1279. Schweitzer, S.H., and D.M. Leslie, Jr. 1996. Foraging patterns of the Least Tern (Sterna antillarum) in North-Central Oklahoma. Southwestern Naturalist 3:307–314. Thompson, B.C., J.A. Jackson, J. Burger, L.A. Hill, E.M. Kirsch, and J.L. Atwood. 1997. Least Tern (Sternula antillarum). No. 290, In A. Poole (Ed.). The Birds of North America Online Cornell Lab of Ornithology, Ithaca, NY. Available online at http//:bna.birds.cornell.edu/bna/species/290. Accessed 13 February 2011. Wilson, E.C., W.A. Hubert, and S.H. Anderson. 1993. Nesting and Foraging of Least Terns on Sand Pits in Central Nebraska. Southwestern Naturalist 38:9–14. Zuria, I., and E. Mellink 2005. Fish abundance and the 1995 nesting season of the Least Tern at Bahía de San Jorge, Northern Gulf of California, México. Waterbirds 28:172–180.