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An Aquatic Bal-Chatri for Trapping Snail Kites (Rostrhamus sociabilis)
Peter J. Mahoney, Kenneth D. Meyer, Gina M. Zimmerman, and Christopher E. Cattau

Southeastern Naturalist, Volume 9, Issue 4 (2010): 721–730

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2010 SOUTHEASTERN NATURALIST 9(4):721–730 An Aquatic Bal-Chatri for Trapping Snail Kites (Rostrhamus sociabilis) Peter J. Mahoney1, Kenneth D. Meyer1,*, Gina M. Zimmerman1, and Christopher E. Cattau2 Abstract -The endangered Rostrhamus sociabilis plumbeus (Florida Snail Kite) has been the focus of several ecological studies emphasizing movements between and within wetland fragments. These studies have required the ability to trap and radiotag free-flying adults without significant risk of injury. We developed and tested a safe alternative to previous methods for trapping Snail Kites as part of a comparative study of VHF and satellite telemetry. The aquatic bal-chatri borrows from historical trap designs with modifications for trapping aquatic birds at the surface of the water. It consists of a square PVC frame with a series of parallel flourocarbon stringers and a mesh basket to restrain the lure species. Nooses are attached to the stringers and used to ensnare the toes of a predatory bird. The trap is held afloat by the PVC frame, with the mesh basket, stringers, and nooses positioned just beneath the surface of the water. After determining effective trap placement in relation to perched birds, we captured 11 kites in 13 days with native Pomacea paludosa (Florida Apple Snail) and exotic Pomacea insularum (Island Apple Snail) as lures. Our results indicated that the aquatic bal-chatri can be used to target specific Snail Kites and recapture previously trapped individuals. This trap design is a safe, efficient, and low-cost alternative to methods previously used for capturing Snail Kites. Additionally, the aquatic bal-chatri is relatively easy to use and appears to have minimal impact on foraging behavior and breeding performance of Snail Kites. Introduction Rostrhamus sociabilis Vieillot (Snail Kite) is a highly specialized raptor found in Central and South America, Mexico, and Cuba with a distinct nonmigratory US population in Florida (Beissinger 1988, Bennetts and Kitchens 1997a, Sykes 1984). R. s. plumbeus (Florida Snail Kite) is listed as endangered by both federal and state governments and is a prominent bioindicator for designing and measuring the success of Everglades restoration efforts (Martin et al. 2007, USFWS 1999). Snail Kites have been the focus of a variety of ecological studies, from the foraging energetics and habitat use of individual birds (Beissinger 1983, 1990; Bennetts and Kitchens 1997a, 1997b; Bennetts et al. 2006; Cattau et al. 2010), to the effects of fragmented landscapes on movements among wetlands (Bennetts 1993; Bennetts and Kitchens 1997a, 1997b, 2000; Martin et al. 2006; Valentine-Darby et al. 1998). A common objective has been to understand the movements, site fidelity, and survival of marked individuals. 1Avian Research and Conservation Institute, 411 NE 7 Street, Gainesville, fl32601- 5545. 2Florida Cooperative Fish and Wildlife Research Unit, University of Florida, Gainesville, fl32611-0485. *Corresponding author - 722 Southeastern Naturalist Vol. 9, No. 4 Radio telemetry is often employed to study fine-scale movements and to identify essential wetlands for the Snail Kite. Researchers have been limited to radio-tagging nestlings (Bennetts and Kitchens 2000, Snyder et al. 1989)—a costly approach due to high juvenile mortality—and the use of a net gun for capturing fledged young and adults (Bennetts and Kitchens 1997a, 2000). The latter method has proved successful, but it requires a highly skilled and experienced operator to reduce the substantial risk of injury (Bennetts and Kitchens 1997a). In the past, the Florida Fish and Wildlife Conservation Commission and the US Fish and Wildlife Service withdrew a net-gun permit after two Snail Kites were injured (J. Rodgers, FFWCC, Gainesville, fl, pers. comm.). Our goal was to develop and test a safe alternative method for trapping Snail Kites as part of a comparative study of VHF and satellite telemetry that required capture of 10 adults during the non-breeding season. We describe the result, its advantages relative to previously used methods, and ways to improve its effectiveness. Field-site Description We tested trap designs and captured Snail Kites on Lake Tohopekaliga and Lake Kissimmee in Osceola County, fl. These two lakes, part of the Kissimmee Chain-of-Lakes region in central Florida, had the greatest densities of Snail Kites during the trapping period, based on concurrent monitoring surveys by the Florida Cooperative Fish and Wildlife Research Unit. Methods Trap design In recent years, the net gun was used to trap free-flying Snail Kites (Bennetts and Kitchens 1997a, 2000; Valentine-Darby et al. 1998). Under appropriate conditions, a skilled operator can capture several Snail Kites in a short period of time. However, the risk of injury or death is high (Bennetts and Kitchens 1997a, Mechlin and Shaiffer 1979), and it requires a significant amount of time for the operator to become proficient. This approach, furthermore, usually focused on heavily used flight paths to and from nesting sites (Bennetts and Kitchens 1997a), potentially impairing nesting performance. Noose-carpets, a traditional falconry method that has proven versatile for trapping many species (Bebe and Webster 2000, Snyder et al. 1989), can be placed on perches, near carrion, or around live prey (i.e., noose harnesses) to capture foraging individuals away from sensitive nest sites. Snyder et al. (1989) captured eight adult Snail Kites in Florida using noose carpets placed on active perches. However, Bennetts and Kitchens (1997a) determined that various traps employing nooses were inefficient, had limited success, and, when used on perches, may have induced sampling bias. We elected to design a simple trap that would minimize risk of injury and could be used year round to capture foraging Snail Kites with little or 2010 P.J. Mahoney, K.D. Meyer, G.M. Zimmerman, and C.E. Cattau 723 no impact on nesting activity. The bal-chatri is a prey-based trap often used to catch foraging raptors (Bebe and Webster 2000, Berger and Mueller 1959, Smith and Walsh 1981). The modern bal-chatri consists of a domed wiremesh cage to which nooses are attached, extending perpendicularly from the dome, with live prey placed inside as a lure. For a prey-choice experiment, Beissinger et al. (1994) demonstrated that Snail Kites in Venezuela will capture snails from trays placed beneath feeding perches. We combined these themes by constructing 61-cm x 61-cm square frames from 1.9-cm (¾-in) polyvinyl chloride (PVC) pipe joined at the corners by 90-degree PVC unions. The PVC was painted with black and brown spray paint (Krylon® Camouflage Spray Paint for Plastic) in order to camouflage the trap at the surface of the water. We drilled sixteen 0.16-cm (1/16-inch) holes on each of two parallel PVC sides and attached thirty-two 0.32-cm (1/8-in) screw-in, stainless steel eyelets (16 per side) (Fig. 1). We connected opposing eye- Figure 1. Bottom view of the aquatic bal-chatri frame showing the layout without nooses. Cutaway without nylon basket on the right. 724 Southeastern Naturalist Vol. 9, No. 4 lets with 7.7-kg (17-lb) test fluorocarbon monofilament (Berkley Vanish®), producing 16 parallel stringers. Nooses were made from 20-cm segments of 7.7-kg (17-lb) test fluorocarbon monofilament by tying overhand knots on one end of each and feeding them individually through a small glass or plastic bead with a hole small enough not to slip over the knot, but large enough to allow two passes of the monofilament (i.e., slightly larger than ≈2x the diameter of the monofilament). The unknotted end was fed back through the bead forming a noose (Fig. 2). Sixteen nooses were then attached to each stringer with a clinch or half-blood knot, totaling 256 nooses per trap. We made a mesh basket from untreated, black nylon window screening and fitted it to the PVC frame to allow for ≈7.6 cm (3 in) of depth at its center. The mesh basket was attached to the frame by wrapping its edges around the PVC and sewing it to itself with monofilament. The stringer eyelets were positioned facing downward (i.e., toward the bottom of the basket) to prevent metallic flash and to ensure that the fluorocarbon stringers and nooses remained beneath the surface of the water (Fig. 2). Trapping sites and conditions Areas within each lake were chosen based on current densities of foraging Snail Kites (S. Stocco, Florida Cooperative Fish and Wildlife Research Unit, Gainesville, fl, pers. comm.). We used canoes, kayaks, and airboats to locate, observe, and trap kites. Before setting out traps in any given area, we observed all the resident kites for a short period (usually 1 to 2 h) to determine as well as possible whether they were nesting and to identify their most frequently used perches. Perches were selected based on frequency of use and suitability of trap placement. Funnel traps were used to collect live snails (Darby et al. 2001) for use as bait. The species composition of snails varied between the two lakes on which we trapped Snail Kites. Our snail traps produced large samples of an exotic, Pomacea insularum d'Orbigny (Island Apple Snail) (Rawlings et al. 2007), on Lake Tohopekaliga, where this dominant species inhabits thousands of acres (P. Darby, University of West Florida, Pensacola, fl, pers. comm.). On Lake Kissimmee, we trapped only small samples of the native Pomacea paludosa Say (Florida Apple Snail) in the areas where Snail Kite Figure 2. Side view of the completed trap with cutaway to show noose layout. Note that the eyelets (·) and stringers (—) are positioned below the PVC frame. Inset shows noose construction, including knotwork and function of the bead. 2010 P.J. Mahoney, K.D. Meyer, G.M. Zimmerman, and C.E. Cattau 725 trapping was conducted. Overall (i.e., natives and exotics combined), the snails used in our traps ranged from about 1.7 to 4.5 cm in length (longest axis; Youens and Burks 2008). Even though our snail traps consistently produced homogeneous samples of the respective species, both species are known to occur within each lake (P. Darby, pers. comm.). Although frozen exotic snails were used initially as lures on both lakes, we eventually used only live snails. However, we were very careful to ensure that exotic snails were used only on Lake Tohopekaliga, and that all traps on Lake Kissimmee were baited only with native snails. We initially lured kites with a simple PVC-and-mesh tray (i.e., without stringers and nooses) containing at least three Apple Snails, by placing it beneath actively used perches. After a short period of trial-and-error, we learned what would elicit a response from a perched kite and began setting out complete traps, consisting of the baited tray (three to 12 snails) and an array of stringers and nooses. The presence of the stringers and nooses did not produce any obvious change in the birds’ behavior. We set each trap in place beneath its respective perch while it was vacant, then moved away to allow the Snail Kite to return. It was not necessary to anchor the trap because adjacent vegetation provided enough resistance to prevent drifting beyond 20 to 25 cm. Typical monitoring locations were at least 50 m from the trap, in or against emergent vegetation or a stand of shrubs with an unobstructed view of the trap. Where possible, we parked our boat beneath another frequently used perch to encourage perching near the trap. Initially, we set the traps at different distances from the kites’ perches and in different positions relative to the wind direction. Trap placement was gradually improved with each successive capture. We occasionally set out two or three traps beneath one or more perches used by the target bird. However, we did so only if all traps could be monitored simultaneously without risk of an unobserved capture. When a bird was caught, the remaining traps were left out only if three or more people were present and one person could constantly watch the other traps. Otherwise, all traps were pulled following a capture. As soon as a kite was ensnared, we approached the trap quickly and carefully to prevent injury to the bird. We loosened the nooses to remove the kite before taking the trap from the water, then covered the bird’s head with a hood while banding, measuring, and attaching the transmitter. All the marked Snail Kites were released at the capture location within 50 min of capture. Results We trapped 10 kites and recaptured one (11 total captures) in 13 days (Table 1). Five kites were trapped on each of the lakes in the study. Of the individuals trapped, five were suspected females (50%) and eight were suspected adults (2+ yrs, 80%) based on plumage and eye coloration (Wheeler 2003). Four of the 11 captures were made using frozen snails (36%). Seven captures (64%) were made on the kites' first attempt, which we defined as 726 Southeastern Naturalist Vol. 9, No. 4 a descent followed by extension of the legs and feet to within reach of the suspended snails. Three kites were captured in two to five attempts (27%), some of which resulted in the kite removing a snail from the trap without becoming ensnared. One kite removed all 15 snails from a trap without getting caught (we captured this bird the next morning with a restocked trap at the same site during its first feeding attempt). Wind direction relative to a line between the perch and the trap was the most important determinant of whether or not a kite approached a trap. Kites quickly responded to motionless traps upwind from their perches. When traps were placed downwind, perched kites usually watched the trap for an extended period of time (10 to 30 min) before flying off to forage elsewhere. In some cases, a bird would fly past the trap and then turn to approach it upwind, often abandoning the effort to return to the perch. If the trap was moved from downwind to upwind of the perch, the same individual often quickly responded by flying to the trap. Wind also affected kite response, and thus trapping success, by moving or rocking the trap. Even with an upwind trap placement, kite response was much lower when winds exceeded 15 km/hr. Snail Kites that approached traps on such days were more likely to abandon their feeding attempt before reaching for a snail when the trap was placed in open water. Although not required on calm days, placing the trap on submerged plants, such as topped-out Hydrilla verticillata (L. f.) Royle (Hydrilla), or within surrounding emergent vegetation (e.g., Panicum or Eleocharis spp.), dampened its movement and resulted in more foraging responses from perched kites. It was important to consider the distance of the trap in relation to the height of the perch and the presence of potential hazards (e.g., barbed wire attached to a fence-post perch). Perched kites were able to see snails at greater distances and shallower angles than expected, and consistently at a horizontal distance equal to the height of the perch. We usually set our traps at roughly this distance, about 2 to 4 m, to allow the bird an easier (i.e., shallower) approach to the trap. If a perched kite did not fly down to a trap, we gradually moved the trap closer until it drew a response. Table 1. Snail Kites trapped using the aquatic bal-chatri in February and March 2007. Sex and age were estimated by plumage coloration. * indicates individual was recaptured to replace the transmitter. Pomacea snail Bird ID Lake Sex Age (years) Vehicle species used as bait Attempt 701 Tohopekaliga Female 3+ Kayak Frozen P. insularum 1st 702 Tohopekaliga Female 3+ Kayak Frozen P. insularum 1st 703 Tohopekaliga Unknown 1+ Kayak Frozen P. insularum 16th 704 Tohopekaliga Female 3+ Airboat P. insularum 3rd 705 Tohopekaliga Male 5+ Airboat P. insularum 2nd 706 Kissimmee Male 2+ Airboat Frozen P. insularum 1st 707 Kissimmee Female 3+ Airboat P. paludosa 1st 708 Kissimmee Female 2+ Airboat P. paludosa 1st 709 Kissimmee Male 3+ Airboat P. paludosa 5th 703* Tohopekaliga Unknown 1+ Kayak P. insularum 1st 710 Kissimmee Unknown 1+ Foot P. paludosa 1st 2010 P.J. Mahoney, K.D. Meyer, G.M. Zimmerman, and C.E. Cattau 727 Discussion The predictable nature and continuous foraging habits of Snail Kites make them an ideal target for traps employing live prey as lures. With relatively little effort (one to two hours), we located Snail Kite foraging areas with habitually used perches. The aquatic bal-chatri placed below actively used perches yielded unexpectedly high capture success—about 0.25 to 1.00 capture per hour—with no logistical or safety complications. The most important considerations when selecting a capture site and trap placement were, in order of priority, 1) trap placement with respect to wind direction (Beissinger 1983, Sykes 1987), 2) presence of surface vegetation, and 3) the distance of the trap from the base of the perch relative to the height of the perch above the surface of the water. There are benefits to using the aquatic bal-chatri over previous Snail Kite trapping methods. Snail Kites returned to the trap for additional attempts even if they had become briefly ensnared in earlier attempts. Unlike most trap designs, the aquatic bal-chatri was functional even after a bird took the lure and escaped, as long as at least one snail remained. One female kite illustrated a second advantage of the aquatic bal-chatri. After this female’s transmitter went silent within a week (charging was precluded by residue on the solar panels), we were able to recapture her and replace the radio, suggesting that retrapping with the aquatic bal-chatri has a relatively high probability of success. Finally, whereas net-gunning often requires a power boat to get close enough for capture, four of our 11 captures employed a kayak, and one capture was by wading from dry land (the other six were from an airboat). Although powerboats can provide access to more birds in a given time period and are often required with other trapping methods, the aquatic bal-chatri can capture birds for projects with limited budgets or where powerboats cannot be used. The aquatic bal-chatri produced 11 captures (including one recapture) in 13 days, despite delays caused by limited availability of both live snails and transmitters that resulted in only 1 to 4 hours of trapping effort per day. With a sufficient supply of live snails and transmitters, a single trapping crew could average 2 to 3 captures per day, well above trapping rates we have experienced with other methods and species of raptors. Our success rate was comparable to that of the net gun (Bennetts and Kitchens 1997a, 2000) in captures per hour (R. Bennetts, National Parks Service, Santa Fe, NM, pers. comm.), although capture rates alone should not be used to legitimize the increased risks of using a net-gun. There are potential hazards to ensnared Snail Kites with this design, including alligator and raptor predation, boat collisions, aspirating water, and drowning. These risks accompany any trapping method targeting aquatic birds, including the net gun. With careful consideration in trap placement and constant vigilance, these dangers can be minimized or avoided. Trauma from moving trap parts, which is a particular risk inherent in use of a net gun, is not a concern with the aquatic bal-chatri. We believe the aquatic bal-chatri 728 Southeastern Naturalist Vol. 9, No. 4 is safe, effective, and efficient relative to methods previously considered or used for trapping free-flying Snail Kites. This trap could be readily adapted for other species of birds that capture aquatic prey by replacing the mesh with material that would contain live fish, using appropriately sized monofilament and nooses, and enlarging the trap area as needed. For example, the trap could be baited with fish to capture Pandion haliaetus L. (Osprey) (Palmer 1988) and Haliaeetus leucocephalus L. (Bald Eagle) (Stalmaster 1987), or with crayfish (Procambarus spp.), frogs, and turtles to capture Buteogallus anthracinus Deppe (Common Black-Hawk) (Schnell et al. 1988), Strix varia Barton (Barred Owl) (Smith et al. 1983), and Circus cyaneus L. (Northern Harrier) (MacWhirter and Bildstein 1996). The aquatic bal-chatri also could be effective for capturing Aramus guarauna L. ( Limpkin), given the prevalence of Apple Snails in their diet (Bryan 2002). Acknowledgments The Florida Cooperative Fish and Wildlife Research Unit and US Geological Survey personnel at the University of Florida provided logistical and equipment support. Trapping and marking were conducted under Station Permit #21465-0 to the Florida Cooperative Fish and Wildlife Research Unit. Primary funding was a grant to K.D. Meyer from the US Fish and Wildlife Service, with additional support from the St. Johns River Water Management District, South Florida Water Management District, and the Felburn Foundation. We thank three anonymous reviewers for helpful comments on previous drafts and Heather Mahoney for producing the illustrations. Literature Cited Bebe, F.L., and H.M. Webster. 2000. North American Falconry and Hunting Hawks. 8th Edition. North American Falconry (privately published). Beissinger, S.R. 1983. 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