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A Modern, Portable Drop Net Can Safely Capture a Suite ofShorebirdsAuthor(s): J. Peter DohertySource: Waterbirds, 32(3):472-475. 2009.Published By: The Waterbird SocietyDOI: http://dx.doi.org/10.1675/063.032.0316URL: http://www.bioone.org/doi/full/10.1675/063.032.0316

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472

A Modern, Portable Drop Net Can Safely Capture a Suite of Shorebirds

J. PETER DOHERTY

214-B 84th Street, Virginia Beach, Virginia, 23451, USA

E-mail: caspiantern@gmail.com

Abstract.—Incorporating lightweight synthetic lines, nylon fish netting and sailboat and fishing hardware, thedrop net is an effective, portable method to trap many different species of shorebirds. During 2006 and 2007 dropnets were used to safely capture more than 265 targeted shorebirds in habitats in North and South America. Shore-birds captured included Calidris sandpipers, Charadrius sandpipers, Buff-breasted Sandpiper (Tringites subruficollis),and dowitchers (Limnodromus). Received 5 January 2008, accepted 23 February 2009.

Key words.—bird banding, bird trapping, Calidris sandpipers, Charadrius sandpipers, dowitcher.

Waterbirds 32(3): 472-475, 2009

Shorebird researchers have adapted anddeveloped a variety of methods to captureshorebirds. Some methods evolved from theearly Dutch “wilsternet” and now include var-ious pull, clap, bow, rocket and cannon nets,some powered by springs, bungee cords andgunpowder. Additional methods include:mist nets, walk-in, funnel and fall traps, daz-zle lighting, noose carpets and drop nets.(Bub 1991; Davis 1981) The method selecteddepends upon the goals and needs of theproject. Some traps are portable while otherslack mobility. Some are invaluable on breed-ing grounds, others are effective on foragingflats or roosting areas. Some are capable ofcatching large numbers of birds at once andneed personnel and equipment to processbirds quickly, while others are capable ofcatching modest numbers of birds periodi-cally without additional resources.

Drop nets have been used to trap prairiechickens and quail (Jacobs 1958), GreaterSage-Grouse (Bush 2008) and phalaropes(Johns 1963) and are discussed and illustrat-ed in Bub (1991). Such drop nets capturedtargeted shorebirds in a range of habitatsduring the northern and austral springs of2006 and the northern summer of 2007. Thenets used materials and hardware which im-proved durability, speed of set-up, adjust-ment and take-down and allowed the net tobe tensioned and released quickly.

METHODS

The portable drop net is a rectangular net suspend-ed in the horizontal plane by tension from a single con-

trol line to a central ring (Fig. 1). The ring is attachedto each of the four corners of the net via lines andblocks which are attached by lanyards to four posts driv-en into the earth. The net drops on the birds when thetension is released manually and gravity brings the netto the ground.

Eleven drop nets were built of green, nylon fish net-ting, with the stretched mesh measuring 2.5 cm or 5.3cm. Such material is more durable than mist netting ofthe same mesh used in early drop nets (Johns 1963).The bolts of netting, which are ~5 m in width of 210 de-nier, 6 ply, thread, were cut into lengths of 11.5 m. No

Figure 1. Overhead view of drop net and rigging.

DROP NET CAPTURES SHOREBIRDS 473

nets were dyed. Black, 0.3cm, parachute cord was woveninto the perimeter of the net and each corner was tiedjoining the cord and netting to allow the net to with-stand the loads generated when the net is raised and settaut. The parachute cord was tensioned along thelength and width of the netting to allow the net to main-tain a nearly rectangular shape when the net was raised.

Sturdy metal fence posts or industrial steel “T” bar, 2m long, were used rather than wooden stakes (Johns1963). Parachute cord was used for the lanyards; thecorner lines were parachute cord and 0.5 cm braidednylon line. The control line was 0.7 cm braided nylonline. The central ring was stainless steel, 5 cm diameter.

Light blocks, commonly used on small sailboats,were secured to the ends of the four lanyards tied to thefour posts rather than rings (Johns 1963). The four cor-ner lines turned on these blocks to eliminate bindingand wear. Further, the use of numerous, small lead splitshot (Johns 1963) was eliminated for environmentalreasons (easily lost and ingested) and to facilitate setupand take down. Instead, six 130 g fishing sinkers mount-ed on six quick-attaching, stainless steel clips wereplaced on the net when set; one on the corner line ateach corner and one at each mid-point on the para-chute cord along the length of the net. These weights al-lowed the perimeter of the net to fall quickly, preventescapes and diminish wind drift of the net. The weightswere removed before the net was taken down.

A 1.25 m length of heavy angle iron was used to an-chor the control line, usually > 100 m from the net. Con-trol line loads were reduced by configuring two lightblocks together with an open-faced clamcleat as a blockand tackle at the post end of the control line. A clam-cleat is a plastic or carbon-fiber cleat which has no mov-ing parts. The cleat grips the line by virtue of the loadon the control line and the v-shape of the cleat’s moldedshape.

Depth perception was negligible from the anchorstake, even through binoculars. Therefore, the decisionto drop the net was tricky. Sticks or brightly-colored tentstakes were placed on the ground outside the near andfar edges of the net to provide a frame of reference. Ifwind was blowing, stake placement allowed for estimat-ed net drift. Stakes were not needed to mark the sides ofthe net, and experience managed any wind-driven sidedrift. If a bird was within a meter or so of the center lineof the net and inside the stakes, the birds were capturedif the control line was released.

The line was removed from the cleat by hand, heldmomentarily while birds were checked by optics and,when appropriate, released by throwing the line to-wards the net, out and away from the holder. Upon re-lease the net was approached immediately. Whentrapping over shallow water, it was occasionally neces-sary to lift by hand a small portion of the net to allow acaptured bird to regain its footing under the net. Ex-traction techniques and processing practices recom-mended for shorebird banding were followed.

Study Areas

Drop nets were used in April and May 2006 duringshorebird migration on playa wetlands of the easternRainwater Basin of south-central Nebraska. These wet-lands are scattered about Clay County, Nebraska andconsisted of drying mud flats or shallow water, varying inarea from <1 to fifty ha. Drop nets were used in Septem-ber 2006 during migration in a wetland on the Paraguay

River near Asuncion, Paraguay. The nets were set overthe water-flat interface and in shallow water. To capturewintering birds during October and November 2006,nets were set on moist, short-grass cattle pastures of 500ha or greater near San Javier, Santa Fe Province, Argen-tina, and along Laguna Mar Chiquita, Cordoba Prov-ince, Argentina, a large, shallow, inland salt lake.During November and December 2006, nets were de-ployed along the edge of a coastal lagoon near La Palo-ma, Uruguay, and in rice fields near Arrozal 33,Uruguay, to capture wintering birds. Nets were used tocapture staging birds on exposed, tidal, sand flats inPleasant Bay, Chatham, Massachusetts during Septem-ber 2007.

RESULTS AND DISCUSSION

Each study targeted only certain speciesof shorebirds for capture. More than 265 tar-geted shorebirds were captured, processedand released without injury. Targeted or not,many species walked under the net, usuallywhile foraging, including Snowy Plover(Charadrius alexandrinus), Collared Plover(Charadrius collaris), Killdeer (Charadrius vo-ciferous), American Oystercatcher (Haemato-pus palliates), Ruddy Turnstone (Arenaria in-terpres), Red Knot (Calidris canutus), Sander-ling (Calidris alba), Dunlin (Calidris alpina),Pectoral Sandpiper (Calidris melanotos),White-rumped Sandpiper (Calidris fusicollis),Baird’s Sandpiper (Calidris bairdii), Semi-palmated Sandpiper (Calidris pusilla), LeastSandpiper (Calidris minutilla), Stilt Sandpip-er (Calidris himantopus), Long-billed Dow-itcher (Limnodromus scolopaceus), Short-billedDowitcher (Limnodromus griseus), Buff-breast-ed Sandpiper (Tringites subruficollis) and Wil-son’s Phalarope (Phalaropus tricolor). ManyLeast and some Semipalmated Sandpiperswere able to escape through the 5.3cm meshnets.

While Southern Lapwing (Vanellus chilen-sis), American Golden Plover (Pluvialis do-minica), Greater Yellowlegs (Tringa melanoleu-ca), Lesser Yellowlegs (Tringa flavipes) andWillet (Catoptrophorus semipalmatus) were de-monstrably wary, each species ventured un-der the drop from time-to-time. Finally, pip-its (Motacillidae) and blackbirds (Icteridae)readily fed under the nets in wet and dryshort-grass habitats and rice fields and wereoccasionally caught as “by-catch” in the2.5 cm mesh nets.

474 WATERBIRDS

Birds appeared to be aware of the netsand lines and avoided them in flight, but thisdid not seem to diminish the effectiveness ofthe nets. Terns (Laridae), swallows (Hirun-dinidae), kingfishers (Alcedinidae), blackbirdsand flycatchers (Tyrannidae) perched on thelines and posts.

After captured birds were removed andthe net raised, Calidris sandpipers, Buff-breasted Sandpipers (Tringites subruficollis),phalaropes and dowitchers often returnedto foraging in the immediate vicinity of thenet within five minutes. As long as there wasat least 75 cm clearance between the groundand the net, foraging shorebirds venturedunder the net. When circumstances allowed,nets were left set overnight to increase thelikelihood of capturing otherwise “net-wary”shorebirds, such as Tringa and Pluvialis.

Scouting to locate sites where the targetshorebirds feed and return to feed oncenets are deployed was essential. When pos-sible, small, isolated areas favored by shore-birds were selected over large, unbroken ar-eas. To evaluate a candidate site, activelyfeeding shorebirds were displaced from thesite before retiring 100m or so to observethe behavior of the displaced shorebirdsand other shorebirds in the immediate ar-ea. If birds returned to the candidate site af-ter a brief absence the likelihood of successwas high.

The dynamic, often ephemeral, nature ofmudflats, tidal estuaries, fresh water wet-lands and coastal wetlands which attractshorebirds caused variation in net capturerates. If the conditions were temporary,caused by tidal stage or a change in wind forexample, they were frequently overcomewithout resetting nets by (a) anticipatingand allowing for change in conditions be-fore the initial set, (b) having an array ofdrop nets set in a pattern which anticipatedthe change, (c) using other capture methodssuch as mist nets, whoosh nets (SpiderTech,Helsinki, Finland), and noose carpets inconjunction with drop nets, as conditions al-lowed. To avoid alarming shorebirds feedingabout drop nets, mist nets or other passivetraps which need periodic tending were notset in close proximity to drop nets.

The use of decoys and electronic callsyielded mixed results. Pectoral Sandpipers(Calidris melanotos), mostly pairs and sin-gles in South America and small groups ofup to six in Nebraska, landed near nets inresponse to their recorded calls and asmall group of “peep” decoys. Some werecaptured. Modest numbers of decoys, nomore than eight, evenly spaced to mimicshorebird feeding patterns, were some-times successful with other species. Insome conditions shorebirds, although ini-tially attracted by the decoys, seemed toavoid the decoys once the birds com-menced foraging. For this reason, decoyswere not placed within the catch zone of anet but, rather, up to ten meters from thecatch zone.

Shorebird feeding patterns often causeda single bird foraging under the net to at-tract others. Thereafter, a group of birds un-der the net occasionally caused a wary spe-cies to venture under the net. Thus, subjectto project requirements, daily parameterswere often established with respect to eachtarget species and a minimum number ofthat species required under the net before adrop.

While the drop net required more train-ing to set up and to adjust than a mist net, itcaptured shorebirds safely long after mistnets were furled because of wind, rain orlight conditions. Also, the drop net was oftenset over shallow water to capture wadingshorebirds; this was not possible with a wetwhoosh net for reasons of bird safety. Over-all, the modern, portable drop net providesa useful technique for a single field ornithol-ogist to capture selectively and safely in vari-ous habitats a variety of shorebirds in reason-able numbers.

ACKNOWLEDGMENTS

J. McCarty and L. Wolfenbarger of the Universityof Nebraska (Omaha) allowed the latitude to experi-ment with shorebird capture techniques, new andold, in the Rainwater Basin. K. Strum and KansasState University provided the opportunity to use the“modern” drop net in South America. B. Harringtonencouraged trying the drop on Red Knots inChatham. S. A. Scoville and J. Fraser made editorialcomments.

DROP NET CAPTURES SHOREBIRDS 475

LITERATURE CITED

Bub, H. 1991. Bird trapping and bird banding: A hand-book for trapping methods all over the world. Trans-lated by F. Hammerstrom, K. Wuertz-Schaefer.Cornell University Press, Ithaca, New York.

Bush, K. L. 2008. A pressure-operated drop net forcatching Greater Sage-Grouse. Journal of Field Or-nithology 79: 64-70.

Davis, P. L. 1981. Trapping methods for bird ringers.British Trust for Ornithology, Tring, Hertford-shire.

Jacobs, K. F. 1958. A drop net trapping technique forGreater Prairie Chickens. Proceedings of the Okla-homa Academy of Science 38: 154-157.

Johns, J. E. 1963. A new method of capture utilizingthe mist net. Journal of Field Ornithology 34: 209-213.