J. Field Ornithol. 78(4):395–400, 2007 DOI: 10.1111/j.1557-9263.2007.00128.x

Use of Willapa Bay, Washington, by shorebirds andwaterfowl after Spartina control efforts

Kim Patten1 and Carol O’Casey

WSU Long Beach Research and Extension Unit, 2907 Pioneer Road, Long Beach, Washington 98631, USA

Received 17 November 2006; accepted 1 May 2007

ABSTRACT. Over the past 15 yr, an invasive cordgrass, Spartina alterniflora, has covered more than 5000 haof tidal mudflats in Willapa Bay, Washington, threatening key shorebird habitat on the Pacific Flyway. Althoughchemical and mechanical control methods have been used to manage Spartina in Willapa Bay, little is known abouthow these methods affect subsequent use by shorebirds and waterfowl. During 2003–2004, four sites were monitoredfor 10-min periods for use by shorebirds [Dunlin (Calidris alpina), Western Sandpiper (C. mauri), Least Sandpiper(C. minutilla), Black-bellied Plovers (Pluvialis squatarola) and Long-billed and Short-billed dowitchers (Limnodromussp.)] and waterfowl [Mallard (Anas platyrhynchos), Gadwall (A. strepera), American Wigeon (A. americana), Green-winged Teal (A. crecca), and Canada Goose (Branta canadensis)]. The four sites were bare mudflat (never infestedwith Spartina), tilled Spartina meadow (free of live Spartina and dead stubble), herbicide-sprayed Spartina meadow(10% living Spartina and 30% dead stubble), and an untreated Spartina meadow. Untreated Spartina meadow wasrarely used by birds, with a mean of seven small Calidris sandpipers (peeps) and 0.8 waterfowl ha−1. Mean numbersof peeps and waterfowl observed using the herbicide-treated Spartina meadow were 62 and 16 ha−1, and the tilledSpartina meadow were 700 and 27 ha−1, respectively. Mean use of the adjacent bare mud site by peeps and waterfowlwas 450 and 11 ha−1, respectively. The untreated Spartina meadow, sprayed meadow, tilled meadow and the baremud site had mean densities of Black-bellied Plovers and dowitchers of 26, 24, 6, and 0 ha−1, and 0.8, 6, 0.3, and 0ha−1, respectively. Our results demonstrate that, within several years of removal of invasive Spartina from mudflatsin a large Washington estuary by either mechanical or chemical means, use by shorebirds and waterfowl will increasedramatically.

SINOPSIS. Uso de la Bahıa Willapa, Washington por playeros y patos luego de un esfuerzopor controlar a Spartina

Durante los ultimos 15 anos la yerba invasiva Spartina alterniflora ha cubierto mas de 5000 ha de lodazales en laBahıa de Willapa en Washington, amenazando un habitat de gran importancia para los playeros que usan la vıa delPacıfico. Aunque se han utilizado controles quımicos y mecanicos para manejar a la yerba, se sabe muy poco de comodichos metodos afectan el uso subsecuente del area por parte de playeros y aves acuaticas. Durante el 2003–2004,se monitorearon cuatro localidades por periodos de 10 minutos ara determinar el uso de parte de Calidris alpina, C.mauri, C. minutilla, Pluvialis squatarola, Limnodromus sp. y anseriformes tales como Anas platyrhynchos, A. strepera,A. americana, A. crecca y Branta canadensis. Los habitats monitoreados incluyen lodazales limpios (nunca cubiertospor Spartina), praderas cultivadas (libres de Spartina viva con organismos muertos), pradera tratada con yerbicida(10% de Spartina viva y 30% de organismos muertos) y lodazal con Spartina sin recibir tratamiento. Las areas queno fueron tratadas fueron utilizadas muy rara vez, con un promedio de 7 playeros y 0.8 patos por ha. El numeropromedio de playeros y patos observados en areas tratadas resulto ser de 62 y 16 ind/ha, respectivamente y en laspraderas cultivadas libres de Spartina se encontraron 62 y 16 ind/ha y 700 y 27 ind/ha, respectivamente. El usopromedio de lodazales desnudos de parte de playeros y patos lo fue de 450 y 11 ha−1, respectivamente. El areade Spartina sin tratar, area tratada con yerbicida, la pradera cultivada y el lodazal desnudo tuvieron una densidadpromedio de P. squatarola y Limnodromus sp. de 26, 24, 6 y 0 ha−1 y 0.8, 6, 0.3 y 0 ha−1, respectivamente. Nuestrosresultados demuestran que si se remueve la Spartina de lodazales en el gran estuario de Washington, ya sea por formaquımica o mecanica, el uso de estas areas por playeros y anseriformes va a incrementar dramaticamente.

Key words: Dunlin, estuary, habitat, herbicide, mudflats, restoration, Pacific Flyway, Western Sandpiper

Spartina alterniflora is a nonnative, perennial,deep-rooted saltmarsh cordgrass that has colo-nized (>5000 ha) the tidal mudflats of WillapaBay, Washington. S. alterniflora spreads clonallyfrom underground rhizomes and disperses longdistances by root fragments and floating seeds.

1Corresponding author. Email: pattenk@wsu.edu

As rhizomes intermingle, clonal circular patchesgrow together to form dense, tall (2 m) meadowsthat entrap sediments, raising the elevation ofthe tidelands and transforming mudflats intoemergent monotypic marshes (Sayce 1988). In-vasive Spartina can act as an “ecosystem en-gineer,” converting vast areas of unvegetatedmudflats into Spartina meadows (Crooks 2002).

C©2007 The Author(s). Journal compilation C© 2007 Association of Field Ornithologists

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396 K. D. Patten and C. E. O’Casey J. Field Ornithol.

Much of the most-preferred shorebird habitat ofWillapa Bay, sheltered upper tidal mudflats, hasbeen displaced by large meadows of Spartina.Willapa Bay is a key shorebird stopover site alongthe Pacific Flyway (Audubon 2004), supportingup to 15.5% of the flyway’s wintering Dun-lin (Calidris alpina) population and an averagespringtime population of over 100000 shore-birds (Buchanan and Evenson 1997). Studies ofshorebird abundance in Willapa Bay in 1991–1995, prior to increased Spartina growth, re-vealed that 44% of the total bird use was intwo areas: the Bear River/Lewis Unit (BRLU) inthe South Willapa Bay region and the WillapaRiver area (Buchanan and Evenson 1997). Peakuse of these sections of the bay by shorebirdsduring winter and spring has declined over60% in the past decade as Spartina meadowshave replaced tidal mudflats (C. Stenvall, pers.comm.).

The long-term ecological impacts of invasiveSpartina, specifically S. anglica, on marshes andthe impact of control efforts on shorebirds havebeen studied primarily in England (Evans 1986,Gray et al. 1997, Lacambra et al. 2004). DuringSpartina expansion, the benthic infauna of thetidal mudflat declines relative to that of nearbytidal mudflats (Millard and Evans 1984). ThisSpartina-associated decline in benthic inverte-brates appeared to particularly impact shorebirdprey, specifically surface feeders, whereas sub-surface feeding invertebrates were not affected(Neira et al. 2006). Following depletion, thereis a decline in tide-edge-feeding species, suchas Dunlins and Sanderlings (C. alba; Goss-Custard and Moser 1988). Spartina also affectswaterfowl, notably wigeon (Anas sp.) and Brant(Branta bernicla), by replacing eelgrass habitat(Gray et al. 1997).

The response of shorebirds to the removal ofSpartina from English estuaries has been unpre-dictable. After herbicides were used to controlSpartina, there was a dramatic increase in shore-bird populations (Evans 1986). However, wherethere was a natural die-off of S. anglica, shore-birds failed to return (Goss-Custard and Moser1988). Although both chemical and mechanicalcontrol methods have been used to manageSpartina in Willapa Bay, little is known abouthow these methods affect habitat restoration forbirds. Given the different control methods usedin Willapa Bay, an opportunity existed to studythe effects of treatment types on shorebird use.

Our objectives were to quantify shorebird useand determine species’ preferences in Spartina-affected tidal mudflats as a function of Spartinacontrol methods and to determine the potentialimpact of the Spartina invasion on the use ofWillapa Bay by migrating birds.

METHODS

Willapa Bay is a large, shallow bar-built es-tuary that covers an area of 347 km2 at meanhigher high water (MHHW) and 191 km2 atmean lower low water (MLLW). The tidal rangebetween MHHW and MLLW is 2.4 to 3.4 m.Our study was conducted at the BRLU of theWillapa National Wildlife Refuge at the southend of Willapa Bay, Washington (46 N, 124 W).Sediment particle size in the BRLU tidal mudflatis between 10 and 100 �m (Boe 2007).

The Spartina meadow at the BLRU (approx-imately 1000 ha) has been established for over10 yr. Different control efforts began at differenttimes in BRLU. From July to October 2000, a200-ha area in the central area of the meadowwas mowed 2–4 cm above the sediment surfaceusing an amphibious double-tracked (0.9 m)vehicle equipped with a 2.8-m flail hammermowing head. During the winter and spring of2001, the meadow was tilled to a depth of 4–6 cm using a front-mounted rototiller on theamphibious vehicle. The site was shallow-diskedduring winter 2002 for additional Spartina con-trol, followed by spot treatment with glyphosate(22 kg/ha) during the summers of 2003 and2004 for control of outliers and new seedlings.This area has been relatively free of Spartina anddead stubble since 2002.

Beginning in 2002, a 50-ha area east of thetilled area was treated with herbicides. Themeadow was broadcast-treated with glyphosate(9 kg/ha) in 2002 and 2003 and imazapyr(1.7 kg/ha) in 2004. As a result, the area wastransformed from a solid meadow with 2-mhigh Spartina to an area approximately 60% freeof Spartina and residual stubble, approximately30% dead Spartina with residual dead stubble,and 10% living Spartina.

We compared use of these two sites by birdsto that of an untreated section (approximately300 ha) of Spartina meadow west of the sprayedsite and a Spartina-free tidal mudflat (approxi-mately 100 ha) located east of the tilled site. Alltreatment sites had similar elevations prior to

Vol. 78, No. 4 Avian Use of Willapa Bay after Spartina Control 397

Spartina invasion. Post-invasion, however, theSpartina-affected tidal mudflat was about 0.3-mhigher with tidal inundation >30 min later thanthe adjacent tidal mudflat. These four treatmentareas were selected for comparison because theywere the first large treated sites in the bay, adja-cent to each other, large enough to be consideredecologically significant units (approximately 50ha), and were easily accessible for unrestrictedviewing.

Bird census. We monitored bird use dur-ing major migration periods from October 2003to December 2004. At each study site, 100 m ×100 m subplots were randomly established using3-m sections of PVC pipe to mark the corners(N = 3 tidal mudflat, tilled and herbicide-sprayed sites, and N = 6 for untreated Spartinameadow). Due to the difficulty of equipmentsetup and regular monitoring in the soft mudconditions, we counted birds in each subplotusing a modified area count method (Bibby et al.2000). A single observer monitored each subplotfrom a dike road approximately 130 m fromthe plots using a spotting scope with a 20–60×lens. All birds already present or landing in thesubplot during a 10-min interval were counted.We visited subplots in random order within a2-h period of the optimal observing time (justprior to tidal coverage or tidal withdraw) andvisits were systematically rotated so that sub-plots were observed at different times of tidalcoverage.

Observations were made at least once a week,with a total of 34, 35, 35 and 31 observationdays for bare mud, tilled, sprayed, and un-treated Spartina meadow, respectively. Density(number per hectare) for each species or groupof birds was determined. When small Calidrissandpipers, including Dunlins, Western Sand-pipers (C. mauri), and Least Sandpipers (C.minutilla), were indistinguishable due to dis-tance or weather conditions, they were groupedcollectively as “peeps” (Withers and Chapman1993). Long-billed and Short-billed dowitch-ers were identified only to genus (Limnodro-mus sp.). Several shorebird species, includingMarbled Godwits (Limosa fedoa), Willet (Catop-trophorus semipalmatus), Semipalmated Plovers(Charadrius semipalmatus), and Greater (Tringamelanoleuca) and Lesser (T. flavipes) yellowlegs,were either not observed in large enough num-bers (< 0.5% of total) or sightings were too infre-quent to make valid inferences about treatment

effects and were not included in our analyses. Foranalysis, nonshorebird species were categorizedas ducks and geese, and the most frequentlyobserved species were Mallard (Anas platyrhyn-chos), Gadwall (A. strepera), American Wigeon(A. americana), Green-winged Teal (A. crecca),and Canada Goose (Branta canadensis). Fewerthan 1% of observed waterfowl were NorthernPintails (A. acuta) and Northern Shovelers (A.clypeata).

Statistical analyses. Overall differencesbetween site treatments were determined usingnonparametric single-factor analysis of variance(ANOVA) of rank-transformed data (Zar 1996,SAS Institute 2006). Data used for analysis ofvisual observations were mean daily subplot den-sities. Daily means were rank transformed, andmeans were compared using Duncan’s multiplerange test.

Although the sites we selected for monitoringpermitted comparison of observations withinthe same time frame, our study design had severallimitations. First, treatment sites were not repli-cated because there were no other large treatedsites in the bay. Second, control treatments werenot implemented at the same time. The tilled sitewas largely free of Spartina for 3 yr before theherbicide-treated site. Therefore, comparisonsbetween sites were limited. Third, obtaining pre-cise counts of shorebirds and waterfowl duringthe 10-min interval was problematic. Countswere made of birds on the ground and any time alanding occurred. To minimize double countingof birds, adjacent plots were scan-sampled simul-taneously. On numerous occasions, due eitherto skittish behavior or flushing by raptors, birdsreturned to the same site. Efforts were made notto double count these birds. For the untreatedSpartina meadow, the tall dense canopy obscuredbirds on the ground under the canopy. Becauseno shorebirds were observed flying into or outof the Spartina canopy on our study plots, weassumed there was little likelihood of missingbirds under the canopy.

RESULTS

Densities of birds observed differed withspecies and among Spartina-treatment types(Table 1). Depending on species or group, shore-birds either used the tilled site significantly morethan all other sites or used the tilled and baremud sites equally and significantly more than

398 K. D. Patten and C. E. O’Casey J. Field Ornithol.

Table 1. Overall mean densities (±1 SE) of shorebirds and waterfowl observed (visual observations) in thefour types of Spartina treatment sites in Willapa Bay, Washington.

Mean bird density (bird ha−1)a

Species or group Bare mud Tilled Sprayed Spartina meadow

Peepsb 450 ± 56a 700 ± 58b 62 ± 13c 7 ± 7dWestern Sandpiper 196 ± 28b 340 ± 39a 50 ± 12c 5 ± 4dDunlin 151 ± 32a 229 ± 36a 5 ± 4b 0bBlack-bellied Plover 26 ± 6a 24 ± 4a 6 ± 2b 0.3 ± 0.3bDowitchersc 0.8 ± 0.3b 6 ± 2a 0.3 ± 0.2b 0bWaterfowld 11 ± 4b 27 ± 8a 16 ± 2b 0.8 ± 0.3c

aMeans with the same letter within a row were not significantly different (Duncan’s multiple range test atthe 0.05% level after nonparametric ANOVA of rank transformation data).

bWestern and Least sandpipers and Dunlin were combined when identification to species was not possible.cWe did not distinguish between Short-billed and Long-billed dowitchers.dWaterfowl included ducks (Mallard, Gadwall, American Wigeon, and Green-winged Teal) and Canada

Geese in an approximate 10:1 ratio.

the sprayed and untreated sites. Waterfowl usedtilled sites significantly more than all other sites,and used bare mud and sprayed sites less of-ten, but to similar degrees. Untreated Spartinameadow was rarely used by either shorebirds orwaterfowl.

DISCUSSION

Our results were similar to those of previousstudies (Goss-Custard and Moser 1988, Grayet al. 1997, Lacambra et al. 2004), with Spartinahaving a negative effect on the use of tidal mud-flats by shorebirds and waterfowl. However, ourresults also demonstrate that efforts to controlSpartina can restore shorebird habitat. Thereappears to be a clear relationship between theamount of vegetative cover and the level ofbird usage. The winter-tilled site, largely freeof Spartina, was used by birds more than theherbicide-treated site that still contained someSpartina, and the herbicide-treated site was usedmore than the Spartina meadow. These dif-ferences may be accounted for by an increasein prey accessibility (Percival et al. 1998) anddecrease in threat from avian predators as veg-etative cover is removed (Dekker and Ydenberg2004). A large thick stand of Spartina is virtu-ally inaccessible for shorebird foraging, while apatchy stand of partially controlled Spartina anddead stubble is accessible, but impairs shorebirds’ability to detect potential predators (Smith andEvans 1973). Numerous studies have reported anegative correlation between shorebird foraging

and vegetation height (Colwell and Dodd 1995,Shepherd and Lank 2004) and a positive cor-relation with distance from shoreline vegetation(Pomeroy 2006). These correlations result fromshorebirds attempting to minimize the risks ofpredation (Shepherd and Lank 2004, Pomeroy2006).

Prey accessibility and vegetative cover, how-ever, do not account for the greater amountof shorebird usage of tilled Spartina meadowthan the bare mudflat. Our unpublished dataon prey density, as well as that of Evans (1986)in England and Kriwoken and Hedge (2000)in Tasmania, indicate that invertebrate densitiesare reduced after Spartina control. Our results,and those of Colwell and Landrum (1993), alsoindicate that prey density on the small spatialscale within an estuary is only weakly correlatedwith shorebird density. A more plausible expla-nation for the greater use of the tilled site maybe temporal and spatial in nature. The tilledSpartina meadow flooded >30 min after theadjacent bare mudflat and was the last accessibleforage area in the bay during tidal inundation.

Differences in the timing of treatments didnot permit us to compare control efforts interms of their efficacy in expediting restoration.We can only infer that, regardless of controlmethod, removal of Spartina results in increaseduse by birds within a few years. From a practicalperspective, tilling of Spartina meadows is cost-prohibitive, requires specialized equipment, andis slow (<0.5–1 ha/d), whereas hundreds ofhectares of Spartina meadows can be aerially

Vol. 78, No. 4 Avian Use of Willapa Bay after Spartina Control 399

sprayed with herbicide per day in a cost-effectivemanner.

Our data indicate that shorebirds and wa-terfowl respond positively to Spartina control,but also suggest that these sites have not beenrestored to fully functional tidal mudflat habitat.Spartina expansion in tidal mudflats can exertphysical, chemical, biotic, and trophic changeson the benthic system, and create a cascade effecton an entire ecosystem (Neira et al. 2006). Forexample, sediment accumulation due to trap-ping of silt by Spartina canopy causes landscape-scale changes in elevation (Swales et al. 2004,Boe 2007). These elevation changes can be fur-ther stabilized if native marsh species becomeestablished. At our study sites, we have observedTriglochin maritimum and Salicornia virginicagrowing 300–600 m into mudflats previouslyinfested with Spartina within 1–2 yr after suc-cessful control. If Spartina-affected mudflatstransition to stable salt marshes, it is unlikelythat they will become functional tidal mudflatsagain. However, if mudflats have not undergonemajor Spartina-induced elevation changes, habi-tat restoration is possible. This would requirethat Spartina be controlled at the initial phase ofits infestation of a mudflat.

Even though use of Spartina-affected mudflatsby shorebirds can be restored after control, littleis known about the long-term implications ofa 1000-ha Spartina meadow succeeding whatwas once prime shorebird habitat. The effects oflarge-scale changes in elevation, sediment com-position, and post-control succession of nativemarsh plant species have yet to be determined.Understanding the impact of habitat restorationon shorebirds will require more detailed, multi-disciplinary, long-term studies.

ACKNOWLEDGMENTS

Our research was sponsored by Willapa WildlifeRefuge, Migratory Birds and Habitat Programs—PacificRegion of U.S. Fish and Wildlife Service, Pacific JointVentures, and Ducks Unlimited. We thank M. Evans forstatistical consulting, and the reviewers and editor for theirvaluable comments and assistance.

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