Effects of Predator Exclosures on Nesting Success of Killdeer

j. Field Ornithol., 53(3):263-268

EFFECTS OF PREDATOR EXCLOSURES ON NESTING SUCCESS OF KILLDEER

BY ERICANOL ANDRONALD J. BROOKS Nestsof shorebirdsare often destroyedby predatorsand in some instancespredation may causesevere local declines in breedingsuccess and in sizeof a breedingpopulation (Hussell and Montgomerie1966, Cartar 1976). Protectingnests with exclosuresmay be one meansof reducingpredator impact on small, threatened,breeding populations of shorebirds.The objectivesof this studywere: (1) to determinethe extentof predationby gulls(Larus spp.) on nestsof Killdeer (Charadrius vociferus),(2) to determineif exclosuresprotect Killdeer nestsfrom predation,and (3) to developrecommendations regarding use of exclosures to protect nestsof shorebirds.

METHODS The studywas conducted on the westernthird of LongPoint, Ontario (42ø34'N, 80ø16'W),a 33-km peninsulaprojecting eastward from the north shoreof Lake Erie. Killdeer nestedon wide sandybeaches and on pebbledislands formed by watercutting through the peninsulafrom Lake Erie to Long Point Bay. The exclosureused in the experimentswas developed from trialsthat tested severaldesigns. The selecteddesign (Fig. 1) was the one that Killdeer entered mostreadily and that seemedmost immune to predators.This exclosurewas constructed of gray, 1.4-cmmesh hardware clothand had eight7 x 12-cmopenings. The sizeof the openingspro- hibited gulls and other relativelylarge predatorsfrom entering,but allowedKilldeer to enter and exit rapidly. When an exclosurewas placed on a nest, all entrances were more than 30 cm from the nest. The indentationsbetween the halvesof the exclosure(Fig. 1) allowed the Killdeer to see their nest from outside the structure. The wire metal platesabove the 2 entrancesin theseindentations made it more difficult for somemammalian predators to reach the nest through theseen- trances. We placed exclosureson 12 neststhat were selectedat random from 29 nestsdiscovered. Each exclosure was held in placeby 12 wire stakes. After placement,each exclosure was observed from a distanceof 50 m, and we recordedhow long it took the returning Killdeer to enter the exclosure and sit on the nest. The time was recorded from when the returning bird first approachedto within 10 m of its nest. All nestswere checked every 2 days.During thesechecks, the observer remainedmore than 3 m from the nest to reduce the possibilityof attractingpredators to the nest. When nestswere lost to predators,re- mains of egg shells,general conditionof the nest, and trackswere recorded to identify the predator (Rearden 1951).

263 264] E. Nol andR. J. Brooks J.Field Ornithol. Summer 1982

J( im

ENTRANCE (lof8) - NEST x

METAL PLATE (1of 2) FIGURE1. Two viewsof exclosureused in the experiments.(a) Lateralview--dotted line indicatesupper surface of metalwire platesabove the 2 entrancesclosest to the nest. (b) Dorsalview--showing location of nestwhen exclosurewas in place.

The calculatednumbers of destroyednests (Mayfield 1975), based on lossrates for eachtreatment, were comparedby a Chi-squaretest. The calculatednumber of destroyednests subtracted from the total number of nestspresent equalled the calculated number of successfulnests. This Vol.•$, •o. $ KilldeerPredator Exclosures [265

TABI.W1. Comparisonof number of nestslost in exclosedand unexclosedtreatments.

Exclosed nests Unexclosed nests

Total nestdays 216 245 Total number of nests observed 12 17 Actual number of nests lost 8 12 Calculated number of nests lost *a 7.6 12.6 Expectednumber of nestslost a 8.4 11.8

* Calculatednumber ofexclosed and unexclosed nests lost was not significantly different than expected,X 2 = .13, P • .05. • After Mayfield (1975). lattervalue, expressed as a percent,was used as a measureof nestsuc- cess. Twelveadditional nests located on the mainlandnear Long Point, but outsidethe studyarea, were monitored until hatching.These nests were visitedevery 2 days,but at eachvisit the eggswere handled. Gull censuseswere conducted from 20 May to 21 July 1978at 5-day intervals.Numbers and speciescomposition of flockswere recorded whenbirds were resting on beachesor nearshore. All gullcensuses took placebetween 1100 and 1500. A seriesof 1-m2 sandplots was constructed at 10-mintervals along thebeach to estimatethe level of activityof mammalianpredators (Bider 1968).Eighteen plots extended 180 m east-westand 10 plotsextended 100 m north-south.Plots were cleareddaily between2000 and 2200 witha woodenboard and were checked the followingmorning between 0700 and 0900.Consequently, only nocturnal and crepuscularactivity were monitored.Only tracksof mammalswere identified.

RESULTS After an exclosurewas placed on a nest,Killdeer approachedthe nest cautiouslyand usuallycircled the exclosurebefore entering it. The mean intervalbetween approach and initial entry was 13.8 +__0.12 min (95% C.I.). At eachof the 12 protectednests, both parentsentered the exclo- sureand appearedto incubatenormally. All observednest losses were caused by predators.There wasno sig- nificantdifference in overalllosses to predatorsbetween nests with and without exclosures(Table 1). On Long Point nestswithout exclosures had a 25.9% calculated nest success.On the mainland, nests without exclosureshad a 64.0% success,and all lossesappeared to be causedby dogs(Canis familiaris). Of 17 unprotectednests on LongPoint, 5 (29.4%)were destroyed by gulls,4 (23.5%)by raccoons,and oneeach (5.8%) by a mustelid,a snake, and a CommonCrow (Corvus brachyrhynchos). The calculatednumber of LongPoint nestslost to gullswas significantly less for exclosedthan for unexclosednests (Table 2). Mammalsdestroyed more exclosedthan 266] E. Noland R. J. Brook6 J.Field Ornithol. Summer 1982

T^BLV,2. Comparisonof number of nestsdestroyed by gulls and mammalsin exclosed and unexclosedtreatments. Total nest-daysand nestsas in Table 1. N is samplesize.

Exclosed nests Unexclosed nests

N = 12 N = 17

Gulls Actual number of nests lost 0 5 Calculated number of nests lost* 0 7.25 Expectednumber of nestslost 3.06 4.34 All mammals Actual number of nests lost 8 5 Calculated number of nests lost 'xs 7.66 7.25 Expectednumber of nestslost 6.45 9.14 Raccoons Actual number of nests lost 6 4 Calculated number of nests lost Ns 6.39 6.10 Expected number of nestslost 5.36 7.59

*Differences betweentreatments were significant(X 2 = 5.01; P (.025). •s Differencesbetween treatments were not significant(P • .05).

unexclosednests, but this differencewas not significant(Table 2). The number of nestsdestroyed by raccoonsdid not differ betweentreatment groups(Table 2). Raccoons(Procyon lotor) destroyed nests by extending their forelimbsinto the exclosureopenings, whereas mink (Mustelavi- son)and long-tailedweasels (M. frenata) were smallenough to go right into the exclosure and at 2 exclosed nests, mink killed one of the in- cubatingadults. The proportion of nestslost to gulls appeared correlatedwith the number of gulls in the study area from 9 May to 12 July (Fig. 2). Al- though the speciesof gull responsiblefor any particular nest losswas unknown, Ring-billedGulls (Larusdelawarensis) and Bonaparte'sGulls (Larusphiladelphia), accounted for 99% of gulls censused. In the study area, tracks of raccoon,mink, long-tailedweasel, and striped skunk (Mephitismephitis) occurred on the sand plots every day from 1 to 21 July 1978. On average,21% of the 28 sandplots (range 7.0-53.0%) had raccoonor mustelidtracks each day during thisperiod.

DISCUSSION Killdeer adjustedreadily to exclosuresplaced over their nests,but the efficacyof exclosuresin deterring predation depended on the species of predatorspresent. Exclosures reduced nest destruction by gulls,and probablywould be effectiveagainst other potentialavian predators (e.g., crows,Corvus spp; jaegers, Stercorariusspp.). However, mammalsde- stroyed64% of exclosedand 43% of unexclosednests. This difference was not significant,but thesehigh rates suggestthat, despiteour pre- Vol.53, No. 3 KilldeerPredatw • Exclosures [267

lOO ß lO

50 / . \/ ./' .

0 o 9-15 16-24 25-2 3-10 11-18 19-26 27-4 5-12 May May - June June -July Number of nests 10 11 8 9 13 9 7 2

F[CURE2. Lossesof Killdeernests in relationto numbersof gullson the studyarea. Numberof nestsbeneath date. Samples from each date are notindependent because the samenests may be in samplesfrom differentdates. Open squares = percentof nestslost to gulls.Closed circles = percentof nestslost to predation.Closed trian- gles= numbersof gulls. cautionsin monitoringthe nests,mammalian predators may have found nestsby followingthe observer'sscent or perhapsby beingattracted by the exclosures themselves. Predationby raccoonsmight be reducedif the entrancesof the exclosureswere smaller. However, the sizeof entranceused in thisstudy requiredthe birdsto lowertheir headsto enter, and smalleropenings might discouragebirds from enteringat all. Tracksaround predator- destroyednests and on our sandplots indicated that mostraccoon pred- atorswere smalljuveniles. Smaller entrances on the exclosureprobably would not deter theseraccoons from reachinginto the nest(Bellrose et al. 1964). Alternatively,if larger exclosureswere used, the entrances would be far enoughfrom the eggsthat raccoonscould not reach the nest. We recommendexclosures to protect nests of shorebirdsfrom avianpredators, but if raccoonsare present,we suggestthat the exclosuresshould be larger than thosedescribed here. The rate of predationon Killdeer eggsin this studywas high com- pared to ratesreported for other temperatezone nestingshorebirds. Ratesof predationon Killdeer nestsranged from 6% (n = 39 nests; Bunni 1959:92) to 38% (n = 18 nests;Mace 1971:53). In these studies, domesticdogs or unspecifiedpredators were responsible for egglosses. For Piping Plovers(C. melodus)percent predation on the eggsranged from 0% (n = 174; Wilcox 1959:142)to 9% (n = 51; Cairns 1977:66). 268] •. Nol andR. J. Brooks j. FieldOrnithol. Summer 1982

On Long Point,high water levels in springand earlysummer may result in a temporarydecline in the amountof availablehabitat for animals using the beaches(e.g., restinggulls, foraging raccoons,and nesting Killdeer). This reductionof habitatmay result in concentrationof both nestingKilldeer and their potentialpredators.

SUMMARY Wire exclosureswere placed over nestsof Killdeer to test whether thesestructures would reduce predation on the nests.Gulls were a major predatorof Killdeer nestson the Long Point studyarea, and the proportion of Killdeer nestslost to gulls appearedrelated to the number of gulls in the area. Nests provided with exclosureslost significantly fewereggs to gullsthan did nestswithout exclosures, but exclosuresdid not reduce the rate of predationby mammalsor the overall nest loss rate. Such exclosuresshould be modified (enlarged)if they are used to protectnests of shorebirdswhen raccoonsare present.

ACKNOWLEDGMENTS We thank A. Lambertand C. Risleyfor assistancein the field. D. J. T. Hussell,A. L. A. Middleton,and D. L. G. Noakesprovided comments and criticismsthroughout the study.Financial support was provided by NSERC (grant no. A 5990) and by the CanadianWildlife Service(con- tracts--KL229-8-7090and KL229-9-4135) to R. J. Brooks.

LITERATURE CITED

BV,LL•OSV,, F. C., K. L. Jo•ssoN, ^NI• T. U. Mv,¾v,•s.1964. Relativevalue of natural cavitiesand nestinghouses for WoodDucks. J. Wildl. Manage.28:661-676. BIDER,J.R. 1968. Animalactivity in uncontrolledterrestrial communities as determined by a sandtransect technique. Ecol. Monogr. 38:269-308. BUNNI, m. K. 1959. The Killdeer, Charadriusv. vociferusL., in the breeding season: ecology,behavior, and the developmentof homiothermism.D.Sc. thesis,Univ. of Michigan, Ann Arbor, Michigan. C^I•NS,W. E. 1977. Breedingbiology and behaviourof the PipingPlover (Charadrius melodus)in southern Nova Scotia. M.S. thesis, Dalhousie Univ., Halifax, Nova Scotia. C^•T^•, R. 1976. The statusof the PipingPlover at Long Point,Ontario, 1966-1975. Ont. Field Biol. 30:42-45. HUSSELL,D. j. T., ANDR. D. MONTGOMERIE.1966. The statusof the Piping Plover at Long Point, 1960-1965. Ont. Field Biol. 20:14-16. M^cv,,T. R. 1971. Nest dispersionand productivityof KilldeersCharadrius vociferu•. M.S. thesis, Univ. of Minnesota. M^¾FIV,LI•, H. F. 1975. Suggestionsfor calculatingnest success. Wilson Bull. 87:459- 466. Rv,^•I•V,N,J. D. 1951. Identificationof waterfowlnest predators. J. Wildl. Manage. 15: 386-395. WILCOX,L. 1959. A twenty-yearbanding study of the PipingPlover. Auk 76:129-152. Departmentof Zoology,University of Guelph,Guelph, Ontario, Canada NIG 2WI. Received9 Mar. 1981; accepted20 Jan. 1982.