The Auk 111(3):579-587, 1994

NOCTURNAL FORAGING BEHAVIOR OF BREEDING PIPING (CHARADRIUS MELODUS) IN NEW JERSEY

KEVIN J. STAINE AND JOANNA BURGER Departmentof BiologicalSciences, Rutgers University, Piscataway, New Jersey08855, USA

ABSTRACT.--Thenocturnal foraging behavior of breeding Piping Plovers (Charadriusmel- odus)was studied in New Jerseyusing a focal-animalapproach in 1989and 1990.More than 30% of the variation in the number of plovers foraging at night was explained by stageof the breeding cycle,tidal stage,and year. The greatestnumbers of adult ploversfed in the intertidal zone during the prenestingand fledgling stagesof the breeding cycle. Piping Ploverswere more likely to be observedfeeding during late ebb and early flood ,than other times. Time devoted to feeding per 2-rain sample was similar at each study site but differed significantlyduring the tidal stages.Pecking rate was higher during late ebb and early flood tides than late flood and early ebb tides. Time devotedto being alert varied dependingon stageof the breedingcycle. Prenesting plovers and individualswith fledglings fed longerand were alert lessper 2-rain samplethan ploversengaged in incubationor brood rearing. The nocturnal peck rate of Piping Plovers was considerablylower than daytime levels.Plovers foraging at night had significantlylower peck rateswhen disturbed.Abun- dance of intertidal polychaetesvaried accordingto tidal stage and, where present,they constitutedthe main food of the plovers. We suggestthat nocturnal foraging is a natural behavior pattern in Piping Ploversalthough it may vary in intensity. Future management should include the assessmentof nighttime recreationaluse of beacheswhere Piping Plovers breed.Received 31 July1992, accepted 25 November1992.

ALTHOUGHSOME , such as many species that shorebirdscannot obtain enough calories of owls, are nocturnal, most are diurnal and during the short daysat thesetimes (Goss-Cus- perform all their activitiesduring the day. Re- tard 1969, 1976, Heppleston 1971, Pienkowski cently, several researchershave examined the 1981, 1982,Myers and McCafiery 1984,Puttick behaviorof shorebirdsboth during the day and 1984, Johnsonand Baldassarre1988). This sug- at night. Night feeding has been documented geststhat the long and warm daysof springand in six species,including Grey Plovers summerafford shorebirdsample time to attend (Pluvialissquatarola; Dugan 1981, Pienkowski to all energy needs without feeding at night. 1982),Ringed Plovers(Charadrius hiaticula; Pien- Robert and McNeil (1989), however, showed kowski 1982), Piping Plovers(C. melodus;Bur- that some shorebirdsspecies wintering, or re- ger 1984), Dotterels (C. morinellus;Kalas 1986), siding year-round in tropical environments, Wilson's Plovers (C. wilsonia;Robert and McNeil which are warmer and therefore less energy 1989), and Semipalmated Plovers (C. semipal- demanding environments,fed at night as well. matus;Robert and McNeil 1989), as well as sev- They suggestedthat the lack of data on night eral other shorebirdspecies (Burger 1984, Goss- foraging in shorebirdswas likely due to tech- Custard 1984, Robert et al. 1989). However, these nological difficulties associatedwith night ob- data,with the exceptionof thosein Kalas(1986), servationand that night feeding may occurall were obtained either during migration or on year. They suggested,as did Dugan (1981), the wintering grounds,and there are no quan- Pienkowski (1982) and Townshend et al. (1984), titative studiesduring the breeding seasonof that increasedavailability of invertebrateprey nocturnal behavior of shorebirds. Furthermore, at night may make it advantageousfor shore- Cairns (1977) stated that, as darkness falls dur- birdsto foragethen. Feedingactivity of several ing the breeding season,adult Piping Plovers shorebird speciesis partly a function of tidal (C. melodus)escort chicks to cover or to their stage(Burger 1984,Burger et al. 1977,Goss-Cus- former nesting territories. tard 1984).The diurnal feeding-activityrhythms Feeding occursat night during both migra- of Piping Plovers during the breeding season tion and winter becauseenergy requirements have been linked to tidal factors and human are high due to long-distancetravel and adverse disturbance(Burger unpubl. data). weather conditions, and because it is assumed Piping Ploversbreed in the interior of North

579 580 ST^II•IœAI•D BURGER [Auk, Vol. 111

America, in the Great region, and along receivesheavy recreationaluse throughout the sum- the Atlantic Coast(AOU 1983,Haig 1985, U.S. mer. The southernmost 300 m of outer beach is used Fish and Wildlife Service 1988).Piping Plovers by off-roadvehicles during the dayand night. In 1989 typically nest on sparselyvegetated beaches or there were 13 nestingpairs of Piping Plovers.In 1990 inland shores (Wilcox 1959, Cairns 1982, there were 12 pairs. North Corson'sis a state park located at the south- Haig and Oring 1985).Piping Ploversare listed ern tip of Ocean City. The nesting area (the south- asthreatened and endangeredboth in the Unit- ernmost 2 km of outer beach and 750 m of inlet beach) ed Statesand (Sidle 1985, Haig 1985). receivesonly minimal artificial light becauseit is lo- Major reasonsfor the decline of the Atlantic cated more than 1.5 km from the nearest residential CoastPiping Plover populationsinclude lossof lighting and over 5.5 km away from concentrated habitat, increased predation, and human dis- light sources.The width of the beach between the turbance (U.S. Fish and Wildlife Service 1988). dunes and waterline varies between 20 and 200 m Of these factors, the effect of human distur- depending on tidal stage. The area receivessimilar bancehas received the greatestattention (Flem- levels of recreational use as Brigantine Beach, but ming et al. 1988, Burger and Gochfeld 1991, much lessvehicular use (only park rangersand staff Burger unpubl. data, Goldin pers. comm.,Grif- during the day). In 1989 there were seven nesting pairs and in 1990 there were six. fin and Melvin pers. comm.). However, all of Whale Beachis approximately1.6 km southof North these studieswere conductedduring the day Corson's.The nesting area is locatedentirely on the and focusedprimarily on adult and chick for- outer beach as there is no inlet. The width of the aging behavior. Since no one has previously beach, depending on tidal stage, varies between 30 studied Piping Plovers at night, the lack of and 100 m. It receivesmuch lesslight from artificial nighttime human disturbancedata on the birds sourcesthan either Brigantineor North Corson'sand is not surprising. Recreationistsdo not congre- recreational use is also lower. There is no vehicular gate en masseon beachesat night, but they are use. In 1989 there were 12 nesting pairs and in 1990 there were 9. nonethelesspresent. Their activitiesrange from The three beaches mentioned above have similar fishing and camping,to walking, jogging, and sloping intertidal zones and similar vegetation. Fur- driving off-road vehicles (Strauss1990). ther descriptionscan be found in Burger (1991). We report on nighttime observationsof Pip- Mantoloking is a beachon New Jersey'snorthern- ing Ploversin New Jerseyduring the 1989and mostbarrier island. The nesting area is comprisedof 1990breeding seasons. Our main objectiveswere 2.4 km of outer beach. It receives minimal artificial to determine:(1) foraging behavior of Piping light and recreationaluse. There are no vehiclesper- Plovers;(2) effectof , study site, and breed- mitted except those of the beach patrol. The width of ing stage on foraging behavior; (3) effect of the beach, 10 to 50 m depending on tidal stage, is nighttime human disturbanceon foraging be- much lessthan any other study site. The slope of the havior; and (4) day and night prey availability. intertidal zone is greater than 20ø which is markedly different from the other three study sites.There were seven nesting pairs in 1990. METHODS Each study site was divided into five oceanfront transects(250 m x 50 m). These transectswere estab- We studied Piping Plovers from 1 April to 15 Au- lishedprior to the nestingseason. Piping Ploverswere gust in 1989and 1990at three New Jerseylocations: observed between 2000 and 0500 EST, three to four Brigantine Beach (39ø22'30"N, 72ø24'30'W), North nights a week using a Lenzar light-image intensifier. Corson's (39ø12'30"N,74ø38'30"W), and Whale Beach The night scopeallowed us to observeindividual (39ø10'30"N,74ø40'W). A fourth study site, Mantolo- Piping Plovers from 100 to 120 m away depending king (40ø03'N,72ø40'W), was examined in 1990 only. on the amount of artificial and natural light (moon- All sites were located on barrier beach islands. light) present. All behaviors normally observed in Brigantine Beach is 1.0 km north of the center of full daylight (e.g. feeding, preening, brooding, dis- Atlantic City. The amount of artificial light emitted tractiondisplays) could be detectedand distinguished from Atlantic City and the surroundingresidences from eachother. The night scopealso made it possible createsan almost perpetual full-moon effect on the to distinguish individual Piping Plovers from other beach.The major nestingarea is on the southernmost shorebirds of similar shape and size (e.g. Semipal- 2 km of outer beach(i.e. beachfacing ocean),and 1 mated Plovers C. semipalmatus;Sanderlings, Calidris km of inlet beach (i.e. perpendicular to oceanfront alba) from over 100 m. and facing inlet waters). The width of the beachbe- During each night of observationwe gatheredtwo tween the dunes and the waterline varies between 35 types of data:(1) abundanceand activity; and (2) for- and 120 m depending on the tidal stage. The area aging behavior. Abundance and activity data were July1994] PipingPlover Nocturnal Foraging 581 used to determine the number of individuals in- (e.g. early ebb tide at BrigantineBeach was sampled volved in a particular activity over the courseof a at 1100 and again at 2200). Once all four tidal stages night (equivalent to 3 h of observations).We spent 1 were sampledfrom each study site a secondset of h censusinga single transectand three transectswere coresamples were obtained in the samemanner. This censusedeach night. Over the duration of the study resultedin 16 coresfrom eachstudy site,one-half of 276 h (92 nights) of censuseswere conducted. which were taken during the day and half at night. We also collected data on foraging behavior using Furthermore,during eachsampling day, one of the a focal- approach (Altmann 1974, Burger un- two samplesobtained was random and the other was publ. data). Once a plover was spotted feeding, its taken in a nonrandommanner (i.e. from sanddirectly behavior during a 2-min time trial was recordedonto beneathwhere a PipingPlover had been feeding only a minicassetteaudio tape.Behaviors recorded for each momentsbefore). Sampling points were randomized 2-min trial included: the time spent feeding, alert, with respectto study site and transectlocation. running or flying undisturbed,running or flying with All sampleswere sievedthrough 2.0-mmand later people or vehicles within 50 m, engaged in conspe- through0.6-mm meshes. Invertebrates were counted, cific aggression,or engaged in other activities (e.g. groupedinto phyla, and preservedin 70% formal- maintenance,loafing); and the number of pecksmade dehyde.Organisms were identified to family or order at the substrate.Prior to each sample we recorded the usinga dissectingmicroscope and field guides(Gos- date, time, temperature,percent cloud cover, moon- ner 1971, 1978). phase,age (adult or chick), and stageof the breeding We calculated means and standard errors for all cycle. variables.Significant differences were determined us- All data were transcribedonto data sheets.During ing chi-squaretests at the 0.05probability level. Krus- the study, 622 2-min foraging trials were conducted kal-Wallistests (yielding an H-value;Sokal and Rohlf on at least 66 individual Piping Plovers. We distin- 1981) were used to determine significantdifferences guished four tidal stages:(1) .•arly flood tide (the 3 h betweengroups (e.g. tidal stages,study sites). A mul- following slacklow tide); (2) late flood tide (the 3 h tiple-regressionanalysis (GLM; SAS Institute 1985) beforeslack high tide); (3) early ebb tide (the 3 h after was performed. All independent variables (e.g. slackhigh tide); (4) late ebb tide (the 3 h before slack moonphase,percent cloud cover, tidal stage)were run low tide). singly and as combinationsin our attemptto deter- The general temporal stagesof the breeding cycle mine the bestmodel explaining variation in the num- of Piping Plovers in New Jerseyare as follows: pre- ber of ploversforaging on a given night. The depen- nesting (late March and April); incubation(late April dent variable was the number of Piping Plovers to early June); with chicks and brood rearing (late activelyforaging during each night. May to early July); with fledgling and postfledgling (early July to the middle of August; (Wilcox 1959, U.S. Fish and Wildlife Service1988). This chronology RESULTS does not take renests into account. Determinations of the stage of the breeding cycle Abundanceand activity censuses.--Overall, 31% were made by knowing the approximatelocations of of the variability in the numberof ploversfeed- the territories of the pairs in each transect.All nests ing on a given night was explained by tidal were monitored daily or weekly. Thus, we knew the stage,stage of the breedingcycle, and year (Ta- total number of nests,adults, chicks,and fledglings ble 1). These variables in combination also ex- over the courseof each seasonat each study site. We plain the most variation when each study site assumedthat plovers' nesting territories were contin- uous(Cairns 1977, Whyte 1985).We alsoassumed that is examined separately (Table 1). At Mantolo- territory size remained unchanged at night, an as- king, however,the numberof ploversfeeding sumptionsupported by a study of wintering birds in at night is associatedwith breeding stageand a relatedspecies, the Grey Plover (Wood 1986).Piping not with tidal stage. On the remaining sites Ploversin our studywere not bandedfor two reasons. during both years,there was a greaterproba- First,because they are designatedfederally asendan- bility of observingforaging plovers during early gered,a banding moratoriumis in effectuntil further flood and late ebb tides than during late flood notice. Second, the light-image intensifier was not and early ebb tides (Fig. 1). Although feeding sensitiveenough, at the distancesit was used,to dis- on the intertidal zone did occurduring late flood tinguish band-color combinations. and early ebb tides, most of the plovers ob- Preyabundance was determined using 4.0-cm-deep served during these tidal stageswere engaged and 12.5-cm-widecore samples.All sampleswere ob- tained from wet or recently wetted intertidal sand. in maintenanceactivities (preening). Coreswere takenduring May, June,and July of 1990. We also examined the number of adult plov- On eachsampling day, a single tidal stagewas sam- ers feeding per night as a function of stageof pled from one studysite during the day and at night the breeding cycle. In each year and over the 582 ST•aNEAND BURGER [Auk,Vol. 111

TABLE1. Factorscontributing to variationin numberof adult Piping Ploversfeeding at night at eachstudy site and over entire study.•

Brigantine Mantoloking North Corson's Whale Beach All Sites Model F 21.30'** 7.80* 15.80'** 23.90*** 29.10'** R 2 0.38 0.08 0.16 0.31 0.31

Variables entered Year ns _b ns 6.7' 8.1' Breedingstage 14.5'** 6.1'** 10.2'** 17.6'** 57.7*** Tidal stage 27.1'** ns 9.9** 26.7*** 26.7***

"ns, P > 0.05; *, P < 0.05; ** P > 0.01; ***, P < 0.001. • Not applicable.Data only collectedfor one year. courseof the study,more ploverswere observed approximately22% and 33% higher than plov- feeding during the early (prenesting)and late ers on North Corson's and Mantoloking. No (with fledglings)stages of the breeding cycle other behaviorsdiffered significantlybetween (Table 2). study sites. We found 66 breeding pairs over the course Piping Plovers devoted significantly differ- of this study (32 in 1989 and 34 in 1990) that ent proportionsof time to foraging depending had feeding territoriesextending into the tran- on tidal stage (H = 24.0, P < 0.0001; Table 3). sect areas.We recorded 2-min foraging trials The number of pecks per 2-min trial was also for at least one member of every pair during significantlydifferent during thesetimes (H = each season. 61.5, P < 0.0001). Plovers made 31% more pecks Feedingbehavior.--Piping Plover foraging tri- during early flood and late ebb tides than dur- als during the 2-min focal observationswere ing early ebb and late flood tides. No other divided into two categories:foraging trials free behaviors differed significantly during tidal of human-related disturbances; and trials when stages. birds were disturbed. Plovers also exhibited variation in their for- At the four study sites for both years com- aging behaviorover the courseof the breeding bined, adult plovers foraging without distur- cycle (Table 4). Plovers fed for significantly bance spent almost identical amounts of the greater proportionsof the 2-min trials during 2-min trials feeding. Although the proportion the prenestingand fledglingstages of the cycle of time spentfeeding was not significantlydif- than during incubationor brood rearing (H = ferent acrossthe study sites,the peck rate was 9.8 P < 0.02). Peckrates also were significantly different (H = 17.5, P < 0.0006). Plovers on greater during the prenestingand fledgling Brigantineand Whale Beachhad pecking rates stages(Table 4). Ploverswere significantlymore alert during the incubationand brood rearing

ß BRIGANTINE ß NO•I•CORSON'S stagesthan during the prenestingor fledgling [] MANTOLOKING [] WHALEBEACH stages.No other behaviorswere significantly different over the courseof the breedingcycle.

80. Feedingcycle and human disturbance.--For both years combined,foraging behavior per 2-min trial was significantly different for disturbed plovers than for those not disturbed (Table 5). Overall, ploversfed longer (H = 89.6,P < 0.001) and made more pecks (H = 26.5, P < 0.0001) per 2-min trial when foraging undisturbed. Ploversdisturbed while foraging spent signif- EARLY FLOOD LATE FLOOD EARLy EBB LATE EBB icantly lesstime in conspecificaggression than TIDAL STAGE when foragingundisturbed (H = 4.1, P < 0.04). Fig. 1. Probabilityof observingat leastone adult Disturbed plovers also spent 25% of the 2-min Piping Plover feeding per hour at night as function foraging trial running or flying from human of tidal stage. related disturbance.This representsa 30-fold July1994] PipingPlover Nocturnal Foraging 583

TABLœ2. Average number of adult Piping Plovers(_SE with n in parentheses)in intertidal zone as function of stageof breeding cycle.

Breeding stage 1989 1990 Total Prenesting 3.40 + 0.31 (18) 3.10 + 0.32 (24) 3.22 _+0.22 (42) Incubation 1.85 + 0.33 (18) 1.73 + 0.20 (24) 1.78 + 0.18 (42) Brood rearing 2.13 + 0.16 (48) 2.37 + 0.09 (48) 2.25 + 0.08 (96) Fledgling 5.00 + 0.33 (48) 4.10 + 0.22 (48) 4.55 _ 0.24 (96)

increasefrom undisturbed plovers that were Breedingcycle.--The number of Piping Plov- not forcedto run or fly from disturbancesat all ersobserved and the behaviorof Piping Plovers (Table 5). foragingat night were stronglyassociated with Preyabundance.--Most of the prey speciescol- particularstages of the breedingcycle. Foraging lected and identified from core sampleswere Piping Plovers were more numerousat night from a single family of bivalve (Donacidae), on the intertidal zone during the prenesting (Spionidae), or (Hippi- and fledglingstages of their breedingcycle than dae). Severalfamilies of gammaridamphipods during either of the other two stages(incuba- alsowere represented.Although the abundance tion and brood rearing). Also, the proportion of these organismsvaried with respectto tide, of each2-min trial devotedto feeding, and the and there were somedifferences between day- peckrate were significantlygreater during these time and nighttime samples,the only regular stageswhile the time spent alert was signifi- patternof variationoccurred in the cantly less (Table 4). This general pattern was (Table6). Their abundancewas higher during observedon all studysites except Mantoloking. early flood and late ebb tides comparedto the Walters (1984) found similar results, with re- other tidal stages,both during the day and at spectto time feeding during the day through- night.Also, nonrandom samples contained sig- out the breedingseason, in four speciesof lap- nificantly greaternumbers of polychaetesthan wings (Vaneflus).This seasonalpattern appears did the randomsamples regardless of tidal stage to be driven by parental constraintsimposed by (i.e. therewere morepolychaetes where plovers certain stagesof the breeding cycle (Walters were foraging than where they were not). 1984,this study). During the prenesting stagePiping Plovers, DISCUSSION and shorebirdsin general, devote time to es- tablishingterritories and mating.Consequent- Sincebreeding stageand tidal stagewere im- ly, individuals do not spend long hours at the portant in statisticallyexplaining variations in nest site and are free to spend more time feed- both abundanceand frequency of night for- ing and lesstime beingalert. Furthermore,dur- aging, we discusseach factor separately.Each ing the prenestingphase, females must obtain breeding stageis discussedwithin the context enough energy reservesto produce a clutch of of the breeding cycle as a whole. four eggs(Welty and Baptista1988). Although

TABLE3. Comparisonof nocturnallyforaging adult Piping Ploversduring eachtidal stage.Each sample was 2-min focal observation (œ + SE).

Tidal stage Early flood Late flood Early ebb Late ebb H a No. samples 139 62 114 220 Time spent (s) Feeding 90.5 + 215 80.3 + 3.82 78.2 + 2.86 92.5 + 0.83 24.0*** Alert 7.1 + 1.31 7.8 + 1.65 10.7 + 1.59 5.9 + 0.89 ns No. pecks 20.1 _+0.96 13.2 + 1.07 14.7 + 0.85 20.6 + 9.03 61.5'** ns, P > 0.05; ***, P < 0.001.Kruskal-Wallis H-test for differencesin proportionof time. 584 $TAINEAND BURGER [Auk, Vol. 111

TABLE 4. Comparisonof nocturnallyforaging adult Piping Ploversduring eachstage of breedingcycle (t _+ SE).

Stageof breeding cycle Prenesting Incubation Brood rearing Fledgling H a No. samples 45 130 141 100 Time spent (s) Feeding 91.6 _+2.60 83.8 _+2.59 82.8 _+2.48 92.7 _+3.01 9.8* Alert 5.0 + 1.56 9.9 + 1.33 13.5 + 1.75 2.3 + 1.08 31.1'** No. pecks 18.7 + 1.03 16.5 + 0.86 15.9 + 0.74 21.2 + 0.93 51.5'**

• *, P < 0.05; ***, P < 0.001.Kruskal-Wallis H-test for differencesin proportionof time. not verified, this increasedrequirement for food to the parental care effort from a distance. Bur- may have resulted in an increasednumber of ger (1991)suggested that, during the day, Pip- females foraging at night. ing Plover pairs exhibit a pattern of alertness The onsetof egg laying brings increasedpa- and vigilanceduring their breedingcycle. Both rental care in the form of incubation duties, and membersof a pair monitor each other's behav- it appearsto decreasethe abundanceof Piping ior, even though they seeminglyare incubating Plovers feeding nocturnally in the intertidal or feeding(Waiter 1984, Burger 1991, this study). zone. This follows, sinceonly one per pair Being more alert at certain times during the is free to be foraging at any one time, sincethe cycle,especially during incubationand brood- other is incubating.We found a 46%decline in ing, may allow for a quicker responsewhen the number of plovers feeding at night during mates need assistance. the incubation stage comparedto the prenest- During the final stage,when pairshave fledg- ing stageof the cycle.With respectto the plov- lings and postfledglings,the number of adult ers' foraging behavior during the incubation ploversfeeding at night morethan doubledfrom stage,individuals on average spent 9% lesstime that of the precedingtwo stagesand reacheda feeding and 50% more time being alert than in level slightly above that observedduring the the prenesting stage. prenestingstage. Furthermore, time spentfeed- After the hatching of young, the time plovers ing per 2-min trial increasedby 11% and the spent being alert during each 2-min foraging time alert decreasedby 60%over that observed trial increasedby 33%over that exhibiteddur- during the brood rearing stage.We link this ing incubation.Since we perceivedno increase changein behaviorto the reductionof parental in either the number of humans or predators care that is associatedwith the fledgling and during this stage of the breeding cycle com- postfledglingstages of the breeding cycle, as pared to the other stages,we assumedthat the well asto staging-upfor the comingmigration. nonbrooding member of a pair is contributing Tidalfactors and prey abundance.--Manyabi-

T^!•i,155. Comparisonof nocturnallyforaging adult Piping Ploverswhen foragingundisturbed, and disturbed by human-related activities (t _+SE).

Undisturbed Disturbed H '

No. samples 534 88 Time spent (s) Feeding 88.1 _+ 1.19 56.3 _+ 1.04 89.6*** Alert 7.5 _+ 0.08 9.4 _+ 0.47 ns Running undisturbed 13.1 _+0.24 16.5 _+0.88 ns Running disturbed 0.0 _+0.0 30.1 _+2.16 _b Conspecificaggression 6.3 + 0.17 3.2 + 0.74 4.1' Other activities 3.8 _+ 0.12 3.6 _+ 0.31 ns No. pecks 18.6 _+ 1.47 13.5 _+0.96 26.5*** ns, P > 0.05; *, P < 0.05; ***, P < 0.001. Kruskal-WallisH-test for differencesin proportion of time. Not applicable. Zero data cannot be used for statisticalcomparison. July1994] PipingPlover Nocturnal Foraging 585

TAnIll 6. Comparisonof invertebrateabundance (individuals per core sample)during eachtidal stage(• + SE).

Tidal stage Early flood Late flood Early ebb Late ebb Night Bivalves 13.1 + 10.9 44.0 _+ 35.2 10.0 + 8.25 34.4 + 27.7 Polychaetes 22.3 + 13.4 4.5 + 6.6 3.4 + 5.2 44.6 + 32.8 Amphipods and mole crabs 49.4 + 9.6 32.6 --- 11.0 51.3 + 30.4 49.8 + 21.6 Day Bivalves 26.6 + 13.9 16.4 _+ 10.4 17.0 + 7.8 21.8 + 16.5 Polychaetes 33.3 + 22.1 2.8 + 3.5 6.5 + 7.1 42.6 + 29.9 Amphipods and mole crabs 66.1 + 38.4 35.8 + 12.1 53.1 + 28.9 48.6 + 22.7 otic factors affect shorebird nocturnal behavior itemsof Piping Ploversis observational.In gen- including shorteneddaylength (Goss-Custard eral Piping Plovers eat marine worms (poly- 1969, Heppleston 1971, Pienkowski 1981, Put- chaetes),, , and mollusks(Bent tick 1984), moonphase(Ralph 1956, Hale 1980, 1929, Palmer 1967, Cairns 1977, Whyte 1985). Milsore 1984), bioluminescence (Robert et al. What Piping Ploversactually eat or prefer to 1989),tidal cycle(Dugan 1981,Robert et al. 1989), eat, however, has not been determined and un- and human disturbance(Burger and Gochfeld doubtedly depends upon where the bird is 1991).These factors have emergedfrom studies feeding(locally throughout its range).The prey of migrating or wintering shorebirdsand only sampling resultsfrom our 1990 seasonindicate Robertet al. (1989),Burger and Gochfeld(1991), that the prey baseof Piping Ploversis limited and Morrier and McNeil (1991) used night- to members of a few marine invertebrate fam- vision equipment to observe and quantify ilies (Table 6). Furthermore,there were no sig- shorebird behavior. nificant differences in overall prey abundance Our resultssuggest that during the breeding asa functionof day or night. Theseresults con- seasonthe behaviorof Piping Plovers,and pos- tradictreports that manyshorebirds feed at night sibly many other shorebirds,is stronglyinflu- becauseof greater prey abundanceor greater enced by tide, regardlessof the time of day. biomass of certain invertebrates, especially Furthermore, in light of this study and others polychaetes(Dugan 1981). However, in our (Burger 1984,Johnson and Baldassarre1988) it study we did not determine the biomassof in- is becomingincreasingly apparent that the be- vertebrates. havior of Piping Ploversis stronglyinfluenced Polychaeteswere the only prey to exhibit a by tidal factorsduring all stagesof their annual distinctpattern with respectto their abundance cycle.We observedthat adult ploversspent sig- and stageof the tidal cycle.Polychaete abun- nificantly different proportionsof each 2-rain dancewas highest in samplestaken during ear- trial feeding depending on tidal stage. They ly floodand late ebbtides compared to the other spent the longest proportions of time feeding tidal stages (Table 6). Moreover, polychaete and exhibitedthe highestpeck rates during ear- abundance from the nonrandom samples was ly flood and late ebb tides (Table 3). This peck higherthan that obtained from the random ones, rate pattern is similar to the pattern of success regardlessof tidal stage. This indicatesthat rate observedin the Common Redshank(Tringa polychaeteabundance is highestwhere plovers totanus);Goss-Custard 1976). The pattern of are feeding. Theseresults suggest that, during abundance(Fig. 1) was similar to that observed the breeding seasonon outer beacheswhere by Burgeret al. (1977) for migrating shorebirds intertidal polychaetesare present,polychaetes feedingon mudflats,but not on outer beaches. are the plovers' major food source. We make This habitat differencemay be in responseto this suggestionbecause polychaetes are not many factorsincluding temporalfactors, prey ubiquitous (Gosner 1971, 1978). This was ap- abundance,prey preference, or prey availabil- parent from samplingthe intertidal at Manto- ity at different times of the annual cycle. loking where we collectedonly 12 individual The majority of information about the prey polychaetes from 16 core samples. Bivalve 586 $TAIIqEAND BURGER [Auk,Vol. 111 abundancewas similarly low. Gammarid am- That is, in the absence of large numbers of phipod and mole crab (Emeritatalpoida) indi- beachgoersat night, foraging plovers may en- vidualsdominated these samples and, thus, seem gage in very long bouts of uninterrupted feed- to be the major food item at that site. ing. However, disturbanceat night reducesthe Comparisonsto a daytimeforaging study.--We peckrate by more than 61%of the daytime level. comparedthe resultsof this foragingstudy with In this regard, any disturbanceat night may be thoseof a daytimestudy conductedon Piping too much. Ploversat the samesites (Burger unpubl. data). Conclusions.--PipingPlovers foraged noctur- In general,plovers spent equivalentpropor- nally at all four study sitessuggesting that it is tionsof time feedingper 2-min trial (day,71%; a usual occurrence.The data also support sug- night, 73%).However, as expected for a species gestionsby Robert and McNeil (1989) and Mor- foragingvisually, the peck rate droppedcon- rier and McNeil (1991) that nocturnal foraging siderablyat night (Pienkowski1982). Indeed, it in shorebirdsis a natural habit. We suggestthat wasalmost halved (day, 34.1 pecks/2 min; night nocturnal foraging in Piping Ploversis an in- 18.6pecks/2 min). There was a similar pattern tegral part of their foraging strategy. Future at night (thisstudy) and during the day (Burger managementof this speciesshould include the unpubl. data) in the time plovers devoted to assessmentof nighttime recreational use on feedingas a functionof stageof the breeding beaches where it breeds. cycle.In both studiesplovers spent more time per 2-rain trial feeding during the prenesting ACKNOWLEDGMENTS and fledglingstages than duringincubation and brood rearing periodsof the cycle. We expressspecial thanks to C. David Jenkinsand Human disturbance.--The effect of human dis- Allison Blades of the New Jersey Endangered and turbanceon foraging behavior hasbeen the fo- Nongame SpeciesProgram for daytime logisticsup- cus of several recent Piping Plover studies port and field assistance.We thank JamesApplegate (Flemming et al. 1988,MacIvor et al. 1990,Bur- and Charles Leck for their support and review of drafts.We are grateful to JoeZurovchak, James Ap- gerand Gochfeld1991, Griffin and Melvin pers. plegateand CassandraLord-Staine for statisticaland comm.). Data from our study shows that the computer assistance. foragingbehavior of Piping Ploverswhen dis- turbed was significantly different than when foragingundisturbed at night (Table 5). LITERATURE CITED People on the beach pose both direct and AL•Iq, J. 1974. Observationalstudy of behavior: indirectthreats to breedingPiping Plovers (U.S. Samplingmethods. Behaviour 49:222-265. Fish and Wildlife Service 1988). 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