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EFFECTS OF HUMAN RECREATION ON THE INCUBATION BEHAVIOR OFAMERICAN OYSTERCATCHERSAuthor(s): CONOR P. McGOWAN and THEODORE R. SIMONSSource: The Wilson Journal of Ornithology, 118(4):485-493. 2006.Published By: The Wilson Ornithological SocietyDOI: http://dx.doi.org/10.1676/05-084.1URL: http://www.bioone.org/doi/full/10.1676/05-084.1

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485

The Wilson Journal of Ornithology 118(4):485–493, 2006

EFFECTS OF HUMAN RECREATION ON THE INCUBATIONBEHAVIOR OF AMERICAN OYSTERCATCHERS

CONOR P. MCGOWAN1,3,4 AND THEODORE R. SIMONS2

ABSTRACT.—Human recreational disturbance and its effects on wildlife demographics and behavior is anincreasingly important area of research. We monitored the nesting success of American Oystercatchers (Hae-matopus palliatus) in coastal North Carolina in 2002 and 2003. We also used video monitoring at nests tomeasure the response of incubating birds to human recreation. We counted the number of trips per hour madeby adult birds to and from the nest, and we calculated the percent time that adults spent incubating. We askedwhether human recreational activities (truck, all-terrain vehicle [ATV], and pedestrian traffic) were correlatedwith parental behavioral patterns. Eleven a priori models of nest survival and behavioral covariates were eval-uated using Akaike’s Information Criterion (AIC) to see whether incubation behavior influenced nest survival.Factors associated with birds leaving their nests (n � 548) included ATV traffic (25%), truck traffic (17%),pedestrian traffic (4%), aggression with neighboring oystercatchers or paired birds exchanging incubation duties(26%), airplane traffic (1%) and unknown factors (29%). ATV traffic was positively associated with the rate oftrips to and away from the nest (�1 � 0.749, P � 0.001) and negatively correlated with percent time spentincubating (�1 � �0.037, P � 0.025). Other forms of human recreation apparently had little effect on incubationbehaviors. Nest survival models incorporating the frequency of trips by adults to and from the nest, and thepercentage of time adults spent incubating, were somewhat supported in the AIC analyses. A low frequency oftrips to and from the nest and, counter to expectations, low percent time spent incubating were associated withhigher daily nest survival rates. These data suggest that changes in incubation behavior might be one mechanismby which human recreation affects the reproductive success of American Oystercatchers. Received 28 July 2005,accepted 24 April 2006.

The effect of human recreational activity onwildlife is an increasingly important area ofresearch (Burger 1981, Burger and Gochfeld1998, Fitzpatrick and Bouchez 1998, Whitta-ker and Knight 1998, Carney and Sydeman1999). Human disturbance has been linked toaltered foraging behavior (Burger 1981, Bur-ger and Gochfeld 1998, Fitzpatrick andBouchez 1998, Rodgers and Schwikert 2003,Stolen 2003) and diminished reproductivesuccess of many waterbird species (Hunt1972, Robert and Ralph 1975, Tremblay andEllison 1979, Safina and Burger 1983, Rhulenet al. 2003). The mechanisms by which humandisturbance lowers reproductive success, how-ever, are poorly understood.

Current data indicate that American Oys-

1 North Carolina Coop. Fish and Wildlife ResearchUnit, Dept. of Zoology, North Carolina State Univ.,Campus Box 7617, Raleigh, NC 27695, USA.

2 U.S. Geological Survey, North Carolina Coop.Fish and Wildlife Research Unit, Dept. of Zoology,North Carolina State Univ., Campus Box 7617, Ra-leigh, NC 27695, USA.

3 Current address: Dept. of Fisheries and Wildlife,302 Anheuser Busch Natural Resources Bldg., Univ.of Missouri, Columbia, MO 65211, USA.

4 Corresponding author; e-mail:cpm4h9@mizzou.edu

tercatcher (Haematopus palliatus) populationsin the Mid-Atlantic states are declining (Ma-whinney and Bennedict 1999, Davis et al.2001). The U.S. Shorebird Conservation Planlists the American Oystercatcher as a ‘‘Spe-cies of High Concern,’’ due, in part, to humanencroachment on breeding habitat (Brown etal. 2001). Evidence that humans are directlyresponsible for American Oystercatcher nestfailure is limited (Davis et al. 2001, McGowan2004); however, human recreation is often as-sociated with lower oystercatcher reproduc-tive success (Hockey 1987, Jeffery 1987,Novick 1996, Davis 1999, Leseberg et al.2000, Verhulst et al. 2001, McGowan 2004).Because American Oystercatcher populationsmay require intensive management in the nearfuture, it is important to understand the rela-tionship between human recreation and oys-tercatcher nesting success (Brown et al. 2001,Davis et al. 2001).

Skutch (1949) hypothesized that higher lev-els of parental activity during the nesting pe-riod might lead to greater rates of predationbecause more activity makes nests more ob-vious to predators. Because American Oyster-catchers are ground-nesting shorebirds that areeasily flushed from their nests (Davis 1999),

486 THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 118, No. 4, December 2006

we similarly hypothesized that human recre-ation might increase the activity of incubatingoystercatchers, thereby leading to increasedpredation rates. Although Skutch’s hypothesishas been tested extensively, conclusions aremixed (Martin 1992, Roper and Goldstein1987, Martin et al. 2000, Tewksbury et al.2002). We believe that nesting American Oys-tercatchers provide a good opportunity to testSkutch’s hypothesis because their nests arerelatively easy to find and monitor, and theyexperience high rates of nest predation (Noland Humphrey 1994, Davis et al. 2001, Sa-bine et al. 2005).

In this study, we used video monitoring torecord human recreational activity and the be-havior of incubating oystercatchers nesting onthe Outer Banks of North Carolina. We askedwhether human recreational activity alteredthe behavior of nesting birds, and whether in-creased parental activity or decreased nest at-tendance were associated with higher rates ofnest failure.

METHODS

Study areas.—We monitored nesting suc-cess of American Oystercatchers at CapeLookout (76� 32� W, 34� 36� N) and Cape Hat-teras (75� 31� W, 35� 24� N) national seashoresin North Carolina during 2002 and 2003. Theseashores comprise �160 km of barrier islandhabitat that supports 90 breeding pairs ofAmerican Oystercatchers. All work at CapeLookout National Seashore was conducted onNorth Core Banks and South Core Banks (seeGodfrey and Godfrey 1976 for site descrip-tion). Cape Hatteras National Seashore com-prises three main islands: Bodie, Hatteras, andOcracoke Islands. These barrier islands arelong, narrow, and bordered by sandy beacheson the ocean side and salt marshes on thesound side. American Oystercatchers nest onthe ocean side beaches, dunes, and adjacentsand flats. Raccoons (Procyon lotor) and feralcats (Felis catus) are common on all islandsexcept Ocracoke, which has no raccoons. Theislands are open to the public and most beach-es are open to vehicles. Approximately650,000 people visit Cape Lookout each year;the visitation rate at Cape Hatteras is consid-erably higher and has increased steadily from1.5 million in 1986 to 2.2 million in 2005(National Park Service 2005). Park visitors

use the beaches for walking, shell collecting,swimming, and fishing, and they drive four-wheel drive passenger vehicles (ORVs) andsmaller, all-terrain vehicles (ATVs) on thebeach. Vehicles are permitted along a networkof unpaved roads behind the primary dunesand anywhere on the open beach, except indesignated areas that are closed to protect veg-etation, nesting sea turtles, and shorebirds, andto prevent erosion.

Data collection.—We located oystercatchernests (n � 268) and, from 15 April until 30July in 2002 and 2003, checked their statusevery 3–4 days until chicks hatched or thenests failed. We used SONY HI-8 video cam-eras to record the incubation behavior of nest-ing adults at randomly selected nests (n � 72).We videotaped nests on Bodie Island and Hat-teras Island (Cape Hatteras National Sea-shore), and on North Core Banks and SouthCore Banks (Cape Lookout National Sea-shore). Nests were filmed for approximately4-hr intervals at least once between the com-pletion of egg laying and hatching. In the ab-sence of human recreational activity, we as-sumed that parental behavior would be naturaland homogenous throughout the incubationperiod. Evidence indicates that both Americanand Black (Haematopus bachmani) oyster-catchers incubate their eggs 90–100% of thetime once the clutch is completed, and that theamount of time spent incubating does not varyduring the incubation period (Nol and Hum-phrey 1994, Andres and Falxa 1995). Verbo-ven et al. (2001) showed that Eurasian Oys-tercatchers incubated 85–90% of the time atundisturbed nests, and that the percentage oftime spent incubating was constant betweenthe end of the laying period and hatching.Studies of other shorebird species indicatesimilar incubation patterns (Norton 1972), al-though Cartar and Montgomerie (1987) foundthat nest attendance of White-rumped Sand-pipers (Calidris fuscicollis) may vary daily,depending on weather or other environmentalfactors.

Novick (1996) reported that human activityon South Core Banks at Cape Lookout Na-tional Seashore was distributed ‘‘fairly even-ly’’ throughout the day and was greater onweekends (Friday–Sunday) than on weekdays.Novick (1996) also reported that humans con-centrated around activity centers, such as the

487McGowan and Simons • HUMAN RECREATION ALTERS INCUBATION BEHAVIOR

ferry dock, the lighthouse, and the ocean in-lets at the north and south ends of South CoreBanks. Our nests were filmed between 07:00and 14:00 EST, on both weekdays and week-ends, which we believe provided an unbiasedrepresentation of human disturbance and pa-rental activity patterns at each nest.

Each video camera was housed in a weath-erproof plastic container attached to a metalstand, and placed approximately 5 m from thenest to avoid disturbing incubating birds.Most cameras faced the ocean and recordedactivity both in the vicinity of the nest and onopen beach beyond the nest. Sometimes cam-eras were placed at nests located in the dunesor other locations where the ocean-side beachwas not visible. In these cases, we directedcameras toward the most likely source of hu-man recreation (e.g., the dune road at CapeLookout). The area sampled by the videocamera was different for each nest due to dif-ferences in the surrounding landscape; there-fore, detection probabilities for human activ-ities were heterogeneous among nests. We re-viewed tapes in real time to count the numberof trips by incubating birds to and from thenest per hr, and the percent time that adultsspent incubating. Herein, the term ‘‘trip’’ re-fers to a bird leaving or returning to its nest.We also counted the number of ORVs, ATVs,and/or pedestrians passing each nest per hr.

Statistical analyses.—We used the Mayfield(1961, 1975) method to estimate daily nestsurvival rates and hatching success for allnests monitored. We applied the Mayfield es-timate to entire clutches and did not considerindividual egg survival. Heterogeneity in sur-vival probabilities during the incubation stagewas not considered, and the midpoint rule wasused to designate the time of failure and timeof hatching for nests that failed or hatched be-tween visits. We considered nests successfulif at least one egg hatched, and failed whenall eggs were lost. Partial nest failure was notconsidered in this study.

Each time a bird left its nest we estimatedthe time between departure and the time atwhich the probable causal event occurred.Possible causal factors included: ATV, ORV,pedestrian, and airplane traffic, as well as in-teractions between territorial pairs and ex-changes in incubation duties. We report thesedata as the percent of nest departures for

which one of the above causal factors fol-lowed. We also report the percent of observedhuman recreational activities that were pre-ceded by a bird leaving its nest.

We used linear regression models (Neter etal. 1996) to determine whether human recre-ational factors were correlated with oyster-catcher parental activity. Trips per hr and per-cent time spent incubating were modeled asdependant variables, with number of ORVs,ATVs, and pedestrians passing a nest per hrserving as the independent variables.

For camera-monitored nests, we used thelogistic exposure method to estimate dailynest survival (Shaffer 2004). We used SAS(ver. 9.1; SAS Institute, Inc. 2003) to generatesurvival estimates and to test competing mod-els of nest survival with parental behaviors ascovariates (Shaffer and Thompson in press).We tested 11 a priori models (Table 1) thatmodeled trip rate and percent time incubatingas both continuous and categorical variables.We used two methods for categorizing thedata: one purely statistical and one based onbehavioral observations. For statistical cate-gorical models, we split the data for numberof trips/hr (Tripcat) and percent time incubat-ing (Inccat) into low and high categories, us-ing the median value of each as the cut-offpoint (Tripcat1: �3.69 trips/hr � low, �3.69trips/hr � high; Inccat1: �85% � low, �85%� high). For the second method (biologicalcategorical models), we used the average val-ues from seven nests that had no evidence ofhuman disturbance; we then divided the datainto a new set of low and high categories. Inthis case, undisturbed nests averaged 2.25trips per hr. Therefore, we used three trips perhr as a conservative estimate of oystercatchernest site activity in the absence of human dis-turbance (Tripcat2: �3.0 trips/hr � low, �3.0trips/hr � high). Time spent incubating by un-disturbed birds averaged 90% of the obser-vation period; thus, we used 90% as the cut-off point to categorize nests as low or high interms of percent time spent incubating (Inc-cat2: �90% � low, �90% � high). We mod-eled each categorical variable separately andin a model that included both trip rate andpercent time incubating (Table 1). One modelincluded a year effect, and we tested a nullmodel (null) that assumed constant survivalover the season. We used an information the-

488 THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 118, No. 4, December 2006

TABLE 1. Eleven candidate models used to examine the relationship between daily nest survival and pa-rental incubation behaviors of American Oystercatchers nesting on the Outer Banks of North Carolina in 2002and 2003.

Candidate model Model covariates

Global Continuous Year, trips to and from the nest per hr, percent incubation time

Year Year

Models with statistically categorized data (splitting low and high data at the median value)

Global categorization 1 Year, tripcat1, inccat1a

Tripcat1 inccat1 Tripcat1, inccat1Tripcat1 Tripcat1Inccat1 Inccat1

Models with biologically categorized data (splitting data at the average value for undisturbed nests)

Global categorization 2 Year, tripcat2, inccat2Tripcat2 inccat2 Tripcat2, inccat2Tripcat2 Tripcat2Inccat2 Inccat2

Null No covariates, assumes constant survival

a Inccat1, inccat2, tripcat1, and tripcat2 are categorical variables into which nests were categorized as low or high in terms of percent time adult birdsspent incubating (inccat) or the number of trips adults made to and from the nest/hr (tripcat), according to the criteria that follow: inccat1: �85% � low,�85% � high; inccat2: �90% � low, �90% � high; tripcat1: �3.69 trips/hr � low, �3.69 trips/hr � high; tripcat2: �3.0 trips/hr � low, �3.0 trips/hr� high.

oretic approach to rank the models from mostto least supported, based on Akaike’s Infor-mation Criterion (AIC)—using AICc, �AICc,and Akaike weights (wi); Burnham and An-derson 2002). Means are reported �SE.

RESULTS

We monitored 185 nests at Cape Lookoutand 83 nests at Cape Hatteras. The overallMayfield estimate of daily nest survival was0.92 � 0.006 at Cape Lookout and 0.94 �0.007 at Cape Hatteras. The highest daily nestsurvival rates were recorded at Cape Hatterasin 2003 (0.96 � 0.008), and the lowest wererecorded at Cape Lookout in 2002 (0.90 �0.007); these were the only year and locationcomparisons that were significantly different(Z � 4.83, P � 0.001).

We filmed 72 nests for a total of 320.18 hrand a mean of 4.45 � 1.19 hr per nest. Mostnests were filmed once for 4 hr, but somewere filmed twice before they hatched orfailed. We excluded one nest from the analysiswhere it appeared that the bird’s behavior wasaffected by the presence of the video camera.Of the 72 nests filmed, chicks successfullyhatched from 19 and 53 nests failed. Sixty twopercent of nest failures were due to mamma-lian predation (n � 32), 28.5% failed for un-known reasons (n � 15), and 11% were lost

to weather, human destruction, or abandon-ment (n � 6).

Though not true experimental controls,there were seven nests at which we observedno human disturbance during filming. Birds atthose nests incubated for 90% � 0.033 of thefilming period and made 2.25 � 0.60 trips/hrcompared to 82% � 0.017 incubation and3.66 � 0.17 trips/hr at all other nests. Thenumber of trips/hr at undisturbed nests wassignificantly lower (t � 2.27, P � 0.026) thanat all other nests. The percent of time spentincubating at undisturbed nests was not sig-nificantly greater (t � 1.34, P � 0.19) than itwas at disturbed nests.

We recorded 539 instances in which incu-bating birds departed their nests. Of those in-stances, ATVs were filmed within 3 min ofnest departure on 136 occasions (25%) andORVs were filmed 92 times (17%) within 3min of departure. We recorded a total of 284ATVs, 62% (n � 177) of which passed by anest within �3 min of a bird departing itsnest. We observed 1,466 ORVs pass by filmednests, but only 11% (n � 168) passed by with-in 3 min of a bird leaving its nest. Groups orindividual pedestrians were filmed 19 times(4%) within 10 min of nest departures. Of allthe 110 pedestrians that we observed, 33% (n� 36) passed by within 10 min of a bird de-

489McGowan and Simons • HUMAN RECREATION ALTERS INCUBATION BEHAVIOR

FIG. 1. The effect of all-terrain vehicle (ATV)beach traffic on incubation behavior of American Oys-tercatchers on the Outer Banks of North Carolina dur-ing the 2002 and 2003 breeding seasons: (A) relation-ship between the percent of time spent incubating andthe average number of ATVs passing per hour (�1 ��0.037, P � 0.025), and (B) relationship between thenumber of trips to and from the nest per hr and thenumber of ATVs passing per hr (�1 � 0.749, P �0.001).

TABLE 2. Candidate models examining the relationship between daily nest survival and parental incubationbehaviors of American Oystercatchers nesting on the Outer Banks of North Carolina in 2002 and 2003. Modelsare ranked in descending order of support based on Akaike’s information criteria AICc, �AICc, and Akaikeweights (wi).

Model Log-likelihood No. parameters AICc �AICc wi

Tripcat2a �159.62 2 323.27 0.00 0.28Null �161.08 1 324.16 0.89 0.18Tripcat2 inccat2a �159.62 3 325.29 2.02 0.10Inccat1 �160.68 2 325.39 2.11 0.097Inccat2 �160.77 2 325.56 2.29 0.089Tripcat1 �160.98 2 325.99 2.72 0.072Year �161.07 2 326.17 2.90 0.066Tripcat1 inccat1 �160.26 3 326.56 3.29 0.054Global categorical2 �159.56 4 327.18 3.92 0.040Global categorical1 �160.24 4 328.54 5.28 0.020Global continuous �261.36 4 530.79 207.52 0.000

a Inccat1, inccat2, tripcat1, and tripcat2 are categorical variables into which nests were categorized as low or high in terms of percent time adult birdsspent incubating (inccat) or the number of trips adults made to and from the nest/hr (tripcat), according to the criteria that follow: inccat1: �85% � low,�85% � high; inccat2: �90% � low, �90% � high; tripcat1: �3.69 trips/hr � low, �3.69 trips/hr � high; tripcat2: �3.0 trips/hr � low, �3.0 trips/hr� high.

parting its nest. Eight percent (n � 44) of nestdepartures were associated with territorial dis-putes and 18% (n � 108) with the exchangein incubation duties. Eight departures (1%)were associated with low-flying airplanes thatpassed within 3 min of nest departure. For theremaining 29% (n � 154) of nest departures,no disturbances, territorial interactions, or in-cubation exchanges took place following de-parture.

Regression models showed that there waslittle or no association between ORV trafficand the rate at which incubating oystercatch-ers made trips to and from their nests (�1 �0.018, P � 0.064) or the percent time theyspent incubating (�1 � 0.0006, P � 0.57).Likewise, pedestrian traffic was not associatedwith a significant reduction in the percent timeincubating (�1 � �0.005, P � 0.75) or birdsmaking more trips to and from their nests perhr (�1 � �0.268, P � 0.079). Increased ATVtraffic, however, was associated with a reduc-tion in the percent time spent incubating (�1

� �0.037, P � 0.025) and an increase in therate of trips to and from the nest (�1 � 0.749,P � 0.001; Fig. 1).

All models except the global continuousmodel received some level of support, but nomodel had overwhelming support (Table 2).The tripcat2 model (i.e., nests divided intolow and high categories based on average triprate for nests with no observed human distur-

490 THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 118, No. 4, December 2006

TABLE 3. Daily survival estimates and hatching probability estimates for nests in two categories of behav-ioral data collected from American Oystercatchers nesting on the Outer Banks of North Carolina in 2002 and2003.

Category No. nestsDaily probability

of survivalLower / upper

confidence intervalsHatching

probability

Median cutoffs

�3.69 trips/hr 37 0.958 0.935 / 0.973 0.314�3.69 trips/hr 35 0.948 0.925 / 0.965 0.240Incubation �85% 32 0.961 0.938 / 0.975 0.338Incubation �85% 40 0.945 0.922 / 0.962 0.218

Zero-observed-disturbance average cutoffs

�3.00 trips/hr 26 0.969 0.946 / 0.982 0.424�3.00 trips/hr 46 0.944 0.924 / 0.960 0.213Incubation �90% 50 0.967 0.945 / 0.980 0.400Incubation �90% 22 0.948 0.926 / 0.964 0.237

bance as the only covariate) had the highestrank of all the models (�AICc � 0.00, wi �0.28). The null model was ranked second(�AICc � 0.89, wi � 0.18), and the modelincorporating both tripcat2 and inccat2 wasranked third (�AICc � 2.02, wi � 0.10). Allthe models with categorical behavioral vari-ables, the year model, and the null model hada �AICc of �7 and weights between 0.02 and0.28 (Table 2). Generally, models with a�AICc of �7 cannot be ruled out, but modelswith weights �0.70 cannot be exclusively ac-cepted (Burnham and Anderson 2002).

The estimated daily survival rate for nestswith �3.69 trips to and from the nest per hrwas greater than the daily survival rate fornests with �3.69 trips to and from the nestper hr (Table 3). That same pattern was ob-served when the data were divided into cate-gories representing nests with �3 trips per hrand �3 trips per hr. Nests in which the parentsincubated for �85% of the observation periodhad higher daily survival probabilities thannests in which incubation percentages were�85%. The same pattern was observed whenwe categorized the data by nests in whichadults spent �90% and �90% time incubat-ing. These data indicated that nests in whichparents made more trips to and from the nesthad a lower daily survival probability, andthat nests where the parents spent more than85–90% of their time incubating had a lowerchance of surviving each day.

DISCUSSIONOur data show clear associations between

human recreation and incubation behavior of

American Oystercatchers. ATV traffic was as-sociated with increased rates of trips to andfrom the nest and reduced time incubating;other forms of human recreation were moreweakly associated with oystercatcher nestingbehaviors. Sixty two percent of the ATVs thatwe observed passed within 3 min of a birddeparting its nest, whereas the same was truefor only 11% of the ORVs that we observed.Birds appear to have habituated to the pres-ence of ORVs (Whittaker and Knight 1998),but they view ATVs (and to a lesser extent,pedestrians) as threats. Peters and Otis (2005)reported that wintering American Oyster-catchers habituated to boat traffic on the in-tercoastal waterway in South Carolina. Otherstudies have shown that birds respond differ-ently to different forms of human recreationaldisturbance (Burger 1981), but most have fo-cused only on changes in foraging behavior(Burger and Gochfeld 1998, Rodgers andSchwikert 2003, Stolen 2003). Our study isone of the few to investigate how human rec-reational disturbance affects incubation be-havior. ATVs are louder and move faster thanORVs and pedestrians, which might explainwhy the birds are affected more by ATV traf-fic (Burger 1981, Burger and Gochfeld 1998).ORVs and pedestrians also tend to stay closerto the firm sand along the water’s edge, whichmeans they generally travel farther from nest-ing birds.

Although the probability of hatching waslow in all nests, regardless of parental activity,we did find evidence that human recreationaldisturbance may reduce the nesting success of

491McGowan and Simons • HUMAN RECREATION ALTERS INCUBATION BEHAVIOR

American Oystercatchers by altering incuba-tion behavior. Analyses based on AIC modelselection indicated that the rate of parentaltrips to and from the nest and the percent timethat parents spent incubating may have af-fected daily nest survival rates. Although nomodel received overwhelming support, noneof the categorical behavioral models could beruled out. The daily survival estimates indi-cated that nesting adults that made fewer tripsto and from the nest had greater daily nestsurvival rates. Conversely, nests where theparents incubated for less time had higher dai-ly survival rates. We hypothesize that mam-malian nest predators, the primary nest pred-ators in this system (Davis et al. 2001), arebetter able to find disturbed nests throughsmell because each time a parent gets up andwalks away from a nest it leaves a scent trailthat raccoons and cats may follow. Our resultsdiffer from those of Verboven et al. (2001),but that is likely because the primary nestpredators in that system were avian predators.

ATV traffic is not the only factor affectingoystercatcher nesting success on North Caro-lina’s Outer Banks. Nest predation is an im-portant determinant of hatching success in theOuter Banks (Davis et al. 2001, McGowan etal. 2005), and relationships between humanrecreation and nest predators are poorly un-derstood. Vehicular traffic also may affect suc-cess during the chick-rearing phase of repro-duction. In the 2003 breeding season, we con-firmed that five chicks from three differentnests were run over by vehicles on the beach-es of South Core Banks at Cape Lookout Na-tional Seashore and Hatteras Island at CapeHatteras National Seashore (McGowan 2004).

The negative association between percenttime incubating and daily nest survival seemscounterintuitive. Conway and Martin (2000)showed that birds balance the costs of egg ex-posure with those of high parental activity.Birds with high levels of nest-predation pres-sure minimize nest-site activity by taking few-er, longer trips off the nest (Conway and Mar-tin 2000). This behavior helps reduce parentalactivity around the nest, but it also reduces theamount of incubation. American Oystercatch-er behavior may reflect a similar trade off;their eggs can tolerate extensive heating andcooling (Nol and Humphrey 1994). In ourstudy, several clutches exposed for approxi-

mately 1 hr at mid day hatched successfully.One videotaped nest hatched successfully,even though the parents incubated for only66.8% of the 4.07-hr observation period. Egghardiness may reflect an adaptation that en-ables parents to reduce nest-site activity. Par-ents that depart their nest and wait until mul-tiple disturbances have passed before return-ing may have greater nesting success than par-ents that return to their nests quickly and flushrepeatedly. Future analyses should assess theeffect that the average amount of time birdsspend off the nest has on nest success.

There were several potential sources ofmeasurement error in our study that might ex-plain why no models were strongly supported.Incubation behavior might vary as birds ha-bituate to disturbance (Whittacker and Knight1998). Because the field of view varied ateach nest, our cameras recorded areas of dif-ferent size for each nest, and we were unableto control for these differences in the analyses.We were also unable to measure the distancefrom the nests to the disturbance recorded onour video. Several studies have shown that theproximity of human disturbance has a majoreffect on the behavioral responses of birds(Burger and Gochfeld 1998, Rodgers andSchwikert 2003). It is likely that in some cas-es, recreational activity recorded by our cam-eras did not elicit a response from the incu-bating bird because the activity was too faraway. Video monitoring is an extremely use-ful tool for studying avian behavior; however,future studies of human disturbance using vid-eo monitoring should entail measuring dis-tances to sources of disturbance. Recordingnests for longer periods of time also wouldalleviate a great deal of uncertainty. Sabine etal. (2005) were very successful in studyingnest success of oystercatchers in Georgia byusing time-lapse videography throughout theincubation period.

Our simplified approach of categorizingnests into low or high levels of parental activ-ity provided a coarse-scale observational mea-sure of behavioral responses to recreation anddisturbance; we expected this to reduce ob-servation errors. Other researchers that haveevaluated the effects of human disturbance onavian behavior used experimental designswith defined treatment groups (Robert andRalph 1975, Tremblay and Ellison 1979, Ver-

492 THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 118, No. 4, December 2006

hulst et al. 2001, Stolen 2003). We studied theeffects of ambient human disturbance causedby park staff and recreational visitors to de-termine whether it was linked to patterns ofnesting success. Future studies of human ac-tivity and oystercatcher nesting success thatcompare the behavior of birds on beachesclosed to vehicle and pedestrian traffic withthe behavior of birds exposed to differenttypes and intensities of human activity areneeded to improve our understanding of thepatterns suggested by this study.

ACKNOWLEDGMENTS

We are grateful to J. R. Cordes and M. Lyons fortheir tremendous contributions to this study. We thankthe National Park Service, the U.S. Fish and WildlifeService, and the U.S. Geological Survey for support-ing this research. We thank M. W. Rikard and the staffof Cape Lookout National Seashore, and S. Harrisonand the staff at Cape Hatteras National Seashore, forall their assistance. We thank J. A. Collazo, K. H. Pol-lock, and F. R. Thompson, III, for assistance on thedesign and analysis of this study, and J. Hostetler andM. S. Pruett for their assistance with data collectionand management. We thank C. C. McGowan for ed-iting this manuscript. We are grateful to three anony-mous reviewers for their comments and suggestionsfor improving this manuscript.

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