0047502

Set-BackDistances to ProtectNesting Colonies fromHuman Disturbance in Florida

JAMESA. RODGERS, JR.,*AND HENRY T. SMITHtt *WildlifeResearch Laboratory,Florida Game and FreshWater Fish Commission,4005 South Main Street, Gainesville,FL 32601, U.S.A. tOfficeof EnvironmentalServices, Florida Department of EnvironmentalProtection, 3900 Commonwealth Boulevard,Tallahassee, FL 32399, U.S.A.

Abstract: Breeding colonial waterbirdsare particularlysusceptible to human disturbancebecause of their high-densitynesting habits. Identified detriments to reproductivesuccess include egg and nestlingmortality, nest evacuation, reduced nestling body mass and slower growth,premature fledging and modifiedadult behaviors.Fifteen species of colonial waterbirdsnesting at 17 colonies in northand centralFlorida were exposed to threedifferent human disturbance mechanisms (HDMs) in order to determinerecommended set-back(RS) distancesfor protecting these mixed-species nesting assemblages. Both intraspecificand inter- specificvariation were observedin flushingresponse distances to thesame human disturbancemechanisms. In general, colonial waterbirdsexhibited greater average flush distances in reaction to a walking approach than to approaching motorboats. Recommendedset-back distances were estimated using aformula based on A themean plus 1.6495 standard deviations of theobserved flushing distances plus 40 meters[RS = exp (|i + 1.64956 + 40)]. In general,a recommendedset-back distance of about 100 metersfor wading bird colonies and 180 metersfor mixed /skimmercolonies should be adequate to effectivelybuffer the sites we studied from human disturbance caused by approach of pedestrians and motor boats. We recommendfollow-up studies to testour model at otherbreeding colonies.

Distancia de alejamientopara protegerde las perturbacioneshumanas a las colonias de aves nidificadorasen Florida Resumen: Las aves acuaticas que habitan en colonias durante el periodo de cria' son particularmente susceptiblesa las perturbacioneshumanas por sus habitos conducentesa una alta densidad de nidos. Los factoresque disminuyenel exito reproductivo,incluyen la mortalidad del huevo y el pichon, la evacuacion del nido, la reduccion de la masa corporal delpichon o crecimeientolento, el abandono prematurodel nido por parte de los pichones y comportamientosadultos modificados. Quince especies de colonias de aves acuaticas que nidificaronen 17 colonias del nortey centrode Florida fueron expuestas a 3 mecanismos de perturbacionhumana diferentes,a los efectosde determinardistancias de alejamiento recomendablespara protegerlas agregaciones mixtas de e'stas especies. Variaciones intra-especificase inter-especificasen las distancias de respuestafrente a los mismos mecanismos de perturbacionhumana En general, las colonias de aves acuacticasexhibieron una mayordistanciapromedio antes de volar en reaccion a la cercania depasos que al acercamiento de una embarcaci6n a motor.La distancia recomendada de alejamiento fue estimada utilizando unaf6rmul9 basada en la media ma's 1.6495 desviacionesstandard de la distancias antes de volar observada ma's 40 m [RS = exp ( + 1.64956 + 40)]. En general,una distancia de alejamiento de alrededor de 100 m para las colonias de aves zancudas y 180 m para las colonias mixtas ("tern/skimmer"),seria adecuada para amortiguar a los sitios que estudiamos de los impactos de las perturbaciones humanas causadas por la aproximacion de caminantes y embarcaciones con motor Recomendamos estudios de seguimientopara probar nuestromodelo en otras colonias de cria.

.?Current address: Florida Departmentof EnvironmentalProtection, Florida Park Service,13 798 S.E. Federal Highway,Hobe Sound, FL 33455, US.A Paper submittedNovember 22, 1993; revisedmanuscript accepted March 16, 1994.

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90 Set-BackDistances for Bird Colonies Rodgers& Smith

Introduction to defendon legal and biological grounds.Therefore, a multispeciesstudy was conducted to determinethe set- Human disturbancecan adverselyaffect wildlife, with back distances necessary to protect nesting colonial colonial breeding being particularlysusceptible wading birds and seabirdsfrom human disturbance. because of their high-densitynesting habits. Several We were interestedin determiningif individualspe- studies have shown both qualitative(Johnson & Sloan cies members of the ground nesting guild (order 1976; Ellison & Cleary 1978; Anderson& Keith 1980) Charadriiformes)and tree nestingguild (orders Pele- and quantitative(jenni 1969; Tremblay& Ellison 1979; caniformesand Ciconiiformes)would exhibit similar Parsons& Burger1982; Kaiser & Fritzell1984) human flushdistances to the same typeof human disturbance. impactson colonial waterbirds.Adverse effects include In Florida,the ground-nestingguild is representedby egg and nestlingmortality (Teal 1965; Schreiber1979; the ,, and skimmers(family Laridae), and the Jeffrey1987), prematurefledging or nest evacuation tree nestingguild is representedby wading birds (fam- (Veen 1977), and reduced body mass or slowergrowth ilies Ardeidae,Threskiornithidae, and Ciconiidae) and of nestlings(Kurry & Gochfeld 1975; Pierce & Simons otherseabirds (families Pelecanidae, Anhingidae, Phala- 1986). Adult behavior also may be altered by distur- crocoracidae). One proposed advantageof colonialityis bance, resultingin alteredforaging patterns (Skagen et its antipredatorrole (Lack 1968), and a primaryadvan- al. 1991) and otherdetrimental effects on reproduction tage of increasedvigilance is to allow the alertedbirds (Gillet et al. 1975; Tremblay& Ellison 1979; Cairns to flee (Krebs 1978). Thus, colonial nestingmay facili- 1980; Safina & Burger 1983). Responses of colonial tate group vigilance,and similar species within each waterbirdsto disturbancemay varywith habitattype, colonial nesting guild may exhibit similar flush dis- physiographyof the colony, food supply,seasonality, tances.In addition,we wanted to identifythe potential and bird species (Manuwal 1978; Ollason & Dunnet of varioustypes of humanactivities for causing wildlife 1980; Erwin 1989). Some researchers also have re- disturbance.Pedestrian and boat trafficare the most ported no significanteffects in relationto frequencyof frequentforms of human disturbance to waterbirdnest- disturbanceon breeding success (Goering & Cherry ing in Florida,especially at colonies on islands and ma- 1971) or variousdegrees of habituationto disturbances rine coastal sites. (Robert & Ralph 1975; Burger 1981a; Burger& Goch- Our primarygoal in this study,however, was to rec- feld 1981). ommend set-backdistances to prevent human distur- The increasingpopularity of outdoor recreationalac- bance of single-species and mixed-species colonies. tivitiesin recentyears has resultedin increasedhuman Herein,we develop a technique to calculate set-back disturbancesof colonial waterbirdbreeding sites. This distances around breeding-birdcolonies and recom- frequentlyhas led to increasedprotection of these sites mend this method as a general model thatmay be ap- by placing them in public ownership and concomi- plied elsewhere for bufferzones specificallydesigned tantly,restricted recreational access to land and water foreach species and location. around these breeding colonies. Conservationperson- nel are facedwith the difficulttask of effectivelybuffer- ing importantwildlife resources from disruptive human StudyArea activitiesdespite increasingdemands for access to pub- lic lands. We definea set-backdistance as a minimum Data were collected at eightwading bird and nine sea- distanceof nonintrusionby humansmeasured from the bird (collectively termed colonial waterbirds)nesting perimeterof a colony thatwill preclude disturbancesto sitesin Floridaduring the spring-summerof 1989-199 1: nestingbirds. Previous recommendationsfor set-backs Dee Dot Ranch (Duval Co.), a mixed-specieswading- aroundnesting colonial waterbirdsas a strategyto alle- bird colony in a sparselytreed freshwater swamp, dom- viate disturbanceshave rangedfrom 50 to 200 meters inantnesting vegetation was cypress(Taxodium disti- fortern (Sterninae) species (Buckley & Buckley 1976; chum); Matanzas Point (St. Johns Co.), two separate Erwin 1989) and 100 to 250 metersfor wading bird tern nesting sites in natural coastal dune habitat, (Ardeidae) species (Vos et al. 1985; Erwin 1989). sparselyvegetated with sea oats (Uniola paniculata); Anderson(1988) suggested a "threshold"estimate of PortOrange (Volusia Co.), a mixed-specieswading-bird 600 metersto protecta Brown (Pelecanus oc- colony on a marinedredged-material island, dominant cidentalis) colony in Mexico. AlthoughFlorida began vegetation was black mangrove (Avicennia germi- protectingcolonial waterbirdnesting sites fromhuman nans); Oaks Mall (Alachua Co.), a mixed-specieswad- disturbancein 1976, the set-backdistances currently ing-birdcolony in a freshwaterswamp, dominant vege- used by a natural-resourcepersonnel to protect avian tationwas southernwillow (Salix caroliniana); Lake colonies in Floridaare not based on regionalempirical Yale (Lake Co.), a mixed-specieswading-bird colony in data. Most set-backdistances were derived from"best a freshwaterswamp, dominant vegetation was cypress; estimates" at the time of posting but were difficult HauloverCanal (BrevardCo.), a mixed-specieswading-

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Rodgers& Smith Set-BackDistances for Bird Colonies 91 bird colony on a coastal dredged-materialisland, vege- ridor(>5 m wide) of sightof the HDM by the nesting tationmostly coastal red cedar (Juniperusvirginiana) birds.The distancebetween the HDM and the nestwas and youpon holly (Hex vomitoria); Pelican Island (In- measuredin metersusing an opticalrange finder (Rang- dian RiverCo.), a mixed-specieswading-bird colony on *ing,Inc., East Bloomfield,New York,U.S.A. models 120 a natural estuarine island, dominant vegetation was and 620) with calibratedaccuracies of ? 0.5 m (-30 black and red (Rhizophora mangle) mangrove;St. Lu- m) to ? 2.0 m (>30 m). Each experimentalbird and cie Intlet State Park (St. Lucie Co.), a tern colony in nestwas at the edge of the breedingbirds under obser- coastaldune habitat;Island DS-3D (HillsboroughCo.), a vation and flushedin advance of other birds risingoff tern and colony on a marine dredged-materialis- their nests. Each test bird was classifledas either an land, little vegetation; Wakulla Springs State Park incubatingadult (with eggs) or brooding adult (with (Wakulla Co.), a cormorantcolony in a freshwaterri- nestlings)based on eitherdirect observations or known pariansite along the banksof Wakulla Springs, primarily colony breedingchronology. Response to a HDM usu- cypresstrees; Holiday Island (Okaloosa Co.), a ternand ally involved alert or agonistic behavior,followed by skimmercolony in disturbedcoastal dune habitatnear risingoff the nest and subsequentflight. Because it was Destin;Navarre Beach causeway(Santa Rosa Co.), a tern difficultto quantifythe initialalert/agonistic response and skimmercolony nestingalong the sparselyvege- distance due to concurrentbreeding activity, we used tated road right-of-way;St. George Island causeway the more readilydetected and easily measured "flush (FranklinCo.), a ternand skimmercolony nestingalong distance"as an index of disturbance.It was definedas the sparselyvegetated road right-of-way;St. George Is- the distancefrom the HDM to the nest at the moment land State Park (FranklinCo.), a terncolony in natural when the bird actuallybegan movementaway fromthe coastal-dunehabitat; Tern Island (FranklinCo.), a tern nest. colony on an oyster-barisland in AlligatorHarbor; and Nestingterns and skimmersmay be more sensitiveto PhippsPoint (Franklin Co.), a ternand skimmercolony human disturbancethan other colonial waterbirdsbe- on a sandy peninsula extendinginto AlligatorHarbor. cause they usually exhibit an "initialpanic" or "initial Furtherinformation for these sites are availablein Port- mass upflight"(hereafter upflight) response when first noy et al. (1981) or Nesbittet al. (1982). disturbed(Palmer 1941; Erwin 1989). An initial up- flight-responsedistance for terns was measured from Methods the HDM to the colony edge. At some colonies, dis- tances for individualflushed birds also were collected This studywas conducted under the Florida Adminis- afterthe ternsreturned to theirnests froman upflight trativeCode, General Purpose WildlifeCode 39-9.002, duringannual surveys.In these situations,the method subsection 2, that permits personnel of the Florida varied fromthe initialupflight data collection in that Game and FreshWater Fish Commissionand theirdes- 2-5 biologistswere generallymoving through a colony, ignated cooperatinginvestigators to disturbbirds for with one person measuringthe distanceabout 1 meter specificpurposes of approvedresearch. Our fieldwork in frontof the others. These individualflushing dis- also followedthe American Ornithologists' Union guide- tanceswere analyzedfor various trends separately from lines for scientistsconducting research on wild birds initialupflight distances because the birds alreadyhad (Oring et al. 1988). The time spentin each colony was been disturbed,but were not used in calculatingthe less thanor equal to the timerequired for normal nest- set-backdistances (see below). monitoringactivities, thereby minimizing disturbance All data were collected between 0700 and 1600 by researcherswhile stillallowing data collection. hours on clear to partlycloudy days,with wind condi- Three types of human disturbance mechanisms tionsless than15 km/hour.Statistical analyses were con- (HDMs) were used to elicit initialflushing responses: ducted withPRODAS (Conceptual Software,Inc., Hous- (1) walking(continuous, 1 step/sec,direct approach by ton, Texas, U.S.A.). Observer bias in measurementsof 1-5 people on foot); (2) canoe (continuous,0.5 m/sec, flushdistances was minimizedby having one or two direct approach by 2 people in a 5.2 meter [17 foot] observerstake most of the measurements. aluminumcanoe); and (3) motorboat(continuous, 0.5 To reduce the effectof autocorrelationbetween the m/sec,direct approach by 2 people in a 4.3 meter[14 firstdisturbance and subsequentflushing events and to foot] aluminumjon-boat with a 30 hp motor of noise minimizeour disturbanceof avian breeding activities, level 80-85 dBA). Our walkingapproach towardnest- we purposelylimited the numberof disturbances to one ing birds essentiallyreplicated Erwin's (1989) rate, or two eventsper species or site at small,compact nest- which we considered a mediumapproach speed. ing colonies. Variancesof statisticsfrom autocorrelated Experimental procedures were applied similarly observationswould be biased because the observations among HDMs. Althoughthe amountand typesof vege- were not independent.A negative(or inverse) relation- tativecover varied among sites,all experimentalnests ship between successive approachesand flushdistances were directlyapproached to allow an unobstructedcor- would suggest that birds acclimated to our repeated

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92 Set-BackDistances for Bird Colonies Rodgers& Smith approaches.Because ofthese restrictions,our data were tances. These histogramsand scatterplots frequently unbalanced (not all combinationsof species, HDMs, yielded right-skeweddistributions. With a threshold sites,and yearswere represented).In thesesituations (n phenomenonsuch as minimumflush distance, this re- < 5), we were unable to analyzesite-year classifications; sult was anticipated. Natural log transformationap- therefore,this source of variabilitywas untested. A peared to normalizedistribution of the data; thus, all greaternumber of disturbanceswere possible at some furtheranalyses were performedon log-transformed largewaterbird colonies; multipletrials were conducted data using parametricstatistical procedures. Variances out of view of the preceding disturbancebecause the and standarddeviation values in the text,Figures 1 and nestinghabitat provided a physicalbarrier to directvi- 2, and calculations recommendingset-back distances sual and auditorycontact with us and between succes- fromback-transformed data were estimatedby applica- sive,individually disturbed birds. For threespecies, we tion of the delta method to obtain the asymptoticdis- were able to collect data to compare the flushdistances tributionas describedby Agresti(1984). between breedingadults and older nestlingscapable of Because most colonies were visitedonly once a year, movementfrom their nests. For nestlings,the approach we could not testfor seasonal variationor habituation. distance also was measured as described above for But for some species and colonies, we were able to adults.For large sampleswe plottednormal probability examinethe effectof successive approacheson flushing values and histogramsfor untransformedflushing dis- distance.For species with a sufficientsample size (n -

Species(sample size) Rank

Pelecaniformes

Double-crestedcormorant (38) A

Brown pelican (63) B

Ciconiiformes

Great blue heron (16) A

Tricolored heron (22) A

Black-crowned night-heron(9) | _ A B

Great egret (I I) B

White ibis (12) | B C

Wood (20) C

Charadriiformes

Least tern- upflight(17) | A

- individual (71)

Black skimmer- upflight(10) A

- individual (47)

I - - - r - I I I X I 0 10 20 30 40 50 60 70 80 90 100 110 120 Distance (m) Figure 1. Distances at which colonial waterbirdsflushed from nests in responseto a walking approach directly toward theirnest (vertical bar = mean; horizontal bar = range; solid horizontal box = + I SD). Species withineach order with thesame letterunder rank are not significantly(p > 0.05) different(ANOVA/Tukey's Multiple Range Test).

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Rodgers& Smith Set-BackDistances for Bird Colonies 93

Species (samplesize) Rank

Pelecaniformes Outboardmotor boat

Anhinga(1l) A

Double-crestedcormorant (12) A

Brownpelican (14) B

Ciconiiformes

Greategret (15) A

Greatblue heron(13) A

Littleblue heron(13) A B

Cattleegret (8) B C

Snowyegret (7) B C

Wood stork (60) | * . C

Tricoloredheron (7) c

Pelecanitormes Canoe

Double-crestedcormorant (5) A

Anhinga (I 8) B

1 ~ ~ ~ 1 0 10 20 30 40 50 60 Distance (m)

Figure2. Distances at which colonial waterbirdsflushed from nests in response to motor boat or canoe ap- proach directlytoward theirnest (vertical bar = mean; horizontal bar = range; solid horizontal box = +1 SD). Species within each order with thesame letterunder rank are not significantly(p > 0.05) different (ANOVA/Tukey'sMultiple Range Test).

5) collected on the same day,we tested fora trendin proaches could be treatedas the experimentalunit and successive flushingdistances and residual autocorrela- thateach flushingresponse was a single datum. tion. Aftereliminating data that exhibitedtrends or auto- correlation,the data were considered independentof Results one another.We combined a species' flushdistances for Data were collected on the flushingdistances of 15 spe- some yearsand siteswith small sample sizes (n < 5) for cies of breedingcolonial waterbirdsin reaction to the some colony-days(data collected on the same day at the threeHDMs at 17 sites in Floridaduring 1989-1991. same colony). We recognizethat this may confound the effectsof HDMs on flushdistances for some species. To WalkingApproach test for differencesamong species within the same HDM, we firsttested for homogeneity of variances using Wood (Mycteria americana) exhibited the Bartlett'sX2 statisticfor sample sizes of at least 5, fol- smallestmean flushdistance (18.4 + 5.5 m), whereas lowed by a t-testor ANOVA/Tukey'sMultiple Range Test GreatBlue Herons(Ardea herodias) possessed the larg- on the subsets (species x HDM, species x species x est mean flushdistance (32.0 + 12.3 m) to a walking HDM). Where colony-daysdid not differin varianceof approach (Fig. 1). There were significant(ANOVA! flushdistances, we concluded that the individualap- Tukey'sM.R.T., p < 0.05) differencesamong some spe-

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94 Set-BackDistances for Bird Colonies Rodgers& Smith

cies of colonial waterbirdsto a walkingapproach that Table1. Recommendedset-back (RS) distancesbetween precluded furtherpooling of the data for the tree- breedingcolonial waterbirds and a walkingor motorboat nestingguild into taxonomicgroups, such as familyand approachdirectly toward the nest. order,higher than the species level. Mean upflightdis- RS Distance (m)a tances forLeast Terns (Sterna antillarum, ? 59.0 23.6 Orderand Species Walking Motor Boat m) and Black Skimmers(Rynchops niger,56.2 ? 30.6 Pelecaniformes m) to a walkingapproach were about twice the mean Brown Pelican individualbird-flush distances (27.9 + 9.4 m and 25.0 + (Pelecanus occidentalis) 76 65 9.6 m, respectively)from their nests afterreturning Double-crested Cormorant froman upflight(Fig. 1). (Phalacrocorax auritus) 96 71 We found significant(p < 0.05) negative relation- Anhinga ships-flush (Anhinga anhinga) 89 distances shorterwith successive on-foot Ciconiiformes approaches-only for LaughingGulls (Larus atricilla) at the Island DS-3D colony (r2 = 0.14,p < 0.04, n = (Ardea herodias) 100 82 31) and Cattle Egrets(Bubulcus ibis) at the Port Or- Great Egret ange colony (2 = 0.66,p < 0.001, n = 12). We found (Casmerodius albus) 91 87 Snowy Egret a significant(p < 0.05) positiverelationship-flush dis- (Egrettathula) 67 tances increasedwith successive on-footapproaches- Tricolored Heron forindividual Black Skimmersat the St. George Island (E. tricolor) 88 59 causeway colony duringthe incubationperiod of 1989 Little Blue Heron and 1990 (r2 = 0.66,p < 0.001, n = 22; r2 = 0.25,p (E. caerulea) 71 < 0.001, n = 56; but not the later respectively), during (Bubulcus ibis) 70 nestlingperiod. We also investigatedautocorrelation Black-crowned Night-Heron (especiallyfirst order) usingautocorrelation plots, plots (Nycticoraxnycticorax) 97 of lagged-regressionresiduals, and Durbin-Watsontests White Ibis (D) forindependence of serialcorrelation among resid- (Eudocimus albus) 76 Wood Stork uals. We detected significant(p < 0.05) positivefirst- (Mycteriaamericana) 65 63 orderautocorrelation only for flush distances of individ- Charadriiformes ual Least Terns during the incubation period at St. Least Ternb George Island causeway (1989: D = 0.906, autocorre- (Sterna antillarum) 154 lation = +0.513, n = 18; 1990: D = 1.543, autocor- Black Skimmerb (Rynchopsniger) 178 relation = + 0.224, n = 77). Consequently,the indi- A vidual flush distances for Least Terns and Black aRS distance was calculated by using theformulaRS = exp (ui + Skimmersat the St. George Island causewaycolony rep- 1.6495 61) + 40 m. Values were rounded to nearestwhole number bRS distancesfor thesespecies were based on the upflightresponse. resentedin Figure 1 are the data collected duringthe nestlingperiod that did not exhibit a significantrela- tionshipor autocorrelationbetween successivewalking Canoe Approach approaches.As the recommendedset-back distances for Double-crested Cormorants(Phalacrocorax auritus) these two species were calculated fromthe largerup- flushedat significantlygreater distances (t = 2.580,p < flightdistances (see Fig. 1 and Table 1), these effectsare 0.05) thanAnhingas (Anhinga anhinga) in our limited discussed here but did not confoundour set-backrec- data set (Fig. 2). ommendations. Comparisonsof HDMs BoatApproach Brown (t = 2.333), Double-crested Cormo- rants(t = 6.168), GreatBlue Herons (t = 5.172), and Brown Pelicans exhibitedthe shortestindividual flush TricoloredHerons (Egretta tricolor, t = 4.351) exhib- distance(4 m) and mean flushdistance (9.4 ? 5.5 m), ited significantly(p < 0.05) shorterflush distances to an whereas Great Egrets(Casmerodius albus) possessed approachingboat thanto walkinghumans, whereas the the longestmean flushdistance (28.9 ? 8.6 m) at the GreatEgret showed similarflush distances (t = 1.174,p approach of a boat (Fig. 2). As with the walking ap- > 0.05) to approacheson footand in a boat (Figs. 1 and proach, there were significantdifferences (ANOVA/ 2). Flushdistances for canoe and motorboat approaches Tukey'sM.R.T., p < 0.05) in the flushdistances among (Fig. 2) were similar(t = 1.370, p > 0.05) for the some species of the tree-nestingguild that prevented Anhinga. pooling these data into taxa higher than the species Double-crestedCormorants exhibited significantly (p level. < 0.05) greaterflush distances than Browrn Pelicans for

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Rodgers& Smith Set-BackDistances for Bird Colonies 95 motorboat (t = 8.291) but notfor walking (t = 1.140). pared to a walldngapproach. Vos et al. ( 1985) reported GreatBlue Herons and GreatEgrets appear to be two of thatmost boatingactivity caused the least disturbance the more sensitivespecies when approachedon footor to Great Blue Herons. Grubb and King (1991) also in a boat (Figs. 1 and 2). found that pedestrian trafficwas the human activity most disturbing to Bald Eagles (Haliaeetus leucoceph- Nestingand InteryearComparisons alus), and Klein (1993) found that human trafficwas Therewere no significant(p > 0.05) differencesin flush more disruptivethan vehicular trafflc to severalspecies distances between incubating and brooding adult of foragingwaterbirds. BrownPelicans (17.2 + 10.1 m [n = 38] and 22.5 + 9.3 We did not detect differencesin the flushdistances m [n = 25], respectively;t = 1.527) and Double- between incubatingand broodingadults. Likewise,Er- crestedCormorants (24.2 + 12.6 m [n = 24] and 33.2 win (1989) reported no significantcorrelations be- + 14.5 m [n = 14], respectively;t = 1.185) for the tween response distancesand nestingphase, thoughhe walking HDM. There were no significant(p > 0.05) did note a weak (p = 0.10) relationshipfor Least Terns differencesin flushdistances collected between 1989 and nestingphase. In our study,three species (Brown and 1990 among adult Brown Pelicans forthe walking Pelicans, Least Terns, and Black Skimmers) demon- HDM (18.0 + 6.3 m [n = 45] and 23.0 + 5.6 m [n = stratedthat a set-backdistance that prevents flushing by 18], respectively;t = 1.383) and individualflush dis- nestingadults also should provide an adequate buffer tances of adult Least Terns (33.4 + 9.9 [n = 18] and zone to prevent flushing(nest evacuation) by older, 26.3 + 10.7 m [n = 53], respectively;t = 1.318), and mobile juvenilebirds. In addition,the upflightdistances individualBlack Skimmersfor the walking HDM (21.5 + were greaterthan the individualflush distances of nest- 8.3 m [n = 22] and 28.5 ? 13.6 m [n = 25], respec- ing Least Terns and Black Skimmers and therefore tively;t = 1.188) approaches. should be used to calculate set-backdistances for these AdultLeast Terns (27.9 + 9.4 m, n = 71) and Black species. Skimmers(25.0 + 9.6 m, n = 47) exhibited signifi- We detected both decreasing distance responses to cantlygreater individual flush distances (t = 1.668 and repeated approaches (among Cattle Egrets,Laughing 1.854, respectively;p < 0.05) thanunattended, mobile Gulls) and increasingdistance responses to sequential nestlingterns (17.2 ? 12.3 m, n = 21) and skimmers approaches (among Black Skimmers),as well as, first- (16.3 + 4.4 m, n = 19) duringa walking approach. order autocorrelatedresponses (among Least Terns) Adult(19.2 ? 7.4 m, n = 63) and nestling(21.3 + 3.5 duringour analyses. It appears that the responses of m, n = 12) Brown Pelicans exhibited similar(t = individualsof these fourspecies to a sequential on-foot 1.523,p > 0.05) flushdistances to walkingapproaches. approach may have been affectedby our previous ap- proaches. Both acclimation to disturbance and in- creased sensitivityto disturbancephenomena should be Discussion considered by researchersin futurestudies. We could Our studydetected interspecificresponse variationto have adjusted for the effectsof first-orderautocorrela- the same HDM among colonial waterbirds,especially tionby multiplyingthe estimatedvariance by a function among the tree-nestingguild. Great Blue Herons and of the estimatedautocorrelation coefficient. Because of Great Egrets generallyexhibited the largestflush dis- the much greaterdistances for the upflightresponses of tances,whereas Brown Pelicans and Wood Storksgen- Least Terns and Black Skimmers,however, we did not erallypossessed the smallestflush distances. Other stud- use data on the flushdistances of individualbirds for ies also have found similar variation among species these two species when estimatingthe recommended (Manuwal 1978; Ollason & Dunnet 1980; Burger & set-back distances as we did for the other colonial Gochfeld1981; Erwin 1989). Muellerand Glass (1988) waterbirds.In addition,we did not estimate set-back noted that some species of waterbirds-Snowy Egrets distancesfor Cattle Egretsand LaughingGulls because (E. thula), Tricolored Herons, and White-facedIbises of a significantnegative relationship in successive ap- (Plegadis chihi)-were more adverselyaffected by dis- proach and flushdistances for these two species. To turbancethan other species. We also found thatsome compensatefor the effectsof acclimation,these species species, such as Brown Pelicans and CattleEgrets, were could be representedby a statisticrelated to the pre- relativelytolerant of human disturbance.This may be dicted regressionvalue of the firstobservation, rather due to their long association with and habituationto thana statisticbased on the species mean. However,we humanactivities in Florida;pelicans frequently associate did not have enough data to do this.Future research by with fishingactivities and "panhandle" at docks and otherinvestigators should consider these factorswhen piers,and Cattle Egretsoften follow farm machinery. designingand testinghypotheses. Severalspecies (Brown Pelican,Double-crested Cor- One proposed advantage of coloniality for single- morant,Great Blue Heron,Tricolored Heron) exhibited species and multi-speciesassemblages or nestingguilds shortermean flushdistances to a boat approach com- is antipredatordefense via early wrarningto colony

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96 Set-BackDistances for Bird Colonies Rodgers& Snith members(see reviews in Burger 1981b; Wittenberger Vos et al. (1985) recommended bufferzones of 250 & Hunt 1985). Least Terns and Black Skimmersexhib- meterson land and 150 meterson waterfor Great Blue ited similarlong upflightdistances, an advantage for Herons.Anderson (1988) proposed a minimumof 600 these ground-nestingguild species. The tree-nesting metersfor Brown Pelicans nestingon an island offthe guild species (Pelecaniformes and Ciconiiformes) west coast of Mexico, but this "minimumthreshold" showed smallerflush distances, however, possibly the value was derivedfrom indirect estimates of human traf- resultof nestingabove groundlevel and securityfrom ficalong footpaths. Schreiber and Schreiber(1978) rec- approach of some mammalianpredators. These tree- ommendedthat Double-crested Cormorants nesting in nestingspecies also exhibitedgreater interspecific dif- single-speciesor mixed-speciescolonies should not be ferencesin theirflush distances. A possible advantageof approached "closer than 75 m." The most extensive these mixed-species nesting assemblages would be studythat specificallyexamined disturbancesto colo- groupvigilance that allows the alertedbirds to fleefrom nial waterbirdswas by Erwin(1989). Based on a mean a predator (Krebs 1978). The intermediate-sizedday (?+SD) flushingdistance formula, Erwin (1989) recom- herons (such as LittleBlue Heron, Tricolored Heron, mended a bufferzone of 100 metersfor Least Ternsand Snowy Egret) that tend to nest under the vegetative wading birds and 200 metersfor Black Skimmersand canopy (see Burger1978) would gain an advantageby Common Terns (S. hirundo). For ternsand skimmers, nestingwith the more vigilant(larger flushdistances) Erwin(1989) used the upflightresponse to recommend Great Blue Herons and Great Egretsthat tend to nest set-backdistances. higherin the canopy. Our attemptsto quantifythe onset of alert and ago- It appearsthat acclimation to tangentialvehicle traffic nisticbehaviors by colonial waterbirdsin response to also occurs among some colonial waterbirds.Although exposure to various HDMs proved very difficult.Be- we made no direct test in this studyto determinethe cause of much concurrentnesting activity, it was not minimumdistance to elicit a flushresponse to a tangen- alwayspossible to detectwhen the bird under observa- tiallymoving vehicle, data fromanother study demon- tion exhibitedan alert/agonisticresponse to the HDM. stratedthat Least Terns (x = 11.0 m, range = 7-15 m, However,observations from blinds indicatedthat nest- n = 103 nests) and Black Skimmers(x = 12.6 m,range ing birds generallybecame agitatedby an approaching = 9-17 m, n = 61 nests) nestedcloser (t = 2.247 and disturbance25-40 meters(n = 11) prior to flushing 3.225, respectively;p < 0.001) to the road edge com- fromthe nest. This distanceis similarto the additional pared to theirindividual flush distances (least tern:x = buffer-zonedistance of 50 metersrecommended by Vos 26.6 m, range = 12-59 m, n = 54 nests; Black Skim- et al. (1985). The additionof 40 meters( + 40 m) to the mer: x~ = 25.0 m, range = 12-44 m, n = 47 nests) in flushingdistances of our sampledpopulations would be responseto walkingapproaches at the St. George Island a conservativeapproach to minimizealert/agonistic re- causewaycolony. The factorlimiting nest site proximity sponses and likewise allow for effectsrelated to varia- to the roadwayat thiscolony was apparentlythe lack of tion in vegetativecover, intraseasonaldifferences, and suitable substratewithin the adjacent grass-covered food supply that mightcause increased stress on the right-of-way.Only rarely did ternsand skimmersnesting colony(Hunt 1972; van der Zande & Vestral1985), and at the St. George Island causeway colony flushin mass other environmentalvariables that can influenceflush due to nearby(about 10-15 meters) tangentialvehicu- distances. lar trafficsuch as large,noisy tractor-trailers.This sug- We estimatedrecommended set-back (RS) distances gests that habituationto some types of human distur- for individualspecies of breeding colonial waterbirds bance is possible for some species at some sites, calculatedfrom the mean and standarddeviation of our especiallywhen breedinghabitat is limitedas forlarids sampledpopulations (Table 1). For a given species, let in Florida.Similar instances of acclimationby colonial Xi representthe observedflushing distance for an indi- waterbirdshave been reportedby Grubb (1978) and vidual nest approach i and Y =- In (Xi). We assumed Anderson(1988). thatthe Xi are independent,identically distributed and follow a lognormaldistribution with the parametersji and a such that ji = E (Yi) and U2 = var (Y1). Using Reconunendationsand Implementation Qo.95 as the ninety-fifthpercentile of this distribution (0.95 = P (Xi S Q095)) the desired RS distance was A major conclusion of our studyis thatall species must consideredto be Qo95 + 40. To estimateQO95 and the be consideredwhen recommendingset-back distances RS, the relationshipbetween percentilesof the lognor- around mixed-specieswaterbird colonies. Association mal and normal distributionswas used. Thus, for the withmixed-species aggregations may even increasethe ninety-fifthpercentile of a standardnormal distribution, flushingdistances for some species (Stinson 1988). Sev- ZO95 = 1.6495 and eral authorshave recommendedset-back distances to protect colonial waterbirdsfrom human disturbance. o.5= exp (> ? 1.6495(X).

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Rodgers& Smith Set-BackDistances for Bird Colonies 97

A Then, the estimated RS distance was calculated as along roadwaysor sounded theirhorns. We have ob- served similarbehavioral responses by Least Terns and A RS = exp (> + 1.6495() + 40, Black Skimmersat the St. George Island causeway col- ony and by Cattle Egretsat an 1-75 colony duringthis where > and a are the sample mean and standarddevi- study. ation for the observed values of Yi = In (Xi), i = Conservationpersonnel must monitorbreeding col- 1, ... , n. We believe that the one-tailed 5% criterion onies forchanges in species compositionso thatthe RS providesa sufficientlyconservative margin in the estab- distancecan be adjustedfor the presence of new, more lishmentof RS distancesfor colonial waterbirdswhile sensitivespecies with greaterflush distances. Likewise, providinga procedure that does not require assump- the perimeterof breedingcolonial waterbirdsmust be tions thatare unreasonablefor our data. monitoredannually so thatthe RS distancereflects cur- Human disturbanceduring wildlife viewing can sub- rentcolony boundaries(see Buckley& Buckley 1972; tly disruptcommunity dynamics (Skagen et al. 1991). Kerns& Howe 1967; Beaver et al. 1980). Managersalso Therefore,at mixed-speciescolonies of waterbirds,the mustknow if a breedingcolony is used as a winterroost mostsensitive species-the most "skittish"species with to determineif the RS distance should be maintained the greatestflush distance-should be used for deter- during the nonbreedingseason. If the RS distance is miningthe RS distance.We furtherrecommend that the discontinuedduring the nonbreedingseason, it should upflightdistances be used to calculate the RS distance be re-establishedseveral weeks prior to the arrivalof formixed ternand skimmercolonies. Due to low sam- breeding birds based on previous monitoringof the ple sizes, we were unable to estimatea RS distancefor breeding chronology of the colony. Effectson the most species disturbedby approach of a canoe. For An- prelayingportion of the breedingcycle associated with hingas,however, the RS distancefor a canoe approach disturbancemay include disruptedoccupation of col- (88 m) is similarto thatof a motorboat (89 m). Thus, ony sites(Conover & Miller1978), subcolonyprelaying we tentativelyrecommend that a RS distancefor canoes abandonment(Safina & Burger1983), or otheradverse and othersimilar vessels be the same as fora motorboat effectson pair-bondestablishment and nest-siteselec- (Table 1). For mixed-speciescolonies thatare subjectto tion behavior. multipleHDMs, our data suggestthat a RS distance of We recommendadditional research to examine the about 100 metersfor wading-birdcolonies and about effectsof variable approach speeds (especially rapid,er- 180 metersfor tern/skimmer colonies shouldprovide an ratic movements),tangential approaches, presence of adequate bufferzone around the populationswe sam- seasonal variationin response to disturbance,and other pled in Florida. types of HDMs (such as jet-skivehicles, aircraftover- We urge conservationpersonnel to use prudence flights,etc.). We realize that there are limits to our when implementingthe RS distancesin Table 1 forsin- method of calculatingestimated RS distances for each gle-species or mixed-species colonies elsewhere. For species and that the values are more subjective than example,on remote islands seldom visitedby humans, impliedfrom the RS equation. Because of the variation terns and other pelagic ground-nestersmay be more in flushdistances among individual birds and species,RS sensitivethan in our study.At the otherextreme, some distancesmay need to be developed on an individual- species may exhibit degrees of acclimationto various colony basis. However, we believe the principles and disturbancesfor shortperiods of time (as with the St. techniquesdeveloped here maybe applied elsewhereto George Island causeway colony). But,we believe accli- serve as a general model for specificdesign of RS dis- mationphenomena should neitherbe used as justifica- tances foreach species, location,and situation. tionfor reducing buffer-zone distances nor forattempt- ing to habituateany species to HDMs afterbirds have colonized a site. Some mitigationmay be possible for Acknowledgments shorterRS distanceswhen physicalbarriers prevent di- rect visual contact between breedingbirds and HDMs Numerousindividuals contributed to the success of this withlow noise levels.Also, some evidence suggeststhat study.We thankthe privateindividuals and public agen- tangentialapproach by a HDM (such as vehiculartraffic) cies thatallowed access to colonial waterbirdsites un- may allow fora shorterRS distance.This effectmay be der theircontrol: J. E. Davis, MerrittIsland N. W. R.,Pel- similarto one observedby Burgerand Gochfeld( 1981 ) ican Island N. W. R., Florida Department of Natural forHerring Gulls (L. argentatus) thatresponded to the Resources, The Nature Conservancy,and the City of potentialthreat of approach by a researcherat greater Port Orange. J. Dodrill, J. Gore, J. B. Miller, S. T. distancesif the approachwas directrather than tangen- Schwikert,and A. Whitehouse assisted with the field tial.Henson and Grant( 1991 ) also noted thatbreeding work and data collection. C. Moore and S. Linda pro- TrumpeterSwans (Cygnus buccinator) onlyreacted to vided valuable statisticalconsultation. We thankS. Nes- common vehicular trafficwhen the vehicles stopped bitt,M. Erwin,C. Futch,P. MacLaren,L. Minasian,D.

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Cairns,and an anonymousreviewer for reading earlier Henson,P., and T. A. Grant.1991. The effectsof human disturbance on draftsof the manuscript.We especiallywish to acknowl- Trumpeter Swan breeding behavior. Wildlife Society Bulletin edge the comments by D. W. Anderson and M. L. 19:248-257. Hunt, G. L. 1972. Influenceof food distributionand human distur- Hunter,Jr., that greatlyimproved our paper. We also bance on the reproductive success of Herring Gulls. Ecology acknowledgethe supportof our agencies and thankthe 53:1051-1061. senior administratorsand staffbiologists at the Florida Jeffrey,R. G. 1987. Influenceof human disturbanceon the nesting Game and Fresh Water Fish Commission and Florida success of AfricanBlack Oystercatchers.South AfricanJournal of Departmentof EnvironmentalProtection that provided WildlifeResearch 17:71-72. us with incentiveto complete this joint-agencyeffort. Jenni,D. A. 1969. A studyof the ecology of four species of herons duringthe breedingseason at Lake Alice,Alachua County,Florida. This paper is derived fromFG&FWFC study number Ecological Monographs39:245-270. 7511. Johnson,R. F.,Jr., and N. F. Sloan. 1976. The effectsof human distur- bance on the WhitePelican colony at Chase Lake NationalWildlife Refuge,North Dakota. Inland Bird BandingNews 48:163-170. LiteratureCited Kaiser,M. S., and E. K Fritzell.1984. Effectsof riverrecreationists on Green-backedHeron behavior.Journal of WildlifeManagement Agresti,A. 1984. Analysisof ordinalcategorical data. John Wiley and 48:561-567. Sons,New York. Kerns,J. M., and J.F. Howe. 1967. Factors determiningGreat Blue Anderson,D. W. 1988. Dose-response relationshipbetween human Heron rookerymovement. Journal of the MinnesotaAcademy of disturbanceand Brown Pelican breedingsuccess. WildlifeSociety Science 34:80-83. Bulletin16:339-345. Klein, M. L. 1993. Waterbirdbehavioral responses to human distur- Anderson,D. W., and J.0. Keith. 1980. The human influenceon sea- bances. WildlifeSociety Bulletin 21:31-39. birdnesting success: conservationimplications. Biological Conser- vation 18:65-80. Krebs,J. R. 1978. Colonial nestingin birds,with special referenceto the Ciconiiformes. in A. Beaver,D. L., R. G. Osborn,and T. W. Custer. 1980. Nest-siteand col- Pages 299-314 Sprunt,IV, J. C. Ogden,and S. editors. National ony characteristicsof wading birds in selected Atlanticcoast col- Winckler, Wadingbirds. AudubonSociety, New York. onies. Wilson Bulletin92:200-220. Buckley,F. G., and P. A. Buckley.1972. The breedingecology ofRoyal Kurry,C. R., and M. Gochfeld. 1975. Human interferenceand gull TernsSterna (Thalasseus) maxima maxima Ibis 114:344-359. predationin cormorantcolonies. Biological Conservation8:23- Buckley,P. A., and F. G. Buckley. 1976. Guidelinesfor the protection 34. and managementof coloniallynesting waterbirds. North Atlantic Lack,D. 1968. Ecological adaptationsfor breeding in birds.Methuen, RegionalOffice, National Park Service, Boston. London. Burger,J. 1978. The patternand mechanismof nestingin mixed- Manuwal,D. A. 1978. Effectsof man on marinebirds: a review.Pages species heronries.Pages 45-58 in A. Sprunt,IV, J. C. Ogden, and S. 140-160 in Wildlifeand people. Proceedingsof theJohn S. Wright Winckler,editors. Wading birds.National Audubon Society,New ForestryConference., Department of Forest and NaturalResources York. and the CooperativeExtension Service, Purdue University, Indiana. Burger,J. 1981a. Effectsof human disturbanceon colonial species, Mueller,A.J., and P. O. Glass. 1988. Disturbancetolerance in a Texas particularlvgulls. Colonial Waterbirds4:28-35. waterbirdcolony. Colonial Waterbirds.11:1 19-122. Burger,J.1981b. A model forthe evolutionof mixed-speciescolonies Nesbitt,S. A.,J. C. Ogden, H. W. Kale, II, B. W. Patty,and L. A. Rowse. of Ciconiiformes.Quarterly Review of Biology 56:143-167. 1982. Florida atlas of breeding sites for herons and their allies: Burger,J., and M. Gochfeld. 1981. Discriminationof the threatof 1976-78. FWS/OBS-81/49.U.S. Fish and WildlifeService, Office of directversus tangential approach to the nestby incubatingherring Biological Services,Washington, D.C. and Black-backedGulls. Journalof ComparativePhysiology and Ollason,J. C., and G. M. Dunnet. 1980. Nest failuresin the Fulmar:the Psychology95:676-684. effectsof observers.Journal of Field Ornithology51:39-54. Cairns,D. 1980. Nestingdensity, habitat structure and humandistur- Oring,L. W.,K P. Able,D. W. Anderson,L. F. Baptista,J. C. Barlow,A. S. bance as factorsin Black Guillemotreproduction. Wilson Bulletin Gaunt,F. B. Gill, and J.C. Wingfield.1988. Guidelines for use of 92:352-361. wild birds in research.Auk 105(1, Suppl.):1A-41A. Conover,M. R., and D. E. Miller.1978. Reactionof ring-billedgulls to Palmer,R. 1941. A behaviorstudy of the (Sterna hir- predatorsand humandisturbances at theirbreeding site. Proceed- undo L.). Proceedingsof the Boston SocietyNatural History 12:1- ings of the Colonial WaterbirdGroup 2:41-47. 119. Ellison,L. M., and L. Cleary. 1978. Effectsof human disturbanceon Parsons,K C., and J. Burger.1982. Human disturbanceand nestling breedingDouble-crested Cormorants. Auk 95:510-517. behaviorin Black-crownedNight Herons. Condor 84:184-187. Erwin,R. M. 1989. Responses to human intrudersby birds nestingin Pierce, D.J., and T. R. Simons. 1986. The influenceof human distur- colonies: Experimentalresults and managementguidelines. Colo- bance on TuftedPuffin breeding success. Auk 103:214-216. nial Waterbirds12:104-108. Portnoy,J. W., R. M. Erwin,and T. W. Custer. 1981. Atlas of gull and Gillet,W. H., J.L. Hayward,and J.F. Stout. 1975. Effectsof human terncolonies: NorthCarolina to Key West,Florida (including pel- activitieson egg and chick mortalityin a Glaucous-wingedGull icans, cormorantsand skimmers).FWS/OBS-80/05. U.S. Fish and colony. Condor 77:492-495. WildlifeService, Office of Biological Services,Washington, D.C. Goering,D. K, and R. Cherry. 1971. Nestlingmortality in a Texas Robert,H. C., and C.J. Ralph. 1975. Effectsof human disturbanceon heronry.Wilson Bulletin83:303-305. the breedingsuccess of gulls.Condor 77:495-499. Grubb,M. M. 1978. Effectsof increasednoise levels on nestingherons Safina,C., andJ.Burger. 1983. Effectsof humandisturbance on repro- and egrets.Proceedings of the Colonial WaterbirdGroup 2:49-54. ductivesuccess in the Black Skimmer.Condor 85:164-171. Grubb,T. G., and R. K King. 1991. Assessinghuman disturbanceof Schreiber,R. W. 1979. Reproductive performanceof the eastern breeding Bald Eagles with classificationtree models. Journalof BrownPelican Pelecanus occidentalis.Natural History Museum of WildlifeManagement 55:500-511. Los AngelesCity Contributions to Science 317:1-43.

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Schreiber,R. W., and E. A. Schreiber. 1978. Colonial bird use and Van der Zande, A. N., and T.J. Vestral.1985. Impactsof outdoor rec- plant succession on dredged-materialislands in Florida. I: Sea reationupon nest-sitechoice and breedingsuccess of the Kestrel. and wading bird colonies. Technical report D-78-14. U.S. Army Ardea 73:90-99. Engineer Waterways Experimental Station, Vicksburg,Missis- Veen, J. 1977. Functionaland causal aspects of nest distributionin sippi. colonies of the SandwichTern (Sterna & sandvicensis Lath.). Be- Skagen,S. K, R. L. Knight,G. H. Orians. 1991. Human disturbancesof haviouralSupplement 20. an avian scavengingguild. Ecological Applications1:215-225. Vos, D. K, R. A. Ryder,and W. D. Grand. 1985. Response of breeding Stinson,C. H. 1988. Does mixed-speciesflocking increase vigilance or Great Blue Herons to humandisturbance in northcentralColora- skittishness?Ibis 130:303-304. do. Colonial Waterbirds8:13-22. Teal, J.M. 1965. Nesting success of egrets and herons in Georgia. Wittenberger,J. F., and G. L. Hunt,Jr. 1985. The adaptivesignificance WilsonBulletin 77:257-263. of colonialityin birds. Pages 1-78 in D. S. Farner,J. R King,and Tremblay,J., and L. N. Ellison. 1979. Effectsof humandisturbance on K C. Parkes,editors. Avian biology, vol. VIII. Academic Press,New breedingBlack-crowned Night Herons. Auk 96:364-369. York.

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