J. FieldOrnithol., 59(3):215-223
FACTORS
INFLUENCING
PREDATION
ASSOCIATED
VISITS
TO ARTIFICIAL
GOOSE
NESTS
M.
MICHELE
VACCA AND COLLEEN
M.
WITH
HANDEL
AlaskaFish and Wildlife ResearchCenter
U.S. Fish and Wildlife Service
7077 E. Tudor Road
Anchorage,Alaska99503 USA
Abstract.--Artificial goosenestswere used to determine what factorsmight increasepredationafter visitsto nestsof CacklingCanadaGeese(Brantacanadensis
minima).We tested
whether leaving the nest uncovered,marking the nest locationwith a flag, or placing the
neston an island or peninsulawould increasethe rate of predation.Predatorsdestroyed
significantlymoreof the nestswith eggsexposedto view (61%) than of the nestswith eggs
covered
with goosedown(35%) (P < 0.05). However,the rate of predationwasonlyslightly
higheramongnestslocatedon peninsulasthan on islandsand equal proportionsof flagged
andunflaggednestswere destroyed.We alsodeterminedthat investigators
attractedpredators
to the studyarea and causedan increasein predationat uncoverednestsimmediatelyafter
the visit. Coveringthe eggswith down essentiallynegatedthe effectof attractingpredators
when visiting the nest.
Amongthe46 nestsdestroyed,
78%weredestroyed
by birdsand22%by mammals.Results
of our studysuggested
that visibilityof exposedeggsrather than nestmarkersprovided
importantcuesto avian predatorsand that islandsprobablyprovidedsomerefugefrom
mammalian predators.Investigatorscan take stepsto minimize their impact on nesting
success
and shouldincorporatea measureof that impact in their studies.
FACTORRS
QUR INFLUYRNRN LA DRPRRDACION,
ASOCIADOS
CON
VISITAS
A NIDOS
ARTIFICIALRS
DR
GANSOS
Resumen.--Se examinaron nidos artificiales de Gansos del Canada (Branta canadensismin-
zma)para determinarlosfactoresque podrianaumentarla depredaci6nluegode las visitas.
Seexperiment6connidosartificialeslocalizados
en islaso peninsulas
y conotrosque fueron
cubiertoso dejadosal descubierto
o su localizaci6nindicadaconuna pequefiabandera.Los
depredadores
destruyeron
la mayoriade losnidosdejadosal descubierto
(61%) y tan solo
afectaronel 35% de los nidosque fueroncubiertosconplum6n de ganso.Sin embargo,la
taza de depredaci6nfue tan soloun pocomils alta en nidos1ocalizados
en peninsulasque
en islas,y de igual magnituden nidosque fuerono no fueronmarcadosconbanderas.Se
determin6que los investigadores
atraen depredadoresal firea de estudioy causanpor
consiguiente
aumentoen la depredaci6n
inmediatamente
despu•sde las visitas,particularmenteen nidosque fuerondejadosal descubierto.
E1 cubrir losnidosconplum6nde ganso,
minimiza el efectode atraer los depredadores.
De los46 nidosdestruidos,78% fueronafectadospor otrasaveay el 22% por mam•feros.
Losresultados
sugierenquela exposici6n
deun nidoa la visibilidadporpartededepredadores
proveea estosde pistasmilsconcretas
que las que le puedanproveermarcadores,
y que las
islasofrecencierta protecci6na la depredaci6npor parte de mamlferos.Los investigadores
debentomaren consideraci6n
el impactodesusvisitasensusinvestigaciones
y tomarmedidas
para minimizar su efecto.
Dramatic declinesin populationsof the CacklingCanada Goose(Branta canadensis
minima) and three other speciesof geesenestingin subarctic
Alaska (Raveling 1984) have recentlycompelledresearchers
to determine
the factorsthat affecttheir productivity.Becauseinvestigatorvisitsto the
nestsof these speciesmay increasepredation, biasesmay result when
estimatingnestingsuccess
for the population(seeNicholset al. 1984 for
215
216]
M. M. Vacca
andC.M. Handel
j. Field
Ornithol.
Summer
1988
reviewof impactsof investigators).Severalstudiesthat estimatethe effects
of visitationon nestingsuccess
havefoundno effect(Gottfried and Thompson 1978, Livezey 1980, Willis 1973), whereas others found nesting
success
significantlydecreased(Bart 1977, Gillett et al. 1975, Lenington
1979, Ollasonand Dunnet 1980, Robert and Ralph 1975, Strang 1980).
The intent of this studywas to determinewhat factorsmight increase
predation after visits to the nestsof Cackling Canada Geese and what
stepsinvestigatorsmight take to minimize their impacton nestingsuccess.
We hypothesized(1) that coveringthe eggswith down,as geesedo when
they leavethe nest(Mickelson 1975), would reducethe rate of predation;
(2) that marking the locationof nestswith flagswould increasethe risk
of predation;(3) that the presenceof the investigatorwould attract predatorsto the nestand predationwould be increased(cf. Strang 1980); and
(4) that visits to nests on islands would cause less of an increase in
predation than visits to nestson peninsulasbecauseof the relative inaccessibilityof islandsto foxes,one of the principal predatorsof geeseand
their eggs(Mickelson 1975). We testedthesefour hypotheses
by analyzing
predationratesof simulatedgoosenestssubjectedto variousexperimental
treatments.
STUDY
AREA
AND
METHODS
Field experiments.--Between15 Jun. and 4 Jul. 1985, we monitored
the fate of 96 artificial goosenestson 12 plots within nestingareasused
by CacklingCanadaGeeseon the Yukon-KuskokwimDelta, Alaska.The
plots were in coastaltundra habitat between3.2 km north and 11.5 km
south of Old Chevak (61ø26'N, 165ø27'W) and were distributed among
drainagesof riversthat emptiedinto Hazen Bay, AngyoyaravakBay, and
KokechikBay. Mickelson(1975) providesa detaileddescriptionof habitats usedby geesewithin this region.
Canada Gooseeggswere simulatedby domesticchickeneggs,which
were smaller but similar in shape and color. The eggswere placed in
nestbowlsthat had beenusedin previousyearsby geese.On eachof the
12 plots,three eggswere placedin eachof eight nestbowls, which were
at least30 m from other natural or experimentalgoosenests.The location
of half of the experimentalnestswas marked by placing a pink, 10 cm
x 12 cm flag on a 1 m wire, 5 m north of the nest bowl. Flagged and
unflaggednestswere at least 400 m apart to reducethe possibilitythat
the flags would attract predatorsto the unmarkednests.Within each
group of flaggedand unflaggednests,two were nest bowls locatedon
islandswithin pondsand two were on peninsulas.We coveredhalf of
thesenestswith goosedown that had been collectedduring past years;
in the remaininghalf of the neststhe eggsremainedexposedto view.
We
varied
the time
intervals
at which
we visited
nests in order
to
calculatethe effectthat visitationhad on predationrates(seeBart 1977).
Four of the twelve experimentalareaswere randomly assignedto each
of three time intervalsbetweeninitial placementof eggsand subsequent
Vol.59,No.•
Visitation
Factors
andPredation
[217
visit: (1) 3-6 h, (2) 21-33 h, and (3) 38-70 h. A nest was considered
depredatedwhen any eggshad been destroyedor removed.
The numberof potentialpredatorswas recordedby scanningthe area
when the nestswere initially setout and at the time they were rechecked.
Potentialpredatorswere GlaucousGulls (Larushyperboreus),
Long-tailed
Jaegers(Stercorarius
longicaudus),
ParasiticJaegers(S.parasiticus),
arctic
foxes(Alopexlagopus),red foxes(Vulpesvulpes)and Americanminks
(Mustela rison).
At eachdepredatednestinvestigators
attemptedto identifythe type of
predator.Predationwas attributedto mammalsif no tracesof eggshells
were found around the nest, if shell fragmentshad tooth marks, or if
tracks, fox fur, or scatwere found near the nest. Predation was attributed
to birds if eggfragmentsor puncturedeggshellsremainedin or near the
nest(Eisenhauer1976; Ely and Raveling1984; M. M. Vacca,pers.ohs.).
Statistical
analyses.--Weuseda seriesof three-waytestsof independence(Sokal and Rohlf 1981) to determinethe influenceon predation
ratesof variouscombinationsof the four experimentalfactors(exposure
time, coveringthe eggswith down, marking the nestswith flags, and
locationof the neston an islandor peninsula).The statisticallysignificant
combinationswere then partitionedusingtwo-way testsof independence
(Sokaland Rohlf 1981) to determinewhich individualfactorssignificantly
influencedpredationrates. Finally, 95% confidenceintervalswere calculatedusingone-tailedt-teststo determinethe magnitudeof the differencesin predationratesamongnestswithin paired treatments•(covered
vs. uncovered,flaggedvs. unflagged,and islandvs. peninsula).We were
ableto considergroupsof nestson eachof the 12 plotsto be independent
samplesbecauseof the balanceddesignof the experiment.Differencesin
the proportionsof nestsdestroyedby avian and mammalianpredators
were testedfor the various experimentaltreatmentsusing Ghi-square
analysis(Sokal and Rohlf 1981).
To determine if the investigator attracted predators to the nest we
calculatedvisitor impact accordingto Bart (1977). First we calculated
the percentactual mortality (ma) and the averagelength of time (ti) of
exposureto predatorsfor clutchesin nestssubjectedto eachof the three
time regimes(3-6 h, 21-33 h, 38-70 h). Using thesefigureswe derived
an averagehourly rate (r) of mortality for the clutchesthat were exposed
for 3-6 h (during which any impacts of visitation would have been
concentrated)as follows: (1 - r) t = (1 - ma). For each of the two longer
time intervalswe then computedwhat the expectedmortality (me)would
have been if this initial rate of mortality had remainedconstant,i.e., if
all mortality observedduring the 3-6 h period had been due to natural
causesand there had been no visitor impact:me= [1 - (1 - r)t]. Gomparisonsof thesepredictedvalueswith the observedmortality rateswere
then made to determine if investigatorshad attracted predatorsto the
nestsand causedan increasein predation immediatelyfollowing their
visitsto the nests.Significantlylower observedrates of mortality than
predictedwould imply that visitor impact had occurred.
218]
M. M. Vacca
andC.M. Handel
J.Field
Ornithol.
Summer
1988
TABLE1. Designand resultsof experiments
showingthe influenceof variousfactorson
the rate of predationof artificialgoosenests.The numberof nestsdestroyedis shown
in relationto thetimetheywereexposed
to predators,
whetheror nottheyweremarked
withflagsor covered
with down,andwhethertheywerelocated
onislands
or peninsulas.
Flagged
Covered
Time exposed(•)
Unflagged
Uncovered
Covered
Uncovered
Is.
Pen.
Is.
Pen.
Is.
Pen.
Is.
Pen. Totals
1
3
0
4
0
4
1
3
0
4
0
4
2
2
2
2
6
26
2
2
3
1
2
2
3
1
0
4
1
3
3
1
2
2
16
16
2
2
2
2
3
1
4
0
3
1
3
1
4
0
3
1
24
8
3-6 h (4.2)
Destroyed
Not destroyed
21-33 h (24.9)
Destroyed
Not destroyed
38-70 h (51.7)
Destroyed
Not destroyed
RESULTS
Effectsof experimentaltreatments.--Predationwas recordedat 48% of
the 96 artificial goosenestswe monitored;among these there were no
instancesof partial predation.The rate of predationvaried greatlyamong
the combinationsof experimentaltreatmentsto which the nestswere
exposed(Table 1).
As expected,the rate of predation increasedsubstantiallywith the
amountof time the nestswere exposedbeforebeingrechecked.Only 19%
of the nestsexposedfor 3-6 h were destroyedwhereas50% and 75% of
those exposedfor 21-33 h and 38-70 h, respectively,were destroyed
(Table 2). Eggs in nestswhich we left uncoveredexperiencedalmost
twice the mortality as thosein nestswhich we coveredwith down. The
rate of predation was only slightly higher among nestslocatedon peninsulasthan amongthoseon islands.Equal proportionsof flaggedand
unflaggednestswere destroyedby predators(Table 2).
Statisticaltestsfor independenceamong these factors (time exposed,
coveringwith down,flagging,andlocationon islandor peninsula)revealed
that there were no significantinteractionsin their influenceon the proportion of nestsdestroyed.Among the six possiblethree-way tests of
independence,
only the three combinationsthat includedexposuretime
as a factor were statisticallysignificant(Table 3). When these combinationswere further partitioned into two-way tests,we found that exposuretime (P < 0.01) and coveringthe nestwith down (P < 0.05) were
theonlytwo factorsthat significantlyinfluencedpredationrates(Table 3).
Becausewe found no significantinteractionsamong the experimental
factors, we were able to calculate confidence intervals for the effect of
each factor on the rate of predation among the 12 experimental areas.
Vol.59,r•o.•
Visitation
Factors
andPredation
[219
T^BLI•2. Percentofartificialgoose
nests
destroyed
bypredators
undervariousexperimental
treatments.
Experimental
treatment
n
Percent
destroyed
Time exposed
3-6 h
21-33 h
38-70 h
32
32
32
19
50
75
Covered
48
Uncovered
48
35
60
On island
48
46
On peninsula
48
50
48
48
48
48
Covering with down
Location
of nest
Marking with flag
Flagged
Unflagged
Under conditionssimilar to thosein this study,goosenestsnot covered
with down after a visit would be expectedto suffer a rate of predation
12-38% (95% C.I.) higherthan that of nestswhoseeggswere covered.
Differencesin depredationof nestsmarked with flags and nestsnot
markedcouldbe expectedto vary from 17%lowerto 17%higher(95%
C.I.) andthelossof nestslocatedon islandscouldrangefrom 14%lower
to 6% higher(95% C.I.) than lossof nestson peninsulas.
We alsotestedif ratesof depredationby avianand mammalianpredatorswereinfluenced
differentlyby the variousexperimental
treatments.
Amongthe 46 nestsdestroyed,
78%weredepredated
by birdsand22%
bymammals.
Mammalianpredators
destroyed
approximately
equalnumbersof uncoveredand coverednests(six and four, respectively;
corrected
x2 = 0.50, df = 1, P > 0.10). Avian predators,however,destroyedmany
more uncoveredneststhan coverednests (23 vs. 13), a differencethat
bordered
onstatistical
significance
(corrected
x2= 2.81,df = 1, P < 0.10).
Approximately
equalnumberson islandsand peninsulas
(19 and 17,
respectively)
weredestroyed
by birds(corrected
x2= 0.139,df = 1, P >
0.5). Nestsonpeninsulas,
however,
weremorevulnerable
to mammalian
predators
thannestson islands(seven
vs.threedestroyed,
respectively),
althoughthis outcomewas not statistically
significant(corrected
x2 =
1.70, 1 df, P > 0.10). Both avianand mammalianpredatorsdestroyed
an equalnumberof flaggedandunflagged
nests(18 eachandfiveeach,
respectively).
Visitorimpact.--Comparison
of percentmortalityamongclutchesin
nestsexposed
to predators
for varyinglengthsof timeallowedusto test
whetherthe investigator
causedan initial increase
in therateof predation
by attractingpredators
to the nestsite.Because
covering
the eggswith
220]
M. M. Vacca
andC.M. Handel
j. Field
Ornithol.
Summer
1988
TABLE3. Statisticalindependence
amongvariousfactorsthat were testedfor their influence
on predation of artificial goosenests.
Significance
Factors a tested in treatments
Three-way testsc
Time x coveringx destroyed
Time x flagging x destroyed
Time x locationx destroyed
Govering x flagging x destroyed
Govering x location x destroyed
Flagging x location x destroyed
Two-way tests
Time x destroyed
Govering x destroyed
Location x destroyed
Flagging x destroyed
df
G value
of treatments b
7
7
7
4
4
4
30.118
25.210
22.176
7.680
6.340
0.836
**
**
**
NS
NS
NS
2
21.680
**
1
1
1
6.074
0.166
0.000
*
NS
NS
aTime exposedto predators;coveredwith down or not; flaggedor not flagged;locatedon
islandor peninsula;destroyed
or not destroyedby predators.
b** = ? < 0.01, * = ? < 0.05, NS = not significant.
cAll three-factorinteractionswere not significant.
down significantlydecreasedthe predation rate, we testedthe groupsof
uncoverednestsand coverednestsseparately for visitor impact.
Among clutchesin uncoverednests,thosein the 16 exposedfor the
shortestperiod of time (3-6 h) suffered31.8% mortality. If this rate had
remained constant,then the mortality for clutchesin uncoverednests
exposedfor longerperiodswould have been 89.2% for thoseexposedan
averageof 24.9 h and 99.0% for thoseexposedan averageof 51.7 h. Since
thesepredictedvaluesare muchhigher than the mortality ratesobserved
during the study(Table 4), it is clear that the investigator'spresenceat
the nestsattracted predators and resulted in a higher rate of predation
immediatelyafter the visit than during subsequenthoursof exposure.
This trend did not hold for neststhat were coveredwith down. Among
the 16 coveredneststhat were exposedfor the shortesttime, 6.3% were
depredated.If this initial rate had remainedconstant,thenpredictedrates
of mortality would have been 31.8% for thoseexposedfor an averageof
24.9 h and 54.8% for thoseexposedfor an averageof 51.7 h. Sincethese
predictedrates are similar to and actually lower than the ratesobserved
(Table 4), coveringthe eggswith down essentiallynegatedthe effect of
attracting predatorswhen initially visiting the nest.
Observationsof predators.--Glaucous Gulls were the most common
predator (• = 12.5, $D = 37.4) countedduring 32 scansof the experimental plots, followedby ParasiticJaegers(• = 0.81, SD = 1.84) and
Long-tailed Jaegers (• = 0.19, $D = 0.74). No minks or foxes were
recordedalthough a food cacheof an arctic fox was found near one of
the plots. The only direct observationsof predation in this study were
Vol.59,No.•
Visitation
Factors
andPredation
[221
TABLE4. Test of visitor impact through comparisonof observedand predictedmortality
ratesa for clutchesin artificial goosenestsleft uncoveredor coveredwith down. For
uncoverednests,higher predictedrates than observedimply that visitor impact did
occur,causingthe rate of predationto increaseimmediatelyafter the investigatorleft
the nest and then to decline
to natural
levels afterward.
Cumulative mortality during two
intervals of exposure
Treatment
Uncovered
Covered
nests
nests
24.9 h b
51.7 h c
Predicted d
Observed
89.2%
62.5%
99.0%
87.5%
Predicted e
31.8%
37.5%
54.8%
62.5%
Observed
a Predictedcumulativemortality was calculatedunder the assumptionthat there was no
visitor impact and that the observedhourly rates of predation for coveredand uncovered
nests visited after the shortest interval (3-6 h, • = 4.2 h, SD = 1.1, n = 32) should have
remainedconstantthroughoutthe longer intervals(cf. Bart 1977).
bAveragetime of exposurefor nestsvisitedafter 21-33 h (n = 32, SD = 3.4).
cAveragetime of exposurefor nestsvisitedafter 38-70 h (n = 32, SD = 10.5).
dCumulativepredictedmortality if initial predationrate observedfor clutchesin uncovered
nestshad remained constant(31.3% over 4.2 h = 8.5%/h).
e Cumulative predictedmortality if initial predation rate observedfor clutchesin covered
nestshad remained constant(6.3% over 4.2 h = 1.5%/h).
two instancesin which ParasiticJaegerswere observedpreying on eggs
in artificial
nests.
DISCUSSION
Resultsof this studysupportthe hypothesis
that visitsby investigators
to goosenestswill, under certain conditions,have a measurableimpact
on predation.Predatorswere attractedto the studyareasby investigators;
however,only the artificial goosenestswith eggsleft exposedsuffereda
significantincreasein predationrates. These nestssufferedalmosttwice
the rate of predationas nestsin which the eggswere left coveredwith
goosedown, a differenceattributable primarily to the higher rate of
destructionof uncoverednestsby avian predators.Our resultsconfirm
contentions
of earlierstudieson the Yukon-KuskokwimDelta thatjaegers
and gulls followed people searchingfor waterfowl nestsand sometimes
tookexposedeggs(Strang1980) and that mostlosses
of CacklingCanada
Gooseeggsresultedfrom suchscavenging
(Mickelson 1975). Other studies
that testedthe effectof leavingwaterfowl nestsexposedfound that visi-
bility..oftheeggsstronglyinfluenced
therateof avianpredation.Gbtmark
and Ahlund (1984) found that after two hours 43% of their uncovered
artificial CommonEider (Somateriamollissima)nestswere destroyedby
avian predatorswhile the coverednestsremained undisturbed.Dwernychukand Boag(1972) foundthat the amountof vegetationconcealing
eggsfrom view was a major factor in the rate of destructionof artificial
duck nestsby avian predators.
222]
M. M. Vacca
andC.M. Handel
J.Field
Ornithol.
Summer 1988
Contrary to our expectations,marking the locationof artificial goose
nestswith highlyvisibleflagsdid not significantly
increasepredation.We
hypothesized
that flaggingthe nestwouldincreasepredationratesbecause
avian predatorscommonlyrely on visual cuesto locatenests(Gottfried
and Thompson1978, Lenington1979, Lill 1974, Strang 1980). Picozzi
(1975) foundthat well-concealedartificial Red Grouse(Lagopuslagopus)
nestssufferedhigher rates of predationby Grows (Corvuscorone)when
their locationswere marked with short canes,and Reynolds(1985) observedthat Sandhill Cranes (Grus canadensis)were attracted to stakes
marking the locationof shorebirdnests.Both of thesestudies,however,
dealt with prey speciesthat normally rely on cryptic nestsfor defense
againstavian predators.In contrast,a Cackling Canada Goosenest is
highly visible to avian predators,even when coveredwith down. Once
incubationhas begun geeseare normally highly attentiveto their nests,
leavingthemonlyfor shortperiods(Mickelson1975). Under undisturbed
conditions,henscovertheir eggswith nestmaterialbeforedeparting,the
ganderguardsthe nest in the hen's absence,and attendinggeeseusually
defendtheir eggsfrom an attackinggull orjaeger(Mickelson1975). Nests
with exposedeggswould indicatethat a defendingadult is not nearby,
either becauseit is during the egg-layingperiod when geeseare less
attentive or becausethe hen has been flushed from the nest. Therefore,
exposedeggsrather than nestmarkerswould be a better indicationto a
gull or jaegerof a potentialfoodsource.
Mammalian predatorswere not a major predator in our study, destroyingonly 10 of 96 artificial nests.Amongthesean equalnumberwere
flaggedand unflaggednests,and approximatelyequal numbers were
coveredand uncoverednests,suggesting
that mammalsare not usingstatic
visualcuesto locatenests,but insteadmay be followinghuman scent,or
are attractedby the investigator.Both Snelling(1968) and Willis (1973)
believedthat mammalianpredatorsfoundnestsby followinghumanscent
or were attractedto the area by flushingbirds or other human activity.
We alsoobservedthat nestson peninsulaswere more vulnerableto mammalianpredationthan nestson islands,indicatingthat islandsmay provide
refugiafrom mammalianpredators.
Our resultsemphasizethat in studiesof nestingsuccess
it is important
to know what predatorsare commonin the study area and what cues
they are likely to use to locatenests.In studieswhere gulls and jaegers
are prevalent,investigatorsmay be able to eliminate the impact of their
visitsto nestsby coveringthe eggswith downbeforetheyleave.However,
becausepatterns of predation may vary with densityof nestsor other
factors,eachstudyshouldincludesomemeasureof the impactof visitation.
ACKNOWLEDGMENTS
This work was supportedby the Alaska Fish and Wildlife ResearchCenter and the
Yukon Delta National Wildlife Refuge. We are grateful to G. DesLaurier, B. A. Hicks,
K. L. Kincheloe,P. E. Seiser,and D. Youkey for their assistance
in the field. We thank J.
Bart for sharinghisstatisticalexpertise,and E. H. Burtt Jr., D. V. Derksen,B. M. Gottfried,
vol.59,•o.3
Visitation
Factors
andPredation
[223
M. R. Petersen,J. S. Sedingerand R. A. Stehnfor their constructive
commentson earlier
drafts.R. A. Stehnis especiallythankedfor hisencouragement
and supportin implementing
the study.
LITERATURE
CITED
BART,J. 1977. Impactof humanvisitationson aviannestingsuccess.
Living Bird 16:187192.
DWERNYCHUK,L. W., AND O. A. BOAG. 1972. How vegetativecover protectsduck nests
from egg-eatingbirds.J. Wildl. Manage. 36:955-958.
EISENHAUER,
D. I. 1976. Ecologyand behaviorof the Emperor Goose(Ansercanagicus
Sewastianov)in Alaska. M.S. thesis,Purdue Univ., Lafayette, Indiana.
EI•Y,C. R., ANDD. G. RAVELING.1984. Breedingbiologyof PacificWhite-frontedGeese.
J. Wildl. Manage. 48:823-837.
GILLETT,W. H., J. L. HAYWARD,
JR., ANDJ. F. STOUT. 1975. Effectsof humanactivity
on eggand chickmor..tality
in a Glaucous-winged
Gull colony.Condor77:492-495.
GOTMARK,F., AND M. AHLUND. 1984. Do field observersattract nest predators and
influencenestingsuccess
of Common Eiders?J. Wildl. Manage. 48:381-387.
GOTTFRIED,B. M., ANDC. F. THOMPSON.1978. Experimentalanalysisof nestpredation
in an old-field
habitat.
Auk
95:304-312.
LENINGTON,
S. 1979. Predatorsandblackbirds:the "uncertaintyprinciple"in field biology.
Auk
96:190-192.
LILL, h. 1974. The evolution of clutch size and male chauvinism in the White-bearded
Manakin. Living Bird 13:211-231.
LIVEZEY, B.C.
1980. Effects of selectedobserver-related factors on fates of duck nests.
Wildl.
Soc. Bull.
8:123-128.
MICKELSON,P. G. 1975. Breedingbiologyof cacklinggeeseand associatedspecieson the
Yukon-Kuskokwim Delta, Alaska. Wildl. Monogr. 45.
NICHOLS,J. D., H. F. PERCIVAL,R. A. COON,M. J. CONROY,G. L. HENSLER,ANDJ. E.
HINES. 1984. Observer visitation frequencyand successof Mourning Dove nests:a
field experiment. Auk 101:398-402.
OLLASON,
J. C., AND G. M. DUNNET. 1980. Nest failures in the fulmar: the effectsof
observers.J. Field Ornithol. 51:39-54.
PICOZZI,N. 1975. Crow predation on marked nests.J. Wildl. Manage. 39:151-155.
RAVELING,
D.G. 1984. Geeseand huntersof Alaska'sYukon Delta: managementproblems
and political dilemmas.Trans. N. Am. Wildl. Nat. Resour. Conf. 49:555-575.
REYNOLDS,
J. D. 1985. Sandhill Crane useof nestmarkersas cuesfor predation.Wilson
Bull.
97:106-108.
ROBERT,H. C., AND C. J. RALPH. 1975. Effectsof human disturbanceon the breeding
success
of gulls. Condor 77:495-499.
SNELLINC,
J. C. 1968. Overlap in feedinghabitsof Red-wingedBlackbirdsand Common
Gracklesnestingin a cattail marsh. Auk 85:560-585.
SOKAL,R. R., AND F. J. ROHLF. 1981. Biometry. W. H. Freeman and Company, San
Francisco.
STRANG,C. A. 1980. Incidence of avian predatorsnear people searchingfor waterfowl
nests.J. Wildl. Manage. 44:220-224.
WILLIS, E. 0.
Auk
1973. Survival rates for visited and unvisited nests of Bicolored Antbirds.
90:263-267.
Received14 Jan. 1987; accepted28 Dec. 1987.