Marbled Murrelet Group Size at Sea as an Index to Productivity
Author(s): Diane Evans Mack, Martin G. Raphael, Fred Cooke and Conrad Thiessen
Source: Northwestern Naturalist, Vol. 85, No. 1 (Spring, 2004), pp. 1-10
Published by: Society for Northwestern Vertebrate Biology
Stable URL: http://www.jstor.org/stable/3536471
Accessed: 19-05-2015 18:34 UTC
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NORTHWESTERNNATURALIST 85:1-10
SPRING 2004
MARBLED MURRELETGROUP SIZE AT SEA AS AN INDEX
TO PRODUCTIVITY
DIANE EVANS MACK1 AND MARTIN G RAPHAEL
PQcific
Northwest
ResearchStation,3625 93rdAve.SW, Olympia,Washington
98512 USA
FRED COOKE AND CONRAD THIESSEN
CentreforWildlifeEcology,SimonFraserUniversity,
8888 University
Avenue,Burnaby,
BritishColumbiaV5A 1S6 Canada
ABSTRACT Populationdemographics
ofmarbledmurrelets
(Brachyramphus
marmoratus)
are
difficult
to measure.Populationsize can be estimatedfromat-seadensities,and productivity
indicesare sometimesderivedfromratiosofhatch-year
to after-hatch-year
birdsobservedduringsystematic
surveyson theocean.However,onecannotdetermine
frommarinesurveysalone
whatproportion
ofa marbledmurrelet
populationsampledat sea is nestingin anygivenyear,
whichwould allow a moremeaningful
interpretation
ofproductivity
ratios.Wehypothesized
thatgroupsize (thenumberofmurrelets
occurring
together
on thewater)couldprovidesuch
an indexif it could be demonstrated
thatsinglemurrelets
detectedon theocean duringthe
incubationphase ofthebreedingseasonrepresent
breedingbirds.Wemonitored
radio-tagged
murrelets
in DesolationSound,BritishColumbia,from29 Maythrough19 June2001,usingan
incubating
pair's distinct24-hon-offoccurrenceon thewaterto determine
nestingstatus.Of
160 murrelet
groupscomprisedof at least 1 individualofknownnestingstatus,therewas a
significantly
greaterpercentage(37%) ofsinglebirdsamongincubating
murrelets
thanamong
non-incubating
birds(20%).Annualvariationin theproportion
ofsinglemurrelets
recordedon
marinesurveysin PugetSoundduringpeak incubation
corresponded
withtheannualproductivityindexin 4 of 5 y from1997to 2001.Our resultssuggestthatgroupsize,especiallythe
proportion
ofsingle-bird
groups,mayhelpassess theproportion
ofmurrelets
thatarenesting.
Multiple-year
comparisons
ofgroupsize withnestingratesareneededtovalidateand interpret
theseresults,and we need to continueto explorenewmethodstomeasuremurrelet
productivity.
Keywords: Brachyramphus
marmoratus,
marbledmurrelet,
groupsize,marinesurveys,telemetry,
nestingstatus,productivity
ratios,DesolationSound,PugetSound,BritishColumbia
Manyseabirds,and mostalcids,are colonial and fecundityis indexed by productivity,
ground-nesters
whose breedingsites can be whichis measuredindirectlyfromcountsat
monitored
forsuchdemographic
parameters
as sea. Currently
themost-frequently
used index
populationage structure,
numberofbreeding ofproductivity
is obtainedfromtheratioofthe
attempts,and fecundity(Gaston and Jones numberofjuvenileto adultmurrelets
observed
1998). The marbled murrelet(Brachyramphus
on theocean (Beissinger1995;Ralphand Long
marmoratus)
poses a greaterchallengebecause 1995;Kuletzand Kendall1998).This measure
activenestsin theforestcanopyare dispersed is confoundedby severalfactors.Thereis an
and adultactivityin theforestis moreoftenat unknownproportion
ofnonbreeders
in murredawn or dusk,makingnestsdifficult
to locate let populationsobservedat sea, which,based
and monitor.
As a result,age structure
is pre- on otherPacificAlcidae,couldrangefrom15 to
sumed fromrelatedspecies (Beissinger1995) >50% annually(Sealy 1975;Ewins 1993;Gaston 1994). Nonbreeders,
likelyconsistingprimarilyof1-to 3-y-oldpre-breeders,
arenotrefromadults as observed
1 Presentaddress: IdahoDepartment
ofFishandGame,555 liablydistinguished
Deinhard Lane, McCall, ID 83638 USA; devansmackG froma slowlymovingsurveyvessel (but see
idfg.state.id.us.
Sealy 1975). Similarly,juveniles (hatch-year)
1
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2 NORTHWESTERNNATURALIST
85(1)
becomedifficult
to distinguish
frommoltedaf- Alternatively,
theuniqueincubation
patternof
ter-hatch-year
birdsby earlyfall (Carterand nestingmarbledmurrelets,
combinedwithour
Stein1995),potentially
resultingin underesti- perceptionthattheproportionof singlebirds
mationof juvenilenumbers.Detectionproba- observedfrommarinesurveyschangedseabilities may differbetween juveniles and sonally,suggeststhatsinglebirdsmaybe a betadults,due to behavioror differential
habitat terindicatorofbreedingactivity.
use (Kuletzand Piatt1999),and movements
Whenincubating,
of
a nestingmarbledmurreafter-hatch-year
and juvenilemurrelets
within let spends about 24 h on the nest,and then
thebreedingseason bias bothcomponentsof spends the next approximately
24 h on the
theratio.Forexample,thereis a steadyincrease oceanwhileitsmateincubatestheegg (Nelson
in numbersofmarbledmurrelets
duringtheinon thewaters 1997;Bradley2002). Therefore,
aroundtheSan JuanIslands,Washington,
be- cubationphaseofthenestingcyclea givenbird
ginningin late Julythatexceedswhatcan be will be on theocean everyotherday. During
attributed
to the appearanceof fledgedjuve- chickrearing,bothparentsmakefeedingvisits
niles and reappearanceof nestingadults that to thenest,buttheyresumea dailyoccurrence
havecompletedincubation(Raphaeland Evans on thewater.The daily on-offincubationpat1997).Mostofthesebirdsareincorporated
into ternsuggeststhatsinglebirdsshouldbe more
theadultand notthejuvenilecomponent
ofthe commonlyobservedon the ocean duringthe
productivity
ratio.Ifjuvenileimmigration
was incubationperiod comparedwith the chickperiods.Further,
beproportional
and followedthesame temporal rearingor post-breeding
capturedearly
pattern,the ratio mightnot be biased but cause manypairs of murrelets
pairs,it
wouldn'tnecessarilyreflect
local production. in thebreedingseasonaremale-female
follows
that
these
pairs
would
be
separated
Kuletzand Kendall(1998) proposedadjustand thesightings
ofsingles
ing productivity
ratiosby using early-season duringincubation,
countsofadultsas thedenominator
(sequential on theocean wouldincrease.
Wesuggestthatgroupsize couldprovidean
vs. concurrent
ratios),minimizingthebias of
alternative
indexto productivity
ifit couldbe
adult post-breeding
movements.
We explored
demonstrated
that
single
murrelets
detectedon
this and otheradjustmentsto marinesurvey
data in Washington
and foundthatour study the ocean duringthe incubationphase of the
populationcould be describedas declining, breeding season representnestingbirds. If
stable,orincreasing
based solelyon themethod true,thiscould lead to a simpleway of estiofcalculatingratios(Raphaeland Evans1997). matingtheproportionofbreedingbirdsfrom
Ideally,the productivity
index should be de- populationssampledwithmarinesurveys.Our
rivedfromknownlocalbreeders,buttherehas objectivesin this study were to (1) observe
beenno methodto determine
frommarinesur- group size in a populationof markedbirds
veysalone whatproportionof thepopulation whose nestingstatuswas known,and (2) retexaminemarinesurveydatato (a)
sampled at sea is nestingin any given year. rospectively
Here we considergroup size (the numberof determineif the frequencyof single birds
groupsdetectedon thewater
murrelets
occurringtogether
on theocean,see amongmurrelet
changed
seasonally,
(b) examinewhetherthe
below formorecompletedefinition)
as a posproportion
of
single
birdsduringtheincubasiblelocal indexto breedingactivity.
tion
period
changed
yearto year,and (c) test
Marbledmurrelets
occuras pairsand singles
of
on theocean morethanany othergroupsize whethertheannualchangein theproportion
singles
during
incubation
corresponded
to
an(Carterand Sealy 1990; Strachanand others
as estimated
1995; Collins 2000; Carten 2001). Studies in nual variationin productivity,
fromjuvenileto adult ratios.Forthistest,we
BritishColumbiahave effectively
demonstratwereprimarily
interested
inconcordance
ofdied thatgroupsof2 marbledmurrelets
captured
rectionsofchangefromyeartoyearin theproearlyin the breedingseason are likelyto be
portionof singlesand in thejuvenileto adult
male-female
pairs(Sealy1975;Vanderkist
1999; ratio.
McFarlaneTranquillaand others,Inpress),and
STUDY AREAS AND METHODS
McFarlaneTranquilla and others(In press)
proposed that counts of pairs early in the
Froman earlierassessmentof marinesurbreedingseasoncouldreflect
breedingactivity. veysin theSan JuanIslands(Fig.1),we had ob-
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SPRING 20QA
EVANS MACK AND OTHERS: MURRELET GROUP SIZE AND PRODUCTIVITY
1-Singles
D - tt17
n
= g50
n
- 17@)
z
Paixs 1B3
n = 1296
or More
|
n= 822
n
= 1614
|
Desolation
g
ffi
Jun ui
l99&
O 25 50 100
Kitometers
NORTH
CC /
m<
1
Great
Puget Sour
fi
Aug
I
un
Jut Aug
1997
fi
Jun Jul
1998
I
Aw
lun Jul Au9
1999
Zun 3Wl Aug
2000
Jun Jwl Aug
2001
FIGU:FtE2. 5easona1 variationin proportionsof
morreletgroup sizes (lt 2, 3 or moreindividuals)
frommarinesurveysin San JuanIsIands)WashiIIgton,was consistent
amongyears.Samplesizes (n) for
the 3 monthstotalare shown above bars foreach
year.
Beginnirlg25 Aprilf telemetrymorlitoring
flightswere conducted daily by helicopterf
pathpassingoverthemainwaFIGURE1. PagetSourEL(includingtheSan }uan Is- withtheflight
lands, JSJ
IS)JWashingtcznr
and Desolation Soundr tersof DesolationSound andton routesalterBritishColumbiafstudyareasA
natirlgby dayfoxrer
themaindrainagesw:ithin
thestudyarea tseeHull and others2001 fora
detailedmap of thisstudyarea). Partof each
se:rveda consistentanrlualpatternof decreas- daytsflightincludeda stop nn a centrallyloing proportionof single marbledmurrelets cated ridgefand all extantfrequencieswere
overtirnefromJuneto Augustcomparedwith scanrwed
fromthislocation.We use<:lthe dispairs or largergroups(Fig. 2). Juner
July;and tinct24-hcon-off
patternto identify
nestingstaAugustlooselycorrespondedto thelatterpart tus (Bradley2Q02).Forour studyJ
murrelets
deofincubaLtiont
chick-rearingt
and thelatterpart tectedconsistently
on alternating
days on the
of thebreedingseasontrespectivelyf
in Wash waterwere assumed to be in the incubating
ington.This observationwas based on data phase of theirbreedingcyclerand locationsof
froma seriesof transectsin the San JuanIs- birdsatinlandnestsitesconfirmed
nestingstalandsrWA (Collins 200Q;Carten2001). Tran- tus.To identify
thebeginningofthechick-:rearsectsweresurveyedduringsequential10-din- ing periodX
we used a 30-d lncubationperied
tervalsthroughoutthe summer.Lengthsof (Nelson1997)in combination
withdailydetectransectstotaled170 km. Observersrecorded tionsoftheradio-taggedadulton thewater.A
thenumberof murrelets
iIl each groupdetect- cessationof the 24-hcen-off
patternin c30 d
ed Groupsweredefinedas birdsincloseprox- identified failed nesters (unlesst for birds
imity(generally1 to2 mofeachother)engaged taggedaftertheyinitiatedincubationr
continin similaLr
behaviLor
and respondingto distur- ued inlandvisitationsuggestedchick-rearing
bancein thesamemanner(f<3r
exampIef
chang- lBradley2OO2])J
and birdsthatneverinitiateda
ingdirectionwhenswimming)+
consistent
on-off
pattern
wereclassifiedas nonTo determineif theseobservedpatternsin nesters.
groupsizes werereflective
ofnestiIlgbehavior) To assess the relationshipbetweengroup
we monitored
a markedpopulationofmarbled size on theocean and nestingstatusrwe visumurrelets
in DesolationSoundtBritishColum- allylocatedtheradio-tagged
birdsfroma small
biarin 2001(Fig.1). Murreletswerecapturedat boat during 29 May through19 June2001. Obnightw1thdip nets (Whitworthand others servationswere concentratedin thecentralpor1997)from20 Aprilthrough
26 May2Q01.Cap- ticonof Desolati<}n Sound, from the west and
turemethodsare detailed in Vanderkist
and southends ofHomfrayand Waddington
chane3thers
(1999)and Lougheedand others(l998)" nelsf respectively,
to the area between the
Radio transmitters
were attachedto 75 birds. northwest
tip oftheMalaspinaPeninsulaand
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' PpplUSl,Wll0.
l,;...l,,,,.,l,, l l,,l lkllUblUlvy
,
4 NORTHWESTERNNATURALIST
85(1)
*
19 May to 3 July
25
southeastCortesIsland.In addition,TobaInlet
and Hernando Island were each surveyed1
timeand theCopeland Islands weresurveyed
we motoredto a
3 times.Duringobservations,
centrallocation,scanned throughfrequencies
which
withan ATS R4000scannerto identify
190
200
180
160
170
140
150
130
120
birdswerepresenton thewaterand theirgen18 Jul
29 Apr 9 May 19 May 29 May 8 Jun 18 Jun 28 Jun 8 Jul
eral location,thenslowlyapproachedeach raDay of Year and Date
dioed birdindividually.The primaryparamehistogramof the average
3.
Frequency
FIGURE
was groupsize at thepointwhen
terofinterest
on any singledate
incubating
of
murrelets
number
the radioed bird was located visually(initial
and day of the year in DesolationSound, 1998 to
group size). We also recordedbehavior(for- 2001,calculatedfromneststartdatesfromtelemetry
aging, swimming,taking flight,avoidance data and assuminga 30-d incubationperiod.Dates
dive), map location,GPS location,and ending withinbracketsdefinethemiddle75% ofnests.
groupsize. Forexample,ifthegroupresponded totheapproachoftheboatand reconfigured
itself(forexample,ifsome or all birdsdove or datesfrom1998through2001 to definea peak
flewor thegroupseparated),we recordedthe incubationperiodthatcouldapplygenerallyto
afterrecon- all years(Fig. 3). We dividedtheremainderof
groupsize forthetargetmurrelet
('endinggroup size'). If conditions thecalendaryearinto3 additionalseasons.
figuration
We used 1997 to 2001 marinesurveydata
allowedand thegroupcouldbe observedfrom
a distance,we observedfor5 to 10 minto de- fromPugetSound(ofwhichthepreviouslyrefremainedin the erencedSan JuanIsland data was a subset)to
termineifthetargetmurrelet
of
group.Repeat observationsof the same indi- examineseasonal changesin theproportion
detectedon thewater,and we
vidual on 1 d wereseparatedby severalhours singlemurrelets
in theproportion
and didn'texceed2 ob- comparedannualdifferences
to avoidautocorrelation
servationsper day per individual.Nestingsta- ofsinglesduringpeak incubationto annualesBecauselargergroups
tus was assignedforeach individualforeach timatesofproductivity.
have higherdetectionprobacould potentially
day it was visuallyobservedon thewater.
study,we re- bilitiesat equivalentdistances,resultingin an
Followingthebreeding-status
ofsinglebirds,we examinedthe
turnedto marinesurveydata fora morede- underestimate
analysis,includinga better relationshipbetweengroup size and detecttailedretrospective
survey
monitoring
of thebreedingseason.To makein- abilityusing2001long-term
definition
ferencesfromgroupsizes recordedon marine data in Puget Sound. Results fromprogram
surveysduringincubationcomparedwiththe DISTANCE(Bucklandand others1993)showed
(MGR and
relationship
restofthenestingseason,we 1stdefineda peak a weak,nonsignificant
incubationperiod.The knownnestingchronol- DEM, unpubl.data).
indexwas calculatedfrom
The productivity
froma limitedsample of
ogy in Washington,
nestsand groundedyoung(Hamerand Nelson juveniletoadultratiosadjustedfortimeofyear
1995),is similarto theincubationperiodiden- ofsurveyfollowingthemethodsofBeissinger
tifiedforDesolationSound (Lougheed2000). (1995), but includingonly Washingtonand
forthe BritishColumbiafledgedatesfromHamerand
Assumingsimilarnestingchronologies
2 areas, we used estimatednest startdates Nelson(1995),augmentedbyfledgedatesfrom
dataand at-seasurfromDesolation Sound telemetrydata from BritishColumbiatelemetry
1998through2001 to delineatea peak incuba- veysinPugetSound,perthemethodsinHamer
countsofjuvetionperiodforeach ofthese4 y. We projected and Nelson(1995).Specifically,
a 30-d incubationfromeach estimatednest nileswereadjustedupwardby theproportion
start date, and froma frequencyhistogram ofyoungthatwouldhavefledgedbytheend of
therangeof days thatencompassed thesurveyperiod,based on a cumulativefreidentified
theincubationperiodofthemiddle75% ofthe quencyof fledgedates fromWashingtonand
data will not always British Columbia. Because the data were
nests.Because telemetry
be available to definethe incubationperiod asymptotic,we divided the cumulativefreeach year,we averaged the peak incubation quencycurveinto3 timeperiodsand calculat|
. -
o
o
z
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-
SPRING 2004
EVANS MACK AND OTHERS: MURRELET GROUP SIZE
AND PRODUCTIVITY
ed a regressionforeachperiodseparately.
The
adjustmentfactors(cumulativeproportionof
nestsfledged)were(0.00204x day ofyear)0.3212fordatesbeforeday 180;(0.02133x day
of year) - 3.8448 for days 180 to 220; and
(0.0044x dayofyear)-0.0798 fordays > 220.
Standarderrorsforthe adjusted ratioswere
calculatedfroma standardformulaforthevariance of a ratio(Manleyand others1993).A1thoughwe used the adjusted ratioin an attemptto compensatefor weaknessesin the
simple calculationof total juvenile to total
adults observed,we were moreinterestedin
changesin thetrendfromyearto yearthanin
theabsolutevalue oftheindex.
Weused Chi-squareteststocompare(1) proportionsofinitialgroupsize bynestingstatus,
and (2) proportions
ofsinglemurrelets
among
seasons.Fornestingstatus,chick-rearing
birds
and failed nesterswere included with nonnesters
torepresent
non-incubating
birds.Nonparametricpost hoc multiple comparisons
wereconductedon the 6 possiblepairingsof
seasons(Marascuiloand McSweeney1977).We
usedSPSS v 10.0.5(SPSS Inc. 1999) forstatisticalanalyses,witha significance
level ((x) of
0.05.
5
100
[
80 o
|
El Single birds
* Multiple birds
|
|
60 -
n
y
40 20
o
Incubating
Birds
Non-Incubating
Birds
FIGURE 4. Proportionsof single and multiple
murreletsamong incubatingbirds (n = 84) compared withnon-incubating
birds(n = 76) in DesolationSound,BritishColumbia,2001.
prised37% of thisgroupcomparedwith13%
non-breeders
(X2= 10.50,P < 0.01).
Incubatingmurrelets
also wereencountered
in pairs (25% of observations)and in larger
groupsof 3 to 17 birds (38%), demonstrating
thatmembersof an incubatingpair will join
otherbirdswhentheirmateis incubating.Exclusiveofchick-rearing
and failednesters,49%
ofnon-nestingmurreletsoccurredin pairs,
12%as singles,and theremaining
in groupsof
3 to 11. Among incubatingmurrelets,16
groups(of 84) reconfigured
duringtheobservation,
resulting
ina different
groupsize bythe
endoftheobservation.
None
of
thesereconfigRESULTS
uredgroups were singles that joined other
Duringthenest-status
study,we visuallylo- groups,
and 4 ofthe6 incubating
birdsinitially
cated43 ofthe75radioedmarbledmurrelets
on
observed
in a groupof2 ended up as a single.
thewaterat least 1 time,recording160 obser- In
comparison,
a similarproportion
(8 of55) of
vations
ofinitialgroupsize. Theseincluded84
groupscontainingnon-nestersreconfigured
observations
ofnestingbirdsin theincubating during
theobservation,
withonly3 endingup
phaseof theirbreedingcycle (hereafterre- assinglesand
none as pairs. None of the 21
ferred
to as 'incubatingbirds', even though groupscontaining
chick-rearing
and failed
their
mate was presumablyincubatingwhen nesters
reconfigured
during
approach.
they
wereobservedon thewater),17 observa- Thepeak
incubation
periodsfromDesolation
tions
of birdsin thechick-rearing
phase,4 of
Sound
telemetry
datafor1998through
2001,refailed
nesters,and 55 ofnon-nesters.
Pairs(54
spectively,
were 20 May to 30 June,inclusive
observations)
and singles (46 observations) (days
140to 181),22 May to 3 July(days142to
comprised
63% ofthetotalsample.Comparing 184),
16 May to 7 July(days 137to 189),and 13
single
birds(groupsize = 1) to all othergroup
May
to 30 June(days 133 to 181). The average
sizes
combined,therewas a greaterproportion of
these periods was 19 May though3 July
of
singlesamongincubatingbirds(37%) than
(days
139 to 184; Fig. 3). Fromthesedateswe
among
non-incubating
birds(20%; n = 160,x2
defined
an earlybreedingseason of April20
=5.74,P = 0.02;Fig.4). Neithergroupwas bithrough
18 May,a latebreedingseason from4
ased
bya smallsampleofindividualmurrelets; July
through15 August,and a fall-winter
sea29
murrelets
comprisedthe incubatinggroup
son
from16 Augustthrough28 February.
compared
with 24 non-incubators.
The relaFrom our retrospective
analysis of marine
tionship
was strengthened
whenwe combined
survey
data in PugetSound,singlesand pairs
chick-rearing
and failednestersalongwithinof
murrelets
represented
88% ofall groupsencubating
birdsas breedingbirds;singlescomcountered.
The proportionof singlebirdsdif-
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n
100-
_
6 NORTHWESTERNNATURALIST
85(1)
TABLE 1. Post hoc pairwise comparisons of the proportionof single murreletsdetected on the water among
seasons, Puget Sound, Washington,1997 to 2001. Significantdifferences(P ' 0.05) are marked with (*).
Estimate
Contrast
Peak incubation-earlynesting
Peak incubation-late nesting
Peak incubation-fall-winter
Early nesting-late nesting
Early nesting-fall-winter
Late nesting-fall-winter
0.429-0.363
0.429-0.288
0.429-0.195
0.363-0.288
0.363-0.195
0.288-0.195
=
=
=
=
=
=
0.066
0.141
0.234
0.075
0.168
0.093
Estimated
variance
95% confidence
interval
0.0008
0.0001
0.0002
0.0008
0.0008
0.0001
-0.0149 to 0.1469
0.1080 to 0.1740*
0.1990 to 0.2690*
-0.0028 to 0.1528
0.0893 to 0.2467*
0.0657 to 0.1203*
DISCUSSION
feredamongseasons (X2= 347.17,P < 0.001),
with the highestproportionduringpeak inOur resultssupportour hypothesisthatthe
cubationand lowestproportion
duringfalland seasonal change in proportionof marbled
winter(Table1, Fig. 5). The changein propor- murrelet
sizes couldbe a digroupsofdifferent
tionof pairs was thereverseof thechangein rectreflection
ofnestingactivity.The proporsingles theproportion
ofpairswas highestin tionof singlebirdsencounteredon thewater
fall-winter
whentheproportion
ofsingleswas was highest during incubation,and single
lowest,was lowest during peak incubation birdsobservedduringthistimeof yearwere
when the proportionof singleswas highest, morelikelyto be incubatingbirds thannonand was intermediateduringearly and late nesters.Thisfitswellwiththemarbledmurreoflarg- let's distinctive24-h incubationpattern,as
nesting(Fig.5). The shiftin proportion
er groupsamongseasonswas variable.
breedingpairs would be separatedduringinThe proportionof singlemarbledmurrelets cubationand thebirdnotactivelyattending
the
duringpeak incubationvaried among years, egg wouldoccuron thewaterwithoutitsmate.
and thisvariationgenerallytrackedchangesin However,incubatingmurreletsin Desolation
the annual productivity
index (Fig. 6). Both Soundalso wereobservedinpairsand inlarger
werehighin 1997,decreasingto a low in 1999, groupsduringtheirstintawayfromthenest.It
and thenincreasingagainin2000.Onlyin2001 is this dynamicthatrequiresfurther
investidid the2 measuresdiverge,whentheproduc- gationand interpretation.
We also recognize
murrelets
oftelemetered
tivityindexincreasedsubstantially
buttheper- thatour observations
were from1 locationduring1 y,but suggest
centofsinglesdecreased.
thatfactorsinfluencing
groupingbehaviorare
notsite-specific.
j * Singles
CL
n = 288
n = 2390
:1 Pairs O 3 or More |
n = 5333
0.14
n = 2763
5Z
so
W
80
1
O
@
60
I
D
a
<-
&
E
20-
*
E
-48
x
E
Peak
Incubation
Late
Nesting
o..o
t
*5
0.08
-46
44
-
o
Early
Nesting
n
-42
Fall - winter
ofmarFIGURE5. Seasonalchangein proportions
bled murrelet
groupsizes (1, 2, 3 or moreindividuals) duringmarinesurveysin PugetSound,Washington,1997to2001.Earlybreedingseason:20 April
to 18 May; peak incubation:19 May to 3 July;late
16
breedingseason:4 Julyto 15 August;fall-winter:
Augustto 28 February(no surveysconductedfrom
1 Marchto 19April).Samplesizes (n) foreachseason
are shownabovebars.
-40
O
38
0.04
Q
- 36
0.02
34
l
0.00
1997
1998
1999
2000
2001
Year
of single
FIGURE6. Annualchangein proportion
marbled murreletsduring peak incubationcomindex,PugetSound,
paredwithannualproductivity
1 sx.
Washington,
1997to 2001.Errorbarsrepresent
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SPRING 2004
EVANS MACK AND OTHERS: MURRELET GROUP SIZE AND PRODUCTIVITY
7
The occurrence
ofincubating
murrelets
with
We used initialgroup size of the radioed
in DesolationSound
murreletsas our parameterfor analysisbe- 1 or moreothermurrelets
ofrelatively
highmurrelet
cause it reflectedconditionson a marinesur- could be a function
vey,wherebyan observerencounters
a murre- densityin thisregion.Althoughwe lackeddiaveragedensitiesin Desolaletgroupat a random,briefpointin timeas the rectcomparisons,
vessel continues along a transect.Ending tionSoundproperduring1996to 1998(excludgroupsize,althoughless characteristic
ofa ma- inginletsand weightedbythenumberofpolyrinesurvey,offered
additionalinsightintothe gons;Lougheed2000)wereup to 4 timeshighbehaviorof incubatingand non-nesting
birds. er than in Puget Sound in 2000 and 2001
None of the 31 incubatingbirds 1st encoun- (Bentivoglioand others2002;Jodice2002),deteredas a singleon thewaterjoined another pendingon yearand stratacompared(forexgroupduringtheshortobservation
period.Ad- ample,6.5 birds/km2in DesolationSound in
ditionally,
an incubatingbirdpaired withan- 1996 comparedwith1.6 birds/km2forPuget
othermurrelet,
presumably
notitsmate,ended Soundin 2000).An incubating
birdleavingthe
up moreoftenthannot as a singlewhen the nestforthewaterwouldbe morelikelyto enpair was approachedor disturbed,causingthe counterothermurrelets
whenbirddensityon
birdsto move.The factthatan incubating
bird thewateris highthanin regionswheremurreoftenseparatedfromthe othermurreletcon- letsare less denseand thusmoredispersed.In
trastswiththerelatively
strongbonddisplayed addition,groupsizes encounteredalong ranbymanyotherpairsthatmaybe 'mated',as ev- domtransects
inDesolationSoundin2001duridencedby repeated(and increasingly
strenu- ing observationsof telemeteredbirds were
ous) vocalizationsupon surfacingto locate largercomparedwiththesame timeperiodin
each otherafterbecomingseparatedduringa PugetSound (x = 2.43,s = 1.98vs. x = 1.93,s
dive,thenswimmingto rejointhe otherbird = 1.22,respectively;
t = -4.94, df = 904,P <
(DEM pers. obs.; Laura McFarlaneTranquilla, 0.01). Largergroupscould be correlatedwith
Simon FraserUniversity,
Burnaby,BC, pers. higherdensities,such as was foundduring
comm.).Our observationssuggestthatincu- morningsurveys(butnotat othertimesofday)
batingbirdswill behavein a solitarymanner in BarkleySound,BC (Carterand Sealy 1990).
morethanbirdsnot incubating,but thisdis- More studyof murreletgroupingbehavioris
tinctionwould not be consistently
apparent neededto determine
ifincubating
birdshavea
froma boat-basedpopulationsurvey,as some similaraffiliation
with otherbirds when the
incubatingbirds would appear with other populationis moredispersed.For example,a
birds.
repeattelemetry
study(thatincludesvisualobOur resultswould be biased if group size servationsof knownnesters)in areas oflower
was predominantly
behavioral(forexample,if murrelet
densitieswouldprovidea good comsome murreletswere 'loners' with a greater parison.
propensityto occur as singles).Sample sizes
The proportionof single birds increased
forindividualsobservedmorethanoncewere when failed nestersand chick-rearing
birds
too small to perform
statistical
tests.We iden- were combinedwithknownincubatingbirds.
tified1 murrelet
(ofthe9 incubatingindividu- On the surfacethis seems contradictory,
as
als thatwere observed4 or moretimes)that failed nestersand chick-rearing
birds could
mighthavebeen exhibiting
a propensity
to be join withtheirmateon thewateroncenestatalone (4 of 6 observations).
However,none of tendancewas no longernecessary.However,
the6 non-incubating
birdsobserved4 or more giventhata chick-rearing
pair staggersitsfish
timesshowedthesamepropensity,
suggesting deliveriestothenest,theadultswouldoccuron
thatoccurrenceas singlesvs. in a groupwas the waterat different
timesforat least some
morereflective
ofbreedingstatus.We did not portionof the nestlingperiod. Known pairs
assess territoriality
(singles repeatedlyob- monitored
withtelemetry
in DesolationSound
servedin thesameplaces) in eitherourmarine did occurtogetheroccasionallyduringchicksurveysor our observationsof telemetered rearing,but not as oftenas duringpre-incumurrelets.Additionalstudiesof murrelet
be- bation (McFarlaneTranquillaand others,In
havioron thewaterwouldhelpclarifytheseis- press).
sues.
Our sampleoffailednesterswas verysmall
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All use subject to JSTOR Terms and Conditions
8 NORTHWESTERNNATURALIST
85(1)
(4 groups,2 ofwhichoccurredas singles),and a generalpeak incubationperiod.Wemaintain
interpretation
of theiroccurrenceas singlesis thatthisaveragewas a reasonablereflection
of
muchmorespeculative.Death of 1 memberof thegeneralincubationactivityin Washington,
thepair could certainlycause nestfailureand as these4 y includeda rangeofmarinecondiseparationofthebirdson thewater,butwheth- tions(warmerand coolersea surfacetemperaer thiswas thecase amongour markedpopu- tures)thatmightinfluence
breeding.
lationis unknown.
Marbledmurrelet
productivity
indicesshow
Our analysisoftheproportions
ofgroupsiz- annual variation (Beissinger 1995; Strong
es encounteredthroughout
the yearin Puget 1995).Productivity
of seabirdscan varyin reSoundwas based on theassumptionofsimilar sponse to oceanic conditions,througheffects
breedingchronology
in Washington
and Brit- on preyand its availability.
Forexample,seaish Columbia,allowingus to definethebreed- bird responsesto El Nino eventshave been
ing season forPuget Sound fromDesolation documented,althoughthe influenceof these
Soundtelemetry
data.Estimatesfrommarbled eventsin theStraitofGeorgiaand PugetSound
murreletnests and groundedchickssumma- are less clearthanin othermarinesystems.In
rizedin Hamerand Nelson(1995)weresimilar yearsoflowproductivity,
we mightexpectfewto thosederivedbyLougheed(2000)fromsev- er nestingattempts,
and, ifgroupsize is a reeral different
methods.The sampleof known flectionof nesting,fewersinglemurrelets
on
fledgingdates in Washingtonwas relatively thewaterduringincubation.
Wefoundthispatsmall,whichlimitedourcomparison.
However, ternin 4 ofthe5 y we compared.Weexpected
index
ongoingtelemetry
studiesin DesolationSound that the variationin the productivity
and ClayoquotSound,BC, have demonstrated mightbe greaterthanin theproportion
ofsinup to a 30-d difference
in thechronologies
of gles detectedat sea because lowernestingefthese2 populationssharingthesame latitude fortshouldresultin lowerjuvenileto adultra(RussellBradley,
PointReyesBirdObservatory,tiosand vice versa,whereasgroupsize didn't
StinsonBeach,CA, pers. comm.),suggesting alwaysindicatebreedingstatusand could revariationwithingeographicareas and caution flecta moreconsistentinfluencesuch as forin our assumptionof similarchronologies
be- aging strategy.However,the magnitudeof
tweenBritishColumbiaand Washington.
changewas similarforeachvariable.Wefound
explaOur use oftelemetry
data,exclusively,
to de- no obviousbiologicalor environmental
finea periodofpeak incubationalloweda 1:1 nationforthedivergenceofthe2 measuresin
temporalcorrespondenceof marine survey 2001.
In summary,group size on the water,and
data withtelemetry
data for4 of the5 y analyzed (1998to 2001),whilealso increasingthe specificallythe proportion of single-bird
samplesize ofneststartdates3-foldfromthat groups,may providean index of nestingatpreviouslypublished(36 neststartdatesfrom tempts,but therelationship
is notpreciseand
The breedingchronology
Hamerand Nelson [1995]forWashington
and needs refinement.
BritishColumbiacomparedwith125 fromte- withinthe studyarea needs to be well doculemetrydata). We defined'peak incubation'as mented,whichcould requireextensivetelemtheperiodwhenthemiddle75% ofnestswere etryor physiologicalstudies.If data are availbeing incubated,eliminatingthe earliestand able,one could redefinetheincubationperiod
latestnests.Lougheed(2000)defineda 'corein- each yearfromneststartdates to correspond
withproductivity
inthatyear.As mencubation'periodfromthemiddle50% ofdates. directly
Bothapproachesattemptto deal withthepro- tionedabove, the extentto whichgroup dyofdenlonged, asynchronousbreeding season. Be- namicsand groupsizes are a function
cause we used thesedates to interpret
group sityshouldbe explored.Lastly,we needtoconsize on thewaterduringmarinesurveysand tinue to explore new methods to measure
productivity.
not to distinguishincubationfromchickrear- murrelet
ing per se, our less conservative
75% provided
ACKNOWLEDGMENTS
a broaderwindowto capturemorebirdsthat
mightbe incubating
whileexcludingthemore
The behavioral observations on which this study
extremeends of thespectrum.Finally,we av- was based could not have been possible without a
eragedthedatesfrom1998to 2001to arriveat large sample of marked birds whose nesting status
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SPRING 2004
EVANS MACK AND OTHERS: MURRELET GROUP SIZE AND PRODUCTIVITY
9
was known. For that,we acknowledge the many finical editors. Ecology and conservation of the
nancial supporters of Simon Fraser University'sCenmarbled murrelet.Albany,CA: USDA ForestSertre for Wildlife Ecology's long-term demographic
vice. General Technical Report PSW-GTR-152. p
studies of the marbled murrelet. We appreciate R
99-109.
Bradley's contributionof nest startdates for 1998 to COLLINS VJ.2000.
2000 summary reportof fieldob2000 from Desolation Sound telemetrystudies. C
servations of marbled murreletsin the San Juan
Williams, B Tarlin,and K Vickersof the Simon Fraser
Islands, Washington.Olympia, WA: USDA Forest
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vations of radioed murreletson the water. N Parker
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facilitated our use of the Desolation Sound field
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