NEST-SITE SELECTION BY SAGE THRASHERS IN SOUTHEASTERN IDAHO

6 Pages·1991·3.3 MB·English

by  Kenneth L Petersen

GreatBasinNaturalist51(3). 1991,pp.261-266
NEST-SITE SELECTION BYSAGETHRASHERS IN SOUTHEASTERNIDAHO
KrmiclliL. i^tc.scn'andl.oiiis15. Hcst"
—
AbstRACT. Nfstsitesseleetetll)\ SatieTiiiasiiers{Oiroscoptcsiiioiilaiiiis)insoutheasternIdahowereeliaraeter-
izedandeomparedwithavailablehabitat. Mierolialjitatswitliin5niofnestshadtallerandmorea^nreiiatedsluni)sand
lessbaref^roundthanthestud\ areainj^eneral. Bigsa.nebrush{Artemisiatridcntatawyomin^oisis)plantsusedlor
nestingwere tallerthan averageavailable shrubs, had greaterfoliage density, were more often living, and more
frequentlyhadbranehesandfoliagewithin30cmoftheground. Nestplacementwasspecificwithrespecttorelative
nestheigiitanddistancefromthetopandperimeterofthesupportshrub. SageThrashersdisproportionatelyused
easterlyexposuresandunderusedwesterlyexposuresfortheirnests.
Keywords: SV/^cThrasher.bi>isaiichrusfi,shnihsteppc IdaJio,iicst-sitcselection.
Choiceofnestsitesrepresentshabitatselec- Sti'dvAreaandMethods
tionon asmall spatial scale(e.g., MacKenzie
etal. 1982, StanflPerand Best 1986, Bekoffet The study area, consistingof25haofsage-
asli.tes19m8a7)y.gAovvaeirlnabtilhietcyoampnodsistniitoanbiolfitbyirodfcnoems-t pblrauisnh1s1hkrumbssotuetphpeofoHnotwhee,uIpdpaehro,SnisaakdemiRniivse-r
mnnities and may strncture species-habitat tered by the U.S. Department ofEnergy as
relationshipsas muchasdo theavailabilityof partoftheIdahoNationalEngineeringLabo-
food and otherresources (Martin 1988). Pre- ratory (INEL). Vegetation is dominated by
sumbably, patterns of nest-site selection big sagebrush (Artemisia tridentata wijomin-
haveevolvedasaresultofselectivepressures ^ensis), green rabbitbrush (ChrysotJiammts
that have ma.\imized nesting success. In viscidifloriis), and scattered bunchgrasses.
particular, predation (e.g.. Murphy 1983, Forbs are sparse and ephemeral, litter accu-
cBellilmeast-eIs(lee.sg.,anPdlePsizcezmyannska198169)78,anFdermgiucsroon- bmaurlea.tiIonns19a8r0e,sfcoaunrt,6.a2n5d-hmaupclhotosf(t2h5e0gmroxun2d5i0s
and Siegfried 1989) have been implicated as m) were established and gridded throughout
major agents in molding nest-site selection at25-mintervalswithsteelstakesaffi.xedwith
strategies. coloredplasticflagging.
TheSageThrasher{Oreoscoptesmontanus) Data were collected during the breeding
is a common breeding bird in sagebrush- seasonsof1980-1984. Nests were locatedby
shrubsteppe communities of the western using a rope-drag technique (Petersen and
United States (Wiens and Rotenberry 1981). Best 1985) to flush adults from their nests.
Previous studies of nest-site selection by Nests also were discovered by observing
Sage Thrashers (Reynolds and Rich 1978, adults feeding young, and many nests were
Rich1978, 1980, Reynolds1981)werelimited foundincidentaltootheractivities. Eachyear
in that only a few parameters (e.g., nest several nests were discovered after being
height, substrate height) were investigated. abandoned, and these were included in the
Ourobjectivewastoprovideamorethorough sample. Nests that had deteriorated or were
analysis ofSage Thrasher nest-site selection, suspected to have been built before the cur-
includingcharacterization ofnest-site micro- rentyearwereexcluded.
habitat, nest substrates, and nest placement Habitat characteristics of each plot were
within substrates. Further, we measured as- (Quantified in June each year by using 20 X
pects ofavailable habitatwith which to com- 50-cm (juadrats (Daubenmire 1959) and line
parenestsites. intercept (Canfield 1941). Each year one to
'DepartmentofBiology,MonmouthCollege.Mnrinioiitli,Illi!
"Departmentof.AnimalEcolo,g\',IowaStateUni\ersit\.,\nies
261
262 GreatBasinNaturalist [Volume51
four quadrats were placed 2.5 or 5 ni from our measurements, we took the measure-
each grid marker (in different locations ments of rabbitbrush, grasses, forbs, litter,
each year); percent coverage ofrabhitbrush, and bare ground soon after a nest had been
grasses, forbs, litter, and bare ground was located. However, thesemeasurementswere
estimated. Additionally, the height of all not recorded fornests abandoned before be-
sagebrush plants included totally orpartially inglocated. Datawereaveragedforeachnest
withinquadratswasrecorded, andthecondi- so that nests were the observational units in
tion of each sagebrush plant was noted as statistical analyses. Wealso recorded thefol-
dead, 25%, 50%, 75%, or 100% living. We lowingmeasurements: (1)heightofeach nest
also qualitatively estimated densityoffoliage (groundtonestrim),(2)distancefromthenest
on the livingportions ofshrubsas low, inter- rimtothetopofthesupportshrub, (3)short-
mediate, or high. Canopy continuity (pres- esthorizontaldistancefrom thecenterofthe
ence or absence of gaps more than 20 cm nest to the perimeter ofthe support shrub,
across)ofeachsagebrushplantwasrecorded, and(4)compassorientationofthenestrelative
and the profile (presence or absence ofany tothecenterofthe supportshrub. Wecalcu-
branches or foliage within 30 cm of the lated relative nest height as the ratio ofnest
ground) ofsagebrush plants greater than 40 heighttotheheightofthesupportshruband
cmtallwasnoted. expressedasapercentage.
Canopy coverage and dispersion of sage- T tests and chi-square analyses were used
brushwereestimatedbylineintercept. Each to compare nest-site features with those of
year 10-25 samples were taken near grid the study area in general. For most t tests,
markers on each plot, different grid markers variances did notdiffersignificantlybetween
being used each year. These samples were the two groups. When variances differed,
regularlyspacedtoprovidean even distribu- we used the t' test (Sokal and Rohlf 1969:
tion of sampling effort across the plot. For 374-375), which relaxes the assumption of
each sample we recorded line intercept of variancehomogeneity.Therewerefewsignif-
sagebrush and distance between adjacent icant variations among years in nest-site fea-
sagebrushplants thatwere interceptedalong turesusedbythrashersorinhabitatfeatures
a5-mtapeextendingineachofthefourcardi- on the study area. Accordingly, data were
nal compass directions. For each sample the pooledforallyears.
coefficientofvariationofintershrubdistances
wasusedasanindexofdispersion;thegreater Results
theindex, the moreclumpedtheshrubs. We
Nest-SiteMicrohabitat
averaged the habitat data (exclusive ofindi-
vidual shrub measurements) for each grid SageThrasherschosenestingareasinwhich
markerandusedthegridmarkersasobserva- sagebrushplantswere significantlytallerand
tionalunitsinstatisticalanalyses. Forindivid- moreclumped thanon thestudyareain gen-
ual shrub measurements (height, condition, eral (Table 1). Percent coverages of sage-
etc.),theshrubsweretheobservationalunits. brush, rabbitbrush, grasses, forbs, and litter
To characterize actual nest sites, we re- within 5 m of thrasher nests were slightly
cordedthesamedataforshrubs supportinga greater than those on the study area in gen-
nest as for those occurring within (]uadrats. eral. Althoughnoneofthesepatternswassig-
We also estimated canopy coverage and dis- nificant, their cumulative effect resulted in
persionofsagebrushalonga5-mtapeextend- significantK less bare groundnearnests than
ingfrom the nest in each ofthe fourcardinal on therestofthestud\-area.
compassdirections. Further,werecordedthe
NestSubstrates
height ofeach sagebrush plant intercepted.
From 1981 to 1984 we estimated coverage of All nests were located in or beneath (on
rabbitbrush, grasses, forbs, litter, and bare the groimd) sagebrush plants. Shrubs se-
groundin20 x 50-cm(juadratsplaced2.5and lectedfornestingaveragedsignificantlytaller
5mfromeachnestineachofthefourcardinal than those representative of the studv area
directions. To minimize the potential con- {t 15.7,df=5079,p<.001). Moreover,the
founding effects ofvegetation change occur- rangeofshrubsizesusedfornestingwasmuch
ring between the time ofnest initiation and narrower than that of the available shrubs
1991 Sa(;i;'1'iiiv\.siii;kNKsi-srii-;Ski.kction 263
Hal)italcharactc )rSaKt''niiaslK' the SD).
\arial) Nr; StiicK
Sagebrushheight(cm) 49± 12(53)-' 41 ± 18(5028)
Sagebnislidispersion' 86 ± 19(53) 77 ± 22(401)
Sagebrushc()\erage(%) 23± 10(53) 22 ± 11(401)
Rabbitbrushcoverage(%) 6±4 (34) 5± 7 (484)
Grasscoverage(%) 9± 9 (34) 8 ± 9 (484)
Forbcoverage(%) 4 ± 7 (34) 3 ± 5 (484)
Littercoverage(%) 7 ± 3 (34) 6 ± 6 (484)
Bareground(%) 50 ± 12(34) 55 ± 21(484)
'^Nestingniicn,lial>italM\v 1(;;< 05,/test).
''Coenkieiitofvariationoil
Further, mostavailableshrubsbearingfoliage
had intermediate foliage density. Although
two-thirds ofthe nest shrubs also had inter-
mediate foliage density, shrubs with high
foliage density were used disproportionately
by thrashers as nest substrates, and shrubs
with lowfoliagedensitywereusedlittle. The
canopycontinuityofshrubsevidentlydidnot
influence shrub selection by thrashers; the
fre(iuencies with which gaps occurred in the
canopies ofnest shrubs and available shrubs
were nearly identical. Finally, a significantly
Representative Sampk greater than expected proportion of shrubs
used for nesting had branches or foliage
n=5.028 within30cmoftheground.
X±SD=41±18
NestPlacementWithinSubstrates
Thrashersplacedtheirnestsdeepwithinor
beneath shrubs (Table 3). Nest height aver-
aged only slightly more than a third of the
substrate height. Further, nests were placed
10 20 30 40 50 60 70 80 90 100>100
horizontally relativelyfarfrom theperimeter
Height(cm) andclosetothecenteroftheshrub. Severalof
Fig. 1. Frecjuency distributions of heights of Sage these measurementsare noteworthybecause
Thrashernestshrubsandarepresentativesampleofsage- oftheirrelativeconstancy;coefficientsofvari-
brushshrubsfromthestudyareaingeneral. ation were small for relative nest height and
distances from the nest to the perimeterand
(Fig. 1). Shrubs less than 50 cm tall consti- thetopoftheshrub.
tuted 73% of all available shrubs; yet no The overall pattern of Sage Thrasher nest
shrubs in this size range were used as nest orientations (Fig. 2) was not significantly dif-
substrates. Indeed, 72% of the nests found ferentfrom auniform distribution (X" = 9.6,
were in or under shrubs greater than 70 cm 7df, p = .22), buteasterly(NE, E, SE)expo-
tall; shrubs in this size range composed only sures were more prevalent than westerly
7%ofallavailableshrubs. (NW, W, SW)exposures. Acomparisonofall
Shrubs used for nestingby Sage Thrashers easterly orientations (combined) to all west-
differedfromavailableshrubsinseveralother erly orientations was significant (X" = 6.1,
respects. Nearlvall shrubs usedbythrashers Idf, p = .02).
were 75% or 100% living (Table 2). This dif-
feredmarkedlyfrom thedistribution ofavail- Discussion
able shrubs among the condition classes, in Sage Thrashers were selective in their
whichmanyshrubswerelessthan75%living. choice ofnest sites. In microhabitats chosen
264 GreatBasinNaturalist [Volume51
Table2. ComparisonsofSageThrashernestslirubswitharepresentatixesampleofsagebrushshrubsfrom the
studyareaingeneral.Valuesrepresentfreciuenciesofoecurreneeandpercentages(inparentheses)ofthetotal.
Nest Representati\(
Variabk sample
Condition <.01
Dead 1(2) 1109(22)
25%living 312(6)
50%hving 2(4) 617(12)
75%living 13(24) 709(14)
100%living 37(70) 2269(45)
Foliagedensity 5.87 .05
Low 3(6) 515(13)
Intermediate 35(67) 2762(71)
High 14(27) 632(16)
Canopycontinuity 0.01
Withgaps 22(42) 2041(41)
Withoutgaps 31(58) 2959(59)
Profile 4.91 .03
Full' 49(92) 1875(80)
Notfull 4(8) 461(20)
rfoli
Table3. Aspectsofnestplacementb\'SageThrashers.
Variable'
1991] SACKTllKASIIKHNKSrsriKSKLKCTION 265
obscurcMiient and cover for thrashers is suii;- differinman\'respectsfromtheaverageavail-
gested also by observations oi platforms of able habitat. Third, variation in theheight oi
twigs in slirul) canopies ai)ove some nests shrubschosen(coefficientofvariation 21%,
(Ricli 19<S(), 1985; personalol)servation). Hicli compared to44% forthe representativesam-
(1980)i)elie\edtliatsomesuclicanopieswere pleof shrubs from the studyarea)andin sev-
old nests, but none that we observed ap- eral ofthe nest placement variables is small
pearedso. In one instance thatweobservcnl, (Table 3). Moreover, the mean nest shrub
twigs were placed in the shrubcanopy about height is similar to means reported in other
oneweekafterthenestwasinitiated. studies (Revnolds and Rich 1978, Rich
Aspectsofnestplacementwithinsubstrates 1980, Reynolds 1981, Castrale 1982). Sage
(e.g., nest height, nestorientation) alsohave Thrashers are characteristic of most sage-
been found to be related to nesting success brush-dominated rangelands in the United
(e.g.. Murphy 1983, Westmoreland and Best States (Wiensand Rotenberry 1981), andbe-
1985) and microclimate amelioration (Hor- cause thrashers have evolved in sagebrush
vath 1964, Rich 1978). In particular, nonran- habitat, thespecificityoftheirnest-siteselec-
dom nest orientation typically is thought to tion should not be surprising. The patterns
reflectresponsesbybirdstoprevailingwinds thatweobservedlikelyhavebeen moldedby
or the radiative environment (e.g., Austin alonghistoryofexposuretoaparticularsuite
1976, Petersenand Best 1985, Ferguson and ofselectiveagents.
Siegfried 1989). Favoringeasterly andavoid-
ing westerly exposures for nests may reflect Acknowledgments
attempts by thrashers to maximize exposure
to the morning sun, shading from the after- WethankPaulSievertandLindaErickson-
noonsun, orboth. Eastwood for assistance in data collection.
It is possible, ofcourse, that factors other Several technicians of the Radiological and
than piedators or microclimate accounted Environmental Sciences Laboratory (RESL)
for the patterns we observed. For example, of INEL also assisted in the field, and the
selection of a large shrub for nesting may RESL staff provided on-site transportation
denotetheneedforstructuralsupportforthe andlodging. WethankTerrellRich, Kimberly
nest.ThetendencyofSageThrasherstoselect A. With, andan anonymous reviewerforcri-
microhabitats with large, clumped shrubs ticjuingearlier drafts ofthe manuscript. This
mightsimplyreflectthespatialdistributionof studywasfundedbytheOfficeofHealthand
areas conducive to robust sagebrush growth. Environmental Research, U.S. Department
Thus, selection ofa large shrub for nesting of Energy, and is a contribution from the
could, de facto, place the nest in an area INEL Radioecology-Ecology Program. Funds
of large, clumped sagebrush. Or, because wereadministeredthroughtheIowaCooper-
thrashersforageprimarilyontheground(per- ative Fish and Wildlife Research Unit, U.S.
sonal observation), clumped shrubs perhaps Fish and Wildlife Service. This is Journal
provide a favorable interspersion of shrubs PaperNo.J-13723oftheIowaAgricidtureand
andopeningsforforagingnearthenest. HomeEconomicsExperimentStation,Ames,
Although the determinants of nest-site Iowa, ProjectNo. 2468.
selection by Sage Thrashers are not known
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935-937'. Accepted15May1991