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The Condor91:848-859
Society 1989
? The CooperOrnithological
RELATIONSHIPSOF SURROUNDING RIPARIAN HABITAT TO
NEST-BOX USE AND REPRODUCTIVEOUTCOMEIN
HOUSE WRENS'
DEBORAHM. FINCH
RockyMountainForestand Range ExperimentStation, 222 South 22nd Street,
Laramie, WY82070
Abstract.I assessedrelationships
nest-siteselection,and reamonghabitatstructure,
threenest-box
productiveoutcomeof HouseWrens(Troglodytes
aedon)by establishing
gridsin riparianwoodlandsin southeastern
Wyoming.Overa 3-yearperiod,37%of the
boxescontained
HouseWrennests;20%contained
unusednestsbuiltbymaleHouseWrens;
and42%wereneverusedby wrens.Directdiscriminant
threebox-use
analysesseparated
groups(unused,1-yearuse,2- to 3-yearuse)andthreenestingoutcomepossibilities
(failure
all years,1-yearsuccess,2- to 3-yearsuccess)alonggradientsof habitatcoverandfoliage
density.Comparedwith unusedboxes,thoseselectedrepeatedly
by HouseWrenswere
locatedin habitatswithsparserunderstories.
Similarly,wrensweremorelikelyto fledge
fromboxeschosenin sparselyfoliatedhabitats.I concludethatcharacteristics
of
offspring
the surrounding
wrenreproductive
habitatinfluenced
outcomeand suggestthatboxesin
of success.Predation
openhabitatswereactivelyselectedbasedon theirhigherprobability
wasthe majorcauseof nestingfailure.By nestingin openhabitats,HouseWrensmaybe
moreadeptat detecting
anddeflecting
intruders
beforenestsarediscovered
and
cavity-nest
destroyed.
Key words: House Wren;dummy nests;foliage density;habitatstructure;nest-box use;
nestingoutcome;predationrisk;riparianwoodlands;secondarycavity-nestingbirds.
INTRODUCTION
Habitatfeaturesat or surroundinga nest site can
significantlyinfluencethe rate of nestingsuccess
through effects on probability of predation
(Westmorelandand Best 1985, Martinand Roper 1988),energyexpensesrelatedto nest or holeentrance orientation (Inouye 1976, Walsberg
1981), and food supply (Martin 1987). If reproductivesuccessis greaterin certainhabitats,then
birdsmay use habitatfeaturesto evaluatequality
of nest sites(Caccamise1977, Montevecchi1978,
Cody 1981). In addition, females may choose
mates on the basis of nest-sitequality(Kendeigh
1941, Askenmo 1984, Slagsvold 1986) and,
therefore,males may construct multiple nests,
which offer females an assortment of nest-site
alternatives(Kendeigh 1941).
In studies of secondary cavity-nesting birds
usingartificialnest sites, disparitiesin surrounding habitat structurehave been used to explain
patternsof nest-boxoccupancy,but the relationship between habitat variation and nesting success has rarelybeen addressed(Nilsson 1984).
When nest boxes are identical in size, interior
color, and hole entrance diameter (Lumsden
1986), nest sites are presumablyselected on the
basis of criteriaexternalto the box itself (but see
Brawn 1988). Such factors may include proximity to nestingconspecifics(Muldalet al. 1985)
or environmentaldisturbance(Zachand Mayoh
1982),food supplyor foragingsubstrate(Franzreb
1977, Hussel and Quinney 1987), probabilityof
predation or competition (Nilsson 1984), features of the nest-box tree (Yahner 1983/1984),
or adjacenthabitatstructure(Willneret al. 1983,
Munro and Rounds 1985, Belles-Islesand Picman 1986a).Habitatcharacteristicssurrounding
the cavity site may be used as cues by nesting
pairsto predictthe likelihood of nesting success
(Nilsson 1984).
Under conditionsof nest-holelimitation,cavity-nesting birds have been shown experimentally to compete for nest boxes and naturaltree
holes (van Balen et al. 1982, Gustafsson 1988).
House Wrens (Troglodytes aedon) are obligate
cavity-nesterswhose abundancecan be limited
by availabilityof nest sites (Yahner 1983/1984,
Belles-Isles and Picman 1986a). Intense competition for nest sites and mates in House Wrens
can lead to increasedaggression,nest usurpation,
bigamy,
killing of adult conspecifics,or infanti24
1989.
Final
23
acceptance
'Received January
cide (Belles-Islesand Picman 1986b, 1987; PicJuly1989.
[848]
USEAND NESTINGOUTCOME 849
NEST-BOX
man and Belles-Isles1988;Freed 1986a, 1986b).
If nest sites are scarce,some individualsmay be
forcedto select low-qualitysites, and thus, nestsite preferencesmay be difficultto detect.
In an effortto reduce the confoundingeffects
of interferencecompetition on nest-site selection, I increasedthe availabilityof nest sites to
House Wrens by establishingnest boxes in riparianwoodlands(see also Drillingand Thompson 1984, Munro and Rounds 1985). I then assessedwhetherhabitatfeaturesat nest boxes used
by wrens differedfrom those at unused boxes.
Further,I determinedwhether the same boxes
selected in multiple years were more likely to
contain successfulnests, and if so, whethersuch
boxes were associated with specific habitat attributes.In contrastto other studies which have
examinedwithin-yearassociationsbetweenhabitat and nest placement (or nesting success), I
definedhabitatdifferencesamongnest sites based
on numbersof yearseach box was used. Because
the nest site ratherthan a site-shiftingbird was
the experimentalunit, this approachproved to
be effective in discerninghigh-qualityhabitats,
i.e., habitatswhere nesting success is favored.
STUDY AREAS
Three study plots were established as grids in
May 1982 in streamside habitats in Carbon
County, southeasternWyoming, at elevations
rangingbetween2,050 m and 2,250 m. Two plots
were alongthe North PlatteRiver, 13 km northwest and 21 km southeastof Saratoga,and one
plot was at Rock Creek, 5 km northeastof Arlington. Narrowleaf cottonwood (Populus angustifolia) dominated the overstory along with
scatteredplainscottonwood(P. deltoides),quaking aspen(P. tremuloides),peachleafwillow (Salix amygdaloides),and Rocky Mountainjuniper
(Juniperusscopulorum).Midstoryvegetationincludedbush willow species(S. exigua, S. lasiandra, S. bebbiana,S. moniticola, S. ligulifolia),
thin-leaf alder (Alnustenuifolia),mountain maple (Acerglabrum), river hawthorn (Crataegus
rivularis),common chokecherry(Prunus virginiana), and western serviceberry(Amelanchier
alnifolia).Understoriesweredominatedby western snowberry (Symphoricarposoccidentalis),
goldencurrant(Ribesaureum),gooseberry(Ribes
spp.), cinquefoil (Potentilla grqcilis, P. fructicosa), wild rose (Rosa woodsii), red raspberry
(Rubusidaeus),red-osierdogwood (Cornusstolonifera), and a variety of grasses, sedges, and
forbs. Shortgrassprairieinterspersedwith sagebrush (Artemisiaspp.) borderedriparianwoodlands.
METHODS
On each of the three plots, 21-22 nest boxes (65
boxes total) were mounted 2 m high on live deciduous trees >10 cm dbh. Boxes were placed
30-35 m apartin gridsthat varied in lengthand
width owingto size of the ripariancorridor.Nest
boxes were built of 1.7-cm thick cedar, 14 x 14
x 28 cm in dimension, with 3.8-cm-diameter
entrancesand latchabletop doors, and were labeled with grid coordinates.To standardizeinfluencesof solar radiationand prevailingwinds
on nest-box choice, I placed all boxes with their
entrancesfacingtowardthe south or southeast
--orientations preferredby some cavity-nesting
species (Pinkowski 1976, Lumsden 1986).
I ascertainedratesof box occupancyand nesting success by checking nest boxes early in the
afternoonevery 2-4 days from mid-May to late
Julyof 1983, 1984, and 1985. Becauseindividual
male House Wrens frequentlyfill multiple cavities with twigs, the appearanceof an egg was
used as evidence of site selection by a nesting
female. Box use by male wrens was recorded
when incomplete twig nests were found but no
eggswere laid. Due to the shortbreedingseason
(May throughJuly)in these high altitudewoodlands,only a few boxes werenestedin twice during a single breeding season. Box reuse by the
same pair of House Wrens could not be distinguishedfrom late box settlementby a new pair.
Such "second" nesting attempts were excluded
from analyses to control for effects of seasonal
variation on reproductivesuccess. Boxes were
cleaned out each Septemberso that new twigs
and nests were not confused with box contents
from the year before.
Vegetationstructurewas sampledin June and
July 1984 at all nest-box sites to determine if
habitat variation affected nest-box choice or
nesting success. The period of vegetation sampling was arbitrarilytimed so that samples reflectedhabitatencounteredbetweenthe periods
of nest construction(Mayand June)and fledging
of young (July and August). At each nest-box
tree, I measured 34 habitat variables using a
point-centered quarter sampling procedure
(Finch 1989). Habitat features were sampled
by dividing each location into four quadrants
orientedby cardinalcompass directionsaround
850
DEBORAHM. FINCH
TABLE 1. Structuralvariablesand transformationsused in analyses.
Mnemonic
acronym
Transformation*
Variable
DBH
SQRT
Tree diameter
CANHT
RCPL
Canopyheight
SHHT
LN
Shrubheight
SHCD
LN
Shrubcrowndiameter
SHDIS
VFDI
SQRT
RCPL
VFD2
SQRT
VFD3
SQRT
VFD4
SQRT
VFD5
LN
EVH
LN
Shrubdispersion
Verticalfoliagedensity in
grass-forblayer
Verticalfoliagedensity in
small shrublayer
Verticalfoliagedensity in
mid-canopylayer
Verticalfoliagedensity in
lower overstory
Verticalfoliagedensity in
upperoverstory
Effectivevegetationheight
WILL
SQRT
Percentwillow
SAP
LN
Percentsaplings
BARE
SQRT
Percentbareground
HERB
SQRT
Grass-forbgroundcover
LOGS
SQRT
Log cover
SHRUB
LN
Shrubcover
COVER
SQRT
Live and dead woody cover
Samplingmethod
Diameter(cm) at breastheight of nearesttrees (>3 cm)
in each quadrant.
Mean height(m) of nearesttrees (or shrubsif no trees
in sample)in each quadrant.
Mean height (m) of nearestshrubs(>1 m tall) in each
quadrant.
Mean diameter(cm) at widest crown of nearestshrubs
(>1 m tall) in each quadrant.
Mean distance(m) to nearestshrub(> Im tall).
Mean numberof vegetationcontactsfallingagainst
verticalrod in <0.3-m interval.
Same as VFDI, but in 0.3- to 1-m interval.
Same as VFD1, but in 1- to 2-m interval.
Same as VFD1, but in 2- to 9-m interval.
Same as VFD1, but in >9-m interval.
Height at which a 20-cm-wideboardis >90% obscuredby vegetationat a distanceof 5 m (Wiens
1969).
Proportionof shrubspecies in distancesample that are
willows.
Proportionof plant species in shrubdistancesample
that are saplings(<3 cm dbh).
Percentof surfacethat is bare, or coveredwith litter,
measuredwith oculartube (Jamesand Shugart
1970).
Percentcover of grassesand forbs measuredwith ocular tube (Jamesand Shugart1970).
Percentcover of logs (>3 cm diameter)measuredwith
oculartube (Jamesand Shugart1970).
Percentcover of small shrubs(< Im tall) measured
with oculartube (Jamesand Shugart1970).
Percentcover of woody plants, saplings,and downed
logs measuredwith oculartube (Jamesand Shugart
1970).
SAcronyms for variable transformations are defined as: SQRT = square root; RCPL = reciprocal; LN = natural log.
the nest-box tree. Pearson's product-moment
correlations were used to assess relationships
among habitat variables. Sixteen of the original
variables were deleted from the analyses because
they were either invariant or highly correlated
with other variables. Within highly correlated
variables, those having a sampling distribution
most closely approaching normality were retained (18 variables, Table 1). Degree of normality was determined by examining normal
probability plots (Afifi and Clark 1984).
DATA ANALYSES
Chi-square analysis was used to test whether the
frequency of nest-box use and reproductive suc-
cess differed among years. Four levels of box use
were identified: (1) nest boxes never used by wrens
in all three study years (NO); (2) boxes used only
by male wrens for stuffing twigs, i.e., dummy
nests (M); (3) boxes selected by nesting pairs during one study year (1YR); and (4) boxes used by
nesting pairs for 2 or 3 years (23YR). Reproductive outcomes were classified as: (1) boxes
with nesting wrens that failed to fledge offspring
all years of use (F); (2) boxes that successfully
fledged at least one wren in one study year (S1);
and (3) boxes that fledged wrens in 2-3 years
(S23). Predation was assumed if nest material
was disturbed, eggs were broken, nestlings were
partially eaten, predator feces were found in the
NEST-BOXUSE AND NESTING OUTCOME
nest, eggsor nestlingswere on the ground,boxes
were damaged or unlatched (probablyby raccoons, Procyonlotor),or nestswereemptybefore
nestlingswere due to fledge.Nests were considered abandonedif nest contents failed to hatch
or fledge,and adultswere no longeractive at the
nest. Partiallosses of clutches and broods were
common, resultingfrom egg puncturingby conspecifics, hatching failure, and nestling starvation. Nests in reduced broods were considered
successfulif the nest remainedactive to the time
of fledging.
Multivariatetechniques were applied in this
studybecausehabitatgradientsand relationships
among multipletreatmentsand multiple habitat
factorsare not readily detected or interpretable
using univariatestatistics (Green 1979, Klecka
1980). To counteractthe criticism that multivariate tests are prone to producesignificantresults even when data contain no relationships
(Rextadet al. 1988),I firstdeterminedthat,within a set of univariatetests, the number of variables that differedsignificantlywas greaterthan
thatexpectedby chance(P < 0.05). Forexample,
for a set of 18 habitat variables,the number of
variables expected to differ by chance alone is
18 x 0.05 < 1.0. Individualhabitatfeatureswere
comparedamong use groups using randomized
complete block analysis of variance (ANOVA),
with the three study plots servingas blocks and
the box site representingthe experimentalunit
(n = 65). This design improves the accuracyof
the comparisonsbetween use classes by eliminatingthe variabilityamong plots (Montgomery
1984) that may arise due to differencesin wren
or predatordensities, and habitats.
I applied multivariate analysis of variance
(MANOVA)to assess whetherhabitatcentroids
differedamong box-use groups or nesting outcomes (Marascuiloand Levin 1983). Direct discriminant analysis (DA) with orthogonal rotation (VARIMAX solution, SPSS, Klecka 1975)
was used to separateand classifynest-boxuse or
fledgingsuccess along axes of habitat structure.
To compensatefor unequalsample sizes among
groups, I based prior probabilitiesfor classificationon proportionsof caseswithineachgroup.
Habitat variables were selected for direct DAs
based on significancelevels (P < 0.1) of the 18
transformedvariablesusedin theunivariateblock
ANOVAs of box use. Additionalvariablesused
in direct DAs were those identifiedby stepwise
DA usingthe Mahalanobisdistancecriterionfor
851
maximizingseparationof groups(Klecka 1975).
Thirteen habitat featureswere selected for the
multivariateanalysesof box-usegroupsand nine
variableswere selectedfor the analysesof reproductive outcomes. Based on univariatetests indicatinghabitatsimilaritybetweenunusedboxes
and boxes with dummy nests, I created a new
use class (NM) by addingthe sampleof dummynest boxes to the unused group;this consolidation improvedsamplesize. Sufficientsamplesize
was defined using Klecka's (1980) criteria that
each group n must be at least two times greater
than the numberof variables.
Box's modificationof Bartlett'stest was used
to evaluatethe homogeneityof variance-covariance matrices (Williams 1983). Because Box's
test is conservative(Green1979),I used an alpha
level of 0.01 to determineif covariancematrices
differed.Discriminantfunctionswere tested for
statisticalsignificance(P < 0.05) by transforming
Wilk'slambdainto a Chi-squarestatistic(Marascuilo and Levin 1983). Tukey's procedurefor
hypothesis testing (Dunnett 1980) was used to
determinethe significance(P < 0.05) of group
separationof meandiscriminantscoresuponeach
individual discriminantaxis. The originalcases
were classifiedinto membershipgroupsto check
the adequacyof the discriminantfunctions(Hand
1981, Williams 1983).
Three sets of data using raw, log-transformed
(LN), and a combinationof squareroot (SQRT),
reciprocal(RCPL),and log-transformedvariates
(Kleinbaumand Kupper 1978) were analyzed.
Resultsusinga combinationof transformedvariates were reportedfor subsequentanalyses because they most closely adheredto statisticalassumptions of normality and equality of
covariance matrices. For ease in interpreting
variables,mean values of raw data were given.
RESULTS
House Wrensnested in 21 boxes (32%of 65) in
1983, 29 (45%)in 1984, and 23 (35%)in 1985
(Table 2). Male wrens deposited twigs into an
additional 15 boxes (23%)in 1983, 13 (20%)in
1984, and 11 (17%) in 1985. Over the 3-year
period,43%of all boxes werenot used by wrens,
20%had male dummynests, and 37%contained
true wren nests.
Occupancyratesof boxes on each plot did not
differamong years for nesting wrens (x2 = 1.27,
df = 4, P > 0.75), male wrens (x2 = 6.29, df =
852
DEBORAH
M. FINCH
TABLE2. Frequency
of nest-boxuse andnestingsuccess,andratesof box occupancy
(%use by plot)and
success(%neststhatfledged onenestling)of HouseWrenson threestudyplotsin 1983,1984,and1985.
_
Yearand plot
Unusedboxes(%)
Male(%)'
HouseWrenuse
Nest (%)
Success(%)b
1983
FooteCamp
Treasure
Island
RockCreek
1984
FooteCamp
Treasure
Island
RockCreek
9 (40.9)
14(66.7)
6 (27.3)
6 (27.3)
2 (9.5)
7 (31.8)
7 (31.8)
5 (23.8)
9 (40.9)
7 (100)
5 (100)
8 (88)
10(45.5)
7 (33.3)
6 (27.3)
3 (13.6)
5 (23.8)
5 (22.7)
9 (40.9)
9 (42.9)
11(50.0)
1 (11)
8 (89)
9 (82)
1985
Foote Camp
TreasureIsland
7 (31.8)
11(52.4)
5 (22.7)
5 (23.8)
10 (45.5)
5 (23.8)
6 (60)
4 (80)
RockCreek
Grandtotal
13(59.1)
62
1 (4.5)
39
8 (36.4)
73
5 (63)
53 (73)
use was countedwhena box was filledwith twigsbut no eggswerelaid.
bSMale
Numberof successesis the numberof usedboxes that fledgedat least one offspring.
4, P > 0.25), and nestingwrensand male wrens
combined (x2 = 4.26, df = 4, P > 0.50) (Table
2). Box occupancywas dependent on whether
the same boxes wereoccupiedthe precedingyear
(1984: x2 = 14.81, df= 4, P= 0.005, and 1985:
2 = 16.06, df = 4, P = 0.003). House Wrens
fledgedoffspringfrom 53 of 73 nests (73%)over
the 3-year period. The rate of nesting success
decreasedfrom a plot average(i + SD) of 96.0
? 6.9% in 1983 to 60.7 ? 43.2% in 1984 and
67.7 ? 10.8%in 1985 (Welch's F = 6.6, P =
0.08). Boxes containingsuccessfulnests in 1983
weremore likelyto be used in 1984 by eitherthe
same or differentbirds,whereasunusedor failed
boxeswerenot likelyto be occupiedthe following
year (x2 = 9.41, df = 2, P = 0.009). Box use in
1985 was similarly related to previous reproductive success(x2 = 10.98, df= 2, P = 0.004).
UNIVARIATE
TESTSOFBOXUSE
(Table 3). Because the number of features expected to differby chance alone was less than 1
at the alpha level of 0.05, I concludedthat patternsof box use weresignificantlyrelatedto habitat differencesand that the use of multivariate
tests was appropriatein subsequentanalyses.
Ingeneral,trendsdetectedusingunivariatetests
weresimilarto those found in multivariateanalyses so univariate results are only briefly described.In sum, nestingwrens selectedboxes in
one or more years in areas with smaller trees,
fewerand largershrubs,fewersaplings,and more
open (herbaceousratherthanwoody)groundand
understorycover than areas containingunused
boxes.
ANALYSIS
OFBOXUSE
DISCRIMINANT
An overallMANOVA for 13 variablesand three
box-use levels (NM, 1YR, 23YR) indicatedthat
Thirteenboxes were never used by wrens;eight the habitatcentroidssignificantlydifferedamong
boxesheldonlydummynestsbuiltby malewrens; use groups (Hotelling'strace = 1.54, F = 2.54,
21 boxes were occupied by nesting wrens in 1 P = 0.0007). Box's test indicated that the coyearonly;and 23 boxes had truewren nests dur- variance matrices computed using transformed
ing 2 or 3 years.Boxes containingmale dummy variablesdid not differin the analysesof box use
nests were locatedin habitatssimilarto those in (P = 0.056) andreproductivesuccess(P = 0.250).
which boxes were never used (17 of 18 habitat Discriminant analysis of 13 habitat variables
features, P > 0.05); therefore,in multivariate producedtwo significantdiscriminantfunctions
analyses,thesetwo classeswerecombined(NM). that distinguishedamong three levels of box use
In pairwisecomparisons,13 featuresdifferedbe- by House Wrens(Table4). The firstdiscriminant
tween various combinationsof wren-useclasses functionaccountedfor 48%of the varianceand
(NM vs. lYR, NM vs. 23YR, lYR vs. 23YR) was most highly correlatedwith SAP, SHDIS,
NEST-BOXUSE AND NESTING OUTCOME
853
TABLE3. Means?-CL of 18 habitatfeaturesmeasuredat nest boxes never used by House Wrensin all three
study years(NO);used only by male wrens(M);used as a wren nest site in 1 year(1YR);or used as a wrennest
site in 2-3 years (23YR). Total sample size = 65.a
No use all years(NO)
Maleuse (M)
(n = 13)
(n = 8)
Habitat feature
x
CL
2
DBH (cm)
CANHT (m)
SHHT (m)
SHCD (cm)
SHDIS (m)
VFD1 (no. hits)
VFD2 (no. hits)
VFD3 (no. hits)
VFD4 (no. hits)
VFD5 (no. hits)
EVH (m)
WILL(%)
SAP (%)
BARE(%)
HERB (%)
LOGS(%)
SHRUB (%)
COVER(%)
27.6
11.4
1.9
120.3
2.9
1.5
0.8
1.0
3.3
0.8
1.0
9.6
4.8
28.8
44.2
3.8
18.2
26.8
7.2
3.2
0.5
38.4
1.6
0.6
0.7
0.6
1.0
0.6
0.5
9.8
5.0
15.6
18.0
3.7
13.5
13.9
34.3
13.1
1.9
107.2
4.0
2.2
0.3
0.7
3.8
0.7
0.6
15.6
6.3
21.9
56.4
6.3
9.3
21.8
Box-useclasses
Nest 1 year(lYR)
(n = 21)
CL
x
14.6
8.1
0.4
54.5
5.6
0.6
0.3
0.4
1.4
0.7
0.5
19.2
7.9
16.8
19.7
7.9
14.5
16.7
23.7
9.7
2.4
149.7
5.8
1.8
0.4
1.0
5.1
1.3
0.4
3.6
1.8
26.7
61.6
5.8
4.2
11.8
Nest 2-3 years(23YR)
(n = 23)
CL
x
5.2
2.7
0.7
28.6
1.9
0.5
0.2
0.6
1.4
0.9
0.3
4.1
3.7
12.0
10.8
4.2
3.3
6.1
24.4
10.3
2.1
148.7
5.3
2.5
0.5
1.1
3.5
0.5
0.4
10.9
0.5
23.3
64.8
2.8
8.6
11.9
CL
Comparisonsb
4.6
3.8
0.4
23.2
1.8
0.7
0.3
0.4
0.8
0.4
0.2
8.5
1.1
10.9
11.2
2.3
5.3
5.3
C
C
NS
D
CD
ABD
NS
NS
BC
BD
D
B
CD
NS
CD
NS
C
CD
a See Table 1 for descriptions of habitat acronyms. Data were backtransformed for ease of interpretation.
b Habitat features were statistically compared
among use classes using randomized block ANOVA. The classes NO and M differed in only one
feature so a new class NM was formed. Habitat features that differed significantly (P < 0.1) between classes are indicated by: A = NO vs. M, B =
1YR vs. 23YR, C = NM vs. 1YR, and D = NM vs. 23YR. NS = not significant.
SHCD, and HERB (Fig. 1). This function, which
described a situation where percentage saplings
decreased as shrub dispersion, shrub size, and
herbaceous cover increased, distinguished unused boxes from those occupied by nesting wrens.
Wrens avoided boxes positioned in habitats with
dense sapling-small shrub understories but frequently nested in boxes at sites with open, herbaceous ground cover and dispersed large shrubs.
Pairwise comparisons using Tukey's test indicated that the means of discriminant scores of
used and unused box sites were significantly separated on the first discriminant function (Table
DISCRIMINANT
ANALYSIS
OF
NESTINGSUCCESS
An overallMANOVA for nine habitatvariables
and three nesting outcome levels indicatedthat
the habitatcentroidswere significantlydifferent
among outcomes (Hotelling'strace = 1.55, F =
2.33, P = 0.009) in House Wrens.Two significant
discriminantfunctionsdistinguishedamongthree
outcome levels in DAs of nine variables(Table
4). The first function accountedfor 44% of the
total variance and was most highly correlated
with VFD5 and VFDI (Fig. 2). This function
5).
The second discriminant function explained
34% of the variance among groups and represented a gradient of decreasing foliage density at
the ground level with increasing foliage density
in the overstory. Means of boxes used 2-3 years
by wrens were significantly separated on the second function from means of boxes used 1 year
as well as from means of boxes that failed (Table
5). Apparently, boxes placed in locations with
denser overstory cover and thinner ground foliage were more often used as nest sites for 1 year
rather than for 2 or 3 years. DA correctly classified 78.5% of all boxes.
TABLE4. Two discriminant
functionanalysesdisandreprotinguishing
amongnest-boxusecategories
ductiveoutcomesin HouseWrens.a
Characteristic
% variance
x2
df
P
Box use
Function
Function
1
2
Nesting outcome
Function
Function
1
2
48.133
60.123
26
0.000
44.259
37.370
18
0.005
34.087
23.353
12
0.025
34.640
15.742
8
0.046
Use categories in House Wrens are (1) box empty or used by male
wrens in all study years, (2) box used by females in 1 year, and (3) box
used by females in 2-3 years. Outcome groups are (1) ailed in all years,
(2) fledged at least one young in 1 year, and (3) fledged at least one young/
year in 2-3 years.
854
DEBORAHM. FINCH
4
Unused
IL
I-.-0
..o.
I L
o
1-year
A
>
r
?c•o
AO
CCt ,
2 *
.
use
A
A
2-3-year
use
* *
?-- La
-
*
•
A
A•A•
oo o
o
o
....J
.. J-"
I*
I
4
IL.-,*
*
2.4
0*
u
-?urn
o00
-4
-2
.
.-
0
DF1
2
4
Low
High
High
High
Percent saplings
Shrub dispersion
Shrub crown diameter
Percent herbaceous cover
High
Low
Low
Low
FIGURE 1. Discriminantscores on two axes derivedfrom discriminantanalysisof box-usegroupsin House
Wrens.Mean vectors for each groupare labeledas 0 = unused, 1 = 1-yearuse, 2 = 2- to 3-yearuse. Habitat
variables,used to interpretgrouppositions, are describedin Table 1.
significantly separated boxes with nesting attempts that always failed from those with successful outcomes one or more times (Table 5).
Compared with successful boxes, failures were
associatedwith denseroverstoryfoliage, sparser
herbaceousfoliagein the surfacelayer,and higher
cover of logs >3 cm diameter.
The second discriminantfunction accounted
TABLE5. Groupmeans and pairwisecomparisonsof discriminantfunctions,priorprobabilities,and classificationratesfrom two sets of discriminantanalysesthat separatedpatternsof nest-boxuse or fledgingsuccess
in House Wrens.
Group
Box use
Unused all years
Nest 1 year
Nest 2-3 years
Nesting outcome
Failureall years
Success 1 year
Success2-3 years
n
Prior
probability Tukey'
Function1
prob
Function2
SD
SD
Tukey"
correctly
classifiedb
21
21
23
0.32
0.32
0.35
A
B
B
-1.39
0.87
0.38
1.21
0.70
0.88
A
B
C
0.03
0.93
-0.87
1.10
0.90
0.99
76.2
85.7
73.9
11
19
14
0.25
0.43
0.32
A
B
C
1.14
0.15
-1.12
1.04
1.05
0.89
A
B
A
0.75
-0.80
0.50
0.68
1.26
0.80
81.8
68.4
78.6
SPairwisecomparisonsof mean discriminantscoresof use and outcomeclasseswereassessedusingTukey'smultiplecomparisontest (Dunnett
1980).
Significant(P < 0.05) differencesbetweenmeansare indicatedby differentletters(e.g.,A vs. B vs. C).
b
Percentage of cases that were correctly classified by the discriminant analyses.
NEST-BOX
USEAND NESTINGOUTCOME 855
4 .tw
r-t
CC
Failure
E
1-year
success
2
-0
S_
c
o
o
>
ccess
0
0
*
2
0*
--o
drr
Ho
1u uor
-'-'f;; c,
o
2-3-year
su
,*
e
??h
Low
A
u
Y-42:
>f
T....
*
tc
0
K
'-
2
4
OAF
-4
-
Low-High--Low
-
-
-2
0
2
High-overstory foliage density-Foliage density grass-forb layer--Percent cover of logs
4
High
Low
High
of discriminant
FIGURE2. Distribution
scoreson twofunctions,basedontheanalysisof nestingoutcomes
in boxesselectedby HouseWrens.Meanvectorsforeachgrouparelabeledas 0 = failure,1 = 1-yearsuccess,
and2 = 2- to 3-yearsuccess.Habitatvariables,usedto interpret
grouppositions,aredescribedin Table1.
for 35%of the total variancein the House Wren
DA and significantlydistinguishedbetweenboxes with 1 year successes and boxes with either
failed attempts or 2-3 year successes (Tables 4
and 5). This resultsuggestedthat successfuloutcomes in reused boxes were more likely to be
repeatedin habitatswheregroundsurfaceswere
bare,shrubslarge,and loweroverstories(VFD4)
sparse (Fig. 2). When both functions are taken
into account, wrens were most reproductively
successfulif they selectedboxes in habitatswith
open canopies and open surfaceswith few decaying logs or dwarf shrubs.The probabilityof
failurewas greatestin habitatswith thick canopy
foliage and heavy nonherbaceousgroundcover.
In this DA, a total of 75% of 44 outcomes was
correctlyclassified(Table 5).
DISCUSSION
NEST-BOX
BYHOUSEWRENS
SELECTION
Boxes used by House Wrensfor nestingdiffered
from unused boxes in that they were located in
open habitats. In experimental woodlands of
Pennsylvaniaand naturalwoodlandsof the Midwest and centralRockyMountains,House Wrens
were reportedto be abundantin areas with low
shrubcover and foliage density (DeGraaf 1987,
Sedgwick and Knopf 1987), selecting natural
cavities in smaller trees and snags than those
used by largercavity-nestingspecies(Staufferand
Best 1982, Gutzwillerand Anderson 1987). In
othernest-boxstudies,House Wrensshowedthe
same tendency to select boxes in sparservegetation(Belles-Islesand Picman 1986a)and, when
856
M. FINCH
DEBORAH
given a choice between boxes in logged or unloggedstands,selectedmoreboxesin loggedareas
with greaterherbaceouscover and fewer shrubs
and saplings(Drillingand Thompson 1984). In
areaswith moreopen fieldand pasture,however,
wrens chose boxes closer to trees, saplings,and
shrubs(Willneret al. 1983, Munroand Rounds
1985). Thus, over a gradientof habitatsranging
from open fields to dense, deciduous forests,
HouseWrensapparentlyselectintermediatesites.
House Wrensforageon the groundand in the
grass-forb-shrublayer (DeGraaf et al. 1985,
Sedgwickand Knopf 1987)and may be attracted
to nesting habitats with accessible surfacesfor
foraging, particularlyif litter and herbaceous
vegetationhave moreinsectspreferredby wrens.
Increasedforagingby House Wrensin loggedand
burnedareas (Franzreb1977) suggestsselection
for open foraging substrate with herbaceous
groundcover. Boxes placedin sparsevegetation
receive greater solar radiation (McComb and
Noble 1981), so microclimatemay play an additional role in nest-box choice, particularlyin
the unpredictable,high altitude climate of Carbon County,Wyoming.Alternatively,box selection may be frequencydependent(Brawn1988).
For instance,if boxes in open habitatsare more
easily found, then such highlyvisible boxes may
be selected more often by wrens.
Male wrensconstructeddummy nests in more
varied sites than those ultimatelyselectedby females. Boxes containing unused dummy nests
were located in habitats similar to those where
boxes remainedempty all 3 years. Indeed, habitat structuresampledat centersof male-defended territorieswas similar to that at randomly
sampled sites (Finch 1989), suggestingthat territoryselectionby male wrensis nonspecialized.
Male House Wrens are aggressivelyterritorial
(Kendeigh1941)and could easily defendseveral
boxes given the box-spacingpatternin this study
(pers. observ.). Nonspecializeduse of boxes by
multiple-nestmales may be adaptive under the
following circumstances.By building nests in
multiple boxes within its territory,a male may
improve its chances of attractinga mate (Kendeigh 1941), even if some boxes are positioned
in low-qualitysites. By matingwith a multiplenest male, a female House Wrencan readilyrenest in a surplusnest hole if the firstattemptfails
because of nest usurpationor predation. High
densities of dummy nests may also protect the
actual nest site from search-strategypredators,
as was demonstratedfor the Marsh Wren, Cistothoruspalustris(Leonardand Picman 1987).
IN HOUSEWRENS
NESTINGSUCCESS
Factors affectingnesting success, such as food
supply, microclimate,and predation,can be influencedby habitatfeaturesat the nest site (Walsberg 1981, Westmorelandand Best 1985, Martin
1987). In this study, I focusedon habitatfactors
that may limit predationrisk because nest predation was the major source of nesting failure
and failurewas associatedwith habitat characteristics.Previousresearchon open-nestingbirds
has shown that increasedfoliagedensitymay reduceriskof nest discoveryby concealingthe nest
(Nolan 1978, Murphy 1983, Westmorelandand
Best 1985), impeding predatortravel or transmission of cues (Bowmanand Harris 1980), or
increasingthe number of possible nest sites a
predatormust inspect(Martinand Roper 1988).
Reproductivesuccessof House Wrensin Wyoming was associatedwith nest sites surrounded
by sparse vegetation and little downed wood,
ratherthan dense foliage and groundcover. Although few studies of cavity-nestingbirds have
considered habitat influences on reproductive
outcome(see Nilsson 1984),Belles-Islesand Picman (1986a) reportedthat productivityof boxnestingwrensin Ontario,Canada,was greaterin
open habitats because nest losses to predators
were minimized.
Over the 3-yearperiod of this study, 18 of 20
unsuccessfulnests were destroyedby predators,
and two were desertedafterpartialclutchesdisappeared.At least four of the depredatednests
showed signs of destructionby House Wrens.
Evidenceof conspecificintrusionincludedpuncturedeggs,nest materialpulledthroughentrance
holes (see Belles-Islesand Picman 1986b), and
observationsof box entryby morethantwo wrens
(all box usersviewed simultaneouslyin vicinity).
Conspecificsarereportedlymajornest destroyers
in House Wrens, pecking eggs and nestlings to
death to confiscatenest sites or mates, or to enhance sexual receptivity in females, allowing
forced extra-paircopulations by males (BellesIsles and Picman 1986a; Freed 1986a, 1986b;
Quinnand Holroyd1989).Predatorsin this study
were undoubtedlysmall (limited by the 3.8-cm
hole entrance)and possibly accustomed to enteringdarkcavities becauseof their own nesting
habits. Small cavity-nestpredatorstypicallyincludewoodpeckersand mice (Nilsson 1984),sec-
USEANDNESTINGOUTCOME 857
NEST-BOX
ondarycavity-nestingbirds (Freed 1987, Butler
and Campbell 1987), weasels (Dunn 1977), and
snakes (Nolan 1959). In riparian habitats of
northcentral Wyoming, bullsnakes (Pituophis
melanoleucussayi) andlong-tailedweasels(Mustelafrenata)were detectedenteringand destroying House Wren nests (L. Scott Johnson and
HenryKermott,pers.comm.). Both of these vertebratespecieswere observedin my study areas.
Becausethe contents of some wren nests in this
studywereremovedwithoutdisturbanceto nesting material,the tree-climbingbullsnakeis a suspected predator.Largepredatorsinclude mammals able to open a box (e.g., unlatchedboxes
and tracksin the vicinity implicate raccoonsin
this study) or poke a sweepingpaw throughan
entrance (e.g., red squirrel, Tamiasciurushudsonicus, filmed by the author with time-lapse
camera).
The restlessactivityand vociferouschatterand
song of House Wrens may attractpredatorsto
nest sites (see also Skutch 1949, Willis 1973).
Cavity nests may be easily approachedby predatorsandconspecificsif densefoliageanddowned
logs conceal their approachfrom nesting adults
(Belles-Islesand Picman 1986a). For the nonsedentaryHouse Wren,the abilityto detectpredators first may be an important aspect of nest
if predatorvisibilityis greater
defense,particularly
around nest sites with little obstructivefoliage.
When a predatoror conspecific is sighted, the
nestingadultscan modify theirbehavioraccordingly, by minimizing activity at the nest, distracting the animal's attention away from the
cavity, or by attacking the intruder. Direct
aggressionis usuallyeffectiveif the intruderis a
conspecific(Kendeigh 1941, Grove 1981). High
visibility may actually be advantageousin territorialdefenseof nest sites againstconspecifics.
In addition, vegetation immediatelyadjacent
to the nest site may facilitatepredationby supplying a bridgeto the nest. Bullsnakeswere observedenteringthreeHouse Wrennestsin northcentral Wyoming by descending from foliage
directlyabove each nest box (L. Scott Johnson,
pers.comm.). House Wrensmay avoid nest sites
where foliage is profuse if close foliage permits
easier nest access by climbing snakes.
Increasesin both nest-boxuse and nestingsuccess of wrenswere associatedwith open surfaces
and open canopies. Thus, boxes that were occupied for multiple years were in habitats associated with greater reproductive success, sug-
gestingthat boxes in open habitatswere actively
selectedbased on theirhigherprobabilityof success. My resultsshowedthatbox use in 1984 and
1985 was relatedto box occupancyand nesting
outcome from the precedingyear. In a study of
marked birds in central Illinois, Drilling and
Thompson (1988) demonstrated that female
House Wrensthat returnedto breedingsites had
producedmore offspringin the previous breeding season than had nonreturningfemales. If
nesting outcome the year before influencedrate
of box reuseby returningbirdsin my study,then
the association between multiple-yearuse and
sparsevegetationmay be an incidentalresult of
the underlyingassociationbetween nesting outcome and habitat.Hence, placementof nests by
House Wrens and other bird species may ultimately depend on age and prior nesting experience (Sonerud 1985, Marzluff1988).
ACKNOWLEDGMENTS
G. J. Sherman,
K. A. Conine,
I thankP. F. Gutzwiller,
C.L.Canaday,
andR.D. Greerforassistance
in checkhabitatcharacteristics,
ing nestboxesandmeasuring
andA. L.Wardforhisencouragement
duringthestudy.
to M. G. Raphaelforadviceon statistical
I amgrateful
design,andJ. D. Brawn,F. L.Knopf,K.J. Gutzwiller,
T. E. Martin,B. A. Maurer,
J. Picman,R. C. Rounds,
J. A. Sedgwick,and R. H. Yahnerfor reviewof the
manuscript.
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