TERRITORY SELECTION BY UPLAND RED-WINGED BLACKBIRDS IN EXPERIMENTAL RESTORATION PLOTS

2006·6.1 MB·English

by  Maria A Furey

3
The Wilson Journal ofOrnithology 118(3):391-398, 2006
TERRITORY SELECTION BY UPLAND RED-WINGED
BLACKBIRDS IN EXPERIMENTAL RESTORATION PLOTS
MARIA A. FUREY1 AND DIRK E. BURHANS24
—
ABSTRACT. We examined territory selection of Red-winged Blackbirds (Agelaius phoeniceus) in experi-
mental treatments with variedgroundcovers anddensitiesofplantedand naturally occurringoaks (Quercusspp.)
used by blackbirds for perching. We also compared vegetation parameters between blackbird territories and
unused (i.e., unoccupied by Red-winged Blackbirds) areas. Although perch densities were greater in blackbird
territories in unplanted controls and oak-planted treatments without redtop grass (Agrostis gigantea) than they
were in unused areas, the low densities of perches in territories planted with redtop grass indicate that perch
density is not limiting above some lowerthreshold. Territories, particularly in treatments with no redtop, tended
to have greatermean grasscoverand tallergrass heights than unused areas. Ourresults areconsistentwithother
studies in finding that Red-winged Blackbirds prefer areas having tall vegetation and dense grass. Received 14
July 2005, accepted21 February 2006.
A large body of observational studies has factors on avian settlement patterns may be
documented relationships between avian masked if measurements are made at inappro-
abundance, orterritory use, and vegetation pa- priate scales (Orians and Wittenberger 1991,
rameters. Examples include studies comparing Pribil and Pieman 1997). For example, Orians
differences among songbird territories withre- and Wittenberger (1991) found that Yellow-
spect to vegetation height or litter depth headed Blackbirds (Xanthocephalus xantho-
(Wiens 1969) and grass or shrub cover (Ro- cephalus) settle according to food supplies at
tenberry and Wiens 1980), and those that re- the scale ofan entire marsh, arelationship that
late avian abundance to vegetation density was not apparent at the territory scale. Simi-
(Orians and Wittenberger 1991) or grass larly, Burhans (1997) found that some factors
(Scott et al. 2002). However, important rela- explaining brood parasitism at the nest-site
tionships between vegetation and habitat use scale were relevant only when considered at
can be obscured if the variation among study the larger scale of habitat.
sites (or plots) is minimal (Orians and Witten- We investigated the role ofvegetation struc-
berger 1991, Pribil and Pieman 1997). One ture in the selection of breeding territories by
way to elucidate habitat variation and distin- Red-winged Blackbirds Agelaiusphoeniceus
(. )
guish factors important in habitat selection is in two experimentally manipulated restoration
by comparing sites that differ explicitly in sites offloodplain oak Quercus spp.) near the
(
terms of vegetation management. For exam- Missouri River. Numerous researchers have
ple, Shochat et al. (2005), Wood et al. (2004), investigated habitat selection by Red-winged
and Murkin et al. (1997) evaluated avian re- Blackbirds (Albers 1978, Joyner 1978, Pribill
sponses among plots that varied with respect and Pieman 1997, Turner and McCarthy
to management regime, and were able to make 1998), and some have examined responses of
clear inferences that may have been obscured Red-winged Blackbirds and other species
had they studied only unmanaged habitats. within plots characterized by differing man-
Even where variation among plots is made agement regimes (Herkert 1994, McCoy et al.
explicit, however, the influences of vegetative 2001, LaPointe et al. 2003); however, our
study is the only one we know of in which
1 Dept, of Forestry, 203 Natural Resources Bldg., more than one factor varied (i.e., perch avail-
Univ. ofMissouri, Columbia, MO 65211-7270, USA. ability and grass cover) among adjoining
2Dept, of Fisheries and Wildlife Sciences, Natural treatment plots within the same sites. These
MO
Resources Bldg., Univ. of Missouri, Columbia, plots varied with respect to densities ofplant-
65211-7270, USA.
3Current address: P.O. Box 7021, Columbia, MO ed trees, which blackbirds used as perches,
65205-7021, USA. and the presence or absence ofa planted cover
4Corresponding author; e-mail: crop. Typically, managed plots in other song-
[email protected] bird studies have been geographically sepa-
391
392 THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 118, No. 3, September2006
rated (Herkert 1994, Swengel 1996, McCoy et Both study sites encompassed three 16.2-
al. 2001); however, our plots shared common ha, adjacent experimental treatments (hereaf-
boundaries to allow comparisons ofhabitat se- ter, “blocks”) that differed with—respect to
lection without the confounding effects ofbe- vegetation tr—eatments. The blocks formerly
tween-site variation. row-cropped were established in 1999 for an
We specifically wished to determine (1) ongoing research project to evaluate the res-
how the availability ofperches and vegetation toration of hard mast (oak acorn; Dey et al.
determines Red-winged Blackbird territory 2003). Oaks were planted at a density of 119
use and density at the treatment scale, and (2) trees/ha (Dey et al. 2003). During our study,
how within-treatment vegetation composition halfofthe planted oaks were >1.5 m high and
and structure in territories would compare were often used as perches by Red-winged
with unused (i.e., unoccupied by Red-winged Blackbirds (MAF pers. obs.). Each site had
Blackbirds) areas. We were particularly inter- three treatment blocks with varying densities
ested in determining the importance of grass of planted and natural perches. (1) “Redtop”
cover and density, because a dense, short-stat- blocks, seeded with a uniform cover ofredtop
ure cover crop of grass (redtop, Agrostis gi- grass, were planted with saplings of swamp
gantea) planted at our sites had suppressed in- white (Quercus bicolor) and pin (Q. palustris)
vading vegetation but was unsuitab—le for nest- oaks distributed in planting units that varied
ing, whereas the common invasive Johnson- in terms of planting methods but had a uni-
grass (—Sorghum halepense), which was also form ground coverofredtop grass (fordetails,
present potentially provided a tall nesting see Dey et al. 2003). The redtop grass pro-
substrate and cover. Because blackbirds in up- duced a low, dense ground cover that largely
land settings prefer dense, tall cover (Albers suppressed invasion by other herbaceous and
1978, Bollinger 1995), we predicted that den- woody vegetation that otherwise may have
sity of blackbird territories would be greater been used as perches or nest sites by Red-
in treatments not planted with redtop. Within winged Blackbirds; thus, redtop blocks con-
treatments, we predicted that blackbird terri- tained some planted oak perches but few or
tories would be characterized by denser, taller no natural perches. (2) “No redtop” blocks
cover than unused areas. Based on previous contained the same configuration ofoak plant-
studies establishing the importance ofperches ings described above for redtop blocks, but
(Joyner 1978, Payne et al. 1998), we predicted they were not seeded with a ground cover;
that densities of Red-winged Blackbird terri- therefore, over time they contained taller,
tories would be greater in treatments planted denser shrubs, trees, and herbaceous vegeta-
with oaks, and that territories would have tion and more “natural” unplanted perches
perches located at greater heights and at great- than redtop blocks. (3) “Control” blocks con-
er densities than unused areas. tained only natural perches, such as invading
forbs and shrubs, and no oak plantings or any
METHODS
of the vegetation treatments listed above.
— —
Study site. Our research was conducted in Delineation of breeding territories. We
central Missouri at two sites located within the identified breeding territories from March to
Missouri River Floodplain. Plowboy Bend May in 2001 and 2002 by monitoring male
Conservation Area (38°48' 5" N, 92° 24' 17" Red-winged Blackbirds exhibiting mating be-
W), a landscape dominated by row-crop ag- haviors, such as the “song spread” (Yasukawa
riculture, is located west of the Missouri Riv- and Searcy 1995) and territory defense. Tode-
er’s main channel within a levee-protected lineate territories, we conducted consecutive
floodplain. Smoky Waters Conservation Area flushing (Wiens 1969), a technique in which
(38° 35' 9" N, 91° 58' 3"W) is located 72 km males are approached and followed until they
southeast ofPlowboy Bend, between the Mis- alight on the perches that define theirterritory.
souri River’s main channel and the Osage Riv- Territories were delineated by identifying and
er. Smoky Waters’ floodplain has not been flagging at least four perches used consecu-
protected since a levee was breached there in tively by each male (mean number ofperches
the 1993 and 1995 floods; thus, it is subject flagged/territory = 7.12 ± 1.—97 SD).
to occasional flooding. Vegetation measurements. Once a breed-
Furey and Burhans • RED-WINGED BLACKBIRD TERRITORY SELECTION 393
ing territory was completely flagged, we re- m increments for each vegetation type of in-
corded the location, species, and height (m) terest (woody, forb, grass, and total vertical
for each perch. We established two 1-m-wide vegetation). Of the vertical vegetation mea-
belt transects in each territory to estimate den- surements, we included only mean total ver-
sity of potential perches (no. stems >1.5 m tical cover, mean vertical grass cover, and
tall/m2 and determined average maximum mean grass height, which was defined as the
)
stem height (m). To establish the first transect, last-recorded increment having grass cover on
the center ofthe territory was visually located the vertical density board. We reasoned that
and staked; then a random azimuth was de- mean total vertical cover was important if
termined to establish the direction ofthe tran- blackbirds were assessing territories based on
sect across the territory. The second transect cover without regard to vegetation type. We
location was established perpendicular to the examined grass cover and height because of
first. Using a 1-m stick held horizontally at the apparent differences in grass cover be-
1.5 m above ground, we walked the territory tween redtop blocks and the otherblock types.
end-to-end along each transect, recording the We also used the vertical vegetation mea-
number of stem contacts and the maximum surements to create a variable called “thresh-
vegetation height (m) at 1-m intervals. Two old nest-cover height,” defined as the lowest
vertical density-board measurements were height at which mean total vertical cover
taken at random locations along each transect, (based on the density board samples) was
resulting in four individual measurements of >60%. The latter value was based upon a
vertical vegetation structure for each breeding 2001 sample of vegetation measured (using
territory. The proportion of vertical vegetation the same vertical density board methodology
was estimated using a 9-increment density described above) at 99 Red-winged Blackbird
board (2.25 m tall X 0.25 m wide). At each nests. At the 99 nests, we determined that the
0.25-m increment, we estimated the proportion mean total vertical cover at nest height
m
of living and dead vegetation from a distance (viewed 15 from the board) was 60%;
of 15 m. We estimated the proportion in each therefore, we assumed that blackbirds select
increment for woody, forb (herbaceous), and nest sites with at least 60% total verticalcover.
grass vegetation and combined them to gener- Typically, total vertical cover approached
ate an estimate of mean total proportion. 100% nearthe ground, but decreased with dis-
We randomly located unused plots (unoc- tance above ground; thus, a high value of
cupied by Red-winged Blackbirds) by using a threshold nest-cover height (i.e., >60%) usu-
UTM
100-m interval grid of (UniversalTrans- ally indicated denser cover below the thresh-
mercator) coordinates placed over the resto- old height, but less cover above. High values
ration sites where there were no active terri- of threshold nest-cover height do not indicate
tories. Sampling of vegetation structure was that vertical cover was denser; rather, they in-
identical to that conducted within blackbird dicate that the vertical height at which cover
territories, with the exception thatbelt-transect equaled or exceeded 60% was greater.
length within a given site was based on the We used a general linear model (PROC
average belt-transect length of all breeding MIXED; SAS Institute, Inc. 2003) to test for
territories found at the—site. differences in territory density among block
Statistical analyses. For each year, we cal- types. We nested block within site as a ran-
culated territory density for each block type dom effect to account for differences in site,
(redtop, no redtop, control) by summing the and included “year” in the model to account
numbers of territories found in each block for additional variation. We used the likeli-
type and dividing by 16.2 ha. If a territory hood ratio test to test the overall model
straddled more than one block type, we placed against a null model that included only the
it in the block type in which the majority of intercept. Ifthe overall model was significant,
its area occurred. we used the LSMEANS statement to examine
We averaged vegetation variables for the whether territory densities varied among the
four samples taken within each blackbird ter- three block types (control, no redtop, redtop);
ritory. For vertical vegetation measurements, we considered differences at P < 0.05 to be
the mean was calculated from all of the 0.25- significant.
n n n
394 THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 118, No. 3, September2006
We analyzed vegetation differences among years. Mean breeding territory area in 2001
block types, by site, using PROC MIXED was 1,667 ± 195 m2 (n = 19), 1,897 ± 221
models as above, again using likelihood ratio m2 ( = 17), and 2,310 ± 464 m2 (n = 10)
tests to compare models against a null model. in redtop, no redtop, and control blocks, re-
Because there were a large number of vege- spectively, and in 2002 it was 1,648 ± 173
ctaotuilodn vbaerisaibglneisf,icfaonrtwbhyicchhoanncee,orwseeveursaeldtetshtes 7m721(±n8=31m4)2,(1,=8048).±We26f9oumn2d(nno=dif1f7e)r,enacneds
sequential Bonferroni method to interpret in territory areaby block type (likelihoodratio
overall model significance (Rice 1989). Al- test: x2 = 2.3, df = 3, P = 0.51). In 2001,
though the sequential Bonferroni test has been mean territory density across both sites was
criticized as overly conservative in circum- 0.71 ± 0.74, 0.67 ± 0.26, and 0.31 ± 0.26
stances where numerous individual tests show territories/ha in redtop, no redtop, and control
moderately significant results (Moran 2003), blocks, respectively. In 2002, mean territory
in this circumstance we feel that it was a suit- density across both sites was 0.46 ± 0.66,
able compromise between having no control 0.56 ± 0.17, and 0.12 ± 0.17 territories/ha;
for type I error and the simple Bonferroni test, there were no blackbird territories in redtop or
which is even more conservative (Rice 1989). control blocks at Plowboy Bend during this
If the overall model was significant, we used year. Territory density did not differ among
the LSMEANS statement to determine wheth- blocks or years (likelihood ratio test: x2 — 5.8,
er territory area and vegetation variables var- df = 3, P = 0.12).
ied among the three block types (control, no We did not find differences among the three
redtop, redtop), by site; within each model, we block types formean perch density, meantotal
considered differences at adjusted P < 0.05 to vertical cover, or mean vertical grass cover
be significant. (Fig. 1A, C, E). The model for mean perch
We also compared parameters ofvegetation height differed significantly from the null
structure between areas occupied (“territo- model (x2 = 39.0, df = 3, adj. P < 0.001),
ries”) and unoccupied (“unused” plots) by but the differences were among years (2001:
Red-winged Blackbirds to describe local veg- 2.16 ± 0.03 m; 2002: 1.82 ± 0.04 m; t =
etation differences affecting blackbird habitat 6.73, df = 74, P < 0.001); there were no dif-
selection within blocks. Because flooding ferences in perch height among blocks (Fig.
events in 2001 prevented us from sampling IB). Similarly, models for mean threshold
unused plots at both sites, only 2002 field data nest-cover height and grass height differed
were used for this analysis, and we removed from null models, but again differences were
territory and unused samples entirely if any among years rather than blocks (threshold
data values were missing. We used a general nest-cover height model: overall x2 = 17.0, df
linear model (PROC MIXED; SAS Institute, = 3, adj. P < 0.01; mean grass height model:
Inc. 2003) with an LSMEANS statement to overall x2 — 28.6, df = 2, adj. P < 0.008; Fig.
calculate means and standard errors for each ID, F). Mean grass height across all territory
variable of interest. We determined that there blocks was greater in 2001 (2001: 0.53 ± 0.02
were differences among territories and unused m; 2002: 0.36 ± 0.02 m; t = 5.53, df = 74,
plots if likelihood ratio tests indicated overall P < 0.001), whereas mean threshold nest-cov-
model significance, based on sequential Bon- er height was shorter in 2001 (2001: 0.40 ±
ferroni adjustments for the six vegetation var- 0.06 m; 2002: 0.63 ± 0.06 m; t = -4.30, df
iables analyzed. If the overall model was sig- = 74, P < 0.001).
nificant, we evaluated multiple comparisons We used samples from 35 Red-winged
among different combinations of block, terri- Blackbird breeding territories and 35 unused
tory, and unused plots (15 comparisons per plots (2002 data only) to compare vegetation
model) with sequential Bonferroni tests to in breeding territories with that in unused
control for type I error. plots ( = 10, 13, and 12 unused plots sam-
pled from both sites combined in redtop, no
RESULTS
redtop, and control blocks, respectively).
We analyzed 81 Red-winged Blackbird Models testing for differences between terri-
breeding territories across both sites and tories and unused plots did not differ from
Furey and Burhans • RED-WINGED BLACKBIRD TERRITORY SELECTION 395
FIG. 1. Vegetation cover (expressed as a proportion), height, and perch density comparisons (±SE) among
treatment blocks at Plowboy Bend and Smoky Waters Conservation Areas, Missouri, 2001-2002.
null models with respect to perch height (Fig. Overall, mean vertical grass cover varied
2B), mean total vertical cover (Fig. 2C), or among combinations of block and territory or
threshold nest-cover height (Fig. 2D). Overall, unused plots (x2 = 21.5, df = 5, adj. P <
mean perch density varied among combina- 0.01). Grass cover was greater in no redtop
tions ofblock and territory or unused plots (x2 territories compared with no redtop unused
= 28.5, df = 4, adj. P < 0.008; Fig. 2A). plots, control unused plots, and redtop terri-
Territories in control blocks had greater perch tories and unused plots (all adj. P ^ 0.004;
densities than in all other block types, al- Fig. 2E). Grass height varied overall among
though there were only four control territories combinations ofblock and territory or unused
in the analysis (all adj. P < 0.005; Fig. 2A). plots (x2 = 15.4, df = 5, adj. P < 0.01). Grass
Perch densities did not differ between other height was greater in no redtop territories than
combinations ofblock and territory or unused in redtop, no redtop, and control unused plots
plots, except that perch densities were greater (all adj. P < 0.004; Fig. 2F).
in no redtop territories than they were in red-
DISCUSSION
top territories and redtop unused plots (no red-
top territories versus redtop territories: t = We found no significant differences in ter-
3.42, df = 61, adj. P < 0.005; no redtop ter- ritory density or area among treatment blocks,
ritories versus redtop unused plots: t = 3.01, nor did we find differences among vegetation
df = 61, adj. P < 0.006). variables by territory treatment block when
396 THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 118, No. 3, September2006
2.5
Control Noredtop Redtop
Control Noredtop Redtop Control Noredtop Redtop
FIG. 2. Vegetation cover (expressed as a proportion), height, and perch density comparisons (±SE) ofRed-
winged Blackbird territories (used) and unused plots at Plowboy Bend and Smoky Waters Conservation Areas,
Missouri, 2002.
2001 and 2002 data were combined. We did tories, perch density could have been an arti-
find differences, however, between territories fact of small sample size, as there were only
and unused plots; generally, blackbird terri- 4 territories in control blocks in 2002 com-
tories were characterized by denser or taller pared to 10 in 2001. However, upon visual
grass cover than unused plots, and territories inspection, we detected similar between-year
in control and no redtop blocks tended to con- differences in mean perch density in redtop
tain more perches than unused plots. blocks (Fig. 1A versus 2A), and in this case
In the analysis of territories versus unused sample sizes were 19 and 14 in redtop terri-
plots, the greater perch density in territory tories in 2001 and 2002, respectively. Such
blocks with no cover crop (no redtop and con- inter-annual inconsistencies in bird-vegetation
trol blocks) compared with those that had a relationships are common and often prevent
cover crop (redtop) may be a reflection ofred- researchers from reaching direct conclusions
top’s ability to suppress invasion by trees and in studies ofavian-habitat associations (Riffell
shrubs. However, perch density did not differ et al. 2001), including studies of Red-winged
among territory blocks or years when data Blackbirds (Erckmann et al. 1990) and other
from both years were combined (Fig. 1A), blackbirds (Orians and Wittenberger 1991).
whereas the territory/unused analysis, which Red-winged Blackbirds may require only a
included only 2002 data, revealedextreme dif- few perches for territory defense. We noted
ferences in perch density among territory that blackbirds typically reused the same
blocks (Fig. 2A). In the case of control terri- perches, sometimes frequenting only four or
Furey and Burhans • RED-WINGED BLACKBIRD TERRITORY SELECTION 397
five perches repeatedly (MAP pers. obs.). It The pattern ofdenser and taller grass cover
may be that perch availability limits blackbird in territories, especially in no redtop blocks,
territory settlement only at some lowerthresh- generally agrees with other findings in studies
old, in which case even territories with very of Red-winged Blackbirds. Bollinger (1995)
low perch densities at our sites (e.g., redtop; believed that blackbirds occupied his upland
Fig. 2A) may have met this requirement. sites due to the availability of suitable nest
Perches have been shown not to limit habitat cover and vegetation with stems strong
use by some songbirds (Vickery and Hunter enough to support their nests; results of other
1995), but at leastone study suggests that they studies also indicate that, where stout plants
are necessary for Red-winged Blackbirds; are available, blackbirds choose them as nest
Joyner (1978) found that even in areas with a sites or for territorial activity (Albers 1978,
preferred grass cover type, blackbir—ds did not Joyner 1978, Turner and McCarthy 1998, Ko-
establis—h territories if fence posts used as bal et al. 1999). Bollinger (1995) found a pos-
perches were totally lacking. itive relationship between presence of grass
In addition to variation in perch density, we and blackbirds, and Camp and Best (1994)
also found differences in grass cover and found a positive relationship between grass
height between territories and unused plots cover and nest densities. Other studies have
within and among treatment blocks. Variable shown that Red-winged Blackbirds favor
grass cover, at least within no redtop blocks, dense vegetation (LaPointe et al. 2003); Al-
suggests that blackbirds may have settled in a bers (1978) found that blackbird territories
non-uniform fashion with regard to grass had significantly taller, denser vegetation than
patches. Although our data did not permit us unused areas, and Bollinger (1995) found that
to relate territories to grass patchiness spatial- Red-winged Blackbirds were most abundant
ly, overall we did not notice obvious patterns in fields with dense cover. However, in a sur-
in territory settlement; there were twopossible vey of Illinois grassland species, Herkert
exceptions; (1) the only two blackbird terri- (1994) found no correlates ofvegetation struc-
tories in the Plowboy Bend redtop block were ture and occupancy by Red-winged Black-
very close to blackbird territories on the ad- birds, which were present on 93% ofhis tran-
joining no redtop block, from which forbs, sects, and Scott et al. (2002) found that black-
shrubs, and Johnsongrass had spread into the birds were negatively associated with grass
redtop block (MAF pers. obs.); and (2) black- cover on reclaimed surface mines in Indiana.
birds tended to avoid settlement along one Although our 2002 data revealed differenc-
edge ofthe Smoky Water control block (MAF es in perch density when comparing territories
pers. obs.). In the second case, we are not sure with unused plots, our results suggest that
why blackbirds avoided the block edge, but perch density does not influence Red-winged
we believe that settlement in redtop at Plow- Blackbird territory selection as long as perch
boy Bend may have been influenced both by density is above some lower limit. However,
the rampant growth of Johnsongrass and by particularly in no redtop blocks, blackbirds
redtop’s ability to suppress Johnsongrass and tended to choose territories that had denser,
other vegetation. Redtop cover was particu- taller grass cover than that observed in unused
larly uniform at Plowboy Bend, where black- plots. This finding is in agreement with other
bird use of the redtop block was minimal, studies, which have shown that Red-winged
whereas the redtop block at Smoky Waters un- Blackbirds appear to favor dense vegetation
derwent extensive invasion of shrubs and (Albers 1978, Kobal et al. 1999), including
forbs (MAF and DEB pers. obs.). Johnson- tall or dense grass cover (Camp and Best
grass, a dense, stout-stemmed grass thatgrows 1994, Bollinger 1995).
m
to 1.8 high, was also used as a nesting sub-
strate, whereas redtop was not. Of more than ACKNOWLEDGMENTS
250 Red-winged Blackbird nests found from
2001-2003, none were anchored in redtop This work was funded through the University of
Missouri’s Center for Agroforestry under cooperative
grass, whereas Johnsongrass was among the agreements 58-6227-1-004 with the Agricultural Re-
five most commonly used nest substrates search Service and C R 826704-01-2 with the U.S.
(DEB unpubl. data). Environmental Protection Agency (EPA). The results
398 THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 118, No. 3, September2006
presented are the sole responsibility of the Principal in Missouri. American Midland Naturalist 145:1-
Investigators and/or University of Missouri and may 17.
notrepresentthepolicies orpositionsoftheEPA. Any Moran, M. D. 2003. Arguments for rejecting the se-
opinions, findings, conclusions, or recommendations quential Bonferroni in ecological studies. Oikos
expressed in this publication are those ofthe author(s) 100:403-405.
and do not necessarily reflect the view of the U.S. Murkin, H. R., E. J. Murkin, and J. P. Ball. 1997.
Department of Agriculture. Equipment and computer Avian habitat selection and prairie wetland dy-
resources support were provided courtesy of Univer- namics: a 10-year experiment. Ecological Appli-
sity of Missouri-Columbia’s Quantitative Silviculture cations 7:1144-1159.
Laboratory and the U.S. Department of Agriculture Orians, G. H. and J. F. Wittenberger. 1991. Spatial
Forest Service North Central Research Station. We andtemporal scales in habitatselection. American
thank F. R. Thompson for statistical advice and M. A. Naturalist 137:S29-S49.
Gold and D. C. Dey for support and professionalcon- Payne, N. E, S. N. Kobal, and D. R. Ludwig. 1998.
siderations. Comments by G. H. Orians, K. Yasukawa, Perch use by 7 grassland bird species in northern
B. B. Steele, P. D. Vickery, andan anonymousreview- Illinois. Transactions of the Illinois State Acade-
ergreatly improved earlierversions ofthe manuscript. my of Science 91:77-83.
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