Reclaimed Coal Mine Grasslands and Their Significance for Henslow's Sparrows in the American Midwest

Abstract

Present methods of surface coal-mine reclamation in the Midwest produce large grasslands, some of which exceed 2,000 ha in extent. Total “mine grassland” production in southwestern Indiana alone is well in excess of 70 square miles (180 km2). Our work in 19 reclaimed coal mines in southwestern Indiana indicates that mine grasslands harbor many Henslow's Sparrows (Ammodramus henslowii). We base that conclusion on point-count and line-transect surveys that yielded between 200–300 singing male Henslow's Sparrows during the 1997 and 1998 breeding seasons. Those survey results imply an uncorrected population density of ∼0.10 males per hectare, and a corrected density of ∼0.16 males per hectare (correcting for undetected males). Extrapolating this corrected density to total habitat coverage suggests an overall population of a few thousand Henslow's Sparrows in the mine grasslands of southwestern Indiana. Small-scale vegetational surveys suggest that much of the within-mine variation in Henslow's Sparrow abundance reflects local vegetative structure, with males preferring sites typically associated with that species of bird: tall, dense grass-dominated vegetation with a substantial litter layer. Management for this kind of vegetative structure could greatly increase the number of Henslow's Sparrows inhabiting reclaimed mines. Midwestern mine grasslands could play a significant role in stabilizing the populations of Henslow's Sparrows and other grassland birds.

Full text

422
The Auk 118(2):422–431, 2001
RECLAIMED COAL MINE GRASSLANDS AND THEIR SIGNIFICANCE
FOR HENSLOW’S SPARROWS IN THE AMERICAN MIDWEST
R
OBB
A. B
AJEMA
,
1
T
RAVIS
L. D
E
V
AULT
,
2
P
ETER
E. S
COTT
,
AND
S
TEVEN
L. L
IMA
3
Department of Life Sciences, Indiana State University, Terre Haute, Indiana 47809, USA
A
BSTRACT
.—Present methods of surface coal-mine reclamation in the Midwest produce
large grasslands, some of which exceed 2,000 ha in extent. Total ‘‘mine grassland’’ produc-
tion in southwestern Indiana alone is well in excess of 70 square miles (180 km
2
). Our work
in 19 reclaimed coal mines in southwestern Indiana indicates that mine grasslands harbor
many Henslow’s Sparrows (Ammodramus henslowii). We base that conclusion on point-count
and line-transect surveys that yielded between 200–300 singing male Henslow’s Sparrows
during the 1997 and 1998 breeding seasons. Those survey results imply an uncorrected pop-
ulation density of ;0.10 males per hectare, and a corrected density of ;0.16 males per hect-
are (correcting for undetected males). Extrapolating this corrected density to total habitat
coverage suggests an overall population of a few thousand Henslow’s Sparrows in the mine
grasslands of southwestern Indiana. Small-scale vegetational surveys suggest that much of
the within-mine variation in Henslow’s Sparrow abundance reflects local vegetative struc-
ture, with males preferring sites typically associated with that species of bird: tall, dense
grass-dominated vegetation with a substantial litter layer. Management for this kind of veg-
etative structure could greatly increase the number of Henslow’s Sparrows inhabiting re-
claimed mines. Midwestern mine grasslands could play a significant role in stabilizing the
populations of Henslow’s Sparrows and other grassland birds. Received 23 August 1999, ac-
cepted 5 February 2001.
F
EW LARGE POPULATIONS
of Henslow’s Spar-
rows (Ammodramus henslowii) are known to ex-
ist, especially east of the Mississippi River
(Pruitt 1996). Our study, however, indicates
that reclaimed surface coal-mine grasslands in
southwestern Indiana harbor such populations
of Henslow’s Sparrows. Grassland birds have
been recorded in the reclaimed surface mines
of Appalachia (Whitmore and Hall 1978, Whit-
more 1980, Allaire 1981, Wray et al. 1982), in-
cluding Henslow’s Sparrows (Peterjohn and
Rice 1991; see also Koford 1999), but the con-
servation potential of mine grasslands in gen-
eral is far from being completely evaluated. In
fact, it appears that large mine grasslands of
the Midwest have gone virtually unnoticed by
avian biologists. Hence, our overall goal was to
evaluate those midwestern mines as potential
habitat for Henslow’s Sparrows, and in doing
1
Present address: Department of Biology, Aquinas
College, Grand Rapids, Michigan 49506, USA.
2
Present address: Department of Forestry and
Natural Resources, Purdue University, West Lafay-
ette, Indiana 47907, USA.
3
Address correspondence to this author. E-mail:
lslima@scifac.indstate.edu
so, derive basic estimates of population density
and size.
The potential importance of mine grasslands
for Henslow’s Sparrows reflects the fact that
midwestern grasslands (native or otherwise)
have declined precipitously in the last 150
years (Samson and Knopf 1994, Warner 1994,
Noss et al. 1995). Against that backdrop of
grassland destruction, the Henslow’s Sparrow
has declined perhaps most significantly of any
grassland bird (Herkert 1995), and more than
most forest-dwelling Neotropical migrants
(Knopf 1994, Peterjohn et al. 1994, Herkert
1995). Although several factors may contribute
to that overall population decline, such as state
of wintering habitat (Plentovich et al. 1999),
disappearance of suitable breeding habitat has
probably been the most critical determinant of
the decline of this species (Askins 1993). That
decline can readily be understood in terms of
the nature of suitable breeding habitat for this
species: large, undisturbed grasslands (Herkert
1994b, Walk and Warner 1999) with relatively
little woody vegetation and a significant litter
layer (Zimmerman 1988; see Herkert 1998).
Such habitat is scarce in the Midwest, where
the Henslow’s Sparrow’s breeding range is con-
centrated (Pruitt 1996, Herkert 1998).
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April 2001] 423Coal Mines and Henslow’s Sparrows
Ironically, surface coal mining in the Mid-
west is a form of environmental disturbance
that produces large blocks of grassland habitat.
Those mine grasslands are found mainly in the
Illinois coal basin (portions of Indiana, Illinois,
and Kentucky) and portions of Ohio, and rep-
resent the main result of mine reclamation
(Brothers 1990). Newly reclaimed mines are
typically stocked with quick growing, cool-sea-
son grasses such as tall fescue (Festuca arundi-
nacea), orchard grass (Dactylis glomerata), and
smooth brome (Bromus inermis), as well as var-
ious legumes such as clovers (Melilotus spp.)
and alfalfa (Medicago sativa), to establish a veg-
etative structure and minimize soil erosion
(Brothers 1990). Tree planting and crop pro-
duction may then follow, but often in only a rel-
atively small portion of the reclaimed area; the
bulk of a reclaimed mine usually remains as
grassland (Brothers 1990). Furthermore, soil
conditions and isolation from forested habitat
(Hardt and Forman 1989) often inhibit woody
plant invasion, allowing those grasslands to
persist for many years with no management.
Several aspects of midwestern mine grass-
lands make them particularly attractive for the
conservation of grassland birds in general.
First, most midwestern mine grasslands exceed
the apparent size thresholds for even the most
area-sensitive grassland species (Samson 1980,
Herkert 1994a, Vickery et al. 1994, Bollinger
1995, Walk and Warner 1999, Winter and Faa-
borg 1999; see also Horn et al. 2000). Second,
although reclaimed mines are not always man-
aged as grasslands, soil conditions may leave
them largely unsuitable for other purposes. Fi-
nally, entire reclaimed mines are often owned
by a single entity, which makes long-term con-
servation of those large grasslands more feasi-
ble. In short, midwestern mine grasslands pro-
vide a unique opportunity for the large-scale,
diverse grassland management approach to
conservation of grassland birds (Herkert et al.
1996, Sample and Mossman 1997).
M
ETHODS
Establishing study sites. Mine grasslands were lo-
cated using satellite imagery and a Geographic In-
formation System (GIS) (ArcInfo) as described in Ba-
jema and Lima (2001). After locating a given mine
grassland, we made contact with the owners, and
then inspected the mine to determine whether it con-
tained suitable Henslow’s Sparrow habitat. Such
habitat consists of relatively dense, undisturbed
grass-dominated vegetation with a significant litter
layer (reviewed by Herkert 1998). If a mine had a sig-
nificant extent of such vegetation, we established
fixed survey routes. Only very recently reclaimed
mines, or a few small mine grasslands converted to
cattle grazing operations, lacked such habitat (Baje-
ma and Lima 2001). We note that our working defi-
nition of ‘‘suitable habitat’’ was meant to be broadly
inclusive so as to not exclude any potential Hen-
slow’s Sparrow habitat. We present a more refined
perspective on habitat suitability below.
Survey methods. We used two survey methods: 5
min, unlimited distance point counts, and unlimited
distance line transects (see Bibby et al. 1992, Ralph
et al. 1993). Point counts were performed in afashion
similar to the Breeding Bird Survey (Price et al. 1995)
and took advantage of the gravel-roadnetworkwith-
in most reclaimed mines. A survey route was estab-
lished by stopping every 0.5 km along mine roads in
suitable Henslow’s Sparrow habitat. That distance
between point counts eliminated the risk of census-
ing the same sparrows twice although still allowed
adequate coverage of an area. During a point count,
all Henslow’s Sparrows detected by song were re-
corded, as was the time of day. Location of the point
count itself was recorded with the use of Garmint
Global Positioning System units (10–30 m accuracy).
A total of 249 points counts were established in 1997,
with 248 points established in 1998; most points
(.98%) were in the same locations both years.
Line transects were conducted in the larger ‘‘road-
less’’ units of Henslow’s Sparrow habitat. Transects
began at least 200 m from the nearest point-countlo-
cation, and were walked slowly at ;2 km per hour.
Exact shape and length of a given transect were dic-
tated by terrain and extent of suitable habitat, but
most traversed a rectangular pattern in a large field.
GPS units were used to map the location of each
transect and Henslow’s Sparrow detections. Overall,
a total of 36 transects (average length 51,220 m,
range 300–4,200 m) covering 44.0 km were estab-
lished in 1997, with 39 transects covering 45.5 km in
1998 (average length 51,170 m, range 300–4,200 m);
enhanced access to some mines allowed us to estab-
lish a few new transects in 1998, otherwise all tran-
sects were in the same locations across years.
We conducted our surveys from mid-May through
mid-July during both 1997 and 1998 breeding sea-
sons, which largely ensured that all detected Hen-
slow’s Sparrows were on established breeding terri-
tories (Herkert 1998). During 1998, we performed
three complete rounds of surveys(Round1,12May–
6 June; Round 2, 28 May–23 June; Round 3, 23 June–
10 July). A single survey round, spread over the en-
tire breeding season, was performed during 1997
(mid-May to mid-July). We confined our daily sur-
veystoa5hperiodbeginning 0.5 h before sunrise,
the period during which Henslow’s Sparrows sing
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424 [Auk, Vol. 118B
AJEMA ET AL
.
most actively (Heller and Hughes 1997, Koford 1999).
Survey work was postponed during inclement
weather (high wind and rain). Surveys were con-
ducted by a crew of five experienced observers dur-
ing each breeding season.
We also surveyed pasture and hay-field habitats.
These areas constitute under 15% of most reclaimed
mines, but as much as 45% in two mines (Bajema and
Lima 2001). We confined these surveys to fields that
were intensively managed for hay but uncut at the
time of censusing, and areas that were actively
grazed by cattle. We used roadside (5 min) point
counts only, with 50 and 54 points established in
1997 and 1998, respectively. As before, three survey
rounds were completed in 1998, with a single survey
round in 1997.
Singing behavior, correction factors, and detection dis-
tances. Observations made during extended visits
to specific point-count locations (in 1997) strongly
suggested that our surveys were missing many non-
singing male Henslow’s Sparrows. At one time, there
may be three or four males singing at a site, whereas
several minutes later only one or two may be singing.
Point counts significantly longer than 5 min could
have detected many of those missed males (see also
Gutzwiller 1991, McShea and Rappole 1997), but lon-
ger counts were not feasible given logistical con-
straints imposed by remoteness of many mines and
amount of territory that we surveyed. Instead we de-
vised a correction factor for missed males.
Correction factors are implied in the basic theory
of point counts (e.g. Barker et al. 1993), although ex-
amples of their use in estimating avian abundance
are few. Our particular correction factor involved de-
termining the proportion of time (s) that the typical
male will sing at least once during a 5 min interval
(the duration of an individual point count). With s
determined, the true number of males (M) at the typ-
ical point-count location is related to the number of
males detected at that location (m)bym5sM, which
rearranged to M5m/s. In other words, the factor
correcting for missed males is 1/s. Robb et al. (1998)
describe an analogous correction factor applicable to
spot-mapping surveys for Henslow’s Sparrows.
To estimate the value of s, we chose 25 count lo-
cations that were known to harbor Henslow’s Spar-
rows and recorded whether or not a given male sang
at least once during each of 12 consecutive 5 min pe-
riods (1 h). The proportion of these 12 periods during
which a male sang is an estimate of s. We first esti-
mated son a per-site basis by combining data from
all males detected at a given site. Because the number
of males detected during a given1hobservationpe-
riod was not correlated significantly with the site-
specific estimate of s(Spearman rank correlations
ranging between 20.04 and 0.10, P.0.6), we aver-
aged site-specific svalues to determine our overall
estimate of s. During 1998, we visited ‘‘singingsites’’
at the end of each survey round. We thus determined
svalues for each round of censusing. During 1997,
we quantified singing behavior mainly in the middle
of the breeding season, corresponding to the mid-
point of our single round of censusing. During both
years, hour-long observation periods were evenly
distributed throughout the 5 h morning surveying
period.
We were able to make visual contact with almost
all singing males, hence monitoring multiple indi-
viduals was feasible. We nevertheless excluded data
from a given male if there was any confusion about
its identity during the 1 h observation period. We
also excluded data taken from males so distant that
we could not reliably detect each (brief) act of sing-
ing. Following the 1 h observation period, we mea-
sured distances to singing perches of all detected
males. We did this at about 50% of observation sites.
Vegetational mapping and surveys. Large-scale pat-
terns in vegetative cover within each mine were
mapped with the aid of recent, large-format aerial
photographs obtained from the controlling mining
companies. On-the-ground observers used those ae-
rial photographs to record the following habitat
types: suitable Henslow’s Sparrow habitat (as de-
fined earlier), grazed grassland, intensively hayed
grassland, forest, shrubland (presence of several
young trees ,10 cm diameter at breast height
[DBH]), open water, and barren ground (usually ac-
tively mined areas). The resulting maps were then
scanned into the GIS, which was then used to deter-
mine the coverage of general grassland habitat and
suitable Henslow’s Sparrow habitat (see Bajema and
Lima 2001 for details). That GIS also contained the
locations of all transects and point counts.
During the 1998 field season, we characterized
vegetative patterns on a finer scale withinapparently
suitable Henslow’s Sparrow habitat. Our goal was to
determine which vegetative components of that hab-
itat (if any) were most closely associatedwith site oc-
cupation by Henslow’s Sparrows (see also DeVault
1999, T. DeVault et al. unpubl. data). To accomplish
that goal we chose, at a given mine, the 5–10 most
populated point-count locations and an equal num-
ber of locations at which Henslow’s Sparrows were
apparently absent. We did that within mines large
enough to provide an adequate sample of such
points. Overall, we sampled vegetation at 104 point-
count locations.
We characterized each of those 104 locations by
measuring vegetation at 6 positions along a transect
perpendicular to the road, starting 20 m off the road,
with 20 m spacing between positions (see Millenbah
et al. 1996). At each sampling position, we measured
visual obstruction (a measurement of horizontal cov-
er, in decimeters; Robel et al. 1970) and the maxi-
mum height of vegetation using a Robel pole. We
then estimated overall percentage cover of grasses
and forbs by species, as well as the percentage cover
of dead litter and bare soil as per Daubenmire (1959)
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April 2001] 425Coal Mines and Henslow’s Sparrows
T
ABLE
1. Locations, ownership, and characteristics of the 19 reclaimed surface-mine study sites in south-
western Indiana.
Mine Ownership
b
County
c
Grassland (ha)
a
Overall Suitable
Universal
Snow Hill
Centerpoint
Saline City
Chinook
Cass
Dugger
Hymera
Minnehaha
Hillenbrand 1
d
Hillenbrand 2
d
Phoenix
Alford
Cup Creek
Petersburg
Westfield
Ayrshire
Lynnville
Squaw Creek
Peabody, private
Peabody
Private, County
Private
Midwest, IDNR
Kindill
IDNR
Private
Kindill, IDNR
IDNR
IDNR
Black Beauty
Kindill
Private
Kindill
Kindill
Amax, IDNR, priv.
Peabody
Peabody
Vermillion
Vigo
Clay
Clay
Clay
Sullivan
Sullivan
Sullivan
Sullivan
Greene
Greene
Daviess
Pike
Pike
Pike
Pike
Warrick
Warrick
Warrick
2,630
270
150
220
1,580
350
550
180
1,040
440
200
670
590
110
730
1,040
3,180
1,830
1,000
1,140
270
110
190
1,220
310
440
170
890
400
200
550
560
100
680
810
1,060
1,500
900
Totals 16,760 11,500
a
‘‘Overall’’ grassland habitat covers all grassland types; ‘‘suitable’’ habitat refers specifically to Henslow’s Sparrow habitat.
b
Amax 5Amax Coal Company; Black Beauty 5Black Beauty Coal Company; Kindill 5Kindill Mining, Inc.; IDNR5controlled or managed
by the Indiana Department of Natural Resources; Midwest 5Midwest Coal Company; Peabody 5Peabody Coal Company; Private 5held in
private ownership.
c
Counties (and mines) listed from north to south.
d
The Hillenbrand Fish and Wildlife Area occurs as two geographically separate reclamation units.
and Patterson and Best (1996). All six subsamples
were averaged, for each vegetational measure, to
characterize the vegetation at a given point count.
Those average measures were compared between oc-
cupied and unoccupied points using simple pair-
wise comparisons. Individual plant species were in-
cluded in our analysis only if their canopy cover
exceeded 1%.
Statistical considerations. Parametric statistics
were used whenever appropriate, but nonparametric
statistical procedures were necessary in many in-
stances. Observations from individual point counts
and transects were treated as independent estimates
of Henslow’s Sparrow abundance, with observations
across rounds (1998 surveys) treated as repeated
measures for a given location. Singing data from in-
dividual observation points (averaged across all
males at a given location) were similarly treated as
independent estimates of singing propensity. We il-
lustrate temporal effects in Henslow’s Sparrow de-
tections using point-count data only; identical tem-
poral trends were also seen in transect data in all
cases. Means are given 6SE. All statistical analyses
were performed using the SPSS statistical package
(Norusˇis 1993).
R
ESULTS
Mine grassland characteristics. We located 19
reclaimed mine grasslands in southwestern In-
diana (Table 1) in which the apparent extent of
suitable Henslow’s Sparrow habitat warranted
establishment of survey routes. Extent of gen-
eral grassland habitat within those mines varied
from 110 to 3,180 ha (Table 1). On average, 68%
of grassland habitat was suitable for Henslow’s
Sparrows at a coarse scale of analysis. However,
relative coverage of suitable habitat varied
among mines (Table 1), largely reflecting the ex-
tent of hayed or grazed grassland. There was
very little change between years in configura-
tion and amount of suitable Henslow’s Sparrow
habitat, which summed to 11,500 ha during the
1997 and 1998 breeding seasons. Other grass-
land types such as pasture and hayfield were
similarly stable over the two-year period.
Basic survey results. Our survey work in
suitable habitat detected a total of 252 male
Henslow’s Sparrows during the 1997 breeding
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426 [Auk, Vol. 118B
AJEMA ET AL
.
T
ABLE
2. Basic (uncorrected) survey results by
round for the 1998 breeding season. Totals indicate
number of males detected using a given survey
technique for a given round. The dates of the sur-
vey rounds are: Round 1, May 12–June 6, Round 2,
May 28–June 23, Round 3, June 23–July 10.
Survey
round
Point counts
Total Males/point
Line transects
Total Males/km
Round 1
Round 2
Round 3
137
99
109
0.557 60.057
0.400 60.051
0.436 60.058
150
104
102
4.42 60.714
2.73 60.534
2.54 60.543
season (131 from point counts, 121 from tran-
sects; those numbers exclude the many addi-
tional males detected during our study of sing-
ing behavior, see below). Point counts detected
an uncorrected average of 0.478 60.052 males
per point. One or more males were detected
during 31.3% of point counts. An (uncorrected)
average of 3.04 60.780 males per kilometer
were detected during line-transect surveys.
The 1998 surveys yielded similar overall re-
sults, which are presented by census round in
Table 2 (recall that only one round was con-
ducted during 1997). There were significant
differences across survey rounds in number of
males detected per point count (Friedman re-
peated measures ANOVA on ranks, x
2
512.6,
df 52, P50.0018), owing entirely to higher
number of detections in Round 1 (pairwise
comparisons, Student-Newman-Keuls method,
P,0.05). A similar survey round effect is also
evident in the transect data (Table 2).
Number of male Henslow’s Sparrows detect-
ed tended to decline with time of day. How-
ever, there was no significant time-of-day effect
in 1997 (Spearman rank correlation, r
s
50.010,
n5249, P50.877) or in any survey round dur-
ing 1998 (20.046 #r
s
#20.020, P.0.45).
We did not detect any male Henslow’s Spar-
rows in grazed and hayed grasslands. Our sur-
vey effort in those habitats involved a cumu-
lative total of 212 point counts over the two
breeding seasons. We have nevertheless made
casual observations of a few male Henslow’s
Sparrows in hayed areas. However, we virtu-
ally never detected that species in areas with
recent or active cattle grazing.
Singing behavior, correction factors, and detec-
tion distance. Our study of singing behavior
showed that male Henslow’s Sparrows sing in-
termittently. When observed for 12 consecutive
5 min intervals, the proportion of intervals dur-
ing which the typical male sang (the value s)was
0.595 60.039 (1997 breeding season; 70 males
observed over 25 observation periods). There
was no correlation between that measure of
singing tendency and time of day (Pearson’s r5
0.279, n525, P50.177). The inverse of that av-
erage proportion (or 1/s) yields the correction
factor of 1.68, implying that the true number of
males at a site with Henslow’s Sparrows was on
average 68% greater than suggested by the ac-
tual number of males detected during 1997 sur-
veys. Very similar results were obtained in 1998
with an overall estimate of sat 0.610 (188 males
observed during 73 observation periods over 3
rounds), implying a correction factor of 1.64.
Male Henslow’s Sparrows tended to sing some-
what less as the (1998) breeding season pro-
gressed, with average estimates of sat 0.632 6
0.039, 0.608 60.034, and 0.586 60.032 for
Rounds 1–3, respectively. However, a one-way
ANCOVA, with time-of-day as the covariate, in-
dicated no significant effect of survey round (F
50.566, df 52 and 69, P50.570) or time of day
(F52.214, df 51 and 69, P50.141) on the sing-
ing tendencies of males.
The validity of our correction factor can be
assessed by determining the ratio of males de-
tected after 1 h of observation to the number of
males detected during the most recent (preced-
ing) ‘‘official’’ 5 min point count for that loca-
tion. With sestimates in excess of 0.5, the ex-
pectation of 1/smales at a given site should be
realized by the twelfth observation interval(i.e.
every male should have sung within the hour).
During 1997, the number of males detected at
a given location was on average 1.63 60.126
times that detected during the preceding offi-
cial point count; that value is not significantly
different from the expected value of 1.68 (t-test,
t520.29, df 524, P50.779). Similarly, during
1998, number of males detected increased by a
factor of 1.59 60.133, which was also not sig-
nificantly different from the expected value of
1.64 (t-test, combining data from all three seg-
ments of the breeding season; t520.30, df 5
53, P50.766).
A plot of distances to detected males (Fig. 1)
suggested that detections faded appreciably by
125 m from the observer, with only one male
detected beyond 160 m. The detection of birds
beyond 125 m required unusually good listen-
ing conditions. We thus take 125 m as our work-
ing estimate of detection radius. Note also that
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April 2001] 427Coal Mines and Henslow’s Sparrows
F
IG
. 1. Frequency histogram of detection distanc-
es for male Henslow’s Sparrows. Data were obtained
during a study of singing behavior in which visual
contact was made with most males (1998 breeding
season only).
T
ABLE
3. Density estimates for male Henslow’s Sparrows during the 1998 breeding season, determined ac-
cording to survey round. Uncorrected and corrected densities are given as the mean 6SE males per ha,
and are based on a detection radius of 125 m. The correction factor given applies only to point counts.
Survey
round
Point counts
Uncorrected
males per ha
Correction
factor
Corrected
males per ha
Line transects
(Uncorrected)
males per ha
Round 1
Round 2
Round 3
0.116 60.012
0.083 60.010
0.091 60.012
1.58
1.64
1.71
0.184 60.019
0.137 60.017
0.156 60.020
0.177 60.029
0.109 60.021
0.102 60.022
few Henslow’s Sparrows were detected at dis-
tances ,50 m from the observer, suggesting
that males close to observers were silent or
moved away to sing at greater distances. If
close-by males did remain silent, then one
could easily devise a correction factor to ac-
count for absence of detections around an ob-
server. Because we could not distinguish be-
tween these two possible hypotheses for the
lack of nearby detections, we conservatively as-
sumed that all males within 125 m were equal-
ly likely to be detected.
Density estimates. We derived estimates of
Henslow’s Sparrow density by year and by sur-
vey round (1998). For 1997, our point counts
yielded 0.478 60.052 males per count. Assuming
a detection radius of 125 m, we arrive at a point-
count estimate of0.097 60.014 males per hectare.
With a correction factor of 1.68, our corrected
point count density is 0.164 60.023 males per
hectare. Our line transects yielded 3.04 60.779
males per kilometer. With a transect width of 250
m, those estimates translate into a density of
0.122 60.031 males per hectare. Our correction
factor applies specifically to point counts, hence
we leave transect estimates uncorrected. Density
estimates for 1998 were determined in the same
manner for each survey round (Table 3). The 1998
point count estimates (using round-specific cor-
rection factors) ranged from about 0.14 to 0.18
males per hectare, with an overall breeding sea-
son average of 0.159 males per hectare. The 1998
line-transect estimates (uncorrected) ranged
from 0.10 to 0.18 males per hectare, with an over-
all average of 0.129 males per hectare. Note fur-
ther that the 1998 average point-count density
compares closely to the 1997 value of 0.164 males
per hectare (which also covered the entire breed-
ing season), suggesting a stable population over
the two-year period; the corresponding values of
0.129 and 0.122 males per hectare for the 1998
and 1997 transect data support the same
conclusion.
We used corrected point-count density esti-
mates to derive a rough estimate of number of
Henslow’s Sparrows actually inhabiting those
mines. Assuming a density of ;0.16 males per
hectare and 11,500 ha of suitable habitat (Table
1), there are about 1,800–1,900 males in the
overall population. If densities away from the
road network are greater than those at (road-
side) point counts, as suggested by generally
higher (uncorrected) density estimates from
transects (see above and Table 3), then overall
population size might be about 10–20% larger
than the above estimate.
Fine-scale vegetation analyses. So far, we have
defined suitable Henslow’s Sparrow habitat
solely in terms of gross vegetative structure.
Within that basic habitat type, however, Hen-
slow’s Sparrows were relatively dense in some
areas and largely absent in others. For instance,
pooling the results of three survey rounds dur-
ing 1998, only 47% of our point counts pro-
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428 [Auk, Vol. 118B
AJEMA ET AL
.
T
ABLE
4. Vegetation structure and composition at point-count locations occupied or unoccupied by male
Henslow’s Sparrows (1998 breeding season). All values are means 6SE. Significant associations (P,0.05)
with Henslow’s Sparrow occupation are highlighted.
Variable
Unoccupied sites
N543
Occupied Sites
N561 tP
Male Henslow’s Sparrows
a
Visual obstruction (dm)
Max. vegetation height (dm)
—
3.39 60.20
8.60 60.28
3.51 60.30
3.23 60.18
9.32 60.25
—
0.61
1.85
—
0.540
0.066
Litter depth (cm)
% ground cover by litter
3.80 60.31
70.9 63.49
5.38 60.36
79.1 62.06
3.13
2.16
0.002
0.033
% Canopy cover by:
Grass (all species) 57.5 64.14 68.1 63.12 2.08 0.040
Tall fescue 30.8 66.64 33.2 63.55 0.41 0.684
Smooth brome 7.47 62.16 15.8 62.66 2.29 0.024
Orchard grass
Japanese brome
b
Kentucky blue grass
c
7.63 62.5
5.93 61.62
1.49 60.78
7.59 61.99
4.32 61.17
3.13 60.75
0.01
0.83
1.48
0.99
0.411
0.141
Broomsedge 0.63 60.39 4.11 61.14 2.50 0.014
Grass species diversity 1.70 60.10 2.01 60.08 2.37 0.020
% Canopy cover by forbs
Forb species diversity
d
31.0 63.70
2.55 60.18
24.0 62.93
2.29 60.12
1.50
1.68
0.138
0.096
a
Sum of males detected over three survey rounds.
b
Bromus japonicus.
c
Poa pratensis.
d
No individual forb species were associated with Henslow’s Sparrow occupation (all P.0.10).
duced one or more (up to 11) singing males.
Those results suggest that some sites were less
suitable for Henslow’s Sparrows than others.
To determine which aspects of mine grassland
vegetation (if any) were associated with pres-
ence of Henslow’s Sparrows, we compared sites
occupied by Henslow’s Sparrows to those at
which they were apparently absent (Table 4).
Simple pairwise comparisons of vegetation
structure and composition indicate that litter
depth and cover were significantly (positively)
associated with Henslow’s Sparrow presence.
Both the percentage of canopy cover by grass
and grass species diversity were also positively
associated with Henslow’s Sparrow presence.
Regarding individual grass species, only cano-
py cover of smooth brome and broomsedge (An-
dropogon viginicus) showed a significant associ-
ation with presence of those sparrows. Finally,
Henslow’s Sparrow presence was marginally as-
sociated with both taller vegetation (positively)
and forb species diversity (negatively).
D
ISCUSSION
Reclaimed coal mine grasslands of south-
western Indiana provide habitat for a substan-
tial number of Henslow’s Sparrows. Corrected
density estimates averaged about 0.16 males
per hectare, which implies a population of male
Henslow’s Sparrows approaching 2,000. Such
an estimate is probably conservative. One
might argue that density estimates from the
earlier survey round (0.184 males per hectare,
Table 3) provide the best estimate of male den-
sity, in which case the overall population might
exceed 2,000 males. This estimate would rise
still further if roadless areas harbor greater
densities of males than point-count locations
by roads. Finally, if one assumes a roughly
equal sex ratio, the overall population of Hen-
slow’s Sparrows was probably approaching
4,000 adults. Whichever is the best density es-
timate, those mine grasslands clearly contain a
large number of Henslow’ Sparrows, and pos-
sibly a significant proportion of the global pop-
ulation of this species (Pruitt 1996).
Our estimates of male density are sensitive
to the detection radius used to determine sur-
vey coverage. We assumed a detectionradius of
125 m, on the basis of actual detection distanc-
es. A detection radius of 150 m has been sug-
gested for Henslow’s Sparrow (e.g. Robb et al.
1998), but our detections of singing males were
well in decline at that distance (Fig. 1). If one
feels that a detection radius of 150 m is never-
theless warranted, then our density estimates
would drop by ;30%. Our density estimates
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April 2001] 429Coal Mines and Henslow’s Sparrows
also include relatively large correction factors
(1.58–1.71) for nonsinging males likely to have
been missed by our censuses. Our data show
clearly that standard point counts missed sev-
eral males. A similar realization led Robb et al.
(1998) to develop a correction factor applicable
to spot-mapping surveys for that species.
How do Henslow’s Sparrow densities on re-
claimed coal mines compare to those observed
in non-mine grassland habitats? A few pub-
lished estimates of Henslow’s Sparrow density
make some comparisons possible. Herkert
(1994b) reported densities of about 0.22–0.28
males per hectare in large prairie fragments in
Illinois near their peak suitability for this spe-
cies. Robb et al. (1998) reported an uncorrected
density of 0.55 male Henslow’s Sparrows per
hectare at Jefferson Proving Ground in south-
eastern Indiana; that grassland habitat is
uniquely suited to Henslow’s Sparrows and har-
bors relatively few grassland species overall. In
the prairie fragments of southwestern Missouri,
Swengel (1996) reported a density of ;7 males
per kilometer of transect. Our comparable un-
corrected mine-grassland estimates of ;0.10
males per hectare and 3.1 males per kilometer of
transect suggest that mine grasslands support a
lower density of Henslow’s Sparrows than those
other habitats. However, some of our ‘‘suitable’’
mine grassland habitat (very broadly defined)
may have been only marginally suitable for
Henslow’s Sparrows. If we focus only on suit-
able habitat as defined by Henslow’s Sparrow
occupation (47.5% of point counts in 1998), then
our overall estimate of Henslow’s Sparrow den-
sity increases to ;0.20 males per hectare. That
density is more comparable to that in Herkert’s
large, high-quality grassland fragments (0.22–
0.28 males per hectare), but still considerably
lower than Robb et al.’s estimate of 0.55 males
per hectare at Jefferson Proving Ground; only
5% of our occupied point counts, and 3.1% of
our total transect length produced densities
above 0.55/ha. Similarly, only 9% of our total
transect length produced the 7 (or more) males
per kilometer observed by Swengel (1996).
Overall, a substantial portion of undisturbed
mine grassland habitat is probably more suit-
able for grassland birds other than Henslow’s
Sparrows, such as Grasshopper Sparrows (Am-
modramus savannarum), Eastern Meadowlarks
(Sturnella magna), Dickcissels (Spiza americana),
and Field Sparrows (Spizella pusilla) (T. DeVault
1999; DeVault et al. unpubl. data).
What constitutes high- or low-quality habitat
in these mine grasslands? At a coarse level of
analysis, Henslow’s Sparrows clearly avoided
grazed and intensively hayed areas. The avoid-
ance of sparse vegetation typical of grazed areas
is well established for Henslow’s Sparrows (Her-
kert 1998). We suspect that intensively hayed ar-
eas are unattractive to Henslow’s Sparrows be-
cause of the relative lack of a significant litter
layer (R. A. Bajema pers. obs.). At a finer scale
of analysis, Henslow’s Sparrows were associat-
ed with vegetational features often associated
with that species: tall, dense grass with relative-
ly few forbs and a substantial litter layer (Her-
kert 1998). Few individual plant species wereas-
sociated with Henslow’s Sparrow occupation,
with the exception of smooth brome and broom
sedge (see also Robb et al. 1998). Notable for its
lack of a statistical effect on Henslow’s Sparrow
abundance was tall fescue, a grass often avoided
by wildlife in general (e.g. Barnes et al. 1995).
However, tall fescue is the most common plant
in reclaimed mines (Brothers 1990; see also Ta-
ble 4), and its ubiquity may have made its effects
difficult to detect. We suspect that the positive
association between Henslow’s Sparrows and
grass-species diversity reflects the positive ef-
fect of grasses that are not fescue. We should
note, however, that smooth brome (one of the
‘‘positive’’ non-fescue grasses) has been associ-
ated with negative effects on other wildlife spe-
cies (Wilson and Belcher 1989).
An important unanswered question is wheth-
er these mine grasslands represent source or
sink habitat (as per Donovan et al. 1995; Trine
1998) for Henslow’s Sparrows. We suspect that
mine grasslands are reasonably productive plac-
es for that species. An ongoing study of nesting
success of grassland birds in reclaimed mines
indicates relatively high daily nest-survival val-
ues for ground-nesting grassland birds in gen-
eral, due in part to extremely low rates of brood
parasitism (,1% of nests, S. L. Lima unpub.
data). That lack of brood parasitismprobably re-
flects the large size of mine grasslands and their
relative isolation from forested areas (Johnson
and Temple 1986, 1990; Burger et al. 1994; see
also Hahn and Hatfield 1995).
It is indeed ironic that Henslow’s Sparrows
should find a refuge in a habitat produced as a
result of profound environmental disturbance of
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430 [Auk, Vol. 118B
AJEMA ET AL
.
surface mining. It is perhaps that incongruity
that has diverted biologists’ attention away from
investigating reclaimed coal mines as habitat for
grassland birds. Reclaimed coal-mine grass-
lands nevertheless have many features that lend
themselves well to conservation of grassland
birds, not least of which is their sheer size. With
management explicitly focussed on those areas
as important grassland habitats, reclaimed sur-
face mines of the Midwest may be a major factor
in conservation of Henslow’s Sparrows and oth-
er species of grassland birds.
A
CKNOWLEDGMENTS
Edward Galligan and Ronald Lange provided
highly competent assistance in our field surveys for
Henslow’s Sparrows, as did Laura Bakken inour veg-
etation surveys. During the early stages of this work,
Jay Withgott provided valuable pilot survey data,
and Jim Herkert and Rolf Koford provided valuable
advice. Forest Clark of the U.S. Fish and Wildlife Ser-
vice kindly provided access to the Indiana Gap Anal-
ysis land coverage GIS data base, and members of the
Indiana State University Department of Geography,
Geology, and Anthropology helped greatly with GIS
methodology. Bill McCoy of the Patoka River Na-
tional Wildlife Refuge provided much appreciated
logistical support for our work in southern Indiana.
We are indebted to the following individuals for
granting full access to reclaimed mine properties:
Don Mann of Kindill Mining; Allen Eicher and Scott
McGarvie of Peabody Coal; Mike Jacquemine and Jeff
Mueller of Amax Coal; and Bernie Rottman of Black
Beauty Coal. The comments of three anonymous re-
viewers improved the manuscript considerably. Our
work was supported by a grant to S.L.L. and P.E.S.
from the U.S. Geological Survey Biological Resources
Division’s Species at Risk Program, and a grant to
R.A.B. and S.L.L. from the Indiana Academy of
Science.
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