Habitat variable Life requisite Cover types

Percent canopy
cover of grasses

Mean height of
the grass canopy
during the
growing season
Food/ —————— Grassland -————————— HSI
Percent of ground Reproduction Shrub Savanna
surface that is
bare or lightly
covered with litter

Percent canopy cover
of vegetation taller
than the dominant
grass stratum

Figure 4. Relationship of habitat variables, life requisites, and cover
types in the lark bunting HSI model.

Definitions of variables and suggested field measurement techniques (Hays
et al. 1981) are provided in Figure 5.

Model assumptions. This model was developed to assess the habitat suit~
ability of grasslands and shrub savannas for supplying the food and reproduc-
tive needs of lark buntings. The model is not intended to produce outputs
that reflect actual population densities at any particular time, but rather it
attempts to estimate the potential of a site to supply the habitat requirements
as defined above, regardless of nonhabitat variables influencing populations.
Model variables and relationships are based on information obtained from
studies disjunct in time, space, techniques, and objectives. As such, the
model is a collection of hypotheses and should not be interpreted as statements
of proven cause and effect. Users should not hesitate to make refinements in
the model that are perceived to correspond to, or better represent, localized
conditions, if local data sets or authorities on the species’ requirements
substantiate the changes.

11


Hansell 1980). Territories with two nesting females had the lowest light
values, whereas bachelor territories had the least amount of shading. When
shading was artificially increased by attaching plastic leaves to alfalfa
plants or decreased by plucking alfalfa leaves the mating status of males was
altered (Pleszczynska and Hansell 1980). Areas improved to a bigamous cover
quality were settled by bigamists; areas improved or impoverished to a monoga-
mous level were chosen by monogamists; and areas altered to a bachelor quality
were settled by bachelors. Reproductive success was negatively correlated
with increased light penetration at the nest (Pleszczynska 1978). Early
nesting favored survival because illuminance was lowest before vegetation
began to dry and wilt.

Nesting success in lark buntings can be affected by the rate of brood
parasitism by brown-headed cowbirds (Molothrus ater). Lark buntings are not
heavily parasitized (Friedmann 1963), possibly because their blue eggs contrast
sharply with the cowbird's speckled eggs. Nevertheless, in 18 nests examined
by Allen (1874), three contained one cowbird egg, two contained two cowbird
eggs, and one had three cowbird eggs. Hill (1976) reported that, in Kansas,
15.5% of 142 lark bunting nests were parasitized and only 7% of the parasitized
nests fledged young.

Interspersion and Composition

Based on Pleszczynska's (1978) data, we estimated the average territory
size of the lark bunting to be about 0.2 ha. In the Pawnee Grasslands, where
densities were lower, average territory sizes estimated from the multiflush
technique and location-mapping were 0.5 ha and 0.75 ha, respectively
(Creighton, pers. comm.). Giezentanner (1970a) indicated that lark bunting
territory size was difficult to measure accurately because unmated males often
displayed within the boundaries of another male's territory.

Breeding densities of lark buntings fluctuate greatly from year to year
(Wiens and Dyer 1975). In the shortgrass prairie of the Pawnee Grasslands,
densities ranged from 7.2 to 13.8 birds on six 8.1 ha plots (Giezentanner
1970a). Wiens (1971) reported an average of 125 buntings/100 ha on heavily
grazed winter plots at the Pawnee site and only 20 to 50 buntings/100 ha on
heavily grazed summer plots. In Oklahoma, bunting densities were only 20
birds/100 ha on grazed and ungrazed plots in shortgrass prairie. Wiens and
Dyer (1975) indicated that relative frequencies of lark buntings were higher
in shortgrass (56% of census) and shrub (40%) habitats than in mixed shrub-
steppe (29%), mixedgrass (5%), and tallgrass (0%). Highest densities were
reported by Pleszczynska and Hansell (1980) who recorded 22 territorial male
buntings on a 4 ha field of alfalfa in South Dakota.

Lark bunting males form seminomadic flocks during the breeding season,
which leads to a high degree of clumping (Rotenberry and Wiens 1976). Flocking
behavior in males may be in response to localized food supplies such as swarm-
ing grasshoppers (Shotwell 1930; Welch 1936). The gregarious behavior of both
sexes is especially prominent on the wintering grounds, where flocks of
hundreds have been observed (Baumgarten 1968).


Pleszczynska, W.K., and R.I.C. Hansell. 1980. Polygyny and decision theory:
testing of a model in lark buntings (Calamospiza melanocorys). Am. Nat.
116(6) :821-830.

Porter, R.D., and H.J. Egoscue. 1954. The lark bunting in Utah. Wilson
Bull. 66(3):219-221.

Quillin, R.W. 1935. New bird records from Texas. Auk 52(3):324-325.

Rand, A.L. 1948. Birds of southern Alberta. Natl. Mus. Can. Bull. 111.
105 pp.

Roberts, T.S. 1936. The birds of Minnesota, Vol. 2. University of Minnesota
Press, Minneapolis.

Rotenberry, J.T., and J.A. Wiens. 1976. A method for estimating species
dispersion from transect data. Am. Mid]. Nat. 95(1):65-78.

Ryder, R.A. 1980. Effects of grazing on bird habitats. Pages 51-66 in R.M.
DeGraff, tech. coord. Workshop proceedings: management of western forests
and grasslands for nongame birds. U.S. For. Serv. Gen. Tech. Rep. INT-86.

Shotwell, R.L. 1930. <A study of the lesser migratory grasshopper. U.S.
Dept. Agric. Tech. Bull. 190. 27 pp.

Spendelow, J.A. 1980. Lark bunting banded in Connecticut. N. Am. Bird
Bander 5:145.

Strong, M.A., and R.A. Ryder. 1971. Avian productivity on the Pawnee site in
north-central Colorado. U.S. IBP Grassland Biome Tech. Rep. 82. Colorado
State University, Fort Collins.

Taylor, S., and V.M. Ashe. 1976. The flight display and other behaviors of
male lark buntings (Calamospiza melanocorys). Bull. Psychon. Soc. 7:527-529.

Thompson, L.S., and D. Sullivan. 1979. Breeding birds of prairie grassland
and shrubland habitats in northeastern Montana. Am. Birds. 33(1):88-89.

Toups, J.A., and M.F. Hodges. 1981. Lark bunting in Mississippi--photographic
documentation. Mississippi Kite 11(1):11-13.

U.S. Fish and Wildlife Service. 1981. Standards for the development of
Habitat Suitability Index models. ESM 103. U.S. Fish Wildl. Serv., Div.
Ecol. Serv. n.p.

Welch, C.M. 1936. Further notes on Montana birds, 1935. Auk 53(2):230-231.

Whittle, C.L. 1922. Miscellaneous bird notes from Montana. Condor 24(3):
73-81.

15




Habitat variable Cover types Suggested technique

Percent canopy cover of G,Ss Line intercept,
grasses (the percent of quadrat

the ground surface that

is shaded by a vertical

projection of grasses).

Mean height of the grass G,SS Line intercept,
canopy during the growing graduated rod_
season (cm) (the mean

distance from the ground

surface to the dominant

height stratum in the

grass canopy, when the

grass canopy is at its

tallest point).

Percent of ground surface G,Ss Line intercept,
that is bare or lightly quadrat, remote
covered with litter [the sensing

percent of the ground
surface that is unvege-
tated or covered with
vegetative litter that
is <5.1 cm (2 inches) in

depth].
Percent canopy cover of G,Ss Line intercept,
vegetation taller than quadrat

the dominant grass stratum
(the percent of the ground
surface that is shaded by

a vertical projection of
crowns of all woody and
herbaceous vegetation,
including mid- and tall-
grasses, taller than the
average canopy height of the
dominant grass stratum).

Figure 5. Definitions of variables and suggested measurement techniques.

12


Fairbanks, R.L., C.R. Legros, D.S. Thorne, and J.K. McBride. 1977. Breeding
bird populations of selected grasslands and weedy fields in north-central
Colorado. Am. Birds 31(1):64-67.

Finzel, J.E. 1964. Avian populations of four herbaceous communities in
southwestern Wyoming. Condor 66(6):496-510.

Friedmann, H. 1963. Host relations to the parasitic cowbirds. U.S. Natl.
Mus. Bull. 233.

Gates, L., M. Hamilton, D. Hamilton, and T. Fairley. 1980. Lark buntings in
Mississippi. Mississippi Kite 10(1):5-6.

Giezentanner, J.B. 1970a. Avian distribution and population fluctuations on
the shortgrass prairie of north-central Colorado. U.S. IBP Grassland Biome
Tech. Rep. 62. Colorado State University, Fort Collins.

1970b. Avian distribution and population fluctuations on
the shortgrass prairie of north-central Colorado. M.S. Thesis. Colorado
State University, Fort Collins.

Giezentanner, J.B., and R.A. Ryder. 1969. Avian distribution and population
fluctuation at the Pawnee site. U.S. IBP Grassland Biome Tech. Rep. 28.
Colorado State University, Fort Collins.

Hays, R.L., C.S. Summers, and W. Seitz. 1981. Estimating wildlife habitat
variables. U.S. Fish Wild]. Serv. FWS/OBS-81/47. 111 pp.

Hill, R.A. 1976. Host-parasite relationships of the brownheaded cowbird in a
prairie habitat of west-central Kansas. Wilson Bull. 88(4):555~565.

Kalmbach, E.R. 1914. Birds in relation to the alfalfa weevil. U.S. Dept.
Agric. Bull. 107.

Kantrud, H.A. 1981. Grazing intensity effects on the breeding avifauna of
North Dakota grasslands. Can. Field-Nat. 95(4):404-417.

1982. Maps of distribution and abundance of selected species
of birds on uncultivated native upland grasslands and shrubsteppe in the
northern Great Plains. U.S. Fish Wild]. Serv. FWS/OBS-82/31. 31 pp.

Kantrud, H.A., and K.L. Kologiski. 1982. Effects of soils and grazing on
breeding birds of uncultivated upland grasslands of the northern Great
Plains. U.S. Fish Wild]. Serv. Wildl. Res. Rep. 15. 33 pp.

. 1983. Avian associations of the northern Great Plains
grasslands. J. Biogeogr. 10(4):331-350.

Langdon, R.M. 1933. The lark bunting. Bird-Lore 35:139-142.

Pleszczynska, W.K. 1978. Microgeographic prediction of polygyny in the lark
bunting. Science 201:935-937.

14


Suitability Index (SIV1)

a b
1.0 1 1 1 = 10 rn n 1
=
0.84 5 2 0.84 -
x
va
0.64 2 0.64 L
0.4 4 5 2 0.44 .
‘a
0.24 5 § 0.24 F
3
0.0 ‘ t T 0.0 T y T
0 25 50 75 100 0 25 50 75 100
Percent canopy cover Mean height (cm) of the
of grasses grass canopy during the
growing season
c
a 1.04 r
sats
2 0.84 r
x
vo
Zz 0.64 L
>
# 0.44 7
5
& 0.24 F
5
0.0 rT T T
0 25 50 75 100
Percent of ground surface
that is bare or lightly
covered with litter
Figure 2. The relationships between vegetation cover variables and

suitability index (SI) values for lark bunting habitat quality.


LARK BUNTING (Calamospiza melanocorys)

HABITAT USE INFORMATION
General

The lark bunting (Calamospiza melanocorys) breeds in native grassland and
shrubsteppe habitat from southern Alberta, Saskatchewan, and Manitoba; south
to northwestern Texas and New Mexico, and east to Nebraska (Bailey and Niedrach
1965; Baumgarten 1968). In the northern Great Plains, lark buntings reach
high populations in a zone extending from southeastern South Dakota to central
Montana, then southerly through the shrubsteppe area of Montana and Wyoming
into the shortgrass area of northeastern Colorado and the southwestern portion
of the Nebraska panhandle (Kantrud 1982). Kantrud (1982) reports sparse
populations in a zone extending northwestward from northwestern Nebraska
to the southwest corner of Montana. The lark bunting previously nested in
Minnesota and Iowa, but the fragmentation of tallgrass prairie apparently
caused its breeding range to shrink westward (Roberts 1936; Baumgarten 1968).
Its movements are irregular during migration, and it occasionally appears as
far east as Connecticut and Mississippi (Gates et al. 1980; Spendelow 1980;
Toups and Hodges 1981), and as far northwest as Oregon and British Columbia
(Baumgarten 1968). The lark bunting winters in southern California and Nevada,
east to north-central Texas and south to central Mexico.

Food

Lark buntings forage primarily on the ground. The diet of adult lark
buntings is approximately 80% animal matter (primarily insects) and 20% vege-
table matter (primarily seeds) during the breeding season (Langdon 1933;
Baldwin 1970). Grasshoppers (Orthoptera) compose from 60% to 80% of the
insect matter, followed by beetles (Coleoptera) (10% to 35%), ants (Formicidae)
(2% to 15%), and miscellaneous material (Kalmbach 1914; Langdon 1933; Baldwin
1970). Baldwin (1970) calculated an index of utilization by dividing the
proportion of each food item in the diet by its proportion in sod samples from
the Pawnee National Grasslands. The analysis revealed that Pawnee lark bunt-
ings, in May, preferred grasshoppers and seeds of Buchloe, Amaranthus,
Lithosperum, and sedges (Carex spp.) while ignoring more abundant scarab
beetles, ants, and seeds of Helianthus and Avena.

The nestling diet is composed entirely of invertebrates (Creighton 1974).
During the early part of the nestling period, lark bunting nestlings in the
Pawnee Grasslands were fed a variety of insect orders fairly equally, but by
the end of summer, orthopterans accounted for 85% of the total dry weight of


Number

FIGURES

Geographic applicability of the lark bunting HSI model in
the contiguous United States ......... cece eee e eee tenet ees

The relationships between vegetation cover variables and suit-
ability index (SI) values for lark bunting habitat quality ...

The relationship between percent canopy cover of vegetation
taller than the dominant stratum of grass and a suitability
index (SI) for lark bunting nesting requirements .............

Relationship of habitat variables, life requisites, and
cover types in the lark bunting HSI model ...........-....00--

Definitions of variables and suggested measurement
TECHNIQUES LoL LLL eee cee eee eee etter

10

11

12


CONTENTS

SO ii
FIGURES ........ ee :
ACKNOWLEDGMENTS ..

HABITAT USE INFORMATION .. 2.2.0.0... ccc cece cccccceveeeeeeeeeceeceveres
General ............, oe
Food ...
Water ..
Cover .. see
Reproduction ..........
Interspersion and Composition .....

HABITAT SUITABILITY INDEX (HSI) MODEL
Model Applicability
Model Description
Application of the Model . .

SOURCES OF OTHER MODELS ......... 2... cece cece eee cece cee ey

ae

REFERENCES 1.0.26. eee eee cece cece cece cc tan ene eeeeeeeteeeeees 13

iv


=> 104 7
=

2 0.84 L
~~

ov

2 0.64 5
Poa4 E
8 0.24 L
5

” 0.0

0 2 50 75 100

Percent canopy cover of
vegetation taller than the
dominant grass stratum

Figure 3. The relationship between percent canopy cover of vegetation taller
than the dominant stratum of grass and a suitability index (SI) for lark
bunting nesting requirements.

The percent canopy cover of grasses (SIV1), mean height of the grass
canopy during the growing season (SIV2), and percent of bare ground (SIV3) are
assumed to be equal in value and are combined with an arithmetic mean in
equation 1. Percent canopy cover of vegetation taller than the dominant grass
stratum (SIV4) is incorporated into equation 1 as the major nest quality
component of the HSI model, and is assumed to be of equal value to the mean of
the three preceding variables. A geometric mean is used to combine SIV4 with
the first three variables because it is assumed that =70% canopy cover of
vegetation taller than the dominant grass stratum will render a site unsuitable
for lark buntings. The HSI is equal to the SIFR.

Application of the Model

Summary of model variables. Four habitat variables are used in this
model to determine a food/reproduction suitability index for the lark bunting.
The relationships between habitat variables, life requisites, cover types, and
an HSI value are summarized in Figure 4.

10


(us



Wiens, J.A. 1970. Avian populations and patterns of habitat occupancy at the
Pawnee site, 1968-1969. U.S. IBP Grassland Biome Tech. Rep. 63. Colorado
State University, Fort Collins.

- 1971. Avian ecology and distribution in the comprehensive
network, 1970. U.S. IBP Grassland Biome Tech. Rep. 77. Colorado State
University, Fort Collins.

- 1973. Pattern and process in grassland bird communities.
Ecol. Monogr. 43(2):237-270.

Wiens, J.A., and M.J. Dyer. 1975. Rangeland avifaunas: their composition,
energetics, and role in the ecosystem. Pages 146-182 in D.R. Smith, tech.
coord. Proceedings of the symposium on management of forest and range
habitats for nongame birds. U.S. For. Serv. Gen. Tech. Rep. WO-1.

Woolfolk, E.J. 1945. Some observations of lark buntings and their nests in
eastern Montana. Condor 42(2):128.

Wunder, B.A. 1979. Evaporative water loss from birds: effects of artificial
radiation. Comp. Biochem. Physiol. 63A(4):493-494.

16


Library

National Wetlands Research Center
U. S. Fish and Wildlife Service

700 Cajundome Boulevard

BIOLOGICAL REPORT 82(10.137) Tatayetter
— MAY 1987

HABITAT SUITABILITY INDEX MODELS:
LARK BUNTING

1 and Wildlife Service
~ % '. Department of the Interior

-US4
no. 82-
10.137



sites occupied by buntings ranged from approximately 5% to 15% (estimated from
Wiens 1973; Creighton 1974). In shrub savanna types used by lark buntings,
shrubs are an important component of nesting habitat (Woolfolk 1945) and tall
plant cover values are probably much higher. Wiens (1970) indicated that lark
buntings were associated with taller vegetation on the winter grazed Pawnee
plot, but the birds tended to occupy areas with generally lower vegetation
structure than that characterizing the plot as a whole. Kantrud and Kologiski
(1982) indicated that tall, weedy annual plants sometimes increased on sites
where bare ground was exposed through grazing or trampling. These plants
formed a sparse canopy of tall vegetation and attracted lark buntings to sites
that would otherwise have been avoided.

In arid, shortgrass areas, heavy grazing is detrimental to lark buntings
(Rand 1948; Finzel 1964; Giezentanner and Ryder 1969; Giezentanner 1970a,b;
Wiens 1971; Ryder 1980). In the northern Great Plains, however, numbers of
lark buntings were highest where grazing was moderate in the eastern and
northern subregions, light in the southern and shrubsteppe, and heavy in the
transition (Kantrud and Kologiski 1983). Intensity of winter grazing had
little effect on breeding lark bunting densities in the Pawnee Grasslands, but
summer grazing intensity had a negative effect (Giezentanner 1970a; Wiens
1971, 1973; Ryder 1980). Lark bunting densities were four to five times
greater on heavily grazed winter plots and lightly grazed summer plots than on
heavily grazed summer plots (Wiens 1971; Ryder 1980). Coverage and density of
forbs, cacti, and shrubs (Artemisia, Atriplex) were greater on the winter
grazed plot (Wiens 1973) as were vertical vegetation density (the total number
of vegetative contacts with a point passed vertically through the vegetation),
effective vegetation height (height at which a narrow board was 90% obscured
by vegetation within 3 cm of the board), and emergent vegetation height
(Wiens 1970). Emergent vegetation is defined here as any plant taller than
the mean height of the dominant grass stratum on the study area. At the
Pantex site in Oklahoma, Jark buntings were slightly more abundant on the
grazed site (Giezentanner 1970b; Wiens 1973), which had higher grass cover and
lower percentages of bare ground and patchiness (Wiens 1973). The ungrazed
site apparently was poor quality bunting habitat regardless of grazing
intensity, presumably because there was not adequate protective cover.

Grazing regimes may have indirect effects on lark bunting densities by
directly affecting other habitat variables. Grazing resulted in decreased
vegetative height on all soils (Kantrud and Kologiski 1982). Highest densities
of lark buntings occurred on plots with short to moderate vegetation heights
(5 to 23 cm), whereas lowest densities were associated with vegetation heights
ranging from 17 to 30 cm. Lark bunting densities peaked on tallgrass plots
that were heavily cropped to 18 cm. In addition, bunting densities were
highest on plots with bare ground coverage ranging from 8% to 13%. Low to
moderate densities were found on plots with relatively lower (e.g., 3% to 8%)
or higher (12% to 25%) percentages of bare ground. The percentage of bare
soil was positively related to grazing intensity on all soils. Apparently,
heavy grazing served lark bunting populations when the original vegetation had
greater canopy coverage and taller grasses than buntings preferred. Increased
canopy height may reduce visibility for foraging or courtship displays.


the nestling diet. In this study, high lark bunting densities were associated
with grasshopper densities of 28 individuals/30 m? (estimated from Bhatnager
and Pfadt 1973; Creighton 1974).

Water

The black plumage of the male Jark bunting may create water balance
problems at high air temperatures. When male lark buntings were irradiated at
10, 30, and 35 °C, evaporative water loss increased 150% at 35 °C compared to
nonirradiated birds at the same temperature (Wunder 1979). The effect of
irradiation was not important at 10 °C. Insects in the diet probably supply
most of the water needed by lark buntings. Behavioral modification may reduce
evaporative water losses (Wunder 1979).

Cover

Lark buntings breed in shortgrass and mixedgrass prairies, grass-shrub
habitats, weedy fallow croplands, and fields of introduced legumes (Wiens
1973; Creighton 1974; Pleszczynska and Hansell 1980; Kantrud 1981; Kantrud and
Kologiski 1982, 1983). Wiens (1973) recorded grassland bird species occurr-
ences at seven International Biome Program (IBP) grassland biome sites and
reported that breeding lark buntings only occurred at the two shortgrass sites
in Colorado and Oklahoma. The Pawnee Grasslands site in Colorado was dominated
by Bouteloua gracilis and Buchloe dactyloides while the Oklahoma site was
dominated by B. gracilis and Aristida longiseta. Buntings in north-central
Colorado were commonly found in Sporobolus cryptandrus grasslands associated
with rabbitbrush (Chrysothamnus nauseosus) (Fairbanks et al. 1977). In the
study by Kantrud and Kologiski (1982), plants that exceeded average mean areal
cover on plots supporting large numbers of lark buntings were Agropyron spp.,
Opuntia polycantha, and Artemisia tridentata. Pleszczynska and Hansell (1980)
reported high densities of lark buntings in a South Dakota field of alfalfa
(Medicago sativa) and sparse grasses. Lark buntings also breed in sagebrush
habitats in Montana (Thompson and Sullivan 1979), greasewood (Sarcobatus
vermiculatus) and saltbush (Atriplex spp.) habitats in Utah (Porter and Egoscue
1954), and Acacia meadows in Texas (Quillin 1935).

Studies by Wiens (1970) and Creighton (1974) support the idea that grass
height and cover, and percentage of bare ground are important habitat variables
affecting lark bunting populations. Creighton (1974) compared habitat
occupancy patterns of four grassland bird species in the Pawnee National
Grasslands and found that buntings preferred areas with greatest total grass
cover (shortgrass, 65.6% and midgrass, 4.7%), and greater sedge, forb, and
shrub cover (7.8%, 7.2%, and 2.1%, respectively). Overall vegetative cover,
including cactus, totalled 89.6%, while the area of bare ground and rocks was
only 10.4%. Wiens (1970) also reported high grass cover estimates (81% on
winter grazed plots and 70% on summer grazed plots) on Pawnee areas occupied
by buntings, but percent of bare ground was somewhat higher (16%).

At the Pawnee site, lark buntings preferred taller (13.4 cm) and denser
(32.2 plants/0.1 m?) vegetation than horned larks (Eremophila alpestris) or
chestnut-collared longspurs (Calcarius ornatus) (Creighton 1974). The percent
canopy cover of vegetation taller than the dominant grass stratum on shortgrass


ACKNOWLEDGMENTS

We thank Philip Creighton, Rod DeWeese, Hal Kantrud, Lowell McEwen, Bret
Peterson, and Ron Ryder for their advice on the development of the model; Hal
Kantrud and A. Lorin Ward reviewed the manuscript. Word processing was provid-
ed by Lorraine Kelly, Mary Armstrong, Carolyn Gulzow, Elizabeth Graf, Dora
Ibarra, and Trish Gillis. A seminar on wildlife and mitigation taught by
Stanley H. Anderson and Wayne Hubert instigated this project. We acknowledge
seminar students for their helpful comments. Funding for the development of
this model was provided by the U.S. Fish and Wildlife Service and the Army
Corps of Engineers.

vi


Summer grazing intensity may affect lark bunting densities negatively,
positively, or not all, depending on vegetation type. In tallgrass types,
heavy grazing can cause an increase in bunting populations, but in shortgrass
prairies, heavy grazing can cause a decline. When vegetation height is taller
than buntings prefer (e.g., >30 cm), heavy grazing can improve habitat quality
by decreasing canopy height. In shortgrass types, heavy grazing is detrimental
because percent of bare ground is drastically increased and food, shade, and
nest site availability are reduced. Thus, lark buntings apparently respond to
a change in vegetation height and cover rather than to grazing intensity per
se. Grazing intensity was therefore not used as a variable in this model.

SOURCES OF OTHER MODELS

No other habitat models for the lark bunting were located in the lit-
erature.

REFERENCES

Allen, J.A. 1874. Notes on the natural history of portions of Dakota and
Montana territories. Proc. Boston Soc. Nat. Hist., Vol. 17.

Anderson, N.L. 1964. Some relationships between grasshoppers and vegetation.
Ann. Entomol. Soc. Am. 57(6):736-742.

Bailey, A.M., and R.J. Niedrach. 1965. Birds of Colorado. Vol. II. Denver
Mus. Nat. Hist.

Baldwin, P.H. 1970. The feeding regime of granivorous birds in shortgrass
prairie in Colorado. IBP, Working Group on Granivorous Birds~-PT section
(Warszawa), 4(1):26-30.

Baldwin, P.H., J.D. Butterfield, P.D. Creighton, and R. Shook. 1969. Summer
ecology of the lark bunting. U.S. IBP Grassland Biome Tech. Rep. 29.
Colorado State University, Fort Collins.

Baumgarten, H.E. 1968. Lark bunting. Pages 638-657 in A.C. Bent and

collaborators. Life histories of North American cardinals, grosbeaks,
buntings, towhees, finches, sparrows and allies. U.S. Nat. Mus. Bull. 237.

Bhatnager, K.N., and R.E. Pfadt. 1973. Growth, density, and biomass of
grasshoppers in the shortgrass and mixed-grass associations. U.S. IBP
Grassland Biome Tech. Rep. 225. Colorado State University, Fort Collins.

Creighton, P.D. 1971. Nesting of the lark bunting in Northcentral Colorado.
U.S. IBP Grassland Biome Tech. Rep. 68. Colorado State University, Fort
Collins.

. 1974. Habitat exploitation by an avian ground-foraging
guild. Ph.D. Dissertation. Colorado State University, Fort Collins.

13


PREFACE

This document is part of the Habitat Suitability Index (HSI) Model Series
{Biological Report 82(10)], which provides habitat information useful for
impact assessment and habitat management. Several types of habitat information
are provided. The Habitat Use Information Section is largely constrained to
those data that can be used to derive quantitative relationships between key
environmental variables and habitat suitability. This information provides
the foundation for the HSI model and may be useful in the development of other
models more appropriate to specific assessment or evaluation needs.

The HSI Model Section documents the habitat model and includes information
pertinent to its application. The model synthesizes the habitat use informa-
tion into a framework appropriate for field application and is scaled to
produce an index value between 0.0 (unsuitable habitat) and 1.0 (optimum
habitat). The HSI Model section includes information about the geographic
range and seasonal application of the model, its current verification status,
and a list of the model variables with recommended measurement techniques for
each variable.

The model is a formalized synthesis of biological and habitat information
published in the scientific literature and may include unpublished information
reflecting the opinions of identified experts. Habitat information about
wildlife species frequently is represented by scattered data sets collected
during different seasons and years and from different sites throughout the
range of a species. The model presents this broad data base in a formal,
logical, and simplified manner. The assumptions necessary for organizing and
synthesizing the species-habitat information into the model are discussed.
The model should be regarded as a hypothesis of a species-habitat relationships
and not as a statement of proven cause and effect relationships. The model
may have merit in planning wildlife habitat research studies about a species,
as well as in providing an estimate of the relative suitability of habitat for
that species. User feedback concerning model improvements and other
suggestions that may increase the utility and effectiveness of this habitat-~
based approach to fish and wildlife planning are encouraged. Please send
suggestions to:

Resource Evaluation and Modeling Section
National Ecology Center

U.S. Fish and Wildlife Service

2627 Redwing Road

Ft. Collins, CO 80526-2899

iii


HABITAT SUITABILITY INDEX (HSI) MODEL

Model Applicability

Geographic area. This HSI model is intended for use within the breeding
range of the lark bunting (Figure 1).

Season. This model was developed to evaluate habitat suitability for the
lark bunting during the breeding season (May through July).

ag

- is,
\

Sa

Figure 1. Geographic applicability of the lark bunting HSI model in
the contiguous United States.


We have assumed that certain measures of plant structure can be used to
evaluate the suitability of a site for both the food and nesting requirements
of lark buntings. For example, lark buntings specialize on grasshoppers as a
food source for both adults and nestlings (Baldwin 1970; Creighton 1974).
Orthopteran density has been linked to percent grass cover (Anderson 1964).
High densities of grasshoppers are associated with grass cover values >60%
(Anderson 1964). In this model, percent canopy cover of grasses between 60%
and 90% is assumed optimal for food, whereas areas with <30% grass cover are
considered unsuitable (Figure 2a).

The intensity of light measured at 10 cm above ground has been correlated
with the number of female lark buntings nesting on male territories
(Pleszczynska and Hansell 1980) and reproductive success (Pleszczynska 1978).
We have assumed that these two nesting parameters (number of females/territory
and success) are functions of habitat suitability. Because light levels can
be difficult to assess in the field, we elected to use the degree of shading
at ground level as a surrogate for the above relationship. We have assumed
that shading is a function of both percent canopy of grasses (SIV1) and the
mean height of the grass canopy during the growing season (SIV2). A mean
height for the dominant grass stratum (i.e., the layer of greatest coverage)
of 8 to 20 cm is assumed optimal. A grass stratum shorter than 3 cm is unsuit-
able because light penetration is too great, and no nest concealment is
provided. A grass canopy with a mean height >50 cm is assumed to be unsuit-
able because of restricted visibility for foraging (Figure 2b).

Lark buntings forage and nest on the ground, and we have assumed that
some bare ground is necessary to facilitate these activities. Optimal condi-
tions are assumed to exist if the amount of the ground that is bare is <15%.
Suitability is assumed to decrease as the percentage of bare ground increases
(Figure 2c). Levels of >60% bare ground are considered undesirable because of
reduced coverage of grasses and increased light penetration.

The percent canopy cover of vegetation taller than the dominant grass
stratum (shrubs, bunchgrasses, tall forbs, cactus, mid- and tallgrasses) is
considered important in evaluating nest site availability for lark buntings.
In some regions, shrubs may be more valuable as nest sites than tallgrasses
and forbs because of their persistence (Woolfolk 1945), but lark bunting
populations can attain high densities in habitats devoid of shrubs (Kantrud
1981). Optimal percent canopy cover of vegetation taller than the dominant
grass stratum is considered to be 10% to 30% (Figure 3). Coverage >70% is
assumed to be unsuitable. If tall plant coverage is <10%, habitat is consider-
ed suboptimal, but not totally unsuitable for lark buntings.

HSI determination. The specific aggregation mechanism chosen for this
model attempts to mimic perceived relationships between the individual criteria
as closely as possible. The suggested equation for obtaining the food/
reproduction suitability index (SIFR) for lark buntings in grasslands and
shrub savannas is as follows:

V2
SEER = [(4 + sive + SIV3 QQ)

) x SIV4


This report should be cited as:

Finch, D.M., S.H. Anderson, and W.A. Hubert. 1987. Habitat suitability index
models: lark bunting. U.S. Fish Wildl. Serv. Biol. Rep. 82(10.137).

16 pp.


50272-101

REPORT DOCUMENTATION | 1}. REPORT NO. z
PAGE Biological Report 82(10.137)

‘3. Recipient's Accession No.

‘4, Title and Subtitie

S Habitat Suitability Index Models: Lark Bunting

5. Report Date

May 1987

6

7. Author(s)
D.M. Finch, S.H. Anderson, and W.A. Hubert

8. Performing Organization Rept. No.

‘9. Performing Organization Name and Address National LCology Center
U.S. Fish and Wildlife Service
Drake Creekside One Building
2627 Redwing Road
Fort Collins, CO 80526-2899

10. Project/Task/Wark Unit No.

11. Contract(C) of Grant(G) No.
(c)

(s)

12. Sponsoring Organization Name and Address National Ecology Center
Research and Development
Fish and Wildlife Service
Department of the Interior
Washington, DC 20240

13. Type of Report & Period Covered

14.

| 15. Supplementary Notes

| 16. Abstract (Limit: 200 words)
;

_ previously developed by U.S. Fish and Wildlife Service.

A review and synthesis of existing information were used to develop a Habitat
Suitability Index (HSI) model for the lark bunting (Calamospiza melanocorys). The model
consolidates habitat use information into a framework appropriate for field application,
and is scaled to produce an index between 0.0 (unsuitable habitat) and 1.0 (optimum

habitat). HSI models are designed to be used with Habitat Evaluation Procedures

[37. Document Analysis a. Descriptors

Birds

Wildlife

Habitatility

Mathematical models
b. Identifiers /Open-Ended Terms.

Lark bunting
Calamospiza melanocorys

Habitat suitability

¢. COSATI Field/Group

eB. Availability Statement 19. Security Class (This Report) 21. No. of Pages
aoe 7 ifi
Release unlimited Unclassified 16
20. Security Class (This Page) (22. Price
Unclassified

(See ANS!-Z39.18) See Instructions on Reverse

OPTIONAL FORM 272 (4-77)
(Formerly NTIS~35)
Department of Commerce


Biological Report 82(10.137)
May 1987

HABITAT SUITABILITY INDEX MODELS: LARK BUNTING

by

Deborah M. Finch
Rocky Mountain Forest and
Range Experiment Station

222 South 22nd Street

Laramie, WY 82070

Stanley H. Anderson
and
Wayne A, Hubert
Wyoming Cooperative Research Unit
Box 3166
University Station
Laramie, WY 82071

National Ecology Center
Fish and Wildlife Service
U.S. Department of the Interior
Washington, DC 20240


Kantrud and Kologiski (1982, 1983) studied the effects of grazing and
soil type on grassland bird populations on 615 plots of native rangeland in
North and South Dakota, Wyoming, Montana, Nebraska, and Colorado. They found
that increased grazing intensity resulted in much lower lark bunting densities
on warm, dry soils. However, the effects of grazing on lark bunting popula-
tions varied greatly among soil types. On cool, moist typic borolls, grazing
had little influence on population density, but on aridic borolls and borollic
Aridisols, moderate or heavy grazing seemed to favor larger populations.
Heavily grazed plots on typic ustolls supported the highest densities of lark
buntings.

Reproduction

Male lark buntings use flight displays to establish and defend territories
and possibly to attract females (Taylor and Ashe 1976). In a South Dakota
alfalfa field, lark buntings were polygynous, forming a dense "colony"
(Pleszczynska and Hansell 1980), but in the Pawnee Grasslands, they were
monogamous with larger territories (P.D. Creighton, Department of Biological
Sciences, Towson State University, Towson, Maryland; pers. comm.). At the
Pawnee site, males established breeding territories in early May and initiated
nests later than other ground-foraging species (Creighton 1971, 1974; Strong
and Ryder 1971). Nesting peaked from late May through mid-June and ended by
mid-July (Creighton 1974). Numbers of lark bunting nests initiated per week
were significantly correlated with grasshopper density (Creighton 1974).
Nesting density was 0.3/ha in Creighton's (1974) study whereas Baldwin et al.
(1969) reported 0.02 to 0.05 nests/ha.

Lark bunting nests are placed on the ground in shallow depressions,
closely associated with protective plant cover (Creighton 1971, 1974).
Woolfolk (1945) reported that 100% of 18 nests found in Montana pastures were
associated with three species of sagebrush (Artemisia tridentata, A. cana, and
A. frigida). In a study near Chugwater, Wyoming, lark buntings placed approx-
imately 80% of their nests under the tall forb, pricklepoppy (Argemone spp.)
(B. Peterson, U.S. Fish and Wildlife Service, Fort Collins, Colorado; pers.
comm.). Creighton (1971) reported that 62.7% of 43 nests were associated with
red threeawn (Aristida longiseta), 23.2% were shaded by rabbitbrush, and the
remaining nests were protected by saltbush (Atriplex canescens), eriogonum
(Eriogonum effusum), and sage (Salvia reflexa). Nests associated with red
threeawn and eriogonum were highly successful (67% and 100% success). Wiens
(1973) reviewed several lark bunting studies from the Pawnee Grasslands and
found greatest nest association (47%) with saltbush, while red threeawn,
eriogonum, and rabbitbrush were secondarily important. Most nests were placed
on the east or southeast side of tall plants, affording protection against the
afternoon sun and the prevailing northwest winds. Lark buntings have also
been reported to nest under tumbleweeds (Cycloloma atriplicifolium) in Montana
(Whittle 1922) and acacia (Acacia spp.) clumps in Texas (Quillin 1935).

Pleszczynska and Hansell (1980) suggested that protective cover from
solar radiation at the nest was the most crucial factor for breeding in lark
buntings. The best predictor of bachelor, monogamous, or polygynous behavior
in an alfalfa field in South Dakota was intensity of light (measured 10 cm
above the ground) on male territories (Pleszczynska 1978; Pleszczynska and


Cover types. This model can be used to evaluate habitat in the following
cover types (terminology follows that of U.S. Fish and Wildlife Service 1981):
Grassland (G) and Shrub Savanna (SS). Lark buntings are apparently also found
in croplands and pasture/hayland (Pleszczynska 1978; Pleszczynska and Hansell
1980; H.A. Kantrud, Northern Prairie Wildlife Research Center, Jamestown, NO,
pers. comm.), but avoid mowed hayland (Kantrud 1981). There is not enough
information in the literature to determine habitat variables for these latter
cover types.

Minimum habitat area. Minimum habitat area is defined as the minimum
amount of contiguous habitat that is required before an area will be occupied
by a species. Territories of male lark buntings range from approximately
0.2 ha to 0.75 ha. Although the minimum habitat area is probably >0.75 ha due
to the gregarious nature of this species, no information was found in the
literature to more accurately define the minimum habitat area. It is the
responsibility of the user to define minimum area to be considered as lark
bunting habitat.

Verification level. This model is intended to provide information useful
for baseline assessments and habitat management where lark buntings are a
species of interest. We have reviewed the lark bunting literature, selected
the criteria described below, supplied values for each (including optimum),
and have suggested an aggregation mechanism. This approach, when carried to
completion, will produce a single index representing the assumed relative
suitability of a site for lark bunting food/reproduction requirements. Our
description of these requirements is based on the assumption that sites
supporting male territories occupied by more than one female represent optimum
feeding/reproduction conditions (our standard of comparison) for lark buntings.
The identified criteria should serve as hypotheses of habitat use by the
species, but their evaluation individually, or in total, against long-term
demographic data, awaits further research.

Comments and suggestions from H. Kantrud, R.A. Ryder, and A.L. Ward on an
earlier draft of this model have been incorporated where possible. Use of the
reviewers' names does not necessarily imply that they concur with each section
of the model, or with the model in its entirety.

Model Description

Overview. All of the breeding habitat requirements of the lark bunting
can be satisfied within grasslands or areas where grasses are the dominant
vegetation. Food, especially invertebrates, is assumed to be the principal
source of water. We have assumed that all cover requirements are met with a
characterization of suitability of nesting cover.

Food/reproduction component. Suitable feeding and nesting sites for lark
buntings can be provided by native grasslands and grass/shrub associations.
This includes prairie grasses with an upper stratum of midgrasses and a dense
understory of shortgrasses. A thin layer of shrubs, bunchgrasses, or mid-
grasses provides shade and is assumed to be an important feature of nesting
habitat.