Northern Prairie Wildlife Research Center
What does a female look for in a nest site? Evolution has favored
those females selecting sites where the chances of producing offspring (and
perpetuating their genes) are maximized. First, a female has to get fertilized
so the nest site must be accessible to males on a booming ground. The actual
nest site must provide shade, concealment cover from predators but, in the
event of discovery by a predator (usually mammals), an unobstructed escape
route must allow upward flushing from the nest. The nest site must provide
some protection from weather factors such as hard rains, hailstorms and flooding.
It must be reasonably close to high protein and energy food resources needed
by the female during the preincubation, laying, and incubation periods. Nests
must also be situated so that vulnerable chicks have ready access to their
specialized habitat needs upon hatching. Nests are generally located within
2 miles (3.2 km) of booming grounds which apparently serve as an orientation
point since some hens return to a given booming ground in subsequent years
(Svedarsky 1988). Females select nest sites within their preincubation home
ranges of about 200 acres (81 ha); often in the same vicinity as the preincubation
range surrounding their successful nests of the preceeding year (Svedarsky
1988). Robert Eng (personal commun.) suggests that prairie chicken
females may not be as opportunistic as waterfowl in exploiting new habitats
after they become associated with a favored nesting area and/or display ground
(a familiar area).
Are nests oriented around booming grounds or vice versa? Both are probably involved to an extent but males are probably more adaptable as to display sites than females are to successful nest sites. Males are only able to perpetuate their genes through females so, evolutionarily speaking, they should maximize the likelihood of encountering and mating with females, and that should be in the vicinity of good nesting cover. Meanwhile, males should display in a setting where their visibility (to females) is maximized along with their security from mammalian and avian predators; that is, being seen and be able to see. In sharptail habitat restoration after strip-mining in southeastern Montana, a square mile of good nesting and brooding cover was established and males established dancing grounds on closely grazed pastures radiating out from this core of cover (Robert Eng, personal commun.).
Brown (1966:220) commented further on the relationship between sharptail
display grounds and the quality of residual cover (used for nesting, brooding,
and roosting) in Montana: "A clear cut relationship appears to exist
between the establishment of new breeding grounds and the estimated cover
density... Without exception, the largest breeding grounds have been located
in areas surrounded by extensive, heavy stands of residual herbage."
In Wisconsin, Westemeier (1971:34) found a significant correlation between
the number of prairie chicken cocks on a booming ground and the amount of
certain habitats (generally grassland/cropland cover types) within a 1 mile
(1.6 km) radius of the booming ground, leading him to suggest that, "The
number of birds using a booming ground may be a useful measure of the productivity
of the surrounding cover." John Toepfer (personal commun.) has
found that most young cocks are recruited to booming grounds near their brooding
Reported measurements (Table 3) of prairie chicken nest characteristics
are quite variable due to the broad geographical range of the species and
different measuring procedures (Buhnerkempe et al. 1984). Commonly, "height",
is given in the literature but it is not clear if that indicates the maximum
height of any part of the vegetation, usually the flowering stems, or the
height of the vegetation which is most functional in concealing a nesting
female. Newell (1987:16) used "height" to essentially indicate "maximum
height" and "effective height" as that level below which all
dots on a cover board were obscured by vegetation; similar to the 100% visual
obstruction reading (VOR) of Robel et al. (1970a). Buhnerkempe et al. (1984)
used a rod marked off in cm intervals and used "maximum height"
as the highest interval touched by standing vegetation and "effective
height" as the height which included the lower 90% of the vegetation
strikes on the sampling rod; also, somewhat comparable to the 100% VOR. They
also used the rod to measure "litter depth" as the height of the
first cm interval seen on the rod from the ground upwards.
In order to compare structural characteristics of prairie chicken nesting covers it is important to use consistent terminology and techniques as well as measure or estimate conditions at the time of nest site selection. Growth or cover removal activities can substantially alter conditions if measurements are taken later in the season.
Table 3. Summary of greater prairie chicken nest characteristics.
|Height1 (cm)||Principal Cover||Predominant Land Use||Apparent Nest Success (N)||Number of Nests||Study|
|16.4 (effective height)||Andropogon scoparius/Poa pratensis||pasture||53% (67)||76 (62 pc, 14 st)||North Dakota (Newell, 1987)|
|29 (100% VOR)||Panicum virgatum||pasture||50%(6)||6||North Dakota (Manske & Barker, 1988)|
|20 (100% VOR)||Andropogon gerardi/Bromus inermis||idle||62%(36)||36||Minnesota (Svedarsky, 1979)|
|45 (mean x height)||Andropogon scoparius||pasture||65%(23)||23||Missouri (Drobney & Sparrowe, 1977)|
|45 (mean x height)||Andropogon scoparius||9||Oklahoma (Jones, 1963)|
|59 (mean x height)||Calamovilfa-longifolia/||pasture||41%(83)||83||Colorado (Schroeder & Braun, 1992)|
|22 (100% VOR)||Sporobolus asper/Andropogon scoparius||33%(38)||38||Texas - Attwater's prairie chicken (Horkel et al., 1977)|
1"Effective height" approximately equals 100% VOR, "average height" probably from 0-50% VOR.
Figure 3 indicates the variation in growth form of 4 general cover
types used by nesting prairie chickens. Consider the variation in levels at
which one would record 0%, 50%, and 100% Visual Obstruction Readings (VOR)
in the four types. Westemeier (Westemeier and Edwards, 1987) working first
in Wisconsin and for the last 26 years in Illinois has examined over 1,000
prairie chicken nests and believes (and Svedarsky concurs) that "ideal"
nesting cover should be relatively dense close to the ground, about 40 cm
(16 in.) tall, and have a growth form similar to smooth brome. This would
have a 100% VOR reading of about 2.7 dm (Svedarsky, 1979). Brome has the majority
of leaves positioned on stems as opposed to the base of the plant and withstands
snow flattening if maintained in a vigorous condition. Interestingly, prairie
chickens recently transplanted from Kansas and Minnesota, to Illinois showed
similar nesting cover preferences (brome, redtop, timothy) as Illinois birds.
Fields where 11 birds successfully nested had late March 100% VOR's of 2.0
(Westermeier, et al. 1995). Svedarsky (1988) working in a Minnesota study
area having a variety of native and non-native cover types, found prairie
chickens strongly preferred brome with 22% of 36 nests found in this cover
which comprised only 2.9% of the study area. This is not to suggest that smooth
brome (an exotic) is favored in an overall sense over native species in ecosystem
management, since it can invade and degrade native prairies in some settings.
The structural features of brome however might be used as a reference in managing
whatever cover a manager has to work with in order to maximize prairie chicken
Plant species composition in itself is not important so long as density
requirements are met, but obviously a given species has a characteristic life
form. Prairie chickens have existed for many years in Wisconsin on "substitute
prairie" dominated by Kentucky bluegrass (Hamerstrom, et al. 1957:12)
and on redtop and timothy seed fields in Illinois (Westermeier, 1980). Quackgrass
is currently a dominant species in the Wisconsin range (John Toepfer, personal
Westermeier (1973) found prairie chickens avoided tall, (>1 m) native
vegetation in Illinois. He believed that a female is most secure in a cover
setting where she can stand and have surveillance of the surrounding area
and readily flush, not run, if threatened. Svedarsky (1979), Yeatter (1943),
Toepfer (1988) and Hamerstrom, et al. (1957) also believed that vegetation
can be too tall and dense for nesting prairie chickens.
Litter is generally not separated from "residual vegetation"
in the literature but would generally be that part of the residual cover which
is horizontally oriented and is more than one growing season old. Residual
cover is all accumulated growth from past years but usually the growth from
the previous year provides the majority of the concealment cover. Svedarsky
(1979) and McKee (1995) found litter amount to be inversely related to nesting
success. Of several nest characteristics, McKee found litter cover the best
single predictor of nest success; nest sites having >25% failing at twice
the rate of nests with <25% litter cover. She related increased litter
to decreased grass and forb cover at nests. Buhnerkempe, et al. (1984) found
litter depths of 5.6 cm in fields containing only unsuccessful nests whereas
fields with only successful nests had litter depths of 4.2 cm. John Toepfer
(personal commun.), in a sample of 57 nests in 1994, found those in
Minnesota CRP habitats (presumably with deeper litter due to no-disturbance
management) experienced lower nesting success (29.7%) compared to those in
native habitats (50.0%); a pattern observed from 1992-1996. Increasing litter
depths may have been one of the factors contributing to prairie chickens declining
on Soil Bank lands in North Dakota, 5-7 years after seeding (Kirsch, et al.,
1973). Excessive litter could indirectly reduce nest success by providing
better small mammal habitat and thus attracting red foxes and/or better conditions
for June beetle larvae and thus attracting striped skunks (Svedarsky, 1979).
In Minnesota, Kimmel et al. (1994) evaluated cover characteristics of warm-season
vs cool-season grasses in CRP plantings after establishment. They found percent
litter cover increased with stand age for cool-season grass (r2
= 0.82) but not so in warm-season plantings (r2 = 0.34). Litter
depth was not related to stand age in either type. VOR's of cool-season
grasses declined with stand age (r2 = 0.71) but increased in warm-season
grasses (r2 = 0.82). This suggests the greater importance in rejuvenation
management of cool-season grasses to maintain optimum residual cover features
in comparison to warm- season cover.
In Illinois, Westemeier (1985) in a sample of 810 prairie chicken nests
found the highest prairie chicken nesting productivity (1.1 successful nests/4
ha) in brome and the lowest in planted prairie grass (0.3). Brome tends to
maintain cover in a more vertical position than prairie grass (in this case,
mostly switchgrass, Indian grass and big bluestem). At least in Illinois,
Westemeier (1985:35) believed brome comes closer to being a "seed it
and leave it" cover than any type used on their sanctuaries. Westemeier
(1988) found nest densities of prairie voles (Microtus orchrogaster)
and southern bog lemmings (Synaptomys cooperi) highest in prairie grass
and lowest in brome, 20.5 and 8.4 nests/10 ha, respectively, and suggested
an attraction for mammalian predators.
Svedarsky (1979) in Minnesota and McKee (1995) in Missouri also found the
extent of woody vegetation lowered prairie chickens nest success. McKee recorded
9 of 11 nests hatched when there was no woody cover but only 9 of 32 when
it was present. Perhaps, woody vegetation adds to the amount of "edge"
and encourages mammals to hunt in an area. It could also serve as perch sites
for potential avian nest predators such as crows and magpies.
Dense vegetation is thought to offer a barrier to predators as nests placed
close to cattle trails experienced higher predator losses than those at some
distance (Capel, 1965). Kirsch (1969) found red fox readily used vehicle trails
as access routes into idle cover. Westemeier (1973:333) however, felt that
redtop seed harvesting operations in Illinois "creates a desirable patchwork
of holes in an otherwise too-thick stand of redtop by leaving, in addition
to the wheel tracks, wads of chaff and stems scattered throughout the stubble."
This was desirable from a prairie chicken nesting use standpoint because of
the "too-thick" stand but Westemeier did not evaluate the wheel
tracks from a predator access standpoint. Duebbert (1969) believed that dense
cover reduced mammalian predator scenting conditions because of minimizing
horizontal movement of air currents.
When do optimum prairie chicken nesting conditions occur after a
new seeding or disturbance management (burning, grazing, mowing) of an established
cover? Westemeier (1973) found the highest prairie chicken nest densities
(1 nest per 7.7 acres or 3.4 ha) in redtop during the second growing season.
Kirsch (1974) suggested that prairie chickens probably do not begin nesting
in new seedings until 2-3 years when residual cover is established but that
stands lose their value between 5 and 7 years after seeding (Kirsch, et al.
1973). Recent field studies by John Toepfer (personal commun.) and
co-workers indicate optimum nesting values of CRP plantings in years 2-4,
thus supporting Kirsch, et al. (1973).
The early availability of good residual cover provides a decided advantage
to birds nesting in an area because it permits early nest initiation and increases
the total period available for nesting which may be important to accommodate
1 or 2 renesting attempts if needed (Kirsch, 1969). Early nests have the potential
to produce more offspring due to larger clutches of first nests and also broods
could hatch before the rainy season, at least in Minnesota (Svedarsky, 1979).
Furthermore, chicks hatching earlier in the season will be older going into
the fall and may have higher survival because of it (John Toepfer, personal
Covers vary with regard to their ability to maintain structure due to compacting
effects of winter snow. This is one of the primary reasons why cover rejuvenation
is critical in the North where substantial amounts of snow occur. Covers dominated
by unmanaged Kentucky bluegrass, for example, tend to flatten more than native
cover dominated by bunch grasses such as little bluestem. Little bluestem
is commonly mentioned as a principal species at prairie chicken nest sites
(Table 3). The importance of bunch grasses being resistant to snow flattening
was emphasized by Brown (1966) in Montana.