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Spring
1998
[TEXT ARCHIVE WEB-PUBLISHED
FEBRUARY 2002.
ORIGINAL PRINT PUBLICATION DATE: SPRING 1998.]
Saving
Habitat for Birds
By
Peter Friederici
It's
a tough time to be a bird. There are lots of bird watchers
around to admire your plumage and song, but for many species
it's increasingly difficult to find a home. Just consider
the downward trends in the nationwide populations of a few
of the species that nest in the Chicago Wilderness region:
Henslow's sparrow. Populations of this grassland species
shrank by an estimated 94 percent in northern and central
Illinois between 1958 and 1983, and by 93 percent continentwide.
American bittern. This secretive wading bird has been declining
by an average of more than two percent a year in the United
States. Red-headed woodpecker. This striking savanna dweller's
population has been dropping in the region at an average
rate of over 1.5 percent a year.
Historically,
many birds declined due to fairly straightforward factors.
Hunting wiped out a few for good (the Eskimo curlew, the
Carolina parakeet) and extirpated others from the Chicago
region (the wild turkey). Uncontrolled pesticide use imperiled
the reproduction of many predatory birds, such as bald eagles
and peregrine falcons. Thanks to restrictions on hunting
and pesticides, and thanks to vigorous reintroduction programs,
some species have bounced back.
Other
threats facing many birds of our region today are more insidious
because they can't be traced so easily to a single source.
For instance, many bird species face greatly reduced quantities
of suitable habitat. It's a problem that comes into sharp
focus in the Chicago Wilderness region, where burgeoning
development has caused the habitats preferred by many birds
to become rare — and where an exceptional diversity of habitats,
with its accompanying diversity of birds, dramatically increases
the complexity of management decisions.
Thanks
to the region's large percentage of protected land and an
active restoration movement, there is opportunity to improve
conditions for many birds. But doing so isn't always easy.
There's no free lunch in nature: burning a woods often enough
to benefit red-headed woodpeckers may decrease populations
of shrub-nesting yellow-breasted chats.
Regional
birding clubs and Audubon groups recently banded together
to form a new Bird Conservation Network to ensure that the
concerns of birders — and the data they can provide — are
incorporated in land management plans. It is birders, after
all, with their long history of volunteer monitoring, who
have been the primary force in documenting the region's
avian richness.
"We
realized that we could be stronger by pooling resources
among the bird clubs, and that we could command more respect
from agencies as a coalition of 8,000 or 10,000 members,"
says Christine Williamson, conservation chair of the Chicago
Ornithological Society. The BCN has been working with Chicago
Wilderness, especially with land-management agencies, to
formulate guidelines for conservation and monitoring of
bird populations.
To
consider the problems birds and land managers face, it's
important to look separately at different habitats. (Remember,
too, that these habitats form a continuum; there's no sharp
line that separates a savanna and an adjacent forest.) It's
also important to have a global perspective. Some birds
are rare here because the core of their range lies elsewhere,
such as the black-throated green warblers that nest in the
area's introduced pine groves. Other species, such as the
red-headed woodpecker or Henslow's sparrow, are uncommon
but in the heart of their range here. Saving their habitat
is likely to be more ecologically significant than saving
that of species more common elsewhere.
"For
some grassland species, we have populations that are potentially
globally significant," says Douglas Stotz, a conservation
biologist at the Field Museum.
Recent
ornithological research in Illinois, some of it described
below, is helping land managers develop priorities for conservation.
This research makes clear that integrating the needs of
humans with those of other species is going to be an ongoing
task for those who wish to maintain the regionıs biological
diversity.
Grasslands
Illinois
is the Prairie State, so it's telling that many once abundant
tallgrass prairie birds have become extremely uncommon here.
In examining their declines, it's not enough to point out
the obvious: almost all Illinois prairies disappeared early
in the state's history. But most grassland birds don't seem
to care whether they're nesting among native or introduced
grasses. It's not the composition, but the structure of
their habitat that matters. In other words, itıs not the
species of grasses but the shape, height, and density that
matter to birds. Many prairie birds thrived even after native
grasslands vanished, because they could feed and breed quite
nicely in pastures of introduced Eurasian grasses.
It
wasn't until after World War II, when soybeans and corn
replaced most Midwestern pastures, that grassland species
began declining precipitously in Illinois and elsewhere.
Data from the Breeding Bird Survey, a volunteer monitoring
effort coordinated by the U.S. Fish & Wildlife Service,
reveal that, since 1966, nesting populations of bobolinks
have dropped by more than 92 percent in Illinois, and by
37 percent across its range. Grasshopper sparrows declined
85 percent in Illinois and 66 percent rangewide; eastern
meadowlarks, 61 percent in Illinois and 53 percent rangewide.
Many
prairie birds have very particular structural preferences.
Upland sandpipers, for example, nest in tall grasses, but
bring their chicks to shortgrass areas to feed on insects
there; originally they probably thrived where bison had
grazed. Bobolinks remain among tall grasses. Loggerhead
shrikes, which prey on insects, rodents, and other birds
from exposed perches, need scattered small trees.
These
birds are also picky about their habitat's extent — they
are extremely "area-sensitive," as ornithologists put it,
and won't nest in grasslands that are too small in area.
For example, upland sandpipers rarely nest on sites of less
than 100 acres. Jim Herkert, a biologist with the Illinois
Endangered Species Board who has conducted extensive research
on grassland birds, did a survey of Henslow's sparrows in
which he found the birds nesting on only one patch of grassland
smaller than 250 acres. "This may be a matter of predation,"
he says. "A lot of predators use woody edges as travel corridors."
In large grassland areas, then, birds have more room to
avoid predators traveling along the edges.
In
Illinois, most prairie restoration projects have been less
than 25 acres in extent. Valuable as those places are for
rare plants — and the human soul — they can't and won't do
much for prairie birds. "A lot of the prairie restoration
work we've done so far has had a botanical focus," says
Herkert. "We're really just now getting to restorations
on a scale that allows us to think about animal life."
The
main site he has in mind in the Chicago Wilderness region
is the new Midewin National Tallgrass Prairie, site of the
former Joliet Arsenal. It will ultimately comprise 19,000
acres — enough space to allow for an almost complete array
of prairie birds. Most of the land is now covered with row
crops or with extensive pastures of Eurasian grasses; thanks
to the latter, the preserve hosts the state's largest population
of nesting upland sandpipers, as well as loggerhead shrikes,
Bell's vireos, bobolinks, and grasshopper sparrows. It's
also near Goose Lake Prairie State Natural Area, a 2,500-acre
swath of mesic grassland that has never been plowed. Goose
Lake is one of only about a half-dozen sites in the entire
range of the Henslow's sparrow where this species nests
reliably in large numbers.
Managers
hope to restore much of Midewin to native grassland managed
with fire and with grazers, possibly including reintroduced
bison and elk. Thanks to the preserve's size, they should
be able to maintain grazed areas for upland sandpipers,
tallgrass patches for bobolinks, occasional trees for loggerhead
shrikes and Bell's vireos.
"At
Midewin we've got the size to get the birds and other animals
back along with the plants," says Herkert. "It's not prairie
now, but there's a lot of grass there already, and the majority
of birds that we would hope to attract to a restoration
are already there. So the goal will be bolstering populations
and getting them to move into native grassland."
Oak
Savannas and Woodlands
The
Chicago Wilderness region is renowned for its oak lands
not because these once-widespread savannas and open woodlands
were or are unknown elsewhere, but because of intensive
restoration here. Thanks to pioneering biological research
by Steve Packard, Ron Panzer and others, and an army of
volunteers, scrubby woodlands are being restored to rich
expanses of grass and wildflowers studded with wide-spreading
oaks.
These
expanses are also forming good homes for a suite of birds
that prefer some space between the trees and shrubs they
call home. Here red-headed woodpeckers and great crested
flycatchers perch on exposed branches and sally out after
flying insects. Baltimore orioles glean caterpillars in
treetops. Indigo buntings and blue-winged warblers forage
lower, in shrubs.
The
Illinois Natural History Survey's Jeffrey Brawn, an ecologist
who has studied bird reproduction in these areas, says there
are about 50 breeding species associated with Midwestern
oak savannas and woodlands. "Some of them," he
says, "have previously fallen through the cracks as
far as conservation goes." There have been some substantial
declines: Breeding Bird Survey data indicate that eastern
wood-pewees declined by 36 percent in Illinois from 1966
through 1991; great crested flycatchers dropped 35 percent.
Many
birds of savannas or open woods are not often found in denser
woodlands and forests. That's due, in part, to their structural
preference. When such lands don't burn, they become too
overgrown for some birds. Even shrub-loving species such
as blue-winged warblers are forced out when thickets grow
into dense collections of trees.
Bird
populations may also suffer as the composition of wooded
areas changes. Little is known about the preference of Midwestern
birds for particular plants, but some open woodland species,
including great crested flycatchers, eastern wood-pewees,
and rose-breasted grosbeaks, show a propensity for foraging
in oaks, which support substantial insect populations.
The
sugar maples and other trees of denser forests apparently
support fewer insects, and hence fewer birds, than oaks.
Ornithologists are not sure why this is so, but Chicago
Wilderness is focusing attention on just this kind of question,
both for scholarly learning and for its practical implications.
"The
viability of many bird species is enhanced by burning,"
says Brawn. "And the birds that come back most readily are
those most strongly associated with oaks." Indeed, Brawnıs
1997 data indicate that 13 of 19 savanna and open woodland
species had greater nesting success in restored or burned
areas than those in unrestored sites.
Restoring
woods with fire may also help nesting birds avoid predation.
Data
are scanty, but Brawn believes populations of common nest
predators such as raccoons and some snakes may diminish
when trees and shrubs are thinned. "We know that rates of
nest predation are lower in restored areas," he says.
One
quality of open woodland birds should allow them to thrive
in the Chicago Wilderness region: unlike prairie species
for some reason, most don't mind small areas. "Historically,
many savannas were probably always fragmented," Brawn points
out.
Their
birds should do well in today's patchwork landscape. Savannas
and oak woodlands are also important to migrant birds. Their
trees tend to leaf out earlier in spring than trees in closed-canopy
forests, a quality that is vital to insect-eating migrants.
The Chicago area is a major migration corridor; birds follow
the shore of Lake Michigan or the north-south river corridors.
"It's
incredible how many birds pack into some areas during migration,"
says Scott Robinson, a forest-bird researcher with the Illinois
Natural History Survey. "Restoring oak woodlands and savannas
is likely to improve conditions for migrants, especially
if a shrub layer is maintained."
Forests
The
birds that live in Midwestern forests — whether dry uplands
or moist bottomlands — have garnered a lot of conservation
concern in recent years, in part due to Scott Robinson's
work. He has studied the reproductive success of forest
songbirds, mainly in central and southern Illinois, and
has found that an astonishingly high percentage of nesting
birds fail to successfully raise young. There are two main
reasons for this.
First,
most Illinois forests are small patches that have a relatively
high proportion of edge to forest interior. Predators that
eat eggs or nestlings, such as crows, grackles, cats, raccoons,
and skunks, tend to thrive along edges. They curtail many
nesting efforts.
Second,
agricultural areas support large populations of brown-headed
cowbirds. Cowbirds don't build their own nests; instead,
they lay their eggs in the nests of other species. Because
cowbird eggs hatch quickly, and because their chicks are
aggressive, many host parents end up unknowingly raising
an adopted cowbird rather than their own young. Savanna
and grassland birds evolved in conjunction with cowbirds — a
Great Plains species that once followed bison herds — but
forest birds did not. They haven't had time to develop defenses
against them. As a result, Robinson's field researchers
have found high rates of cowbird parasitism among wood thrushes,
scarlet tanagers, and other forest species.
Also,
many migrant forest dwellers face deforestation of their
Latin American wintering grounds. Some combination of habitat
changes here and to the south caused ovenbird populations
to shrink by 70 percent in Illinois from 1966 through 1991,
according to the Breeding Bird Survey; Acadian flycatchers
dropped 66 percent. Even red-eyed vireos, adaptable birds
that utilize small forest fragments, dimished by 36 percent.
Much
of the Chicago Wilderness region supports fewer cowbirds
than in rural Illinois, probably because there isn't much
agriculture nearby anymore. But it does support large numbers
of other nest predators, many of which thrive in suburban
environments.
"Very
few tracts in the area are large enough to provide buffering
against predators," says Robinson, who says that large numbers
of deer are probably also responsible for removing ground
cover that could shield the nests of ground-nesting birds.
"There's no direct evidence of that," he says, "but it's
hard to imagine that it doesn't make a difference."
The
upshot is that the Chicago metropolitan area, like the rest
of Illinois, is likely a "population sink" for many forest
species: because nesting success here is very low, birds
disperse into the area from other places where reproduction
is more successful. That may seem like bad news, but it
may not be a big deal in global terms because forest birds
have never had core populations here.
"We're
between the two main regions of forest diversity, the Southeast
and the North Woods," says Douglas Stotz. "We've got bits
and pieces of both, but we're not that important in the
overall picture for breeding forest birds." Stotz and Robinson
agree, though, that some forested areas — especially the
oak woods — are vital to migrants, especially those along
the north-south river corridors and the Lake Michigan shoreline.
Wetlands
Historically,
much of the Chicago region was, in a word, soaked.
Geologically
young, the region was poorly drained, and water stood in
innumerable low spots. Variable rainfall and winter runoff
created an ever-changing landscape of open water, cattail
marshes, sedge meadows, and other wetlands. Breeding birds,
most of them migrants from the south, were able to take
quick advantage of changes by flying to marshes offering
whatever water level they preferred. "They moved around
and looked for the best habitat," says Charles Paine, a
biologist at the Max McGraw Wildlife Foundation in Dundee.
Or
they could. About 90 percent of Illinois' wetlands have
been lost, mainly to draining for farming and development.
A larger proportion has survived in the Chicago Wilderness
region, but still the extent of available habitat is much
reduced. The Illinois Department of Natural Resources says
that about 30 birds on the state's list of threatened and
endangered species rely on wetlands. Among the signal species
are some coveted by birders because of their secrecy, such
as American and least bitterns and king rails.
Paine
has been trying to determine what features affect the presence
and nesting success of marshbirds in the region. His preliminary
conclusions: large marshes support a greater diversity of
birds than small ones; marshes that are part of a wetland
complex have a greater bird diversity than isolated ones;
marshes with diverse habitats have a wider range of birds
than homogenous ones. Different bird species prefer different
water levels, so a healthy diversity relies on a patchwork
of wetter and drier marshes. Beyond that, though, there's
a lot still to be learned, such as what are the minimum
size requirements to nest for various species and which
factors affect nest predation. It's also not known just
how important the region is for the migration of wetland
birds, though Paine has a guess. "South of here there aren't
many wetlands," he says, "so we may be quite important for
migrants."
The
main priority in protecting marsh birds, both breeding and
migrant species, may be stemming the course of future development,
even outside wetlands. When houses, roads, and parking lots
cover previously open land, water runs off more quickly.
That can make water levels in marshes rise precipitously,
rendering them useless for some species, even flooding nests.
Nonpoint pollution, such as oil flushed from parking lots,
is also a pollution hazard that's difficult to combat or
even to monitor. But solving such diffuse problems will
be difficult, especially in wetlands that are also coveted
by people. Cutting back on speedboat use of the Chain O'Lakes,
for example, might keep some waterside nests from being
flooded, but politically it would be difficult.
Peter
Friederici is a freelance writer whose favorite Chicago
Wilderness birding sites include Ryerson Woods and the lakefront.
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