BIOLOGICAL DATA AND HABITAT REQUIREMENTS
WILDLIFE SPECIES: Molothrus ater | Brown-Headed Cowbird 



TIMING OF MAJOR LIFE HISTORY EVENTS : 
Diurnal Activity:  In the Sierra Nevada during the breeding season,
brown-headed cowbirds spend the early morning hours in host-rich
forests, then commute up to 2.7 miles (6.7 km) to prime feeding sites
such as residential developments (especially where bird feeders are
present) and horse corrals.  Flocks of up to 30 brown-headed cowbirds
may develop at feeding areas.  Evening roosting behavior depends on
locality.  In the Sierra Nevada flocks of brown-headed cowbirds roost
in willow (Salix spp.) thickets, away from both feeding and breeding
areas.  The flocks disperse to breeding areas in the early morning
[51,70].  

Spring Migration:  The brown-headed cowbird is a short-distance migrant
within North America.  In the Northeast most brown-headed cowbirds
travel 300 to 340 miles (800-850 km) between breeding season banding and
winter recovery.  Brown-headed cowbirds travel during the day, often as
part of large mixed-species flocks with other blackbirds: red-winged
blackbirds (Agelaius phoeniceus), common grackle (Quiscalus quiscula),
European starling (Sturnus vulgaris), and, less frequently, Euphagus
blackbirds and American robins (Turdus migratorius) [35].  In Maryland
and the District of Columbia, spring movements of brown-headed cowbirds
occur from February 5 to April 25, with a peak of activity from March 10
to April 10 [64].

Mating Systems and Pair Bonding:  Brown-headed cowbird mating systems
have been variously reported as monogamous, polygynous, and polygamous
[35,51].  Rothstein and others [51] reviewed reports on mating systems
and could come to no conclusion.  Different mating systems may occur
with different densities or sex ratios; males guard mates more
frequently in low density populations.  Pair bonding is difficult to
assess due to the difficulty of defining a pair.  Bonding may be brief
or season-long [35].

Nesting:  The brown-headed cowbird is a brood parasite; no nest is built
and the eggs are laid in the nests of other birds (host species).  In
Maryland and the District of Columbia, brown-headed cowbirds lay eggs
from late April to late July, with the peak from early May to early
July.  Extreme dates are April 24 and July 28 [64].  In Missouri
brown-headed cowbirds eggs have been present in nests from mid-April
until mid-July [35].

Nests:  Female brown-headed cowbirds choose the nest, finding nests in
one of three ways. They may perch in a hidden spot, watching for nest
building activity in nearby open areas; walk on the ground watching for
activity; or search noisily on short flights appearing to intentionally
flush potential hosts from nests [35].  Preferred host nests have eggs
present.  Host eggs are usually of smaller volume than brown-headed
cowbird eggs.  Preferred nests are active; eggs are added on a daily
basis by the host female.  Large diameter (greater than 3 inches [7.6
cm]) closed nests are preferred over small diameter (less than 2 inches
[5.1 cm]) open and closed nests.  Large, open nests are least preferred.

Clutch:  Brown-headed cowbirds lay eggs in "sequences" of one to seven
eggs (laid daily with pauses of 1 or more days between eggs) and rest
periods of at least 2 days between sequences throughout the breeding
season [54].

Incubation:  Since incubation depends on host nesting activity it is
difficult to assess; incubation may be as short as 10 days but is
probably 11 to 12 days.  Brown-headed cowbird eggs laid the same day as
host eggs usually hatch first [35].

Development of Young:  Brown-headed cowbird hatchlings are altricial and
remain in the nest.  Growth is rapid.  Brown-headed cowbird nestlings
emerge from the nest onto nearby branches at 8 to 13 days.  Short
flights are attempted at 11 days.  Fledglings continue to be fed by host
parents until 16 to 28 days after leaving the nest [35].

Territoriality:  In the eastern Sierra Nevada, where both hosts and
brown-headed cowbirds are widely dispersed on the breeding areas, there
is no apparent territoriality.  Elsewhere, such as in the eastern states
where brown-headed cowbirds are moderately abundant and competition for
host nests is higher, territorial defense is typical.  At very high
brown-headed cowbird densities, territoriality is apparently not cost
effective and is rarely observed [51].
 
Resistant Hosts:  Vireos (Vireo spp.) and some warblers (i.e., yellow
warbler [Dendroica petechia]) cover brown-headed cowbird eggs with a new
floor and lay their own eggs on top of it.  Yellow-breasted chats
(Icteria virens) abandon nests with foreign eggs [36].  American robins
and gray catbirds (Dumetella carolinensis) usually eject foreign eggs
from the nest.  Instances of brown-headed cowbird nest parasitism in
gray catbird nests apparently occur when the brown-headed cowbird egg is
laid before a gray catbird has laid any of its own eggs and has learned
to recognize its own eggs [34,36].


Fall Migration:  On western slopes of the Sierra Nevada, juvenile
brown-headed cowbirds began to form flocks as independence was achieved.
The first independent juveniles appeared on July 14.  In the same area
all adult brown-headed cowbirds left between July 17 and July 21.  In
Maryland and the District of Columbia, fall movements occur from August
15 to December 10, with peak activity from September 25 to November 1
[64].

Roosting Behavior:  In the nonbreeding season, brown-headed cowbirds
spend the night in large mixed-species roosts with other blackbirds
[40].  Mixed-species roosts in Kentucky may be as large as 5 million
birds, 2 to 5 percent of which are usually brown-headed cowbirds.
Oklahoma roost numbers peak in November with 250 to 900 thousand birds
[35].

Demography:  Males are sexually mature at 1 year but rarely copulate
until they are 2 years old.  Females breed at 1 year [35].  Female
brown-headed cowbirds may lay up to 40 eggs in a season [52].  Stable
populations of brown-headed cowbirds would be maintained by 2.5 percent
survival rate and lifetime fecundity of 80 eggs per female.  Annual
adult survivorship is 48.5 percent for males and 40.4 percent for
females.  The longevity record is 15 years, 10 months for a banded male [35].

Mortality Factors:  Lowther [35] reviewed reports of ectoparasites,
helminth parasites, and infectious bacteria in brown-headed cowbirds.


PREFERRED HABITAT : 
Brown-headed cowbirds prefer open habitats of low or scattered trees
interspersed with grasslands; they usually avoid unbroken forest.  They
prefer, and may require, areas of short grass or bare ground for
foraging [50].  Open coniferous and deciduous woodlands, forest edges,
brushy thickets, agricultural land, and suburban areas are all
acceptable brown-headed cowbird habitat [11,24].  In the Sierra Nevada
brown-headed cowbirds are largely or completely absent from extensive
stands of old-growth forest.  They are rare in meadow and riparian areas
more than 16 miles (10 km) from centers of human activities.  In areas
close to human activities, brown-headed cowbirds were observed in
meadow-edge communities more often than in clearcuts, partially logged
forests, or uncut forests [70].  In Saskatchewan aspen (Populus spp.)
parklands, brown-headed cowbird abundance increased with increasing
grove size; however, brown-headed cowbirds occupied even very small
groves [29].  In the western states the brown-headed cowbird is more
abundant in second-growth than in old-growth forests, and more abundant
in stands surrounded by a large amount of open land [25].  In North
Dakota mixed-grass prairie, brown-headed cowbird density was higher on
plots with shrubs present than on shrubless plots [5].  In Texas
loblolly pine plantations less than 10 years old, brown-headed cowbirds
were more abundant on plots with snags than on snagless plots [12].

In Pennsylvania a mosaic of even-aged aspen stands (various ages) and
mixed-oak stands (either mature or early successional) was assessed for
use of edges by birds.  In mature aspen stands, there was no difference
between brown-headed cowbird use of edge and interior.  In stands that
were cut less than 10 years prior to the survey, brown-headed cowbirds
only occupied edges, and they were not present in recently cut (less
than 2 years) stands.  In mature oak stands, brown-headed cowbird use
was confined largely to edges; in stands cut less than 10 years prior to
the survey a few brown-headed cowbirds were observed in the interior and
none on the edges [72].


COVER REQUIREMENTS : 
Female brown-headed cowbirds use perches to spot potential host nests;
areas with hidden perches are preferred (i.e., shrubs in grass-dominated
communities) [35].

In Ohio autumn roosts were usually in dense stands of deciduous trees on
moist soils; tree crowns were generally narrow, vertical, and dense [40].
In the Southeast brown-headed cowbirds commonly roost in thickets [24].


FOOD HABITS : 
The brown-headed cowbird is a ground gleaner, consuming weed seeds,
grass seeds, waste grain, and insects.  Historically it may have
depended on grazing by large ungulates to create suitable feeding
conditions.  A common foraging technique is to follow large grazing
animals, gleaning seeds and the insects stirred up by the feet of the
grazer [11,39,35].  Brown-headed cowbirds also feed on insects present
on large ungulates; they have been observed walking on resting bison
(Bison bison), picking insects from the hide [39].  In the northeastern
United States plant foods common in brown-headed cowbird diets include
seeds of bristle grasses (Setaria spp.), ragweeds (Ambrosia spp.), oats
(Avena spp.), corn (Zea mays), crab grasses (Digitaria spp.), knotweeds
and smartweeds (Polygonum spp.), paspalums (Paspalum spp.), and
sheepsorrel (Rumex spp.).  In the Southeast plant foods include seeds of
bristle grasses, panic grasses (Panicum spp.), ragweeds, oats, wheat
(Triticum spp.), doveweeds (Croton spp.), corn, and paspalums [36].  A
summary of several dietary studies reveals that approximately 75 percent
of the brown-headed cowbird diet is weed seeds; grasshoppers and beetles
form the larger part of the remaining 25 percent of the diet [35].


PREDATORS : 
In northeastern Arkansas short-eared owl (Asio flammeus) pellets
contained brown-headed cowbird remains [60].  Predators on fledglings
include black racer (Coluber constictor), black rat snake (Elaphe
obsoleta), and blue jay (Cyanocitta cristata).  Brown-headed cowbirds
join in mobbing great horned owls (Bubo virginianus), a potential
predator of both adults and young [35].


MANAGEMENT CONSIDERATIONS : 
Range Expansion:  Historically brown-headed cowbirds were largely
confined to the mid-continental prairies where they presumably followed
herds of nomadic bison, and perhaps pronghorn (Antilocapra americana)
and elk (Cervus canadensis) as well [46].  The brown-headed cowbird
underwent a rapid range expansion with habitat alterations due to forest
clearing, domestic cattle grazing, urbanization, and conversion of
forested habitats to agricultural land [46].  It invaded the Great Lakes
States and the Northeast during the nineteenth century [44].

Root and Weckstein [49] reported that brown-headed cowbird winter range
has recently expanded into Maine and Nova but has contracted elsewhere,
particularly Pennsylvania, Michigan, Wisconsin, Iowa, Montana, and
Washington.  He speculated that the trapping of brown-headed cowbirds in
Michigan (for preservation of the Kirtland's warbler [Dendroica
kirtlandii]) that removed more than 40,000 brown-headed cowbirds was
partly responsible for this unexpected phenomenon.  Breeding Bird Survey
(BBS) data from 1965 to 1979 indicate that brown-headed cowbirds have
been increasing in the Southeast, including the Carolinas and southern
Georgia [44].  Colonization of the Sierra Nevada by brown-headed
cowbirds has taken place in the last 50 years [51].  Expansion into the
western states is due largely to rapid colonization by dwarf cowbirds
[32,50].  Rothstein [50] stated that the separation of breeding and
feeding areas has allowed much of this range expansion.  Since
brown-headed cowbirds can commute up to 3 miles (7 km) between breeding
areas and feeding areas, creation of feeding habitat (i.e., pack
stations, horse corrals, suburban developments, and bird feeders) within
formerly continuous forest has opened new areas for brown-headed cowbird
breeding [50].  In the Sierra Nevada brown-headed cowbird numbers
dropped sharply with distance from pack stations; they were always
present when horses were present [70].

Population Status:  Estimates of the North American population of
brown-headed cowbirds range from 20 to 40 million individuals [63].
Graber and Graber [76] estimated the 1957 brown-headed cowbird
population of Illinois as 1.1 million.  The estimated brown-headed
cowbird population in North Dakota in 1972 was approximately 1 million,
with a maximum density of 15.2 pairs per square mile (5.9 pairs/sq km)
[63].

Brown-headed cowbird populations have increased throughout the twentieth
century in most sections of the United States.  In the Northeast,
brown-headed cowbirds have experienced a statistically significant
population decrease [59].  A ranking system for neotropical migrants
(including brown-headed cowbird) listed the population trend for
brown-headed cowbird in Alaska as unknown (no data), decreasing in
Arizona (uncertain due to small size of the BBS), slightly increasing in
Idaho (also uncertain due to small BBS sample), and sharply increasing
in Montana [75].  In Oregon brown-headed cowbird populations increased
in juniper woodlands between 1899 and 1983, although they decreased
significantly statewide [35,55].  In the Sierra Nevada brown-headed
cowbird populations increased between 1966 and 1985 [43].  Breeding Bird
Survey data for the Midwest suggest a decrease in brown-headed cowbird
populations for the period 1966 to 1981, but an increase from 1982 to
1991 [66].  Significant population increases (BBS data 1966-1987)
occurred in Georgia, North Carolina, Iowa, North Dakota, Utah, and
Colorado.  Significant decreases occurred in Minnesota, Michigan,
Wisconsin, New York, Rhode Island, Ohio, Ontario, West Virginia,
Tennessee, New Brunswick, Oklahoma, and Texas [35].  Another
interpretation of BBS data indicates a generally negative trend for the
United States but a sharply increasing trend for Illinois during the
period 1966 to 1991 [26].

Host Species:  Because of range expansion the brown-headed cowbird has
come into recent contact with many new potential host species, most of
which lack defenses against nest parasitism.  Numbers of parasitized
species have increased in the last half century; new parasitism records
for species continue to be reported making estimation of the number of
parasitized species difficult.  Stewart and Robbins [64] listed 223
parasitized species in the United States.  In their area (Maryland and
the District of Columbia), the species most heavily parasitized were
red-eyed vireo (Vireo olivaceous), song sparrow (Melospiza melodia), and
chipping sparrow (Spizella passerina) [64].  In 1985, Friedmann and Kiff
[73] listed 240 species known to be parasitized, the majority of which
are neotropical migrant songbirds.  Lowther [35] reported that 144
species have actually reared brown-headed cowbird young.  Over the
entire United States the top 17 species with over 100 records of fledged
brown-headed cowbirds are yellow warbler, song sparrow, red-eyed vireo,
chipping sparrow, eastern phoebe (Sayornis phoebe), rufous-sided towhee
(Pipilo erythrophthalmus), ovenbird (Seiurus aurocappilus), common
yellowthroat (Geothylpis trichas), American redstart (Setophaga
ruticilla), indigo bunting (Passerina cyanea), yellow-breasted chat,
red-winged blackbird, Kentucky warbler (Oporomis formosis), willow
flycatcher (Empidonax traillii), Bell's vireo (Vireo bellii),
yellow-throated vireo (V. flavescens), and field sparrow (Spizella
pusilla) [35]

Brown-headed cowbirds reduce host productivity because 1) females remove
one egg from 33 to 90 percent of host nests, 2) brown-headed cowbird
eggs are thick-shelled, often causing breakage of host eggs, 3)
brown-headed cowbird eggs have a shorter incubation period than those of
host species, 4) host nestlings are usually smaller and less aggressive
than brown-headed cowbird nestlings, and 5) brown-headed cowbird
nestlings grow more rapidly, beg louder, and have larger gapes (bigger
mouths) than host nestlings [46].

Threats to Endangered Species:  Brown-headed cowbird parasitism is one
of the variables used in a scoring method to determined species
prioritization for songbird conservation [28].  Several neotropical
migrant songbirds with restricted ranges are endangered at least partly
as a result of brown-headed cowbird nest parasitism, including
Kirtland's warbler [37,48], Bell's vireo [42,48], golden-cheeked warbler
(Dendroica chrysoparia) [48], black-capped vireo (V.  atricapillus), and
willow flycatcher [46,48,53].  The precise effect of nest parasitism on
willow flycatcher has not been clearly established, however.  For willow
flycatcher and most of the other endangered species affected by
brown-headed cowbird parasitism, there is a complex interaction between
direct effects due to habitat loss and indirect effects related to nest
parasitism [50].

Threat to Other Species:  Brown-headed cowbirds pose a potential threat
to many neotropical migrant songbirds.  Brown-headed cowbirds feed
mostly in short grass communities including shortgrass prairie,
pastures, and lawns; and on bare ground.  Feeding areas are enhanced by
human activities [46].  At least 10 species of songbirds have declined
since brown-headed cowbirds have expanded their range into California,
possibly due to brown-headed cowbird nest parasitism [50].  On the
western slopes of the Sierra Nevada, no host species is immediately
threatened, but increased human use of backcountry areas and increased
fragmentation of dense forests will increase potential feeding areas for
brown-headed cowbirds [70].  Low intensity monitoring for neotropical
migrant conservation may include checks for cowbird parasitism every 3
years in management areas; monitoring is recommended for the western
Sierra Nevada [15,70].  Airola [2] recommended that any development
activities should be far from the highest densities of potential host
species.

Parasitism Rates and Habitat Parameters:  Brown-headed cowbird nest
parasitism has been enhanced by shifts in agricultural practice and
farmland structure in this century.  There is a higher rate of
brown-headed cowbird parasitism near field edges with elevated perches
than away from field edges and perches [48].  Brown-headed cowbirds are
often more abundant on edges than in interior.  Parasitism rates are
higher near forest edges and edges of prairie fragments [19].  In the
eastern deciduous forest, numbers of brown-headed cowbirds and rates of
parasitism decrease with distance from forest edges [9].  According to
Brittingham and Temple [9] forest fragmentation leads to higher levels
of nest parasitism by increasing the ratio of forest edge to forest
interior (defined as area more than 990 feet [300 m] from an edge).  In
the Midwest brown-headed cowbird parasitism is negatively correlated
with forest cover [47].  Nest parasitism is low (less than 10% of all
potential host nests) in extensively forested sections of Mark Twain and
Hoosier National Forests (Missouri and Indiana, respectively) [46].
However, Robinson and others [47] reported that in a highly fragmented
landscape with a long history of brown-headed cowbird presence, there
was no appreciable decline in nest parasitism even more than 2,310 feet
(700 m) from the nearest edge.  They speculated that in this area,
brown-headed cowbirds have saturated all available host nests because
brown-headed cowbird populations are high and host populations are
minimal.  In addition, brown-headed cowbird populations in the most
highly fragmented areas may be more limited by host availability than
feeding sites or forest density [47].

In western states and particularly in the Sierra Nevada, availability of
local feeding areas such as livestock corrals and pack stations is
associated with increased levels of brown-headed cowbird nest parasitism
[46].  Bock and others [8] listed the brown-headed cowbird as either
unresponsive or showing mixed or uncertain response to grazing in
grasslands in the western states but showed a positive response to
domestic cattle grazing in shrub-steppe communities.  Robinson and
others [46] concluded the magnitude of edge effect on brown-headed
cowbird parasitism rates varies within and among regions in relation to
landscape level variation in fragmentation and brown-headed cowbird
abundance.

Management Recommendations/Cowbird Control:  Methods for removal of
brown-headed cowbirds from critical breeding habitat for neotropical
migrants include trapping and shooting individuals, and roost kills.
The use of baited decoy traps to capture and kill female brown-headed
cowbirds has reduced the rate of nest parasitism and increased nesting
success for a few species.  It is thought that this action was the key
element in stabilizing populations of Kirtland's warbler [38,46].  In
1971 Mayfield [37] predicted that the Kirtland's warbler would be
extinct by 1980 if brown-headed cowbirds were not controlled.  An
extensive program of trapping and shooting of brown-headed cowbirds was
initiated in 1972.  The Kirtland's warbler population did not rebound,
but remained stabilized (with very low levels of nest parasitism) until
there was a slight population increase in 1991 and 1992 [38].  In other
cases use of decoys and shooting of female brown-headed cowbirds has
effectively reduced nest parasitism rates [32,46].  Removal of
brown-headed cowbirds from riparian habitats occupied by Bell's vireos
has resulted in increased Bell's vireo productivity [22].

It is not clear whether brown-headed cowbird trapping at the landscape
level would reduce nest parasitism in extensively fragmented landscapes
such as the Midwest.  Large-scale elimination of brown-headed cowbirds
at winter roosts may reduce overall numbers of brown-headed cowbirds,
but this approach may not affect target songbird populations in breeding
habitats.  Winter banding programs are needed to determine movements of
brown-headed cowbirds and the feasibility of winter trapping for
population reduction in critical areas [32].

The ethical implications of large-scale eradication of the brown-headed
cowbird, a native songbird, need to be considered [46].  The
brown-headed cowbird is protected under the Migratory Bird Treaty Act
[68].  Trapping and killing female brown-headed cowbirds is at best a
temporary solution to excessive nest parasitism.  The most complete
solution is landscape level management including consolidation of
ownership to preserve large tracts of forest, reduction or elimination
of brown-headed cowbird feeding areas within large tracts, and
minimization of edge [47].  Shape of forest tracts influences the ratio
of edge to interior.  Long narrow areas like riparian woodlands have
very little interior area.  Forest tracts in simple square or circle
shapes have the greatest interior to edge ratios [19].  Riparian
corridors need to be wide [46].  In the case of willow flycatcher in
California, protection of high-elevation riparian areas and meadows from
grazing and/or pack animals is recommended to reduce the threat of
brown-headed cowbird parasitism [53].  Laymon [32] recommended
elimination of grazing near riparian areas and removal of feedlots,
stables, and dairies in critical areas, and reforestation of riparian
areas.  Robbins and others [45] suggested that in the mid-Atlandtic
States, 7,410 acres (3,000 ha) of contiguous forest is the minimum
required for population maintenance of other songbirds in the presence
of brown-headed cowbird parasitism.  Data from moderately fragmented
areas of the Midwest suggest that 49 thousand to 1.2 million acres
(20,000-50,000 ha) may be necessary.  In Texas the Biological Advisory
Team [74] suggested that 4,940 to 12,700 acres (2,000-5,000 ha) are
needed to minimize the effects of brown-headed cowbird parasitism on the
golden-cheeked warbler [46].


REFERENCES : 
NO-ENTRY

Related categories for Wildlife Species: Molothrus ater | Brown-Headed Cowbird