United States

Department of

Agriculture

Forest Service

Rocky Mountain

Research Station

Fort Collins,

Colorado 80526

General Technical

Report RMRS-GTR-1

The Brown-Headed Cowbird and Its

Riparian-Dependent Hosts in

New Mexico

Sara H. Schweitzer

Deborah M. Finch

David M. Leslie, Jr.
Abstract

Schweitzer, Sara H.; Finch, Deborah M.; Leslie, Jr., David M. 1998. The brown-headed cowbird and

its riparian-dependent hosts in New Mexico. General Technical Report RMRS-GTR-1. Fort

Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. 23 p.

Numbers of brown-headed cowbirds (Molothrus ater) are increasing in some regions of North

America, while certain populations of long-distance, neotropical migratory songbirds (NTMs) are

declining. In the Southwestern United States, several species of NTMs nest only in riparian habitats.

The significant decline of two species of NTMs dependent upon riparian habitat, the southwestern

willow flycatcher (Empidonax traillii extimus) and the least Bell’s vireo (Vireo bellii pusillus), is of great

concern. Brood parasitism by the brown-headed cowbird and loss of riparian habitat may be the

primary causes of the decline of these populations. Extant data on the distribution, abundance,

density, and rates of parasitism of the brown-headed cowbird in New Mexico have not been

synthesized and interpreted. Our goal was to collect and review existing data on the brown-headed

cowbird in New Mexico, compare them to data from adjacent western states, and interpret the

findings. We hypothesized that increased human use of riparian habitats in New Mexico had resulted

in increased abundance of brown-headed cowbirds and their parasitism on riparian-dependent

NTMs. Our results suggest that quantitative studies should be conducted to determine the distribution, abundance, density, and rates of parasitism of brown-headed cowbirds in New Mexico’s riparian

habitats because existing data are inadequate. Results of such studies will allow conclusions to be

made about the multiplicative effects of riparian habitat use and modification by human activities on

cowbird and rare NTM populations.

Keywords: brood parasitism, brown-headed cowbird, Empidonax traillii extimus, Molothrus ater,

neotropical migratory songbirds, Rio Grande, riparian, southwestern willow flycatcher

The Authors

Sara H. Schweitzer is an assistant professor of wildlife ecology and management at the D. B. Warnell

School of Forest Resources, University of Georgia, Athens, GA 30602-2152.

Deborah M. Finch is a project leader and research wildlife biologist with the Rocky Mountain Research

Station, 2205 Columbia SE, Albuquerque, NM 87106.

David M. Leslie, Jr. is a unit leader and adjunct professor with the Oklahoma Cooperative Fish and

Wildlife Research Unit, Department of Zoology, 404 Life Sciences West, Oklahoma State University,

Stillwater, OK 74078-3051.

Publisher

Rocky Mountain Research Station

Fort Collins, Colorado

February 1998

Cover photos (left-right): Brown-headed cowbirds, Middle Rio Grande (slide collection of Deborah Finch);

blue grosbeak nest with 3 cowbird eggs (speckled) and 4 grosbeak eggs found at Rio Grande Nature Center,

Albuquerque, NM (photo by Wang Yong); male blue grosbeak captured at mist-net station, Rio Grande Nature

Center, NM (photo by Wang Yong).

You may order additional copies of this publication by sending your

mailing information in label form through one of the following media.

Please send the publication title and number.

Telephone (970) 498-1719

E-mail rschneider/rmrs@fs.fed.us

FAX (970) 498-1660

Mailing Address Publications Distribution

Rocky Mountain Research Station

3825 E. Mulberry Street

Fort Collins, CO 80524-8597
Contents

Introduction ................................................................................................... 1

Methods ........................................................................................................ 1

Riparian Habitats .......................................................................................... 1

Community Description ............................................................................ 1

Importance of Riparian Habitat to Birds ................................................... 2

Human Alterations to Riparian Habitats ................................................... 2

Effects of Human Settlement .................................................................... 3

Cowbird Natural History ................................................................................ 4

Description ................................................................................................ 4

Distribution ................................................................................................ 5

Life History ................................................................................................ 6

Habitat Use ............................................................................................... 6

Cowbirds and Their Hosts in New Mexico .................................................... 6

Historical Overview ................................................................................... 6

Summary .................................................................................................. 10

Management Implications ......................................................................... 20

Acknowledgments ......................................................................................... 20

Literature Cited ............................................................................................. 20

The Brown-Headed Cowbird and its

Riparian-Dependent Hosts in New Mexico

Sara H. Schweitzer, Deborah M. Finch, and David M. Leslie, Jr.

USDA Forest Service Gen. Tech. Rep. RMRS–GTR–1. 1998 1

Introduction

Numbers of brown-headed cowbirds (Molothrus ater)

are increasing in some regions, while certain populations

of long-distance, neotropical migratory songbirds (NTMs)

and other nonmigratory birds are declining. In the Southwestern United States, several species of NTMs nest only

in riparian habitats (Saab et al. 1995). Factors that contribute to the decline in populations of these songbirds include

fragmentation, degradation, and destruction of nesting

habitat. These changes to the nesting habitat of NTMs can

benefit the brown-headed cowbird, a brood parasite. The

cowbird’s habit of laying its eggs in nests of other species

can decrease the productivity of host species significantly.

Increases in the brown-headed cowbird population in

certain regions of the Southwest have been attributed to

human activities that caused riparian habitat loss and

degradation.

The significant decline of two NTM species dependent

upon riparian habitat, the southwestern willow flycatcher

(Empidonax traillii extimus) and the least Bell’s vireo (Vireo

bellii pusillus), is of great concern. Both species are federally and state listed as endangered (State Game Commission 1990, USFWS 1995). Brood parasitism by the brownheaded cowbird and loss of riparian habitat have been

cited as the primary causes of the decline of these populations (USFWS 1995). This review synthesizes the available

information on brown-headed cowbird populations and

uses of riparian habitat in New Mexico. Data from studies

conducted throughout the West and Southwest were incorporated as examples and references where there were

little or no data from New Mexico.

The objective of this review was to summarize existing

data and narratives from studies in New Mexico on:

• riparian communities and activities that have affected them;

• distribution and abundance of populations of brownheaded cowbirds and NTMs using riparian habitat

for nesting;

• rates of parasitism by cowbirds on NTMs; and

• nest success of NTMs with parasitized and

unparasitized nests.

Methods

Our review focused on riparian habitats associated

with the following major river basins within New Mexico:

the Rio Grande, San Juan, Pecos, Gila, and Canadian. An

extensive literature search and data synthesis were conducted on these areas in New Mexico and other Southwestern states (Arizona, southeastern California, and

southwestern Texas). Topics reviewed included: riparian

community ecology, passerine bird species associated with

riparian habitat, human use of riparian habitats (past and

present), and effects of these uses on bird species. The goal

of this synthesis was to provide a single-source reference

of historical review and present conditions of these communities in New Mexico. Special emphasis was placed on

the response of the brown-headed cowbird to changes in

riparian communities in New Mexico. Review of scientific

journals, communication with subject experts, and computer searches of abstracted and indexed literature provided the majority of the material.

Riparian Habitats

Community Description

Riparian communities in New Mexico, as elsewhere,

occur in or adjacent to a drainage and/or its floodplain

and are characterized by species and/or life forms different from those of the immediately surrounding communities (Lowe 1964, Brown et al. 1977). These riparian communities are characterized by broad-leaved, winter-deciduous phreatophytic trees (Gaines 1980). Brown et al. (1977),

in their delineation of riparian communities, described

them as “composed either of constituents peculiar to the

riparian situation, or an extension of a higher, climax

association fingering downward into the drainage way.”

A riparian area may be <1 m wide adjacent to a steeply

banked river or >1 km wide along lowland streams and

rivers (Deusen and Adams 1989). Because riparian habitats are associated with linear waterways, they are ecotonal

with high ratios of edge to surface area (Kauffman and

Krueger 1984).

The most extensive riparian vegetation in New Mexico

is associated with intermittent streams and rivers that

drain the precipitation from mountains (Dick-Peddie 1993).

These vegetative communities evolved to recolonize rapidly because occasional flooding could scour all vegetation including mature trees (Dick-Peddie 1993). Adaptive

mechanisms of riparian plants vary from prolific seed

production, to efficient dispersal, to efficient and rapid

vegetative reproduction.

Dick-Peddie (1993) classified riparian habitats of New

Mexico into four groups:

1. Mature montane riparian vegetation, which often

takes the form of a closed-canopy or gallery forest.
2 USDA Forest Service Gen. Tech. Rep. RMRS–GTR–1. 1998

The closed canopy greatly reduces soil temperature

during the growing season and increases available

soil moisture. Mature trees resist all but the most

extreme scouring by floodwaters and thereby reduce

erosion, contain channel banks, and retard nutrient

loss (Dick-Peddie 1993).

2. Floodplain-plain riparian vegetation, which is typically found on older, meandering river systems where

there are extensive floodplains (e.g., the Rio Grande).

3. Arroyo riparian vegetation, which occupies drainages that dissect bajadas and mesas of New Mexico

(Dick-Peddie 1993).

4. Closed basin-playa-alkali sink riparian habitats,

which are internally drained depressions in varied

watershed sizes. These communities are usually

broad, flat, or gently sloping areas where water

tends to spread. Water moves slowly after light rains

and tends to evaporate, producing an increase in

salinity. Dense stands of native fourwing saltbush

(Atriplex canescens) are common in closed basins.

The studies herein were conducted primarily on

floodplain-plain riparian vegetation.

Importance of Riparian Habitat to Birds

Riparian habitats constitute only about 1% of the landscape in the Southwest (Knopf et al. 1988, Ohmart 1994),

but they support greater wildlife diversity and abundance

than most other community types (Deusen and Adams

1989, Dick-Peddie 1993, Ohmart 1994, Thompson et al.

1994). More than 60% of vertebrates found in riparian

habitats of the Southwest are obligates (Ohmart and Anderson 1982). The San Juan and Gila River Valleys of New

Mexico are extremely rich in avian species. Hubbard

(1977) believed that 16-17% of the breeding bird species of

temperate North America nest in these valleys. About

40% of the 94 bird species known to breed in riparian areas

of the Southwestern United States nest in riparian habitats

of the Rio Grande (Schmidly and Ditton 1979).

Studies in Arizona and California found that riparian

habitats of cottonwood-willow (Populus-Salix spp.) supported more nesting bird species than any other vegetation type (Ingles 1950, Brown et al. 1977). Cottonwoodwillow forests provide a stratified foliage profile and,

when mature, a habitat niche for a shade-tolerant understory of younger or smaller trees, shrubs, vines, and forbs

(Gaines 1980). Habitat diversity and species richness are

greater in riparian areas than in surrounding habitats

because the composition of vegetation tends to be independent of the upland vegetation.

Many researchers have noted that populations of several NTMs have declined over the last 25 years (Robbins et

al. 1989, Terborgh 1989, Askins et al. 1990, Finch 1991,

Petit et al. 1995). In the Southwest, NTMs frequently select

riparian habitat for nesting sites (Carothers et al. 1974,

Ohmart 1994). Hubbard (1977) estimated that 25% of

breeding avifauna in the Gila and San Juan River Valleys

of New Mexico was restricted to riparian habitat. Ten of 32

avian species listed as endangered by the New Mexico

Department of Game and Fish (State Game Commission

1990) depend on riparian habitat for breeding and/or

feeding sites, and four of these are NTMs. Throughout the

Southwest, lowland desert riparian habitats support a

disproportionate number of rare and endangered bird

species (Johnson et al. 1987).

Human Alterations to Riparian Habitats

In 1979, Schmidly and Ditton stated, “exploitation by

man has greatly altered the riparian habitats of the Southwest, and, in the last 100 years, the rate of alteration has

increased significantly.” More than 15 years have passed

since this statement was made and concern for New

Mexico’s riparian communities has increased (Crawford

et al. 1993).

Johnson and Jehl (1994) estimated that “95% of riparian

woodland, the richest ecologic formation for nesting birds

in western North America, has either been degraded or

destroyed in the past century by water management,

agriculture, and domestic livestock grazing.” They stated

that domestic livestock grazing was the most pervasive

threat to riparian habitats and their avifauna and implicated nest parasitism by brown-headed cowbirds as causing population declines in several riparian-nesting bird

species. Degradation or destruction of riparian habitats

and an increase in abundance of cowbirds are synergistic.

Cowbirds are “edge” species (O’Conner and Faaborg 1992)

that benefit from increased access to passerine nests in

fragmented riparian woodland, insects and waste grain

from agricultural fields, and grazing livestock in and

adjacent to riparian habitat (Rothstein 1994).

Riparian habitats benefit human populations by providing water, rich soils for agriculture, lush forage for

domestic livestock, recreational opportunities, and home

sites (Ohmart and Anderson 1986). Because of these benefits, human activities have altered riparian habitats for

centuries (Deusen and Adams 1989, Rothstein 1994). Since

the late 1800s, however, alterations to riparian habitats in

the Southwest have been on larger spatial and shorter

temporal scales than those implemented earlier by American Indian and Spanish inhabitants (Rosenberg et al. 1991).

Many native plant and animal species have not been able

to withstand these recent, extreme changes to their environment (Dick-Peddie 1993).

The riparian forest habitats of New Mexico are experiencing increased impacts from the demands of a growing
USDA Forest Service Gen. Tech. Rep. RMRS–GTR–1. 1998 3

human population. Loss of riparian habitat could result in

loss of about 46% of bird species breeding in the San Juan

Valley (Schmitt 1976) and about 49% of bird species breeding in the Gila River Valley (Hubbard 1971). The Middle

Rio Grande (MRG) Biological Interagency Team completed the Bosque Biological Management Plan in 1993

(Crawford et al. 1993) to mitigate stresses experienced by

the MRG riparian ecosystem. This plan recognized the

need to avoid fragmentation of riparian forests and to

enhance cottonwood communities.

Effects of Human Settlement

Plants and animals in riparian habitats were used by the

local populations of American Indians and Spaniards.

There was little change in the vegetation along the lower

Rio Grande between 1582 and 1846, but by the late 1800s

to early 1900s most of the gallery forests of cottonwoods

had been removed for agriculture, building material, livestock forage, and fuel (Engel-Wilson and Ohmart 1979).

Old settlements have now become urban centers (e.g.,

Albuquerque), and demands on riverine resources have

increased significantly. In response, reservoirs have been

created to provide drinking and irrigation water, and

remaining riparian woodlands have been cleared for agricultural and urban development. More than 2,800 ha of

wetlands of the Middle Rio Grande ecosystem have been

drained for agriculture and development (Funk 1993).

The consequences of human population growth on

riparian resources are similar throughout the Southwest.

Concern about damage to river hydrology and the native

riparian ecosystem from dam construction (e.g., Davis

Dam, Glen Canyon Dam) on the Colorado River (Carothers

et al. 1974) was the impetus for several avian research

projects within riparian habitats of the lower Colorado

River. Results from these studies indicated that species

composition and structural complexity of riparian vegetation (vertical and horizontal) strongly influenced abundance and richness of avian species. Anderson and Ohmart

(1977) assessed the importance of vegetative structure to

bird populations relative to spatial and temporal dimensions in the riparian environment of the lower Colorado

River. Carothers et al. (1974) found greater numbers of

bird species and total populations of nesting birds in

homogeneous cottonwood stands along the Verde River,

Arizona, than in mixed deciduous areas along its tributaries. Their findings demonstrated the importance of spatial

diversity among habitats and foliar height diversity within

habitats to bird species diversity and riparian community

structure.

Woody riparian vegetation has increased down river of

the Glen Canyon Dam on the Colorado River in Arizona

because natural, annual scouring flows no longer occur

(Carothers and Sharber 1976, Brown and Johnson 1985).

This new habitat provides nesting sites for riparian breeding birds where none existed previously. Carothers and

Sharber (1976) found that summer resident bird species

were restricted almost exclusively to the narrow belt of

riparian vegetation along the Colorado River.

The Amistad Dam on the lower Rio Grande caused the

loss of the original shoreline and its riparian vegetation

(Schmidly and Ditton 1979). This habitat alteration affected bird populations that used the riparian vegetation

for nesting, foraging, roosting, and cover during migration.

The floodplain-plains riparian community of New

Mexico has suffered greatly from human activities (DickPeddie 1993, deBuys 1993). Cottonwood and willow regeneration is minimal. Exotic species, notably saltcedar

(Tamarix spp.) and Russian olive (Elaeagnus angustifolia),

are increasing, and dumping, arson, and vandalism from

adjacent urban dwellers are frequent (Dick-Peddie 1993,

deBuys 1993).

The introduction and spread of exotic plant species,

such as saltcedar, throughout the Southwest have affected

plant species composition, richness, and diversity in riparian communities. Saltcedar has replaced native cottonwood and willow trees at many places along the lower Rio

Grande (Schmidly and Ditton 1979). Virtually no cottonwood gallery forests remain between El Paso and Presidio,

Texas, on the lower Rio Grande. Only a few isolated

cottonwoods remain along ditches or canals apart from

tall, dense saltcedar forests (Schmidly and Ditton 1979).

Between 1967 and 1971, about 21,600 ha of riparian

vegetation were removed along the Pecos River (Ohmart

and Anderson 1986). This vegetation had roots in perennial ground water or in the capillary fringe above a water

table (phreatophytic). Because most of these species transpired large quantities of water, managers believed that

water would be saved if the species were removed. However, results from studies along the Pecos River suggest

that an insignificant amount of water is saved if phreatophytes are removed (Ohmart and Anderson 1986).

Along the lower Rio and Pecos River, passerine species

used saltcedar more than they did in the lower Colorado

River system (Hunter et al. 1988). This suggests that native

avifauna can adapt to and benefit from introduced plant

species in some areas.

Alluvial riparian floodplains have been converted to

agricultural fields primarily along larger rivers (Ohmart

1994), and water has been provided to the entire floodplain

via irrigation canals and ditches. Impacts of such agricultural practices on bird populations are diverse. Densities of

some bird species increase in the presence of agricultural

land because they use crops as a food source (Carothers et

al. 1974, Anderson et al. 1984). In Arizona, where cultivated fields were adjacent to riparian habitat, Anderson et

al. (1984) found that when resources in other areas were

scarce, the interface accommodated large densities of
4 USDA Forest Service Gen. Tech. Rep. RMRS–GTR–1. 1998

birds such as the mourning dove (Zenaida macroura),

Gambel’s quail (Callipepla gambelii), white-crowned sparrow (Zonotrichia leucophrys), and loggerhead shrike (Lanius

ludovicianus). Thus, Anderson et al. (1984) concluded that

interface habitat was important in maintaining avian populations that normally inhabit riparian areas or migrate into

the lower Colorado Valley. Rosenberg et al. (1991) suggested that significant increases in populations of species

associated with agricultural fields created from alluvial

riparian floodplains (e.g., cowbirds, European starlings

[Sturnus vulgaris], and house sparrows [Passer domesticus])

may have adverse effects on populations of sensitive avian

species. Also, most croplands do not provide suitable

nesting habitat for NTM birds (Petit et al. 1995).

In the far West, the settlement of the Central Valley of

California in the late 1800s significantly changed the nesting avifauna in forested riparian habitats (Gaines 1980).

Riparian vegetation removal was one of the first significant alterations to the Central Valley (Katibah 1984). Today, the banks of the Sacramento River and its tributaries

are bordered by many remnants of the once extensive

riparian woodland (Thompson 1980). Breeding populations of at least 11 bird species, including the willow

flycatcher (Empidonax traillii) and Bell’s vireo (Vireo bellii),

declined significantly, while breeding populations of the

brown-headed cowbird and the European starling increased significantly in response to riparian woodland

cutting and agriculture development (Gaines 1980). The

population of brown-headed cowbirds increased through

natural range expansion. The European starling, introduced to America’s east coast in the 1800s, adapted well to

human-disturbed habitats and native bird species’ nest

cavities.

Along the Colorado, Gila, Salt, and Santa Cruz Rivers of

Arizona, Hunter et al. (1987) found that loss of mature

broadleaf trees (e.g., cottonwoods) and snags coincided

with the decline of large raptors and cavity nesters. Loss of

mixtures of broadleaf trees and shrubs at low elevation

river systems resulted in the decline or absence of nine

NTMs (Hunter et al. 1987) that had lost their preferred nest

and perch sites. In addition, changes in plant composition

and structure at these sites may have coincided with

microhabitat changes (e.g., temperature and humidity)

that were unacceptable to many nesting avian species

(Hunter et al. 1987).

Unmanaged grazing by domestic livestock in riparian

habitat is frequently detrimental to avian communities

(Saab et al. 1995). Riparian areas attract livestock because

they provide shade and nutritious, palatable forage (Ames

1977, Kauffman and Krueger 1984). Because grazing varies so much in its local intensity, its impact on breeding

birds is not uniform nor easily defined (Kauffman and

Krueger 1984). Mosconi and Hutto (1982) examined the

effects of heavy grazing (2.5 cow-calf units/ha) and light

grazing (0.3 cow-calf units/ha) in riparian areas on bird

populations. Although there were no significant differences in total bird densities between sites, bird species

composition and foraging guilds were significantly different. Flycatchers were most affected by grazing. The foraging guilds affected most were ground-foragers and foliage-gleaners. Passerines associated with riparian habitats

are generally vulnerable to overgrazing. Cowbirds benefit

from foraging with grazing animals by catching insects

that are flushed (Ryder 1980). Livestock grazing in and

adjacent to riparian habitat likely would enhance the

quality of such sites for cowbirds, and provide them with

greater access to nests of flycatchers and other NTMs, as

well as improved foraging opportunities.

Riparian vegetation is used by numerous resident and

short- and long-distance NTMs during migration, breeding, and wintering because of its species composition and

structure. These communities also support human activities associated with an increase in abundance of some bird

species and a decrease in others. The challenge faced by

the land manager is to balance human uses of riparian

areas with the needs of dependent communities of native

flora and fauna, and with the recovery of rare and endangered species (e.g., the southwestern willow flycatcher and

least Bell’s vireo). The brown-headed cowbird presents a

special challenge because this species profits from human

activity while most long-distance migratory birds decline,

and the cowbird frequently selects long-distance migrants as

hosts for its offspring (Friedmann and Kiff 1985), further

decreasing recruitment success of long-distance migrants.

Cowbird Natural History

Description

Cowbirds are in the family Emberizidae (warblers,

tanagers, grosbeaks, sparrows, and blackbirds) and the

subfamily Icterinae. Adult cowbird males weigh from 40

to 50 g and adult females are 10% smaller than males

(Lowther 1993). The length of both adult males and females averages 19 cm from tip of bill to tip of tail (National

Geogr. Soc. 1987). Cowbirds are terrestrial or semiterrestrial birds, and all but one genus are parasitic (Friedmann

1929). The nonparasitic Agelaioides and the parasitic

Tangavius genera are distributed in South America. The

Molothrus genus contains four parasitic species that are

distributed in North and South America and the West

Indies (Friedmann 1929). Species of Molothrus are characterized by plumage dimorphism with dark males and

lighter-colored females. Male brown-headed cowbirds

(M. ater) are blackish with a greenish metallic sheen and

brown head and neck (Friedmann 1929, Lowther 1993).
USDA Forest Service Gen. Tech. Rep. RMRS–GTR–1. 1998 5

Females are brown, do not have a metallic sheen, and

sometimes are indistinctly streaked below (Friedmann

1929, Lowther 1993). Although three species of Molothrus

breed in the United States (brown-headed cowbird, shiny

cowbird [M.bonariensis], and bronzed cowbird [M. aeneus]),

the brown-headed cowbird is the most abundant and

widely distributed (Lowther 1993).

Because Molothrus spp. are obligate brood parasites,

they depend on avian host species to incubate their eggs

and rear their young. Brown-headed cowbirds parasitize

at least 220 avian species (Friedmann and Kiff 1985), but

results from several studies suggest that they select certain

hosts, perhaps returning to the species that raised them

(Post and Wiley 1977, Walkinshaw 1983, Friedmann and

Kiff 1985). Mayfield (1965) hypothesized, however, that

the apparent selectivity of cowbirds for certain hosts may

result from failure of observers to find parasitized nests

abandoned by a host early in the incubation period.

Three subspecies of the brown-headed cowbird are

recognized: the dwarf cowbird (M. a. obscurus), Nevada or

sagebrush cowbird (M. a. artemisiae), and brown-headed

cowbird (M. a. ater) (Freidmann 1929, Laymon 1987,

Rothstein 1994). Distributions of the dwarf and sagebrush

subspecies are farthest west, primarily California, Arizona, and Nevada (Bailey 1928, Friedmann 1929,

Friedmann 1971). The sagebrush cowbird also is found in

the Great Basin and Western Great Plains regions of the

U.S. (Lowther 1993). The sagebrush and dwarf cowbirds

may be closely related. Grinnell (1909) believed that the

sagebrush cowbird was derived from the dwarf cowbird

and not from the brown-headed cowbird that is found east

of the Mississippi River. Bailey (1928) stated that the dwarf

cowbird was relatively common in the southwestern corner of New Mexico. Hereafter, unless otherwise noted,

reference to the brown-headed cowbird refers to the

M. a. ater subspecies.

Distribution

The brown-headed cowbird is endemic to the shortand mixed-grass prairies of North America (Great Plains

and Great Basin regions; Mayfield 1965, Rothstein 1994),

where they developed a commensal relationship with

native herds of bison (Bison bison) and other large ungulates. Originally, cowbirds were known as buffalo birds

(Friedmann 1929). As these large animals grazed, they

flushed insects and reduced the amount of vegetative

ground cover, increasing the availability of insects and

seeds. Bailey (1928:660) described the brown-headed cowbird as a “fairly common breeder over much of the lower

parts of New Mexico except the southwestern corner.”

Her data and references indicated that New Mexico was

the western edge of the brown-headed cowbird’s range.

Before settlement, the brown-headed cowbird was rare

in the tallgrass prairies and unbroken tracts of forest in the

Eastern United States (Mayfield 1965). These habitats did

not provide adequate foraging opportunities for cowbirds. By the late 1700s, the Eastern forests had been

opened substantially by loggers and farmers. In addition,

herdsmen opened pathways to the West and began to

increase the number of cattle, sheep, and swine grazing

and trampling the tallgrass prairie (Mayfield 1965). The

cowbird expanded its range and increased in abundance

in response to these activities (Mayfield 1965, Brittingham

and Temple 1983, Friedmann and Kiff 1985).

In the West, Spanish explorers and missionaries traveled north from Mexico and established small settlements

in the Lower Colorado River Valley and along the Middle

Rio Grande in the late 1500s to 1600s (Bailey 1928, Rothstein

1994). The Spaniards brought livestock with them, which

may have enabled the dwarf cowbird to become common

by the early 1900s along the Colorado River, in the Tucson,

Arizona area, and farther east into southern Texas

(Rothstein 1994). Hanna (1928) first reported observations

of dwarf cowbirds in the San Bernardino Valley, California in 1918, and by the late 1920s, he stated that the

“continual increase of the dwarf cowbird ... caused me

considerable alarm.” In the San Gabriel River Valley,

California, Rowley (1930) noted an apparent increase in

dwarf cowbird numbers from 1920 to 1929. The sight of a

cowbird had been uncommon and only seven parasitized

nests were found from 1920-1924, but from 1925-1929,

Rowley (1930) found 21 parasitized nests in the San Gabriel

River Valley. During a banding study near Phoenix, Arizona in 1942, Neff (1943) commented that dwarf “...cowbirds were so numerous as to be a nuisance about the

traps....”

By the 1900s, the Nevada cowbird, a derivative from the

dwarf cowbird subspecies (Grinnell 1909), was widespread

throughout the Great Basin and adjoining parts of Oregon

and Washington east of the Cascades (Laymon 1987,

Rothstein 1994). This expansion may have been influenced

by Anglo-American settlement of the region (Rothstein

1994).

Currently, the continued fragmentation of habitat by

intensified agriculture, livestock production, timber harvest, and urbanization has provided cowbirds with better

access to foraging sites and nests of forest interior hosts.

While brown-headed cowbirds were originally restricted

primarily to the central grasslands, they are now ubiquitous throughout North America. Likewise, the distribution of the dwarf and Nevada cowbird populations has

increased in the far West. Analyses of the U.S. Bird Banding Lab’s Breeding Bird Survey data from New Mexico

determined that brown-headed cowbird numbers have

increased in New Mexico from 1968 to 1994 (Mehlman

1995). Mehlman concluded that the increasing cowbird

population in New Mexico was probably a result of the

increasing human population and urbanization, changes

in agricultural practices, or possibly a change in cowbird
6 USDA Forest Service Gen. Tech. Rep. RMRS–GTR–1. 1998

distribution from the midwestern prairies to the Southwest. Mehlman did not attribute the increasing population

trend to the presence of domestic livestock because livestock

operations have been present in New Mexico since the

Spanish settlements were established in the 1600s.

Life History

Brown-headed cowbirds are short distance migrants

within North America. In the winter, cowbirds travel and

form large roosts with other icterids, but they are frequently

found at the margins of such aggregations (Lowther 1993).

Spring migration to breeding habitat begins in March, and

first eggs are laid in hosts’ nests in mid-April (Lowther

1993). Egg-laying continues until mid-July (Lowther 1993)

when foraging flocks are formed and migration to winter

habitat begins. Brown-headed cowbird eggs are oval; average length and width are 21.45 x 16.42 mm (n=127; Bent

1965). Eggs have a white to grayish-white background

color overlain by brown/gray spots which are usually

more dense at the larger end of the egg (Lowther 1993).

The incubation period for cowbird eggs ranges from 10

(Briskie and Sealy 1990) to 12 days (Nice 1953). The host

species provides all care for the egg(s). Cowbird eggs are

laid synchronously with the host’s eggs but usually hatch

first because of their relatively short incubation period. In

addition, cowbirds frequently parasitize hosts whose eggs

are smaller than their own; thus, the host inadvertently

provides more warmth to the cowbird egg, resulting in its

hatching first (Friedmann 1929:187).

Cowbirds are altricial (naked, blind, move little, dependent on adults for food) and nidicolous (remain in the

nest). Nice (1939) observed the development of a cowbird

chick and reported the following: short flight at 11 days,

hopped at 11 to 14 days, walked at 15 days, pecked at insect

at 14 days, drank at 16 days, and picked up food at 17 days.

Chicks leave the nest at about 8 to 13 days and are independent at 25 to 39 days (Woodward and Woodward 1979).

Male and female cowbirds are sexually mature at one

year, but males may not breed during their first year

(Lowther 1993). Studies in Canada found that a single

female may lay >40 eggs per breeding season (Scott and

Ankney 1980). Scott and Ankney (1980) used the product

of the probabilities of survival from egg to fledging, fledging to independence, and independence to breeding to

estimate that the probability of survival of a cowbird egg

to adult cowbird was about 0.03. A stable population is

maintained with this survival rate and a lifetime fecundity

of 80 eggs per female (Scott and Ankney 1980).

Habitat Use

Brown-headed cowbirds use different habitats for breeding, roosting, and feeding. Breeding ranges of female

cowbirds overlap host breeding areas, and ranges of male

cowbirds overlap those of females. The primary intent of

male cowbirds is to gain access to breeding females, while

female cowbirds must gain access to hosts’ nests (Rothstein

et al. 1986). Aggression or expression of territoriality by

cowbirds stems from competitive access to breeding females or to hosts’ nests.

Female cowbirds are active but generally silent in breeding ranges in the morning (Rothstein et al. 1984). They are

highly mobile and perch or fly high above the ground

(Rothstein et al. 1984), presumably searching for hosts’

nests. Breeding ranges are rarely used for foraging

(Rothstein et al. 1984), but Yokel (1986) found that cowbirds feed opportunistically on cicadas in breeding ranges

in a California riparian habitat. Morning ranges of males

frequently overlap female breeding ranges (Yokel 1986),

and males often travel with females (Rothstein et al. 1984).

Males are active and vocal in the morning and in feeding

ranges.

Sizes of female breeding ranges are associated with

density of hosts’ nests. Rothstein et al. (1980) determined

that cowbird density was greater in riparian habitat than

in open coniferous forests of the Sierra Nevada. Rothstein

et al. (1984) suggested that cowbirds preferred dense

riparian vegetation.

Cowbirds feed in flocks up to 7 km from breeding

ranges (Rothstein et al. 1984). They are ground-feeders

that walk among and near large grazing animals to feed on

insects that are flushed and seeds that are exposed in the

grazed vegetation and bare ground. They also feed in

agricultural fields, livestock paddocks or corrals, and urban yards and feeders. Rothstein et al. (1984) found that

female cowbirds used feeding sites from afternoon (>1115

hr) to dusk (>1815 hr) and were relatively sedentary at

these sites. After dusk, females tended to roost in breeding

ranges. Males also commuted to feeding sites in the afternoon but tended to move among sites more frequently

than females. Commuting between morning breeding

ranges and afternoon feeding sites may not be necessary if

forage is abundant near breeding ranges.

Cowbirds and Their Hosts in

New Mexico

Historical Overview

The first systematic survey of New Mexico fauna began

in 1903 under the direction of C. Hart Merriam and Vernon

Bailey (Bailey 1928). Bailey’s book summarizes this systematic survey and earlier field notes. Merriam and Bailey’s
USDA Forest Service Gen. Tech. Rep. RMRS–GTR–1. 1998 7

surveys found that the brown-headed cowbird was a

fairly common breeder over much of the low elevation

region of New Mexico, except the southwestern corner at

altitudes from 1,100 to 2,000 m. The dwarf cowbird was

found only in the southwestern corner of New Mexico.

A few cowbirds were seen with almost every herd of

cattle in the 1900s (Bailey 1928). At Mesilla Park, hundreds

of cowbirds formed flocks of adults and fledglings in late

July. Cowbirds migrated through the southern half of

New Mexico from September to October to their wintering

sites in Mexico. All cowbirds left New Mexico in the winter

and returned in late April (Bailey 1928). Bailey noted that

cowbirds parasitized 90 species of birds smaller than

themselves. Nests of the willow flycatcher (Empidonax

trailli brewsteri [formerly the Traill’s flycatcher]) and two

vireo species were listed as recipients of cowbird eggs.

Ligon (1961) published the second comprehensive book

on the birds of New Mexico. He stated that the brownheaded cowbird was common and widespread, and that it

posed a “serious limiting factor in the populations of

smaller birds.” He explained that cowbirds congregated

in large flocks in the fall and often did considerable damage to unharvested head grains. Ligon (1961) reported that

cowbirds were found statewide and at altitudes up to

2,400 m in the summer. The bronzed cowbird was reported to breed in the extreme southwestern corner of

New Mexico. One of four nests of the willow flycatcher

that were found in riparian habitat along the lower Rio

Grande was parasitized by a cowbird (Ligon 1961). The

least Bell’s vireo, described as a very rare species, was

found only in the southwestern corner of New Mexico, but

no mention was made of its nests being parasitized.

Hubbard (1971) surveyed the summer birds and habitat of the Gila River Valley, New Mexico and reported that

the brown-headed cowbird was uncommon to common

throughout the valley. The vermilion flycatcher

(Pyrocephalus rubinus), Lucy’s warbler (Vermivora luciae),

and yellow-breasted chat (Icteria virens) were cowbird

hosts. The Bell’s vireo (Vireo bellii arizonae) was considered

rare to uncommon in the riparian habitat and valley mesquite. The bronzed cowbird (Molothrus aneus loyei) was an

occasional to irregular summer resident of the Gila River

Valley and was classified as an addition to the avifauna

since the late 1950s to early 1960s. Hubbard (1971) noted

that the brown-headed cowbird was most abundant in

riparian habitats, which are important population centers

for most of the passerine species. Hubbard (1971) recorded 143 bird species in the Gila River Valley at the

height of the nesting season; 112 probably nested in the

valley.

Schmitt (1976) conducted a similar study in the San

Juan Valley in northwestern New Mexico. The brownheaded cowbird was casual to regular throughout the

valley and was fairly common to locally common in riparian woodland and adjacent habitats. The willow flycatcher

was an occasional breeding bird in riparian woodlandshrubland habitat. He recorded 105 species of birds in the

San Juan Valley; 100 nested there.

Wauer (1977) conducted a survey along the lower Rio

Grande of western Texas. He concluded that the riparian

habitat of this area provided a significant migration corridor within the surrounding arid habitat. The mourning

dove, verdin (Auriparus flaviceps), northern oriole (Icterus

galbula), brown-headed cowbird, and house finch

(Carpodacus mexicanus) exhibited the greatest selection for

riparian habitat. The Bell’s vireo was abundant during the

breeding season and was unaffected by the abundance of

brown-headed cowbirds. The bronzed cowbird was a

summer resident within the riparian zones of Big Bend

National Park since 1969. The primary hosts for the bronzed

cowbird were hooded (Icterus cucullatus) and orchard (I.

spurius) orioles (Wauer 1977).

Elephant Butte Marsh, one of the largest remaining

wetlands in the Middle Rio Valley, was the study site for

Hundertmark’s (1978) survey of avifauna during the 1970

to 1975 breeding seasons. In his annotated list of breeding

birds, he stated that the brown-headed cowbird was regularly found in riparian habitats and was fairly common

overall. He found cowbird eggs in nests of willow flycatcher, Lucy’s warbler, and red-winged blackbird

(Agelaius phoeniceus). He found that the willow flycatcher

was fairly common in riparian woodland or shrubland

and that nearly all nests were over water. Of the 64 species

known or believed to breed in Elephant Butte Marsh,

Hundertmark (1978) found that 42% were restricted to

riparian habitat.

Surveys along the Gila River during a six-month period

in 1975 by Baltosser (1986) indicated that 112 species used

the habitat, and that it was an important spring migration

corridor. The Gila River Valley supported a great number

of bird species in a relatively small area.

Freehling (1982) provided some of the first quantitative

data on the density of breeding birds in New Mexico. He

recorded 35 species of breeding birds, including the brownheaded cowbird (tables 1, 2, 3), during the breeding seasons of 1979 and 1980 along the Middle Rio Grande.

Hildebrandt and Ohmart (1982) conducted a study along

the Pecos River from 1979-1981 (table 1, figure 1) to determine the quality of the habitat for breeding, migrating,

and wintering birds. Riparian habitat was selected over

other habitats by breeding birds, but birds were most

abundant in the Pecos River Valley in autumn. Hildebrandt

and Ohmart (1982) suggested that the abundance of brownheaded cowbirds (table 2) could be used as an indicator of

the nesting potential of a habitat for small to mediumsized birds that build open-cup nests, which are typical

cowbird hosts. The blue grosbeak (Guiraca caerulea) had

the greatest rate of parasitism in this study (Hildebrandt

and Ohmart 1982).
8 USDA Forest Service Gen. Tech. Rep. RMRS–GTR–1. 1998

NEW

MEXICO

PRIMARY

STUDY

AREA

SECONDARY

STUDY

AREA

SECONDARY

STUDY

AREA

TEXAS

20

Oria

Lake McMillan

Lake Arthur

Fort Sumner

Roswell

Carlsbad

Malaga

Artesia

Pecos

20

Arroyo del

Macho

Salt Cr.

Rio

Pecos

River

Hondo

Rio

Rio

Felix

Penasco

Delaware Cr.

NEW

MEXICO

PRIMARY

STUDY

AREA

SECONDARY

STUDY

AREA

SECONDARY

STUDY

AREA

TEXAS

20

Oria

Lake McMillan

Lake Arthur

Fort Sumner

Roswell

Carlsbad

Malaga

Artesia

Pecos

20

Arroyo del

Macho

Salt Cr.

Rio 

Pecos

River

Hondo

Rio

Rio

Felix

Penasco

Delaware Cr.

Figure 1. Area of New Mexico studied by Hildebrandt and Ohmart (1982).

Surveys by Hink and Ohmart

(1984) along the Middle Rio

Grande (figure 2) during 1981 and

1982 found 277 bird species in

bosque and adjacent agricultural

habitats. More than 60% of the

bird species known to occur in

New Mexico were found within

these habitats. Breeding birds

tended to concentrate in cottonwood forests of the valley.

Invasion and proliferation of

saltcedar within the Pecos River

Valley provide woody structure

where little to none existed previously (Hunter et al. 1988). Several bird species have increased

their distribution from Texas to

the Pecos River Valley of New

Mexico. Bird species that used

cottonwood-willow habitat for

nesting habitat within the lower

Rio Grande Valley have persisted,

in lower numbers, by nesting in

saltcedar habitat (Hunter et al.

1988).

Hoffman (1990) conducted breeding and spring migration season

studies in Rio Grande Valley State

Park, within the Middle Rio

Grande Valley, from 1987 to 1990

(table 1). He determined that

brown-headed cowbirds were

relatively common during the

summer breeding season (45/

100 acres; tables 2, 3). The

most common breeding birds

were black-chinned hummingbirds (Archilochus alexandri),

mourning doves, black-headed

grosbeaks (Pheucticus melanocephalus),

and American robins (Turdus

migratorius); none of these are

common hosts for brown-headed cowbirds. Hoffman

(1990) recorded 130 species during his study; 69 probably

nested in the study area.

During winter 1992 and spring through summer 1993,

Thompson et al. (1994) surveyed bird species at five different habitat types within the Middle Rio Grande Valley

(table 1, figure 3). They recorded 162 species along their

transects. Species richness was greatest in sites where

Russian olive and saltcedar were codominants. They detected >98% of the neotropical migratory species that

were present in this area historically. The brown-headed

cowbird was not considered common in any of the five

habitat types surveyed (tables 2, 3). The greatest numbers

of resident and riparian-dependent bird species were

within undeveloped riparian habitat, followed by drainage channel habitat. Data from this study indicated that

populations of bird species that are well-adapted to human activities and landscapes were increasing (e.g., European starling, house sparrow, rock dove [Columba livia],

cattle egret [Bubulcus ibis]). Species associated with young

successional stages dominated by willows (e.g., Bell’s

vireo, painted bunting [Passerina ciris], southwestern willow flycatcher, hooded warbler [Wilsonia citrina], and

yellow warbler [Dendroica petechia]) were declining.
USDA Forest Service Gen. Tech. Rep. RMRS–GTR–1. 1998 9

GBG

GBC

GBC

Albuquerque

Isleta Pueblo

Bernalillo

GBC

Espanola

San

Ildefonso

Pueblo

Cochiti Pueblo

GBC

GBC GBG

GBG

GBG

GBC San Acacia Diversion Dam

MIDDLE RIO

GRANDE

CHIHUAHUAN DESERTSCRUB

GREAT BASIN GRASSLAND (GBG)

GREAT BASIN 

CONIFER WOODLAND 

(GBC)

GBG

Figure 2. Area of New Mexico studied by Hink and Ohmart (1984).
10 USDA Forest Service Gen. Tech. Rep. RMRS–GTR–1. 1998

Rio 

Rio G

ra

nde

Alamosa

Conejos

River

Red

Fe

Embudo

River

Trincheras Cr.

Cr.

Cr.

Cr.

LaJara

Grande

Platoro Reservoir

Heron

Reservoir

El Vado

Reservoir

Nambe

Falls

Reservoir

Abiquiu

Reservoir

Cochiti

Reservoir

Galisteo

Reservoir

Santa Fe

Pena

Blanca

Alamosa

COLORADO

NEW MEXICO

Albuquerque

Belen

Middle Rio Grande

Conservancy

District

San Juan/Chama

Diversion Site

Socorro

Truth or Consequences

N

During the 1993 breeding season, an effort was

made to estimate the abundance and distribution of

the southwestern willow flycatcher within the Rio

Grande Basin of New Mexico. This effort was in

response to the proposal to list the southwestern

willow flycatcher as a federally endangered species

(USFWS 1993; listed as endangered in March 1995,

USFWS 1995). Schwarz (1993) conducted surveys

within the Cibola National Forest (table 1) and found

one willow flycatcher pair. Mehlhop and Tonne

(1994) conducted surveys from the San Acacia Diversion Dam to the southern end of Bosque del

Apache National Wildlife Refuge (NWR) within the

Middle Rio Grande Valley (tables 1, 2). They found

seven willow flycatcher pairs and three territorial

males. They recorded two cases of parasitism by

brown-headed cowbirds. Schwarz (1994) continued

surveys in the Cibola National Forest during the

1994 breeding season (table 1) and recorded 24 brownheaded cowbirds on eight riparian transects and an

average of 32 (±3 SE) species per sample site (tables

2, 3). Two southwestern willow flycatchers and no

least Bell’s vireos were recorded.

Studies on the species composition and abundance of migratory songbirds by Finch et al. (1995) at

Bosque del Apache NWR within the Middle Rio

Grande Valley (table 1) found that the brown-headed

cowbird was one of the most abundant species during spring migration (tables 2, 3). They caught 244

individual cowbirds in mist nests; 9.2% of all birds

captured (Finch et al. 1995).

Summary

Avian studies in New Mexico, beginning in the

late 1970s, reported quantitative estimates of bird

species composition, abundances, and densities

(table 1), and several studies related these estimates

to habitat characteristics. The results, however, are

difficult to compare among studies because the estimates of abundance and density frequently are in

different units, and methods used to survey and

census birds are inconsistent or not standardized.

Although none of these studies focused on the brownheaded cowbird, cowbirds were included, and their

abundance and/or density and distribution can be

examined within each survey location (table 2) and

ecological region (table 3).

Figure 3. Thompson et al. (1994) surveyed the avian

community of the Rio Grande Basin from just north of

Santa Fe (Velvarde, Rio Arriba County) to just south of

Las Cruces (Mesquite, Dona Ana County), New Mexico.
USDA Forest Service Gen. Tech. Rep. RMRS–GTR–1. 1998 11

Table 1. Avian surveys conducted within major river drainages of New Mexico.

Author Season Year County General area of study Description of area

Freehling 1982 Breeding 1979-80 Bernalillo Middle Rio Grande Valley Dominant woody species was Russian

olive (Elaeagnus angustifolia)

U.S. Bureau of Land Fall migration 1980 Socorro Middle Rio Grande Valley Mesquite (Prosopis spp.) rolling upland,

Management 1981 shrub piedmont, riparian woodland,

and mixed shrub-grassland valley

Hoffman 1990 Breeding 1987-89 Bernalillo Middle Rio Grande Valley Rio Grande State Park, City of Albuquer

que; woody species included cotton

wood Populus spp.), Russian olive,

saltcedar (Tamarix spp.), and coyote

willow (Salix gooddingii)

Farley et al. 1994 Spring migration, 1991-92 Socorro, Middle Rio Grande Valley 30-yr-old cottonwood, 2-yr-old evegetabreeding Sierra ted cottonwood, 3-yr-old revegetated

cottonwood, and 5-yr-old cottonwood

riparian woodlands

Thompson et al. 1994 Spring migration, 1993-94 Rio Arriba, Rio Grande floodplain Cottonwood overstory and Russian

breeding, Los Alamos, olive understory; cottonwood overstory,

fall migration Santa Fe, and Russian olive and willow under

Sandoval, story; cottonwood overstory, and

Bernalillo, saltcedar, Russian olive, willow

Valencia, understory; cotton wood and saltcedar

Socorro, overstory, and mesquite or no understory;

Sierra, Russian live, pecan, cottonwood/ willow,

Doña Ana mesquite/saltcedar overstory

Finch et al. 1995 Spring migration 1994 Socorro Middle Rio Grande Valley Agricultural fields; cottonwood,

mesquite, and saltcedar riparian

woodlands

Mehlhop and Tonne Breeding 1994 Socorro Middle Rio Grande Valley Mature cottonwood stands and large

1994 expanses of saltcedar; few stands of

willow

Maynard 1994 Breeding 1994 Rio Arriba, Rio Grande, Coyote Creek, Mature cottonwood, saltcedar, Russian

Los Alamos, Rio Chama, Rio Puerco, olive, willow, alder (Alnus spp.) riparian

Sante Fe, Rio Nutria, Pescado River, woodlands; small areas of marsh

Sandoval, Bluewater Creek,

Bernalillo, Pecos River, Gila River

Valencia, and its tributaries

Socorro,

Sierra,

Doña Ana,

Grant

Baltosser 1986 Spring migration, 1975 Grant Lower Gila River Valley Cottonwood, box-elder (Acer negundo),

breeding willow riparian woodland; sandy river

bottom

Hildebrandt and Spring migration, 1979-81 Pecos River Valley; Four-winged saltbush (Atriplex

Ohmart 1982 breeding, Primary study area: canescens), cottonwood, Roswell

fall migration, south to Loving, NM; saltcedar, areas cleared of saltcedar,

winter secondary study areas: and honey mesquite

Ft. Sumner south to (Prosopis glandulosa) woodlands

Roswell, NM, and

Loving, NM south to

Pecos, TX

Schwarz 1993, 1994 Breeding 1993-94 Socorro Riparian habitats within Willow, marsh areas, cottonwood,

Cibola National Forest; poplar riparian habitats

tributaries to the Rio Grande
12 USDA Forest Service Gen. Tech. Rep. RMRS–GTR–1. 1998

Table 2. Brown-headed cowbird densities estimated during avian surveys in riparian and adjacent habitats of New Mexico.

Study/location or habitat Date of survey(s) Density (N/40 ha) Original units (if not N/40 ha)

Freehling 1982

Isleta South May - July 1979 33.5

Isleta North May - July 1979 27.5

Montano May - July 1979 50.5

Sandia May - July 1979 16.2

mean (SE) 31.9 (7.2)

Isleta South Aug. - Sept. 1979 0.5

Isleta North Aug. - Sept. 1979 2.1

Montano Aug. - Sept. 1979 2.1

Sandia Aug. - Sept. 1979 0.9

mean (SE) 1.4 (0.4)

Isleta South May - July 1980 38.2

Isleta North May - July 1980 48.5

Montano May - July 1980 36.6

Sandia May - July 1980 30.8

mean (SE) 38.5 (3.7)

U.S. Bureau of Land Management 1981

Mesquite rolling upland 29 May 1980 16 40/km2

Shrub piedmont 10 June 1980 72.4 181/km2

Riparian habitat 17 June 1980 8 20/km2

Mixed shrub, grass valley 8-10 July 1980 6 15/km2

Hoffman 1990

Cottonwood, Russian olive, June-Aug. 1987,

saltcedar, coyote willow 1989 44.4 45/100 acre

Farley et al. 1994

30-yr-old cottonwood riparian forest April - May 1992 3.0 individuals detected

2-yr-old revegetated cottonwood site late May - Jun 1992 1.0 individual detected

3-yr-old revegetated cottonwood site late May - Jun 1992 2.7 individuals detected

5-yr-old revegetated cottonwood site late May - Jun 1992 1.8 individuals detected

30-yr-old cottonwood riparian forest late May - Jun 1992 2.6 individuals detected

Thompson et al. 1994

Stratum 1 Apr. - May 1993 6 individuals detected

Stratum 2 Apr. - May 1993 19 individuals detected

Stratum 3 Apr. - May 1993 12 individuals detected

Stratum 4 Apr. - May 1993 11 individuals detected

Stratum 5 Apr. - May 1993 13 individuals detected

Stratum 1 Jun. - Aug. 1992, 1993 10 individuals detected

Stratum 2 Jun. - Aug. 1992, 1993 46 individuals detected

Stratum 3 Jun. - Aug. 1992, 1993 64 individuals detected

Stratum 4 Jun. - Aug. 1992, 1993 57 individuals detected

Stratum 5 Jun. - Aug. 1992, 1993 69 individuals detected

Stratum 1 Aug. - Oct. 1992, 1993 0 individuals detected

Stratum 2 Aug. - Oct. 1992, 1993 1 individual detected

Stratum 3 Aug. - Oct. 1992, 1993 0 individuals detected

Stratum 4 Aug. - Oct. 1992, 1993 1 individual

(on the 1 marsh habitat transect)

Stratum 5 Aug. - Oct. 1992, 1993 0 individuals detected

Finch et al. 1995

Agricultural fields April - May 1994 0.72 birds/survey/ha

Cottonwood April - May 1994 0.71 birds/survey/ha

Mesquite April - May 1994 0.84 birds/survey/ha

Saltcedar April - May 1994 0.77 birds/survey/ha
USDA Forest Service Gen. Tech. Rep. RMRS–GTR–1. 1998 13

Mehlhop and Tonne 1994

San Acacia #1 June 1994 10.6 6/22.6 ha

McNierney #2 June 1994 29.8 5/6.7 ha

Lemitar #3 June 1994 23.9 20/33.5 ha

Lemitar Willow #4 June 1994 140.8 25/7.1 ha

Middle Socorro #5 June 1994 18.1 23/50.9 ha

Socorro Thicket #6 June 1994 45.7 8/7 ha

South Socorro Saltcedar June 1994 40.0 20/20 ha

Long Stretch San Antonio Section #8 June 1994 20.0 50/100 ha

Bosque del Apache North #8A June 1994 5.5 20/146 ha1

Bosque del Apache South #8B June 1994 5.0 18/145 ha

San Antonio Willow #9 June 1994 42.9 3/2.8 ha

Northern Parula Willow #10 June 1994 38.1 4/4.2 ha

Mosquito Coast #11 June 1994 21.3 16/30 ha

Val Verde #12 June 1994 17.3 13/30 ha

Lower Val Verde Site #13 June 1994 12.6 22/70 ha

WIFL Condo Site #13A June 1994 24.2 23/38 ha

Supra-train trestle tamarisk traillii tract June 1994 3.2 65/806 ha

cattail site #15 June 1994 55.8 7/5.02 ha

Baja Mesa Peak #16 June 1994 28.4 3/4.22 ha

San Marcial/Ft. Craig June 1994 12.1 32/106 a

dike ditch #18 June 1994 28.9 13/18 ha

west bank willow stand #19 June 19942 52.9 9/6.8 ha

upper Elephant Butte state park #20 June 19942 5.2 10/77 ha

Ft. Craig #21 June 19942 5.9 5/34 ha

Lower Ft. Craig #22 June 19942 6.4 4/25 ha

Ft. Craig cemetery #23 June 19942 19.0 4/8.4 ha

Milligan Gulch #24 June 19942 11.1 36/130 ha

Sheep Canyon #25 June 19942 30.4 7/9.2 ha

San Acacia #1 July 1994 5.3 3/22.6 ha

McNierney #2 July 1994 11.9 2/6.7 ha

Lemitar #3 July 1994 7.2 6/33.5 ha

Lemitar Willow #4 July 1994 28.2 5/7.1 ha

Middle Socorro #5 July 1994 6.3 8/50.9 ha

Socorro Thicket #6 July 1994 22.9 4/7 ha

South Socorro Saltcedar July 1994 12.0 6/20 ha

Long Stretch San Antonio Section #8 July 1994 19.2 48/100 ha

Bosque del Apache North #8A July 1994 12.9 47/146 ha

Bosque del Apache South #8B July 1994 13.1 47.5/145 ha3

San Antonio Willow #9 July 1994 85.7 6/2.8 ha

Northern Parula Willow #10 July 1994 76.2 8/4.2 ha

Mosquito Coast #11 July 1994 34.7 26/30 ha

Val Verde #12 July 1994 40.0 30/30 ha

Bell’s WIFL Beach #12A July 1994 24.5 3/4.9 ha

Lower Val Verde Site #13 July 1994 21.1 37/70 ha

WIFL Condo Site #13 July 1994 19.0 18/38 ha

Supra-train trestle tamarisk traillii tract July 1994 3.4 68/806 ha

cattail site #15 July 1994 87.6 11/5.02 ha

Baja Mesa Peak #16 July 1994 56.9 6/4.22 ha

San Marcial/Ft. Craig July 1994 19.2 51/106 ha

dike ditch #18 July 1994 28.9 13/18 ha

west bank willow stand #19 July 1994 47.1 8/6.8 ha

upper Elephant Butte state park #20 July 1994 22.9 44/77 ha

Ft. Craig #21 July 1994 226.7 34/6 ha

Lower Ft. Craig #22 July 1994 11.2 7/25 ha

Ft. Craig cemetery #23 July 1994 23.8 5/8.4 ha

Table 2. Cont’d.

Study/location or habitat Date of survey(s) Density (N/40 ha) Original units (if not N/40 ha)
14 USDA Forest Service Gen. Tech. Rep. RMRS–GTR–1. 1998

Milligan Gulch #24 July 1994 18.8 61/130 ha

Sheep Canyon #25 July 1994 130.4 30/9.2 ha

Maynard 1994

Los Ojos Fish Hatchery 2 & 21 July 1994 14 individuals counted

Velarde 15 June & 9 July 1994 2 individuals counted

Chamita 14 June & 9 July 1994 5 individuals counted

San Juan Pueblo Bridge 15 & 17 June 1994 1 individual counted

Orilla Verde 20 June & 6 July 1994 3 individuals counted

Tierra Azul 17 July 1994 8 individuals counted

Santa Clara Oxbow 18 June 1994 4 individuals counted

Coyote Creek State Park 19 July 1994 7 individuals counted

Black Rock, Zuni Pueblo 20, 22 Jun, 4 July 1994 8 individuals counted

Gila Lower Box, USDOI, BLM 16, 17 Jun, 2 July 1994 4 individuals counted

Ft. West Ditch June & July 1994 45 individuals counted

1830 Low Flow Channel 30 June & 19 July 1994 2 individuals counted

Pump Canyon Exclosure (BLM) 18 June 1994 15 12/80 acre

La Plata #9 (BLM) 16 June 1994 2 1/40 acre

Gallegos Tract (BLM) 25 June 1994 5 6/120 acre

Valdez (San Juan River); (BLM) 25 June 1994 12 10/80 acre

La Plata (BLM) 17 June 1994 3 7/200 acre

Bloomfield Tract (San Juan River)

(BLM) 25 June 1994 10 2/20 acre

La Plata (BLM) 17 June 1994 2 2/80 acre

La Plata (BLM) 16 June 1994 3 4/120 acre

Quintana Ditch, Rio Chama 22 June 1994 1/3000 sq-ft area

Rio Chama Canyon (Rd. 151) 24 June 1994 5 3/23 ha

Black Canyon; Hyde State Park 8 July 1994 6/4-mi transect

Benedictine Monastery and Monastery

Lake (along Pecos River) 7 July 1994 2/1-mi transect

Pecos Nat. Monument; Glorieta River 30 June 1994 2/2-mi transect

Pecos Nat. Monument; Pecos River 7 July 1994 3/1.8-mi transect

Pescado River (Zuni Pueblo) 22 June 1994 2/0.75-mi transect

Rio Puerco; La Ventana 10 July 1994 12/0.75-mi transect

North Corrales Bosque #1 11 & 27 Jun 1994 7/1.6-mi transect

Corrales flooded willows #2 11 & 27 Jun 1994 198 4/2 acre

Middle Corrales Bosque #3 11, 21, 27, 29 Jun 1994 30/4.5-mi transect

Rio Grande, Alameda Islands, North 11 July 1994 9/2-mi transect

Alameda #4 14 June 1994 30/2.2-mi transect

Corrales Unit #5 3 July 1994 11/1-mi transect

San Francisco Hot Springs to Gorilla

Springs; Gila National Forest 25 June 1994 5/3.5-mi transect

Laney Allotment; San Francisco River;

Gila National Forest 10 & 12 June 1994 5/4.2-mi transect

Trout Creek; US For. Serv. Allotment;

Gila National Forest 14 & 19 June 1994 3/6.5-mi transect

Lee Russell Canyon; Gila National Forest 18 June 1994 1/1.2-mi transect

Kerr Springs Canyon; Gila National Forest 18 June 1994 3/1.3-mi transect

Havre Gulch Allotment; San Francisco

 River.; Gila National Forest 24 June 1994 5/2-mile transect

LH Ranch Allotment; San Francisco

 River.; Gila National Forest 26 June 1994 5/1.75-mi transect

West Fork Gila River; Forest Service 28 June 1994 6/3-mi transect

Heart Bar Wildl. Area; West Fork Gila;

 New Mexico Dep. Game & Fish 27 June 1994 4/1.4-mi transect

West Fork Gila Reach #5; Gila National

 Forest 1 & 2 June 1994 20/7-mi transect

Table 2. Cont’d.

Study/location or habitat Date of survey(s) Density (N/40 ha) Original units (if not N/40 ha)
USDA Forest Service Gen. Tech. Rep. RMRS–GTR–1. 1998 15

Blue Creek, West Bank Blue Creek #1 & #2;

 Corral #3; Picnic Canyon (BLM; private) 16 June 1994 6/3-mi transect

Nichol’s Canyon (BLM; Lower Gila Box) 2 July 1994 2/0.75-mi transect

Fort West Ditch (The Nature Conservancy) 3 July 1994 4/1.3-mi transect

Gila Bird Area (Forest Service) 29 June 1994 9/4-mi transect

Polonias Creek (South Fork) (BLM; private) 6 July 1994 14 5/34 acre

Percha Creek (BLM) 1 July 994 14 3/20.7 acre

Tierra Blanca Creek (BLM) 15 July 1994 108 3/2.75 acre

Sevilleta NWR; La Joya State Game Refuge 21-27 June 1994 16/5-mi transect

Bosque del Apache Unit 18A 10 June 1994 8 in a 15-by-60-m area

Bosque del Apache; East side of Rio

 Grande 14 June 1994 24 individuals counted

Bosque del Apache; South side half of

 unit #6 13 & 17 June 1994 7 individuals counted

Bosque del Apache 13 July 1994 17 individuals counted

Bosque del Apache; Unit 17 13 June 1994 5 individuals counted

Bosque del Apache; 13b1 & 14b 16 June 1994 9 individuals counted

Bosque del Apache; 13c, 13b1, 13a, &

 13b2 17 June 1994 8 individuals counted

Three Rivers Campsite; Lincoln Nat. Forest 2 June 1994 200 4/0.8 ha

Laborlita Canyon (BLM; private) 13 June 1994 3/2-mi transect

Chance Canyon; Lincoln National Forest 8 June 1994 28 1/3.5 acre

Baltosser 1986

Lower Gila River, riparian woodland April 1975 9

Lower Gila River, riparian woodland May 1975 20

Lower Gila River, riparian woodland June 1975 43

Hildebrandt and Ohmart 1982

C/S type AT-VI4 Mar. - May 1980 1

Aug. - Sep. 1980 3

C/S type CW-I Mar. - May 1980 6

Jun. - July 1980 3

C/S type CW-II Dec. 1979 - Feb. 1980 3

Mar. - May 1980 9

C/S type Saltcedar III Mar. - May 1980 7

Jun. - July 1980 13

C/S type Saltcedar IV Mar. - May 1980 13

Jun. - July 1980 18

Aug. - Sep. 1980 4

C/S type Saltcedar V Mar. - May 1980 5

Jun. - July 1980 10

C/S type Saltcedar VI Mar. - May 1980 10

Jun. - July 1980 17

Aug. - Sep. 1980 2

C/S type DC-strips Mar. - May 1980 12

Jun. - July 1980 16

C/S type OC-V/VI Mar. - May 1980 1

C/S type HM-V Dec. 1979 - Feb. 1980 3

Jun. - July 1980 10

C/S type HM-VI Mar. - May 1980 1

Jun. - July 1980 1

C/S type GC-VI Jun. - July 1980 1

C/S type “misc.” Mar. - May 1980 7

Jun. - July 1980 17

Aug. - Sep. 1980 1

Table 2. Cont’d.

Study/location or habitat Date of survey(s) Density (N/40 ha) Original units (if not N/40 ha)
16 USDA Forest Service Gen. Tech. Rep. RMRS–GTR–1. 1998

Schwarz 1994

Copper Canyon, Magdalena Mountains June 1993 2 individuals counted

Sawmill Canyon, Magdalena Mountains June 1993 4 individuals counted

Sawmill Canyon, Magdalena Mountains June 1994 3 individuals counted

Tajique Canyon, Manzano Mountains June 1993 9 individuals counted

Tajique Canyon, Manzano Mountains June 1994 5 individuals counted

Perra Canyon, Manzano Mountains June 1993 7 individuals counted

Perra Canyon, Manzano Mountains June 1994 7 individuals counted

Mills Canyon, Kiowa/Rita Blanca

 National Grassland June 1993 6 individuals counted

Mills Canyon, Kiowa/Rita Blanca

National Grassland June 1994 4 individuals counted

Perico Creek, Kiowa/Rita Blanca

National Grassland June 1993 10 individuals counted

Perico Creek, Kiowa/Rita Blanca

National Grassland June 1994 5 individuals counted

Manzano Mountains June 1994 2 individuals counted

Pole Canyon, Zuni Mountains June 1994 1 individual counted

1 Mean of two counts: 21 June and 5 July 1994.

2 Counts conducted during first survey period, but 2-4 July.

3 Mean of two counts: 6 and 15 July.

4 Definitions of avain survey locations.

C/S: community/structure type.

AT-VI: four-winged saltbush; variable foliage height, usually little volume above 1.5 m, includes all areas recently disturbed by

plowing and burning, and areas with very sparse vegetation.

CW-I: cottonwood-dominated communities, trees 12-18 m high, with substantial amount of foliage above 12 m.

CW-II: cottonwood-dominated communities, scattered trees above 9 m but lacking substantial foliage above 12 m.

Salt Cedar-III: saltcedar dominated communities, very few trees above 9 m but having substantial foliage between 4.6 and

7.6 m.

Salt Cedar-IV: saltcedar dominated communities, little foliage above 4.6 m, dense between 1.5 and 4.6 m.

Salt Cedar-V: saltcedar dominated communities, little foliage above 3 m, generally rather sparse, often with open areas between trees or groups of trees.

Salt Cedar-VI: saltcedar dominated communities, variable foliage height, usually little volume above 1.5 m, includes all areas

recently disturbed by plowing and burning, and areas with very sparse vegetation.

DC-strips: uncleared strips of saltcedar approximately 15-20 m wide adjacent and parallel to Pecos River.

OC-V/VI: cleared communities dominated by various annual and perennial weeds and shrubs, little foliage above 3 m, generally

rather sparse, often with open areas between trees or groups of trees, and variable foliage height, usually little foliage above

1.5 m, includes all areas recently disturbed by plowing and burning, and areas with very sparse vegetation.

HM-V: honey mesquite dominated communities, little foliage volume above 3 m, generally rather sparse, often with open areas

between trees or groups of trees.

HM-VI: honey mesquite dominated communities, variable foliage height, usually little volume above 1.5 m, includes all areas

recently disturbed by plowing and burning, and areas with very sparse vegetation.

GC-VI: cleared communities dominated by grasses, variable foliage height, usually little volume above 1.5 m, includes all areas

recently disturbed by plowing and burning, and areas with very sparse vegetation.

Table 2. Cont’d.

Study/location or habitat Date of survey(s) Density (N/40 ha) Original units (if not N/40 ha)
USDA Forest Service Gen. Tech. Rep. RMRS–GTR–1. 1998 17

Table 3. Densities (N/40 ha) of brown-headed cowbirds estimated during avian surveys in New Mexico.

Author Region Date Mean density SE

Freehling 1982 Middle Rio Grande Valley May - July 1979 31.9 7.16

Aug. - Sept. 1979 1.4 0.41

May - July 1980 38.5 3.69

U.S. Bureau of Land Management Middle Rio Grande Valley 17 June 1980 8 not a mean

1981

Hildebrandt and Ohmart 1982 Pecos River Valley Dec. 1979 - Feb. 1980 6 not a mean

March - May 1980 72 not a mean

June - July 1980 106 not a mean

August - September 1980 10 not a mean

October - November 1980 0 not a mean

Hoffman 1990 Middle Rio Grande Valley June-early Aug., 1987 & 1989 45 not given

Farley et al. 1994 Middle Rio Grande Valley spring 1992 31 not given

summer 1992 21 0.4

Thompson et al. 1994 Rio Grande floodplain summer 1992 & 1993 52 not a mean

summer 1992 & 1993 1222 not a mean

Schwarz 1994 Cibola National Forest;

tributaries to the Rio Grande summer 1993 382 not a mean

summer 1994 242 not a mean

Finch et al. 1995 Middle Rio Grande Valley spring 1994 0.763 0.03

1 mean no. individuals

2 no. individuals

3 bird/survey/ha

In addition to estimates of abundance and species

composition, habitat associations have been examined.

More birds were found in riparian habitats with mid- and

overstory woody vegetation than in adjacent uplands.

Hunter et al. (1988) and Thompson et al. (1994) found that

exotic woody vegetation, such as saltcedar and Russian

olive, were used by some bird species. Birds associated

with human activities and habitats (e.g., agricultural fields)

were increasing, while birds associated with mid-story

willows were declining (Thompson et al. 1994). Woody

riparian habitats supported many bird species during

both migration and breeding seasons.

We found only one study on the nesting ecology of

cowbirds and their hosts in New Mexico, and it was

conducted in upland habitat (table 4). All other studies on

rates and impacts of brown-headed cowbird parasitism

have been conducted elsewhere in the Southwest and

southern California (table 4). Nesting ecology studies

conducted in riparian habitats elsewhere in the Southwest

and West indicate that cowbird parasitism affects the

nesting success of hosts. Rates of parasitism ranged from

0 to 100%. Although rates of parasitism in habitats with

and without cattle varied, they did not exhibit a noticeable

trend. When cowbirds were trapped intensively in least

Bell’s vireo habitat, rates of parasitism dropped from 47%

to 0% (Griffith and Griffith 1993). Results from the parasitism studies reported in table 4 can not be compared

directly because different methods of estimating nesting

success were used. In addition, nest success rates estimated by directly counting the number of nests with ≥1

fledglings produced relative to the number of nests with

≥1 eggs, are biased by not considering nests, eggs, and

fledglings not found by the observer (Mayfield 1961).

Bird species responded differently to restoration of

native riparian vegetation along the lower Colorado River;

densities of most permanent resident species increased

quickly (<1 yr) in response to vegetation changes (Anderson et al. 1989). Revegetation benefitted insectivorous

more than granivorous species, and early-season more

than mid- or late-season breeders. Insect collections from

blooming riparian vegetation along the upper Colorado

River indicated that some exotic plant species (e.g.,

saltcedar, white sweetclover [Melilotus alba]) successfully

attracted insects (Carothers and Sharber 1976). However,

insect densities were lower on saltcedar than on native

plant species when not blooming, suggesting that saltcedar

has not been incorporated evenly into the riparian ecosystem. Densities of insect species declined markedly from
18 USDA Forest Service Gen. Tech. Rep. RMRS–GTR–1. 1998 Table 4. Rates of brood parasitism by the brown-headed cowbird on host species in the Southwest and southern California. Success Method used Habitat Host Rate of of parasitized to calculate State Location type species Year parasitism (%) nests (%) nest success Texas1 Kendall County Upland: cedar brakes; Golden cheeked 1962 57.6 27.3 % of young fledged ash juniper canyons warbler 1963 1964 Texas2 Fort Hood, Upland: Texas hill Black-capped a) 1987-90 a)mean range a) 2.4-25.5 not provided central Texas country; gypsum vireo (no trapping) 61.6-90.9 canyons b) 1991, 1992 b)38.8, 29.2 b) 42.2, 48.4 (trapping) Texas3 Kerr Wildlife Upland:Texas hill Black-capped a) 1986 a)71 not provided not provided Management Area country; gypsum vireo (no trapping) canyons b) 1993 b)55 (no cattle), (trapping) 11 (cattle) Colorado4 Arapaho National Riparian:shrub Willow flycatcher 1985 40.7 18.2(parasitized) ≥1 flycatcher Wildlife Refuge, willows, floodplain of 1986 56.3 (not fledged per nest North Park Illinois River parasitized) New Mexico5 East of Sangre de Upland:pinyon pine, Solitary vireo 1992 100 (no cattle) 33.3 (no cattle) Mayfield 1961, 1975 Cristo Mountains one-seed juniper, 1993 80 (cattle) 7.7 (cattle) short-grass prairie Western tanager 1992 92 (no cattle) 33.3 (no cattle) Mayfield 1961,1975 1993 78 (cattle) 54.5 (cattle) Blue-gray 1992 63 (no cattle) 44.4 (no cattle) Mayfield 1961, 1975 gnatcatcher 1993 75 (cattle) 25.0 (cattle) Chipping sparrow 1992 13 (no cattle) 43.8 (no cattle) Mayfield 1961, 1975 1993 0 (cattle) 54.5 (cattle) Rufous-sided 1992 0 (no cattle) 59.1 (no cattle) Mayfield 1961, 1975 towhee 1993 25 (cattle) 60.0 (cattle) Western wood- 1992 0 (no cattle) 64.3 (no cattle) Mayfield 1961, 1975 peewee 1993 14 (cattle) 85.7 (cattle) Arizona6 Colorado River: Riparian: 389 km Willow flycatcher 1982-1987 50 (

n=8 nests) not provided not provided

Grand Canyon corridor; Glen Canyon (5 yrs)National Park, Dam to Diamond Bell’s vireo 1982-1987 7 (

n=57 nests) not provided not provided

Glen Canyon Creek Blue-gray 1982-1987 32.1 not provided not providedRecreation Areas gnatcatcher (

n=28 nests)

Lucy’s warbler 1982-1987 23.1 not provided not provided

(n=13 nests)

Yellow warbler 1982-1987 22.7 not provided not provided

(n=22 nests)

Common 1982-1987 55.6 not provided not provided

yellowthroat (

n=9 nests)

Yellow-breasted 1982-1987 10.8 not provided not provided

chat (

n=37 nests)

Blue grosbeak 1982-1987 60 not provided not provided

(n=5 nests)

Overall rate 17.9 not provided not providedfor study area: (

n=207 nests)
USDA Forest Service Gen. Tech. Rep. RMRS

–GTR–1. 1998 19

California7 southern Riparian: coastal and Least Bell’s vireo 1977 50 21 no. nests

California desert slope; low- 1978 (n=14 nests) producing

growth vegetation ≥1 fledgling,

÷ total no. nests

California8 The Nature Riparian:moist, low- Willow flycatcher 1987 68 (n=19) not provided not provided

Conservancy’s lying areas; thickets with dense willow

Kern RiverPreserve

California9 Marine Corps Riparian habitat Least Bell’s a) 1981, 1982 a) 47 not provided not provided

Base Camp vireo (no trapping)Pendleton b) 1987 b) 17

(trapping)c) 1990 c) 0(trapping)

California10 The Nature Riparian:willow- Southwestern 1989, 1990, 60.2 26.1 (no. nests successConservancy’s cottonwood willow flycatcher 1991 fully fledging ≥1

Kern River woodland SWWF) ÷ (total no.Preserve nests found with

≥1 egg)

USDA Forest Riparian:willow- Southwestern 1989, 1990, 78.6 17.9 same as aboveService, South cottonwood willow flycatcher 1991Fork Wildlife Area woodlandThe Nature Riparian:willow- Southwestern 1993, 1994 17.4 41.6 same as aboveConservancy’s cottonwood willow flycatcher (trapping)

Kern River woodlandPreserveUSDA Forest Riparian:willow- Southwestern 1993, 1994 56.3 43.8 same as aboveService, South cottonwood willow flycatcher (trapping)

Fork Wildlife Area woodland

1Pulich 19762Hayden et al. 19933Armstrong 19934Sedgwick and Knopf 19885Goguen 19946Brown 19947Goldwasser et al. 19808Harris 19919Griffith and Griffith 199310Whitfield 1995

Table 4. Cont’d.

 Success Method used

Habitat Host Rate of of parasitized to calculate

State Location type species Year parasitism (%) nests (%) nest success
20 USDA Forest Service Gen. Tech. Rep. RMRS–GTR–1. 1998

the more richly vegetated riparian zone to the terrace and

talus slope (Carothers and Sharber 1976).

Densities of the brown-headed cowbird in New Mexico

during the breeding season (May-July) for the last 15 years

range from about 8 to 106 birds/40 ha, and average about

45.9 ± 16.3 SE birds/40 ha (table 3). Most earlier accounts

stated that the brown-headed cowbird was fairly common

to common during the breeding season, especially in

riparian habitats.

An analysis of Breeding Bird Survey (BBS) data collected from 1966 to 1995, indicated a significant (P<0.05)

2.5% increase in number of cowbirds per year for New

Mexico (U.S. Breeding Bird Lab, Worldwide Web Site

1997). The shorter term (1966 to 1979 and 1980 to 1995)

trend analyses are not significant (H0: ß1=0; P>0.05). The

results of various surveys conducted throughout riparian

and upland habitats reported in table 3 do not appear to

support the BBS suggestion that cowbird populations

have increased over the last 30 yr. However, surveys

conducted during independent studies of riparian communities and those conducted for the BBS used different

counting methods and were located in different habitats,

so the results are not directly comparable. Perhaps the

cowbird population increased since the late 1960s, but not

in all habitat types of New Mexico.

We found no published studies conducted in New

Mexico to determine rates and impacts of cowbird parasitism on riparian-dependent host species. Riparian habitats

in New Mexico have been greatly reduced, and populations of host species would be impacted further by increasing rates of parasitism. The least Bell’s vireo and southwestern willow flycatcher, both endangered, are ripariandependent, long-distance migratory songbirds and frequent hosts for the brown-headed cowbird throughout the

Southwest (table 4). Currently, biologists are collecting

data on the abundance and distribution of these hosts in

New Mexico. Future studies should quantify cowbird

abundance and distribution, and rates of parasitism on

hosts, especially hosts listed as rare, threatened, or endangered.

Management Implications

Data yielded by research projects will provide guidance for managers concerned with restoration of populations of rare avian species and their habitats. If cowbirds

are a significant factor affecting small, isolated populations of rare species, then immediate management actions

should be taken (Schweitzer et al. 1996). However, habitat

enhancement, restoration, and preservation must be incorporated into any management plan to address the

ultimate problem facing migratory songbirds in the Southwest— habitat loss (Laymon 1987, Robinson et al. 1993,

Robinson et al. 1995).

Management that mitigates the loss of riparian habitat

functions due to invasion of exotic plant species has been

successful in the lower Colorado River Valley. Anderson

and Ohmart (1979, 1984) enhanced and established native

riparian vegetation on disturbed sites along the lower

Colorado River in Arizona and California. Subsequently,

avian density and diversity in their renovated sites were

greater than in unmodified sites. High horizontal and

vertical foliar diversity enhanced avian species diversity

and abundance (Anderson et al. 1979). Vegetation structure was more important to birds in winter than in summer

(Anderson and Ohmart 1980). Anderson and Ohmart

(1980) found that successful habitat improvement plans

must consider the seasonal bird-vegetation relationships

of both permanent and migratory species. Briggs et al.

(1994) suggested that research on the effectiveness of

riparian revegetation has been too small scale (one study

site) and short-term (2 to 3 years post-restoration); they

encouraged long-term studies that considered survival of

planted trees and perennial plants.

Before populations of brown-headed cowbirds are removed from an area, and before extensive habitat restoration plans are implemented in efforts to improve conditions for rare neotropical migratory songbirds, surveys

and quantitative studies must determine whether these

actions are necessary. Our review of existing literature

suggested that data on parasitism rates are lacking and

that songbirds may adapt to and use exotic vegetation in

riparian habitat.

Acknowledgments

We greatly appreciate the assistance provided by individuals, private organizations, and state and federal agencies in supplying data, reports, and publications. In particular, we wish to thank W. Yong (USFS), S.O. Williams,

III (NMDGF), The New Mexico Natural Heritage Program, H.R. Schwarz (USFS), M.J. Whitfield (Kern River

Res. Ctr.), D.W. Mehlman , and B.C. Thompson (NMSU).

D.H. White, D.G. Krementz, P. Mehlhop, and J. Kelly

provided helpful comments on an earlier manuscript

draft. Funding for this project was provided through

Cooperative Agreement 28-C4-853 between Oklahoma

State University and the Albuquerque Laboratory of the

Rocky Mountain Research Station.

Literature Cited

Ames, C.R. 1977. Wildlife conflicts in riparian management: grazing.

Pages 49-51 in R.R. Johnson and D.A. Jones, tech. coords. Proceed-
USDA Forest Service Gen. Tech. Rep. RMRS–GTR–1. 1998 21

ings of the symposium, importance, preservation, and management

of riparian habitats. USDA For. Serv., Rocky Mountain For. and

Range Exp. Stn., Gen. Tech. Rep. RM-43, Fort Collins, CO.

Anderson, B.W., and R.D. Ohmart. 1977. Vegetation structure and bird

use in the lower Colorado River Valley. Pages 23-34 in R.R. Johnson

and D.A. Jones, tech. coords. Proceedings of the symposium, importance, preservation, and management of riparian habitats. USDA

For. Serv., Rocky Mountain For. and Range Exp. Stn., Gen. Tech.

Rep. RM-43, Fort Collins, CO.

Anderson, B.W., and R.D. Ohmart. 1979. Riparian revegetation: an

approach to mitigating for a disappearing habitat in the Southwest. Pages 481-487 in G.A. Swanson, tech. coord. The mitigation

symposium: national workshop on mitigating losses of fish and

wildlife habitats. USDA For. Serv., Rocky Mountain Forest and

Range Exp. Stn., Gen. Tech. Rep. RM-65, Fort Collins, CO.

Anderson, B.W., and R.D. Ohmart. 1980. Designing and developing a

predictive model and testing a revegetated riparian community

for southwestern birds. Pages 434-450 in R.M. DeGraaf, tech. coord.

Proceedings of a workshop: management of western forest and

grassland nongame birds. USDA For. Serv., Intermountain For. and

Range Exp. Stn., Gen. Tech. Rep. INT-86, Ogden, UT.

Anderson, B.W., and R.D. Ohmart. 1984. Avian use of revegetated

riparian zones. Pages 626-631 in R.E. Warner and K.M. Hendrix,

eds. California riparian systems: ecology, conservation, and productive management. Univ. Calif. Press, Los Angeles.

Anderson, B.W., R.D. Ohmart, and J. Disano. 1979. Revegetating the

riparian floodplain for wildlife. Pages 318-331 in R.R. Johnson and

J.F. McCormick, tech. coords. Proceedings of the symposium, strategies for protection and management of floodplain wetlands and

other riparian ecosystems. USDA For. Serv., Gen. Tech. Rep. WO-12,

Washington, D.C.

Anderson, B.W., R.D. Ohmart, and H.A. Allen, Jr. 1984. Riparian birds

in the riparian/agricultural interface. Pages 190-195 in R.E. Warner

and K.M. Hendrix, eds. California riparian systems: ecology, conservation, and productive management. Univ. Calif. Press, Los

Angeles.

Anderson, B.W., W.C. Hunter, and R.D. Ohmart. 1989. Status changes

of bird species using revegetated riparian habitats on the lower

Colorado River from 1977 to 1984. Pages 325-331 in D.L. Abell, tech.

coord. Proceedings of the California riparian systems conference:

protection, management, and restoration for the 1990s. USDA For.

Serv., Gen. Tech. Rep. PSW-110, Berkeley, CA.

Armstrong, B. 1993. The cowbird control program on the Kerr Wildlife Management Area. Page 62 in T. Cook, S. Robinson, S. I.

Rothstein, J. Smith, and S. Sealy, eds. Abstracts of the North

American research workshop on the ecology and management of

cowbirds. 4-5 November, The Nature Conservancy of Texas, Austin.

Askins, R.A., J.F. Lynch, and R. Greenberg. 1990. Population declines

in migratory birds in eastern North America. Current Ornithol.

7:1-57.

Bailey, F.M. 1928. Birds of New Mexico. New Mexico Dept. Game and

Fish, Santa Fe. 792pp.

Baltosser, W.H. 1986. Seasonal analysis of a southwestern New Mexico

riparian bird community. Western Birds 17:115-131.

Bent, A.C. 1965. Life history of North American blackbirds, orioles,

tanagers, and allies. Dover Publications, New York, NY. 549pp.

Briggs, M.K., B.A. Roundy, and W.W. Shaw. 1994. Trial and error:

assessing the effectiveness of riparian revegetation in Arizona.

Restoration Manage. Notes 12:160-167.

Briskie, J.V., and S.G. Sealy. 1990. Evolution of short incubation

periods in the parasitic cowbirds, Molothrus spp. Auk 107:789-794.

Brittingham, M.C., and S.A. Temple. 1983. Have cowbirds caused forest

songbirds to decline? Bioscience 33:31-35.

Brown, B. T. 1994. Rates of brood parasitism by Brown-headed Cowbirds on riparian passerines in Arizona. J. Field Ornithol. 65:160-168.

Brown, B.T., C.H. Lowe, and J.F. Hausler. 1977. Southwestern riparian

communities: their biotic importance and management in Arizona. Pages 201-211 in R.R. Johnson and D.A. Jones, tech. coords.

Proceedings of the symposium, importance, preservation, and management of riparian habitats. USDA For. Serv., Rocky Mountain For.

and Range Exp. Stn., Gen. Tech. Rep. RM-43, Fort Collins, CO.

Brown, B.T., and R.R. Johnson. 1985. Glen Canyon Dam, fluctuating

water levels, and riparian breeding birds: the need for management compromise on the Colorado River in Grand Canyon. Pages

76-80 in R.R. Johnson, C.D. Ziebell, D.R. Patten, P.F. Ffolliott, and

R.H. Hamre, tech. coords. Riparian ecosystems and their management: reconciling conflicting uses. USDA For. Serv., Rocky Mountain For. and Range Exp. Stn., Gen. Tech. Rep. RM-120,

Fort Collins, CO.

Carothers, S.W., R.R. Johnson, and S.W. Aitchison. 1974. Population

structure and social organization of southwestern riparian birds.

Am. Zool. 14:97-108.

Carothers, S.W., and N.J. Sharber. 1976. Birds of the Colorado River.

Pages 109-135 in S.W. Carothers and S.W. Aitchenson, eds. An

ecological survey of the riparian zone of the Colorado River between

Lees Ferry and the Grand Wash Cliffs, Arizona. Final Res. Rep., U.S.

Nat. Park Serv., Grand Canyon Nat. Part, AZ.

Crawford, C.S., A.C. Cully, R. Leutheuser, M.S. Sifuentes, L.H. White,

and J.P. Wilber. 1993. Middle Rio Grande ecosystem: bosque

biological management plan. Final Rep., Biological Interagency

Team, Albuquerque, NM. 291pp.

Deusen, M.S., and P.W. Adams. 1989. Riparian areas: fish and wildlife

havens. Woodland fish and wildlife publication. World Forestry

Center, Portland, OR. 7pp.

Dick-Peddie, W.A. 1993. New Mexico vegetation: past, present, and

future. Univ. New Mexico Press, Albuquerque. 244pp.

deBuys, W. 1993. Moving from diverse viewpoints to results. Pages

255-260 in B. Tellman, H.J. Corthner, M.G. Wallace, L.F. DeBano,

and R.H. Hamre, tech. coords. Riparian management:common

threads and shared interests. A western regional conference on river

management strategies. USDA For. Serv., Rocky Mountain For. and

Range Exp. Stn., Gen. Tech. Rep. RM-226, Fort Collins, CO.

Engel-Wilson, R.W., and R.D. Ohmart. 1979. Floral and attendant

faunal changes on the lower Rio Grande between Fort Quitman

and Presidio, Texas. Pages 139-147 in R.R. Johnson and J.F.

McCormick, tech. coords. Strategies for protection and management of floodplain wetlands and other riparian ecosystems. USDA

For. Serv., Gen. Tech. Rep. WO-12, Washington, D.C.

Farley, G.H., L.M. Ellis, J.N. Stuart, and N.J. Scott, Jr. 1994. Avian species

richness in different-aged stands of riparian forest along the Middle

Rio Grande, New Mexico. Conservation Biol. 8:1098-1108.

Finch, D.M. 1991. Population ecology, habitat requirements, and conservation of neotropical migratory birds. USDA For. Serv., Rocky

Mountain For. and Range Exp. Stn., Gen. Tech. Rep. RM-205, Fort

Collins, CO.

Finch, D.M., G.L. Wolters, and W. Yong. 1995. Plants, arthropods, and

birds of the Rio Grande. Pages 133-164 in D.M. Finch and J.A.

Tainter, tech. eds. Ecology, diversity, and sustainability of the

Middle Rio Grande Basin. USDA For. Serv., Rocky Mountain For.

and Range Exp. Stn., Gen. Tech. Rep. RM-GTR-268, Fort Collins, CO.

Freehling, M.D. 1982. Riparian woodlands of the Middle Rio Grande

Valley, New Mexico: a study of bird populations and vegetation

with special reference to Russian-olive (Elaeagnus angustifolia).

Res. Comp. Rep., Region 2, U.S. Fish Wildl. Serv., Office of Environment, Albuquerque, NM. 35pp.

Friedmann, H. 1929. The cowbirds: a study in the biology of social

parasitism. Charles C. Thomas, Springfield, IL. 421pp.

Friedmann, H. 1971. Further information on the host relations of the

parasitic cowbirds. Auk 88:239-255.

Friedmann, H., and L.F. Kiff. 1985. The parasitic cowbirds and their

hosts. Proc. Western Found. Vert. Zool. 2:227-302.

Funk, R. 1993. Rio Grande Valley State Park. Pages 159-161 in B.

Tellman, H.J. Cortner, M.G. Wallace, L.F. DeBano, and R.H. Hamre,

tech. coords. Riparian management: common threads and shared

interests; a western regional conference on river management strategies. USDA For. Serv., Rocky Mountain For. and Range Exp. Stn.,

Gen. Tech. Rep. RM-226, Fort Collins, CO.
22 USDA Forest Service Gen. Tech. Rep. RMRS–GTR–1. 1998

Gaines, D.A. 1980. The valley riparian forests of California: their

importance to bird populations. Pages 57-85 in A. Sands, ed.

Proceedings of the symposium, riparian forests in California: their

ecology and conservation. Univ. California, Davis.

Goguen, C. B. 1994. The effects of domestic livestock grazing on

nongame birds in northeastern New Mexico. M.S. Thesis, Texas

Tech Univ., Lubbock.

Goldwasser, S., D. Gaines, and S. R. Wilbur. 1980. The Least Bell’s Vireo

in California: a de facto endangered race. Am. Birds 34:742-745.

Griffith, J. T., and J. C. Griffith. 1993. Brown-headed Cowbird trapping

and Least Bell’s Vireo recovery on Marine Corps Base Camp

Pendleton, 1983-1993. Page 30 in T. Cook, S. Robinson, S. I. Rothstein,

J. Smith, and S. Sealy, eds. Abstracts of the North American research

workshop on the ecology and management of cowbirds. 4-5 November, The Nature Conservancy of Texas, Austin.

Grinnell, J. 1909. A new cowbird of the genus Molothrus; with a note

on the probable genetic relationships of the North American

forms. Univ. Calif. Publications Zool. 5:275-281.

Hanna, W.C. 1928. Notes on the Dwarf Cowbird in Southern California. Condor 30:161-162.

Harris, J. H. 1991. Effects of brood parasitism by Brown-headed

Cowbirds on willow flycatcher nesting success along the Kern

River, California. Western Birds 22:13-26.

Hayden, T. J., D. J. Tazik, R. Melton, and J. Cornelius. 1993. A landscape

perspective of cowbird control and conservation of endangered

Black-capped Vireo (Vireo atricapillus) on Fort Hood, Texas.

Pages 26-27 in T. Cook, S. Robinson, S. I. Rothstein, J. Smith, and S.

Sealy, eds. Abstracts of the North American research workshop on

the ecology and management of cowbirds. 4-5 November, The

Nature Conservancy of Texas, Austin.

Hildebrandt, T.D., and R.D. Ohmart. 1982. Biological resource inventory (vegetation and wildlife), Pecos River Basin, New Mexico and

Texas. Final Rep., U.S. Bureau Land Manage., Tempe, AZ. 162pp.

Hink, V.C., and R.D. Ohmart. 1984. Middle Rio Grande biological

survey. Final Rep., U.S. Army Corps Engineers Contract No.

DACW47-81-C-0015. 160pp. + appen.

Hoffman, S.W. 1990. Bosque biological monitoring program: bird

population surveys in Rio Grande Valley State Park (1987-1990).

Contract 8901186, City of Albuquerque, NM. 52pp.

Hubbard, J.P. 1971. The summer birds of the Gila Valley, New

Mexico. Nemouria: Occasional Papers of the Delaware Museum of

Natural History, No. 2. 35pp.

Hubbard, J.P. 1977. Importance of riparian ecosystems: biotic considerations. Pages 14-18 in R.R. Johnson and D.A. Jones, tech. coords.

Proceedings of the symposium, importance, preservation, and management of riparian habitats. USDA For. Serv., Rocky Mountain

Forest and Range Experiment Station., Gen. Tech. Rep. RM-43, Fort

Collins, CO.

Hundertmark, C.A. 1978. Breeding birds of Elephant Butte Marsh.

New Mexico Ornithol. Soc. Pub. No. 5. 17pp.

Hunter, W.C., R.D. Ohmart, and B.W. Anderson. 1987. Status of breeding riparian-obligate birds in southwestern riverine systems. Western Birds 18:10-18.

Hunter, W.C., R.D. Ohmart, and B.W. Anderson. 1988. Use of exotic

saltcedar (Tamarix chinensis) by birds in arid riparian systems.

Condor 90:113-123.

Ingles, L.G. 1950. Nesting birds of the willow-cottonwood community

in California. Auk 67:324-330.

Johnson, R.R., L.T. Haight, and J.M. Simpson. 1987. Endangered habitats versus endangered species: a management challenge. Western

Birds 18:89-96.

Johnson, N.K., and J.R. Jehl, Jr. 1994. A century of avifaunal change in

western North America: overview. Pages 1-3 in J.R. Jehl, Jr. and

N.K. Johnson, eds. A century of avifaunal change in western North

America. Stud. Avian Biol. No. 15.

Katibah, E.F. 1984. A brief history of riparian forests in the Central

Valley of California. Pages 23-29 in R.E. Warner and K.M. Hendrix,

eds. California riparian systems: ecology, conservation, and productive management. Univ. Calif. Press, Los Angeles.

Kauffman, J.B., and W.C. Krueger. 1984. Livestock impacts on riparian

ecosystems and streamside management implications: a review.

J. Range Manage. 37:430-438.

Knopf, F.L., R.R. Johnson, T. Rich, F.B. Samson, and R.C. Szaro. 1988.

Conservation of riparian ecosystems in the United States. Wilson

Bull. 100:272-284.

Laymon, S.A. 1987. Brown-headed cowbirds in California: historical

perspectives and management opportunities in riparian habitats.

Western Birds 18:63-70.

Ligon, J.S. 1961. New Mexico birds and where to find them. Univ. New

Mexico Press, Albuquerque. 360pp.

Lowe, C. H. 1964. Arizona’s natural environments: landscapes and

habitats. Univ. Arizona Press, Tucson. 270pp.

Lowther, P. E. 1993. Brown-headed Cowbird (Molothrus ater). In A.

Poole and F. Gill, eds. The Birds of North America, No. 47. Philadelphia: The Academy of Natural Sciences; Washington D.C.: The

American Ornithologists Union. 24pp.

Maynard, W.R. 1994. Summary of 1994 survey efforts in New Mexico

for Southwestern Willow Flycatchers (Empidonax traillii extimus).

Final Rep., New Mexico Dep. Game and Fish, Santa Fe, NM. 48pp.

Mayfield, H.F. 1961. Nesting success calculated from exposure. Wilson Bull. 73:255-261.

Mayfield, H.F. 1965. Chance distribution of cowbird eggs. Condor

67:257-263.

Mayfield, H.F. 1975. Suggestions for calculating nest success. Wilson

Bull. 87:456-466.

Mehlhop, P., and P. Tonne. 1994. Results of surveys for the southwestern willow flycatcher. U.S. Army Corps Engineers, Albuquerque,

NM. 27pp.

Mehlman, D.W. 1995. Notes on the increase of the brown-headed

cowbird in New Mexico. Western Birds 23:59-63.

Mosconi, S.L., and R.L. Hutto. 1982. The effect of grazing on the land

birds of a western Montana riparian habitat. Pages 221-233 in

Proceedings of the symposium, wildlife-livestock relationships. Univ.

Idaho, Moscow.

National Geographic Society. 1987. Field guide to the birds of North

America. Second ed. National Geographic Society, Washington,

D.C. 463pp.

Neff, J.A. 1943. Homing instinct in the Dwarf Cowbird in Arizona.

Bird-Banding 14:1-6.

Nice, M.M. 1939. Observations on the behavior of a young cowbird.

Wilson Bull. 51:233-239.

Nice, M.M. 1953. The question of ten-day incubation periods. Wilson

Bull. 65:81-93.

O’Conner, R. J., and J. Faaborg. 1992. The relative abundance of the

Brown-headed Cowbird (Molothrus ater) in relation to exterior

and interior edges in forests of Missouri. Missouri Acad. Sci.

26:1-9.

Ohmart, R.D. 1994. The effects of human-induced changes on the

avifauna of western riparian habitats. Pages 273-285 in J.R. Jehl, Jr.

and N.K. Johnson, eds. A century of avifaunal change in western

North America. Studies in Avian Biol. No. 15.

Ohmart, R.D., and B.W. Anderson. 1982. North American desert

riparian ecosystems. Pages 433-479 in G.L. Bender, ed. Reference

handbook on the deserts of North America. Greenwood Press,

Westport, CT.

Ohmart, R.D., and B.W. Anderson. 1986. Riparian habitat. Pages 169-

199 in A.Y. Cooperrider, R.J. Boyd, and H.R. Stuart, eds. Inventory

and monitoring of wildlife habitat. USDI, Bureau of Land Manage.,

Washington, D.C. 858pp.

Petit, L.J., D.R. Petit, and T.E. Martin. 1995. Landscape-level management of migratory birds: looking past the trees to see the forest.

Wildl. Soc. Bull. 23:420-429.

Post, W., and J.W. Wiley. 1977. Reproductive interactions of the shiny

cowbird and the yellow-shouldered blackbird. Condor 79:176-184.
USDA Forest Service Gen. Tech. Rep. RMRS–GTR–1. 1998 23

Pulich, W. M. 1976. The golden-cheeked warbler: a bioecological

study. Texas Parks & Wildl. Dept., Austin. 172pp.

Robbins, C.S., J.R. Sauer, R.S. Greenberg, and S. Droege. 1989. Population declines in North American birds that migrate to the neotropics.

Proc. Nat. Acad. Sci. 86:7658-7662.

Robinson, S.K., J.A. Grzybowski, S.I. Rothstein, M.C. Brittingham, L.J.

Petit, and F.R. Thompson. 1993. Management implications of

cowbird parasitism on neotropical migrant songbirds. Pages 93-

102 in D.M. Finch and P.W. Stangel, eds. Proceedings of a workshop:

Status and management of neotropical migratory birds. USDA For.

Serv., Rocky Mountain Forest and Range Exp. Stn., Gen. Tech. Rep.

RM-229, Ft. Collins, CO.

Robinson, S.K., S.I. Rothstein, M.C. Brittingham, L.J. Petit, and J.A.

Grzybowski. 1995. Ecology and behavior of cowbirds and their

impact on host populations. Pages 428-460 in T.E. Martin and D.M.

Finch, eds. Ecology and management of neotropical migratory

birds. Oxford Univ. Press, New York, NY.

Rosenberg, K.V., R.D. Ohmart, W.C. Hunter, and B.W. Anderson. 1991.

Birds of the Lower Colorado River Valley. Univ. Arizona Press,

Tucson. 416pp.

Rothstein, S. I. 1994. The cowbird’s invasion of the far west: history,

causes, and consequences experienced by host species. Pages 301-

315 in J.R. Jehl, Jr. and N.K. Johnson, eds. A century of avifaunal

change in western North America. Stud. Avian Biol. No. 15.

Rothstein, S.I., J. Verner, and E. Stevens. 1980. Range expansion and

diurnal changes in dispersion of the brown-headed cowbird in the

Sierra Nevada. Auk 97:253-267.

Rothstein, S.I., J. Verner, and E. Stevens. 1984. Radio-tracking confirms

a unique diurnal pattern of spatial occurrence in the parasitic

brown-headed cowbird. Ecology 65:77-88.

Rothstein, S.I., D.A. Yokel, and R.C. Fleischer. 1986. Social dominance,

mating and spacing systems, female fecundity, and vocal dialects

in captive and free-ranging brown-headed cowbirds. Current

Ornithol. 3:127-185

Rowley, J.S. 1930. Observations on the Dwarf Cowbird. Condor

32:130-131.

Ryder, R. A. 1980. Effects of grazing on bird habitats. Pages 51-66 in

R.M. DeGraaf, tech. coord. Proceedings of a workshop: management of western forests and grasslands for nongame birds. USDA

For. Serv., Intermountain For. and Range Exp. Stn., Gen. Tech. Rep.

INT-86, Ogden, UT.

Saab, V.A., C.E. Bock, T.D. Rich, and D.S. Dobkin. 1995. Livestock

grazing effects in western North America. Pages 311-353 in T.E.

Martin and D.M. Finch, editors. Ecology and management of Neotropical migratory birds. Oxford University Press, New York.

Schmidly, D.J., and R.B. Ditton. 1979. Relating human activities and

biological resources in riparian habitats of western Texas. Pages

107-116 in R.R. Johnson and J.F. McCormick, tech. coords. Strategies

for protection and management of floodplain wetlands and other

riparian ecosystems. USDA For. Serv., Gen. Tech. Rep. WO-12,

Washington, D.C.

Schmitt, C.G. 1976. Summer birds of the San Juan Valley, New Mexico.

New Mexico Ornithol. Soc. 4:1-22.

Schwarz, H.R. 1993. Willow Flycatcher survey on the Cibola National

Forest. USDA For. Serv., Albuquerque, NM, Ann. Rep. 14pp.

Schwarz, H.R. 1994. Cibola National Forest breeding bird survey

report for 1994. USDA For. Serv., Albuquerque, NM, Ann. Rep.

25pp.

Schweitzer, S.H., D.M. Finch, and D.M. Leslie, Jr. 1996. Reducing

impacts of brood parasitism by brown-headed cowbirds on riparian-nesting migratory songbirds. Pages 267-276 in D.W. Shaw and

D.M. Finch, tech. coords. Desired future conditions for southwestern riparian ecosystems: bringing interests and concerns together.

USDA For. Serv., Rocky Mountain For. and Range Exp. Stn., Gen.

Tech. Rep. RM-GTR-272, Fort Collins, CO.

Scott, D.M., and C.D. Ankney. 1980. Fecundity of the Brown-headed

Cowbird in southern Ontario. Auk 97:677-683.

Sedgwick, J. A., and F. L. Knopf. 1988. A high incidence of brownheaded cowbird parasitism of willow flycatchers. Condor

90:253-256.

State Game Commission. 1990. Handbook of species endangered in

New Mexico. Regulation No. 682. New Mexico Dept. Game Fish,

Santa Fe, NM.

Terborgh, J. 1989. Where have all the birds gone? Princeton Univ.

Press, Princeton, NJ. 207pp.

Thompson, B.C., D.A. Leal, and R.A. Meyer. 1994. Bird community

composition and habitat importance in the Rio Grande system of

New Mexico with emphasis on neotropical migrant birds. Res.

Comp. Rep., U.S. Fish and Wildl. Serv., Region 2, Div. Wildl. and

Habitat Manage., Albuquerque, NM. 83pp. + appen.

Thompson, K. 1980. Riparian forests of the Sacramento Valley, California. Pages 35-38 in A. Sands, ed. Proceedings of the symposium,

riparian forests in California: their ecology and conservation. Univ.

California, Davis.

U.S. Bureau Land Management. 1981. Integrated habitat inventory

classification system (IHICS) or terrestrial and aquatic wildlife

species for the Stallion Planning Unit. Soccorro District, Bureau

Land Manage., Soccorro, NM. 195pp. + appen.

U.S. Fish Wildlife Service. 1993. Endangered and threatened wildlife

and plants; proposed rule to list the Southwestern Willow Flycatcher as Endangered with critical habitat. Fed. Register 58:39495-

39522.

U.S. Fish Wildlife Service. 1995. Endangered and threatened wildlife

and plants; final rule determining endangered status for the

southwestern willow flycatcher. Fed. Register 60:10694-10715.

Walkinshaw, L.H. 1983. Kirtland’s Warbler: the natural history of an

endangered species. Cranbrook Inst. Sci., Bloomfield Hills, MI.

207pp.

Wauer, R.H. 1977. Significance of Rio Grande riparian systems upon

the avifauna. Pages 165-174 in R.R. Johnson and D.A. Jones, tech.

coords. Proceedings of the symposium, importance, preservation,

and management of riparian habitat. USDA For. Serv., Rocky Mountain For. and Range Exp. Stn., Gen. Tech. Rep. RM-43, Fort Collins,

CO.

Whitfield, M. J. 1995. Draft manuscript: Results of a Brown-headed

Cowbird control program for the Southwestern Willow Flycatcher.

Kern River Res. Ctr., Weldon, CA.

Woodward, P.W., and J.C. Woodward. 1979. Survival of fledgling

brown-headed cowbirds. Bird-Banding 50:66-68.

Yokel, D.A. 1986. The social organizations of the Brown-headed

Cowbird in the Owens Valley, California. Pages 164-172 in C.A.

Hall, Jr. and D.J. Young, eds. Proceedings of the symposium, national historical White-Inyo Range, eastern California and western

Nevada and high altitude physiology. Univ. Calif., Los Angeles.
The Rocky Mountain Research Station develops scientific information

and technology to improve management, protection, and use of forests

and rangelands. Research is designed to meet the needs of National

Forest managers, federal and state agencies, public and private

organizations, academic institutions, industry, and individuals.

Studies accelerate solutions to problems involving ecosystems,

range, forests, water, recreation, fire, resource inventory, land

reclamation, community sustainability, forest engineering technology,

multiple use economics, wildlife and fish habitat, and forest insects and

diseases. Studies are conducted cooperatively, and applications can be

found worldwide.

Research Locations

Flagstaff, Arizona

Fort Collins, Colorado*

Boise, Idaho

Moscow, Idaho

Bozeman, Montana

Missoula, Montana

Lincoln, Nebraska

* Station Headquarters, 240 West Prospect Road, Fort Collins, CO 80526

Reno, Nevada

Albuquerque, New Mexico

Rapid City, South Dakota

Logan, Utah

Ogden, Utah

Provo, Utah

Laramie, Wyoming

The U.S. Department of Agriculture (USDA) prohibits discrimination in all its

programs and activities on the basis of race, color, national origin, gender,

religion, age, disability, political beliefs, sexual orientation, and marital or familial

status. (Not all prohibited bases apply to all programs.) Persons with disabilities

who require alternative means for communication of program information (Braille,

large print, audiotape, etc.) should contact USDA’s TARGET Center at (202)

720-2600 (voice and TDD).

To file a complaint of discrimination, write USDA, Director, Office of Civil

Rights, Room 326-W, Whitten Building, 14th and Independence Avenue, SW,

Washington, D.C. 20250-9410, or call (202) 720-5964 (voice or TDD). USDA is

an equal opportunity provider and employer.

Printed on recycled paper