Research

Managing Grasslands for Breeding Bird Diversity
by Steven Hoekman and Joe Ball, Montana Cooperative Wildlife Research Unit

Breeding birds across the North American prairies share a common ecological problem: a critical shortage of suitable and productive nesting habitat. Extensive habitat loss has resulted from conversion of grasslands to crop production, and suitability of much remaining grassland has been degraded by haying, intensive grazing, and invasion by alien species. Many species of grassland-nesting birds are experiencing population declines, and loss of suitable nesting habitat is likely involved.

Much management of grassland bird habitat has focused on limiting disturbance in order to provide tall, dense, relatively monotypic stands of grass for upland-nesting ducks. Fundamental reevaluation of this practice is necessary, however, because demographic benefits to ducks have often been less than expected. Also, integrated planning and management of habitat for diverse migratory species are a growing emphasis for conservation programs, such as the North American Waterfowl Management Plan's Prairie Pothole Joint Venture.

Detailed knowledge of preference by each species is necessary to develop "targets" for habitat management. We therefore sought to identify the vegetation features influencing nest site selection by birds nesting in managed grasslands of west-central Montana. We examined nest site selection at multiple spatial scales around nests by comparing density of vegetation, heterogeneity of vegetation, and plant species composition for common species of game and nongame birds.

Bird species selected nest sites based primarily on vegetation immediately (0.5-10 meters) around the nest, and each species selected nest sites with unique vegetation characteristics (Figure 1). Species showed strong differences in nest sites with respect to vegetation density, primarily near ground level around nests and immediately above the nest. In addition, some species preferred grasses in homogenous stands but others nested in more heterogeneous bunchgrasses with clumped growth form.

These strong differences in nest site preference among species suggest that providing an increased diversity of vegetation structure will provide suitable nesting habitat for more bird species on a given tract of grassland. Furthermore, increasing vegetation diversity at a relatively fine spatial scale likely could provide suitable nesting habitat for many species without detriment to others because most selection involved a small area around the nest.

Managers can increase fine-scale diversity in vegetation density and structure by carefully applying disturbance such as grazing, burning, and haying (and carefully monitoring the results). In grassland restoration on limited areas, where providing nesting for a diverse assemblage of grassland-nesting species is a priority, seeding grass species with diverse growth forms may help to create distinct microhabitats and more closely mimic natural prairie vegetation.

Knowledge of nesting habitat selection for a variety of species enables habitat managers to attract bird species that are habitat limited or that otherwise benefit from nesting in managed habitat (i.e., species that might otherwise nest in "ecological traps" such as hayfields, where reproduction or survival is low). Providing increased diversity of grassland density and structure at a fine spatial scale may improve nesting habitat for a variety of bird species, at little or no detriment to any other species or groups.

For more information, contact Steven Hoekman, Montana Cooperative Wildlife Research Unit, U. S. Geological Survey, University of Montana, Missoula, Montana 59812, (406) 243-5372, shoekman@selway.umt.edu.

50 Ducks Make Spring Migration with Carry-on Baggage
by Tildy La Farge, Ducks Unlimited, Inc.

Northern pintail ducks, elegant in form and acrobatic in flight, are being watched and monitored by satellites. Female pintails have been outfitted with back-pack satellite transmitters, which allow scientists to track their migratory movements from the Central Valley of California, the most important pintail wintering area in the world.

Dr. Bruce Batt, chief biologist at Ducks Unlimited, says, "Pintails were once the second most abundant ducks, after mallards. In the 1950s, they peaked at 10.3 million birds, versus 10.4 million mallards. They've gone down to about 3 million in four decades. We need to find out what has happened. If you look at common ducks, the pintail is one of just two species that is seriously below population goals of the North American Waterfowl Management Plan."

Typically, pintails that winter in California migrate to the Canadian prairies and to Alaska to breed in the spring. In recent years, fewer and fewer have arrived on the prairies, while the Arctic population has been relatively stable. Scientists suspect the pintails may be overshooting the prairies in search of more hospitable breeding habitat. Dry conditions, intensive farming, avian disease, and predation all put stress on the pintail population and limit the species' reproductive success.

Michael Miller, a scientist at the Western Ecological Research Center of the U.S. Geological Survey in Dixon, California, leads the team analyzing data collected during the study, which began in January. The back-pack transmitters send signals to satellites. Receivers attached to the satellites allow the downloading of data, which is sent via e-mail to Miller's team.

Each transmitter, weighing about 20 grams (less than 3 percent of a female pintail's body mass), relays a broad spectrum of information, including date, time, and location (latitude/longitude) of the ducks, as well as estimates on location quality, the temperature of the transmitters, and activity levels of the ducks.

"Once we determine the migration routes and spring-use areas that link the Central Valley wintering areas with specific nesting areas, we hope to design field studies that address the issue of whether or not winter habitat, and its effect on pintail body conditions, influences productivity of nesting hens," says Miller. The information also may allow managers to determine the proportion of pintails exposed to perennial botulism, a serious population stressor.

"Right now," Batt explains, "we have a lot of information about pintails and the key aspects of their biology. With satellite technology, we can build upon that knowledge. We're going to pursue these and other studies until we figure out what's wrong. Until then, we'll continue our work on the ground, restoring and enhancing habitat pintails use throughout the annual cycle."

For more information, contact Tildy La Farge, Ducks Unlimited, Inc., (901) 758-3859, mlafarge@ducks.org. The results of "Discovery for Recovery," with an accompanying biologist's journal and online forum, are accessible at website www.ducks.org.