James R. Herkert

Area Sensitivity in North American Grassland Birds: Patterns and Processes

Grassland birds have declined more than other bird groups in North America in the past 35–40 years (Vickery and Herkert 2001, Sauer et al. 2008), prompting a wide variety of research aimed at understanding these declines, as well as conservation programs trying to reverse the declines (Askins et al. 2007). Area sensitivity, whereby the pattern of a species’ occurrence and density increases with patch area (Robbins et al. 1989), has been invoked as an important issue in grassland-bird conservation, and understanding the processes that drive area sensitivity in grassland birds is a major conservation need (Vickery and Herkert 2001). Here, we review the literature on North American grassland bird species that is relevant to the following questions. (1) What is the

EGG REJECTION BY COWBIRD HOSTS IN GRASSLANDS

Abstract We tested Field Sparrows (Spizella pusilla), Vesper Sparrows (Pooecetes gramineus), Lark Sparrows (Chondestes grammacus), Grasshopper Sparrows (Ammodramus savannarum), Dickcissels (Spiza americana), Eastern Meadowlarks (Sturnella magna), and Western Meadowlarks (S. neglecta) to determine whether the low level (<10%) of observed parasitism by Brown-headed Cowbirds (Molothrus ater) on these grassland hosts is a result of egg rejection. Western Meadowlarks rejected 78% of artificial and real cowbird eggs, Eastern Meadowlarks rejected 36% of artificial cowbird eggs, and Dickcissels rejected 11% of artificial cowbird eggs. None of the other hosts regularly rejected cowbird eggs. Thus, egg rejection may account for some, but not all, of the low level of observed parasitism on grassland hosts in the Midwest. Meadowlarks were also tested with nonmimetic eggs, and the remaining hosts were tested with undersized mimetic and nonmimetic eggs when possible. All hosts, with the exception of the Field Sparrow, demonstrated some level of rejection of the nonmimetic eggs. These results suggest that some grassland hosts, which apparently have been in contact with cowbirds the longest, have evolved some form of rejection behavior that might have selected for mimetic eggs in cowbirds. The intermediate levels of rejection by both species of meadowlarks also may indicate that rejection is increasing in these populations.

Ecology and conservation of grassland birds of the Western Hemisphere

We summarize population trends for grassland birds from 1966 to 1996 using data from the North American Breeding Bird Survey. Collectively, grassland birds showed the smallest percentage of species that increased of any Breeding Bird Survey bird group, and population declines prevailed throughout most of North America. Although 3 grassland bird species experienced significant population increases between 1966 and 1996, 13 species declined significantly and 9 exhibited nonsignificant trend estimates. We summarize the temporal and geographic patterns of the trends for grassland bird species and discuss factors that have contributed to these trends. LA CONDICI6N DE LA POBLACI6N DE AVES DE PASTIZAL EN AMBRICA DEL NORTE UTILIZANDO EL BREEDING BIRD SURVEY DE NORTEAMfiRICA, 1966-1996 Sinopsis. Resumimos las tendencias poblacionales para las aves de pas&al desde 1966 hasta 1996 utilizando datos de1 Breeding Bird Survey de NorteamCrica. Colectivamente, las aves de pas&al tuvieron el menor porcentaje de especies que aumentaron entre todos 10s grupos de aves en el Breeding Bird Survey. Prevalecieron las disminuciones de poblaciones de estas aves en la mayoria de Norteamtrica. Aunque 3 especies de aves de pastizal experimentaron importantes aumentos poblacionales entre 1966 y 1996, 13 especies disminuyeron significativamente y 9 manifestaron estimaciones de tendencias no significativas. Resumimos 10s patrones temporales y geog&ficos de las tendencias para especies de aves de pastizal y analizamos 10s elementos que han contribuido a estas tendencias.

Response of Bird Populations to Farmland Set-Aside Programs

The role of agricultural landscapes in biodiversity conservation has been largely ignored despite their potential role in conserving declining species. Within agricultural landscapes, set-aside programs may offer the most promising conservation opportunities due to the large area involved in these programs. I explored the relationship between set-aside effect size--on the basis of data from field studies of birds in cropland and set-aside fields--and population changes following establishment of these fields. Species whose abundance was most strongly influenced by the establishment of set-aside lands also tended to show positive changes in population trends following broad-scale implementation of the set-aside program. This relationship was strongest for grassland obligate birds, a group of birds experiencing broad-scale population declines throughout North America. There is now increasing evidence that set-aside lands within the United States are providing population-level benefits to grassland birds. Nevertheless, there are also increasing concerns about the stability of these set-aside lands in the face of increased demand for crops and rising commodity prices. If set-aside area within the United States is allowed to decline, additional declines of birds and perhaps other taxa within agricultural landscapes seem likely.

Conservation of Grassland Birds in North America: Understanding Ecological Processes in Different Regions: "Report of the AOU Committee on Conservation"

—Many species of birds that depend on grassland or savanna habitats have shown substantial overall population declines in North America. To understand the causes of these declines, we examined the habitat requirements of birds in six types of grassland in diff erent regions of the continent. Open habitats were originally maintained by ecological drivers (continual and pervasive ecological processes) such as drought, grazing, and fi re in tallgrass prairie, mixed-grass prairie, shortgrass prairie, desert grassland, and longleaf pine savanna. By contrast, grasslands were created by occasional disturbances (e.g., fi res or beaver [Castor canadensis] activity) in much of northeastern North America. The relative importance of particular drivers or disturbances diff ered among regions. Keystone mammal species—grazers such as prairie-dogs (Cynomys spp.) and bison (Bison bison) in western prairies, and dam-building beavers in eastern deciduous forests—played a crucial, and frequently unappreciated, role in maintaining many grassland systems. Although fi re was important in preventing invasion of woody plants in the tallgrass and moist mixed prairies, grazing played a more important role in maintaining the typical grassland vegetation of shortgrass prairies and desert grasslands. Heavy grazing by prairiedogs or bison created a low “grazing lawn” that is the preferred habitat for many grassland bird species that are restricted to the shortgrass prairie and desert grasslands. Ultimately, many species of grassland birds are vulnerable because people destroyed their breeding, migratory, and wintering habitat, either directly by converting it to farmland and building lots, or indirectly by modifying grazing patt erns, suppressing fi res, or interfering with other ecological processes that originally sustained open grassland. Understanding the ecological processes that originally maintained grassland systems is critically important for eff orts to improve, restore, or create habitat for grassland birds and other grassland organisms. Consequently, preservation of large areas of natural or seminatural grassland, where these processes can be studied and core populations of grassland birds can fl ourish, should be a high priority. However, some grassland birds now primarily depend on artifi cial habitats that are managed to maximize production of livestock, timber, or other products. With a sound understanding of the habitat requirements of grassland birds and the processes that originally shaped their habitats, it should be possible to manage populations sustainably on “working land” such as catt le ranches, farms, and pine plantations. Proper management of private land will be critical for preserving adequate breeding, migratory, and winter habitat for grassland and savanna species. Received 12 December 2006, accepted 24 April 2007. Resumen.—Muchas especies de aves que dependen de habitats de pastizal o savana han mostrado disminuciones signifi cativas en sus poblaciones en Norte America. Para poder entender las causas de estas disminuciones examinamos los requerimientos de habitat de aves 8E-mail: raask@conncoll.edu. Coauthors are listed alphabetically. 9U.S. Department of Interior (retired). ORNITHOLOGICAL MONOGRAPHS NO. 64 2 Bird species that depend on grassland and shrubland have declined in many regions in eastern and central North America during the past century, and open-country species frequently outnumber woodland species on state lists of endangered and threatened species (Askins 1993). Grassland birds, in particular, appear to be in trouble; during the past 25 years they have shown “steeper, more consistent, and more geographically widespread declines than any other behavioral or ecological guild” of North American birds (Knopf 1994:251). An analysis of continent-wide population trends on Breeding Bird Survey (BBS) routes between 1966 and 2002 showed that only 3 of 28 species of grassland specialists increased signifi cantly, whereas 17 species decreased signifi cantly (Sauer et al. 2003). Although some species that showed a continent-wide population decline were increasing in particular regions, many species (including Bobolink [Dolichonyx oryzivorus] and Eastern Meadowlark [Sturnella magna]) declined throughout almost their entire breeding range. Population declines of grassland birds have occurred not only in the northeastern United States (Vickery 1992), where regenerating forest has replaced much of the farmland that dominated the landscape in the 18th and 19th centuries (Norment 2002), but also in the Midwest (Herkert 1995) and the Great Plains (Knopf 1994), the historical centers of abundance and diversity of grassland birds. Population declines have also occurred in grassland birds in South America (Vickery et al. 1999b), Europe (Newton 1998), and other parts of the world (Goriup 1988). Although declines in particular grassland bird populations can be att ributed to a wide variety of factors, such as habitat fragmentation, nest parasitism, pesticides, and invasion by woody vegetation (Peterjohn and Sauer 1999), an overriding cause of regional declines appears to be agricultural intensifi cation. Because most natural grasslands were converted to farmland or are used as ranchland, grassland birds now largely depend on habitats that are managed for agricultural production. Although farmland and pasture may provide good breeding or wintering habitat for some grassland bird species (e.g., Wunder and Knopf 2003), their suitability oft en declines as agriculture becomes more effi cient. Agricultural intensifi cation involves a shift toward monocultures that support fewer natural species (Matson et en seis regiones del continente. Habitats abiertos originalmente se mantenían por conductores ecológicos (procesos ecológicos continuos y perdurables), como sequía, pastoreo, y/o fuego como en praderas de pastizal alto, mediano, y corto, pastizal desértico y sabana de pino de hoja larga. En contraste, los pastizales se crearon por disturbios ocasionales (fuego o actividad de castores) en el noreste de Norte America. Especies claves de mamíferos (como perrito de las praderas y bisonte en las praderas del oeste y castores en bosque deciduos del este) jugaron un papel crucial, y frecuentemente no apreciado, manteniendo sistemas de pastizales. Mientras el fuego fue importante en prevenir la invasión de especies leñosas en praderas de pastizal alto y mediano, el pastoreo jugo un papel mas importante en mantener la vegetación típica de pastizales cortos y desérticos. Alta presión de pastoreo por perrito de las praderas y bisontes crearon una capa de “césped pastoreado” que es el habitat preferido por algunas especies de aves de pastizal que están restringidas a las Grandes Planicies y pastizales desérticos. Muchas especies de aves de pastizal estan vulnerables porque la actividad humana ha destruido sus habitats de anidacion, migración e invernacion directamente mediante la conversión a áreas de cultivo o construcción, o indirectamente mediante la modifi cación de patrones de pastoreo, supresión de fuego, o interfi riendo con otros procesos ecológicos que originalmente mantenían el pastizal abierto. El entendimiento de los procesos ecológicos que mantenían el sistema de pastizal es sumamente importante para esfuerzos de mejoramiento, restauración, o creación de habitats para aves de pastizal y otros organismos. Consecuentemente, de alta prioridad debería de ser la preservación de grandes áreas de pastizal natural o seminatural donde estos procesos se podrían estudiar. Sin embargo, muchas especies de pastizal ahora dependen principalmente de habitats artifi ciales manejados para maximizar la producción de ganado, madera, u otros productos. Con un claro entendimiento de los requerimientos de habitat de aves de pastizal y los procesos que originalmente moldearon sus habitats seria posible mantener sus poblaciones en terrenos manejados como ranchos ganaderos, granjas, y plantaciones de pino. El manejo apropiado de tierras privadas será critico para la preservación adecuada de areas de habitat de anidacion, migración e invernacion para aves de pastizal y savana. CONSERVATION OF GRASSLAND BIRDS 3 al. 1997), and the channeling of more primary production toward food or fi ber. In a broad sense, intensifi cation would include conversion of rangeland to cropland, and the shift to the use of exotic grasses and forbs in pastures and hay meadows. A suite of factors associated with agricultural intensifi cation tend to degrade grassland bird habitat: these include increased use of pesticides, removal of natural fi eld edges, spring plowing, land drainage, replacement of mixed farms with farms dominated by one crop, harvesting or mowing earlier in the season when birds are still nesting, and higher stocking rates for livestock (Newton 1998). Murphy (2003) showed that population changes in grassland birds in eastern and central North America between 1980 and 1998 were highly correlated with changes in agricultural land use in their breeding areas. The most important factor was loss of rangeland, which was associated with negative population trends for 12 species of grassland birds. Rangeland is used for livestock production, but it generally is not managed as intensively as cropland or pasture and it is dominated by native species of plants, providing habitat for a diverse group of grassland birds (Peterjohn 2003). In the Midwest, population declines in several species of grassland birds are highly correlated with declines in the combined area of pasture and hay meadow (Herkert et al. 1996). Our goal is to provide recommendations for halting and reversing the decline in grassland birds. In some cases, this may be accomplished by restoring natural grasslands, but in other cases it is more realistic to try to promote farming and ranching methods that make the land both economically productive and biologically diverse. In either case, it is impo

Conservation Reserve Program Benefits on Henslow's Sparrows Within the United States

Abstract Henslows sparrow (Ammodramus henslowii) is one of North Americas fastest declining songbirds. Population declines combined with a small global population have led to heightened conservation concern. I used data from the North American Breeding Bird Survey to assess the impact that the Conservation Reserve Program (CRP) has had on Henslows sparrows throughout their United States breeding range. My analysis suggests local Henslows sparrow population trends are correlated with CRP enrollment, with populations increasing more in areas with relatively high local CRP enrollment, and that CRP appears to be playing a significant role in reversing long-term population declines.

Response of Northern Harriers and Short-Eared Owls to Grassland Management in Illinois

We studied habitat selection by northern harriers (Circus cyaneus) and short-eared owls (Asio flammeus) on 2 intensively managed grassland sanctuary complexes in southeastern Illinois to determine the influence of grassland management on nest placement. Northern barriers showed a tendency (P < 0.001) to locate their nests in fields not disturbed by recent management activities (within the last 12 months). Overall, northern harrier nest placement was not influenced by whether fields were dominated by native or nonnative grasses (P = 0.307), Short-eared owls were irruptive breeders on these grasslands, and habitat selection could only be assessed in 1 year (1990). In 1990, all short-eared owl nests were located in fields disturbed by grassland management activities within the last 12 months. Although northern harriers are believed to be area-sensitive in the Midwest, grassland area did not influence nest-site selection for northern harriers on our sanctuaries (P = 0.622). Northern harriers and short-eared owls have generally contrasting habitat preferences. To provide appropriate habitats for both species, a mix of both managed and idle grasslands must be provided.

The use of Social Cues in Habitat Selection by Wetland Birds

Abstract. Wetlands are dynamic and can be destroyed and created quickly by natural forces. Therefore, birds inhabiting these wetlands may need to locate new suitable sites quickly. We investigated the cues wetland birds use when selecting a breeding site. Many species may use both habitat (e.g., vegetation structure) and social cues (presence of conspecifics and/or heterospecifics) when selecting a location for breeding. Using a two-species occupancy-modeling approach, we found certain wetland birds more likely to occur with other species, suggesting the presence of heterospecifics may influence settlement. Species that preferred wetlands with a roughly 50:50 ratio of open water to vegetation (hemi-marsh) occurred more frequently than expected with the Pied-billed Grebe (Podilymbus podiceps), suggesting these species may use the presence of grebes when selecting a habitat. Conversely, Yellow-headed Blackbirds (Xanthocephalus xanthocephalus) do not appear to use the presence of other species when selecting a breeding location. Previous research supports this finding, in that the number of young produced at a site the previous year (patch reproductive success) was important in how this species selected its breeding sites. Because settling on the basis of patch reproductive success requires occupancy or visiting a site the previous year, individuals new to a population must use other cues such as habitat. In this population inexperienced Yellow-headed Blackbirds were more prone than experienced individuals to colonize recently created wetlands. Several wetland species we investigated used social cues to select breeding sites, and this behavior may help explain the occurrence and distribution of wetland birds.

ORGANOCHLORINE PESTICIDES ARE NOT IMPLICATED IN THE DECLINE OF THE LOGGERHEAD SHRIKE

Abstract I compared pesticide levels in the eggs of Loggerhead Shrikes (Lanius ludovicianus) collected from Illinois in 1995–1996 with those reported for the state in 1971–1972. Pesticides were detected in 19 of 21 (90%) eggs from 1995–1996. DDE was the most frequently detected pesticide, occurring in 17 of 21 eggs. DDT was the second most frequent pesticide detected and was found in nine eggs. Mean DDE levels in shrike eggs from 1995–1996 were 79% lower than in eggs from 1971–1972. Unlike most bird species for which pesticide levels are known to have been reduced (whose populations have tended to increase), Loggerhead Shrike populations have declined during the period in which pesticide levels within their eggs has been reduced. These data suggest that organochlorine pesticides are likely not driving current Loggerhead Shrike declines in Illinois. Los Pesticidas Organoclorados no Están Implicados en la Disminución de Lanius ludovicianus Resumen. Comparé los niveles de pesticidas en huevos de Lanius ludovicianus colectados en Illinois entre 1995 y 1996 con los niveles de pesticidas reportados para el estado entre 1971 y 1972. Se detectaron pesticidas en 19 de 21 huevos (90%) entre 1995 y 1996. El pesticida detectado con mayor frecuencia fue DDE, el cual se detectó en 17 de 21 huevos. DDT fue el insecticida detectado con la segundo mayor frecuencia, y fue encontrado en nueve huevos. Los niveles medios de DDE encontrados en los huevos de 1994– 1996 fueron un 79% más bajos que en huevos de 1971–1972. A diferencia de la mayoría de las especies de aves para las cuales se sabe que los niveles de pesticidas se han reducido (cuyas poblaciones han tendido a aumentar), las poblaciones de L. ludovicianus han disminuido durante el período en que los niveles de pesticidas hallados dentro de sus huevos han sido reducidos. Estos datos sugieren que probablemente los pesticidas organoclorados no están causando la disminución actual de las poblaciones de L. ludovicianus.