Robert A. Askins

Population Trends in Grassland, Shrubland, and Forest Birds in Eastern North America

There is growing evidence that many species of birds have declined throughout the eastern United States during the past 40 years. In many cases this decline involves groups of species that share the same habitat and have similar ecological requirements, indicating that there may be a few general underlying causes for these population changes rather than a unique explanation for each species. I examined the evidence for population declines in three important ecological groups: forest migrants, grassland species, and shrubland species. These three groups of relatively specialized birds include a large proportion of the songbird species in eastern North America.

Relationship between the regional abundance of forest and the composition of forest bird communities

Abstract We surveyed bird populations in 46 forest tracts in Connecticut, USA, to determined how the distribution of birds is related to forest area, isolation from other forest, and vegetation structure. Both the density and species richness of forest-interior birds tend to be lower in smaller forests. Some species show a significant tendency to be absent from small forests, while others are present in small forests but have higher densities in large forests This pattern is apparently not related to vegetation structure. Sites that are more isolated from other forests also tend to have fewer forest-interior birds. Forest area is the best predictor of the density and species richness of forest-interior birds for small forests, while isolation is the best predictor for large forests. Our results indicate that maintenance of a diversity of forest-interior birds will require preservation of large areas of forest.

Impact of the spread ofPhragmites on the distribution of birds in Connecticut tidal marshes

Dense monocultures ofPhragmites australis (common reed) have been rapidly expanding in Connecticut’s tidal wetlands at the expense of cordgrass (Spartina spp.) and cattail (Typha spp). Bird and vegetation surveys in 40 salt and brackish marshes showed that there were significantly fewer species of birds and state-listed species inPhragmites-dominated wetlands than in short-grass marshes. Seaside Sparrow. Saltmarsh Sharp-tailed Sparrow, and Willet, three marsh specialists adapted to nesting in short graminoids, had low frequencies in plots dominated byPhragmites. Marsh Wren and Swamp Sparrow, however, are marsh specialists that prefer tall, reedy vegetation, and both species had significantly greater densities at sites with morePhragmites or cattail. Although the bird communities of cattail sites andPhragmites sites were similar, the abundance of Virginia Rails was positively correlated with percent cover of cattail but notPhragmites. The extent of pools was positively related to bird species richness in short-grass meadows but not inPhragmites plots. InPhragmites-dominated wetlands, the height and density of reed stands may inhibit bird use of any pools that are present. Muskrats create pools that may enhance bird species richness, but populations of this mammal have dwindled during the same time period thatPhragmites increased in connecticut’s marshes. Although a few species may benefit from reed invasion, it has a negative impact on some marsh bird species that have already declined. These findings support the continued need for marsh restoration and the control of common reed.

Relationship between habitat area and the distribution of tidal marsh birds

Abstract To assess the relationship between marsh area and relative abundance of tidal marsh bird species, we surveyed birds on 86 circular plots in 40 salt and brackish tidal marshes in Connecticut. We measured marsh area in two ways: the amount of contiguous marsh vegetation not interrupted by broad barriers (>500 m of open water or >50 m of upland habitat) and by narrow barriers (>30 m of open water or >10 m upland). We determined the relationship between marsh area and the relative abundance of particular species (mean number of individuals per survey plot) with linear or logistic regression. When the broad barrier definition was used, we found that all three species of short grass meadow specialists, Willets (Catoptrophorus semipalmatus), Seaside Sparrows (Ammodramus maritimus), and Saltmarsh Sharp-tailed Sparrows (A. caudacutus), were less abundant or absent in survey plots in smaller marshes. The Seaside Sparrow and Willet also showed a significant tendency to be less frequent in smaller marshes when the narrow barrier definition was used. In contrast, species that used a wider range of wetland types, as in the Virginia Rail (Rallus limicola), Marsh Wren (Cistothorus palustris), and Swamp Sparrow (Melospiza georgiana), were equally frequent on plots in marshes of different areas. Our results are consistent with the hypothesis that fragmentation of marsh systems with artificial habitat causes a decline in the density of short grass meadow specialists in the remaining patches of appropriate habitat.

Impact of Hurricane Hugo on bird populations on St. John, U.S. Virgin Islands

Bird populations were surveyed in Virgin Islands National Park, St. John, U.S. Virgin Islands in 1987 (two years before Hurricane Hugo hit the Virgin Islands) and in 1990 (four months after the hurricane). Abundance was estimated using the fixed-radius point count method. The average number of individual permanent residents per survey point was significantly lower after the hurricane in both moist forest and dry evergreen woodland. Most of the resident species that showed substantial population declines feed primarily on fruit or nectar, a pattern which is consistent with the results of several other recent studies of the effect of hurricanes on forest bird communities. Also, an insectivorous winter resident (Parula americana) had significantly lower densities after the hurricane. Although many areas within the park were defoliated after the hurricane, most trees remained standing and they began to produce new leaves within a few weeks. Our results suggest that even relatively mild storm damage can result in a marked reduction in the numbers of some species of birds due either to mortality or dispersal from the affected area.

Positive Relationships Between Association Strength and Phenotypic Similarity Characterize the Assembly of Mixed-Species Bird Flocks Worldwide

Competition theory predicts that local communities should consist of species that are more dissimilar than expected by chance. We find a strikingly different pattern in a multicontinent data set (55 presence-absence matrices from 24 locations) on the composition of mixed-species bird flocks, which are important subunits of local bird communities the world over. By using null models and randomization tests followed by meta-analysis, we find the association strengths of species in flocks to be strongly related to similarity in body size and foraging behavior and higher for congeneric compared with noncongeneric species pairs. Given the local spatial scales of our individual analyses, differences in the habitat preferences of species are unlikely to have caused these association patterns; the patterns observed are most likely the outcome of species interactions. Extending group-living and social-information-use theory to a heterospecific context, we discuss potential behavioral mechanisms that lead to positive interactions among similar species in flocks, as well as ways in which competition costs are reduced. Our findings highlight the need to consider positive interactions along with competition when seeking to explain community assembly.

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

Flocking behavior of migratory warblers in winter in the virgin Islands

We assessed the flocking behavior of birds on St. John and St. Thomas, U.S. Virgin Islands, with systematic surveys along trails in moist forests. Winter residents (species breeding in North America and that winter in the Virgin Islands), all of which were warblers, comprised 91% of the individuals found in 28 flocks but only 49% of solitary individuals. The average flock size was 4.0 individuals of 3.1 species, and did not differ between St. John and St. Thomas even though the average forest tract on St. John (1,000 ha) was much larger than on St. Thomas (62 ha). Northern Parula (Parula americana) and Black-andwhite Warbler (Mniotilta varia), the most common species in flocks, occurred in 76% and 79% of the flocks, respectively. Northern Parula flocked significantly more frequently on St. Thomas than on St. John, but no other species showed a difference in flocking behavior between the two islands. Each flock typically included one individual of each species.

The Importance, Effects, and Ethics of Bird Collecting

— 690 — The Auk, Vol. 127, Number 3, pages 690−695. ISSN 0004-8038, electronic ISSN 1938-4254. uf6d9 2010 by The American Ornithologists’ Union. All rights reserved. Please direct all requests for permission to photocopy or reproduce article content through the University of California Press’s Rights and Permissions website, http://www.ucpressjournals. com/reprintInfo.asp. DOI: 10.1525/auk.2010.09199 Kevin WinKer,1,8 J. Michael reed,2 Patricia escalante,3 robert a. asKins,4 carla cicero,5 Gerald e. houGh,6 and John bates7

Restoring Forest Disturbances to Sustain Populations of Shrubland Birds

more shrublands. S the 1970s, conservationists and land managers have been concerned about the future of songbirds in eastern North America. Most of their attention has focused on species that nest in temperate-zone forests and migrate to the tropics to spend the winter (Askins et al., 1990; Rappole, 1995). Many of these species are threatened by a combination of habitat fragmentation in their northern breeding areas and extensive deforestation in their wintering areas in the West Indies and in Central and South America (Sherry and Holmes, 1995; Rappole, 1995). Recently, conservationists have also become concerned about grassland birds because many grassland species have suffered serious population declines in eastern North America, and several species are now listed as endangered or threatened in some states (Vickery, 1992; Askins, 1997). However, another group of birds that includes many declining species--the shrubland or thicket specialists--has largely been ignored. Among the species that require early-successional woody habitats, only species that are federally endangered, such as the Kirtland’s warbler (Dendroica kirtlandii) of Michigan and the black-capped vireo (Vireo atricapillus) of Oklahoma and Texas, have received much attention from conservationists. Many species of shrubland birds that are still widespread have suffered continuous, steep population declines during the past few decades. If these trends continue, additional species will eventually require the often expensive and politically contentious rescue efforts needed to save endangered species. To prevent this, we need to sustain and restore their habitats before these birds become extremely rare. Although they now largely depend on highly artificial habitats such as fallow and abandoned farmland, there is considerable evidence that appropriate habitats for these species existed in eastern North America before European settlement. Until recently, fires, beavers and other natural disturbances created shrubby openings in the forest, but modern forest management tends to eliminate these sources of disturbance. As a result, low, woody vegetation has become increasingly patchy and localized, leading to the decline of an entire set of habitat specialists. Shrublands can be restored relatively easily, either by reintroducing the natural causes of disturbance or by simulating them. Also, shrubland birds are welladapted for finding and colonizing even isolated areas of favorable habitat. Hence, the greatest barrier to sustaining viable populations of these species may not be the difficulty of the solutions, but the failure to recognize that there is a problem. Conservation efforts in eastern North America have traditionally focused on forests and wetlands, and the importance of early-successional habitats for maintaining regional biological diversity has largely been ignored.