Higher Bird Abundance Research Articles

The confluence of landscape context and site-level management in determining Midwestern savanna and woodland breeding bird communities

Species distributions are determined by complex interplays between multi-scale factors. Conservation management, however, often occurs at a single scale of the site level. This is true for bird communities of restored savannas and mixed woodlands in the central U.S. In this region, many historic open-canopy oak savanna habitats have become closed canopy mixed woodlands due to loss of landscape-scale disturbance from fire and grazing. Site-level management efforts return some mixed woodland habitats back to savanna through fire and mechanical thinning. Savanna and woodland historically formed complex mosaic landscapes at the ecotone between prairies and Eastern deciduous forests and now exist within landscapes that vary in amount of open (e.g., perennial grassland and row crop agriculture) and woodland habitat. To understand the interplay between site and landscape level factors in savanna restoration, we sampled the breeding bird community in four combinations of site and landscape: restored savanna in open landscapes, restored savanna in woodland landscapes, and closed canopy woodland in both landscapes. We found that the outcome of site-level savanna restoration depended on the surrounding landscape. Compared to other treatment types, restored savannas in open landscapes supported a distinctive bird community characterized by high species richness, bird abundance, and percent of ground feeders, shrub nesters, and edge species. Both savanna and woodland sites in the open landscape had a higher percent of species of conservation concern, while at both site and landscape levels, woodland was associated with a higher percent of area sensitive species and habitat specialists. Our results suggest savanna restoration efforts should focus on sites that exist either in open country or on edges where closed canopy forest meets open country. This strategy would combine site and landscape level benefits of savanna restoration for avian diversity, while also preserving the conservation benefits of large tracts of intact forest.

Bird species diversity in riparian buffers, row crop fields, and grazed pastures within agriculturally dominated watersheds

In response to concern about the loss of ecosystem services once provided by natural riparian systems, state and federal agencies have established incentive programs for landowners to convert sensitive lands from agricultural to conservation uses. Enhancement of wildlife habitat, while identified as a function of such systems, has often been of secondary importance to soil conservation and water quality objectives. Though greatly important, little consideration has been given to how specific species will respond to the design and management of riparian buffers or other conservation lands. This study compared avian communities within a chronosequence of riparian buffers established on previously cropped or pastured land with those of the nearby matrix land cover types (row crop fields and an intensively grazed pasture). The riparian buffers consisted of native grasses, forbs, and woody vegetation established at three different times (2, 9, and 14+ years prior to survey). At each site, 10 min point counts for breeding birds were conducted using 50 m fixed radius plots, which were visited eight times between May 15 and July 10, 2008. A total of 54 bird species were observed over all of the study sites. The re-established riparian buffers in this study had higher bird abundance, richness, and diversity than the crop and pasture sites. These results suggest that re-establishing native riparian vegetation in areas of intensive agriculture will provide habitat for a broad suite of bird species, but that specific species will reflect successional stage, horizontal and vertical vegetative structure, and compositional diversity of the buffer vegetation. These results emphasize the importance of matching buffer design and management to species requirements if the objectives are to attract specific target species or species groups.

Collision fatality of raptors in wind farms does not depend on raptor abundance

Summary The number of wind farms is increasing worldwide. Despite their purported environmental benefits, wind energy developments are not without potential adverse impacts on the environment, and the current pace and scale of development proposals, combined with a poor understanding of their impacts, is a cause for concern. Avian mortality through collision with moving rotor blades is one of the main adverse impacts of wind farms, yet long‐term studies are rare. We analyse bird fatalities in relation to bird abundance, and test several factors which have been hypothesized to be associated with bird mortality. Bird abundance was compared with collision fatality records to identify species‐specific death risk. Failure time analysis incorporated censored mortality data in which the time of event occurrence (collision) was not known. The analysis was used to test null hypotheses of homogeneity in avian mortality distribution according to several factors. There was no clear relationship between species mortality and species abundance, although all large‐bird collision victims were raptors and griffon vulture Gyps fulvus was most frequently killed. Bird mortality and bird abundance varied markedly among seasons, but mortality was not highest in the season with highest bird abundance. Mortality rates of griffon vultures did not differ significantly between years. Bird collision probability depended on species, turbine height (taller = more victims) and elevation above sea level (higher = more victims), implicating species‐specific and topographic factors in collision mortality. There was no evidence of an association between collision probability and turbine type or the position of a turbine in a row. Synthesis and applications. Bird abundance and bird mortality through collision with wind turbines were not closely related; this result challenges a frequent assumption of wind‐farm assessment studies. Griffon vulture was the most frequently killed species, and species‐specific flight behaviour was implicated. Vultures collided more often when uplift wind conditions were poor, such as on gentle slopes, when thermals were weak, and when turbines were taller at higher elevations. New wind installations and/or repowering of older wind farms with griffon vulture populations nearby, should avoid turbines on the top of hills with gentle slopes.

Associations Among Breeding Birds and Gambel Oak in Southwestern Ponderosa Pine Forests

Abstract: Ponderosa pine (Pinus ponderosa) forests with Gambel oak (Quercus gambelii) are associated with higher bird abundance and diversity than are ponderosa pine forests lacking Gambel oak. Little is known, however, about specific structural characteristics of Gambel oak trees, clumps, and stands that may be important to birds in ponderosa pine‐Gambel oak (hereafter pine‐oak) forests. We examined associations among breeding birds and structural characteristics of Gambel oak at a scale similar in size to individual bird territories in pine‐oak forests in northern Arizona and western New Mexico, USA. Avian species richness and occurrence of some bird species were associated with specific growth forms of Gambel oak. Estimated probability of Virginia's warblers (Vermivora virginiae), black‐headed grosbeaks (Pheucticus melanocephalus), and red‐faced warblers (Cardellina rubrifrons) occurring at points increased with increasing density of pole‐sized Gambel oak 7–15 cm in diameter at breast height. We also found evidence that large Gambel oak trees (≥23 cm dbh) were associated with increased occurrence of yellow‐rumped warblers (Dendroica coronata) at points. Some avian associations with oak were influenced by characteristics of ponderosa pines. For example, bird species richness was positively associated with the abundance of large Gambel oak when density of large pine trees ≥23 cm in diameter at breast height was low. Because large oak trees are rare and their numbers are thought to be declining, efforts should be made to retain and promote growth of additional oaks in this size class. Forest management practices that maintain forest openings, such as prescribed burning, could promote growth of pole‐sized Gambel oak, which appears important to some bird species in pine‐oak forests.

Land use and habitat gradients determine bird community diversity and abundance in suburban, rural and reserve landscapes of Minnesota, USA

Bird species’ community responses to land use in the suburbanizing Twin Cities, Minnesota, USA, were contrasted among reserves, rural lands, and suburbs. For each land use type, bird composition, diversity, and abundance were recorded for 2 years in ≈99 plots in three sampling units (each ≈4500 ha). A habitat gradient defined by canopy structure (grasslands to savannas to forests) was influenced by land use, so ≈300 plots were used to characterize simultaneous variation in bird communities along land use and habitat gradients. At broad scales (aggregate of 33 plots covering ≈4500 ha) suburbs supported the lowest bird richness and diversity and rural landscapes the most, with reserves slightly below rural. Although reserves were like rural lands in diversity of bird communities, they supported more species of conservation concern, particularly of grasslands and savannas. Differences among land use types varied with habitat structure. Suburbs, rural lands, and reserves had similar forest bird communities, but differed in grassland and savanna bird communities. The extensive rural forests are important for the region’s forest birds. Suburban grasslands and savannas had low shrub abundance, low native bird richness and high non-native bird richness and abundance. However, total bird richness and diversity were as high in suburban as in rural and reserve plots because high native richness in suburban forests and high non-native species richness in suburban grasslands and savannas compensated for lower native richness in suburban grasslands and savannas. Bird conservation here and in the Midwest USA should protect rural forests, expand grasslands and savannas in reserves, and improve habitat quality overall.

Spatial and temporal variation in fruit use by wildlife in a forested landscape

We monitored production and removal rates of fruit from 22 common plant species over 2 years in five habitats of a managed landscape in South Carolina (USA). Our long-term goal is to determine the importance of fruit as a resource for vertebrates and to provide recommendations for management of key species and habitats. This study lays the foundation for that goal by documenting fruit production and availability, variation in use by wildlife, and how these factors vary by plant species, habitat, and season. Six species produced >1 kg dry mass of pulp per hectare per year. Vertebrates consumed ≥50% of fruits in 17 of the 22 plant species. Fruit loss to insects and microbes was generally small and varied significantly among seasons, being lowest in fall and winter. The length of time ripe fruit survived on plants varied among species from 3 to 165 days. Survival time of fruits did not vary significantly among habitats but was significantly shorter in the summer than in fall or winter. Approximately, half the species produced fruit in the fall and winter and these fruits were primarily consumed by over-wintering wildlife. This pattern is inconsistent with the general belief that fruit production in the eastern United States is timed to correspond with periods of high bird abundance during fall migration. Production and consumption of winter fruits deserves further attention from forest managers, as relatively little other food is available in winter, energy demands of over-wintering birds are high, and current management practices often reduce fruit availability of key species (e.g., Myrica cerifera). We suggest that fruit is more important than generally realized in maintaining vertebrate diversity in temperate forests and that the focus of managers on hard mast production should be broadened to include fruiting plants.

Conservation of temperate forest birds in Chile: implications from the study of an isolated forest relict

We characterize the bird assemblage of an isolated relict temperaterainforest (Santa Ines) in terms of its structure, composition anddynamics. The relict character and long-term isolation of this forest remnant,immersed in a matrix of semiarid scrub, provide a natural experiment to assessthe consequences of long-term isolation and forest area reduction for temperatebird species. Bird surveys were conducted in a forest fragment and thesurrounding scrub matrix between April 1999 (austral autumn) and October 2000(austral spring) on a seasonal basis. Within the forest fragment wedistinguished two microhabitat types: creeks and areas far from creeks (i.e.slopes). A total of 36 species were recorded in the study site, of which 21were inside the relict forest. Highest bird abundance and richness wererecorded during winter and spring, and these were always higher in creek plotsrather than slope plots. Comparisons between this assemblage and thosedistributed in the continuous temperate forest (located more than 900km southwards) indicate that this bird assemblage shares moresimilarities, with regard to bird species composition, to southern temperateforest localities and to other relict forests, than to nearer scrub habitats.However, there are eight species, endemic to temperate forests in southern SouthAmerica, missing from our system. In this regard, our analyses indicate thatthese eight endemic species should be of great conservation concern and willlikely be the ones that will go extinct first if the fragmentation of thetemperate forest continues.

Effects of Deterrents on Avian Abundance and Nesting Density in Electrical Substations in Oklahoma

Power outages caused by wildlife at electrical substations can be a major financial burden for utility companies and cooperatives. Snakes, presumably preying on nesting birds, have historically been the major cause of wildlife-related outages at Western Farmers Electric Cooperative (WFEC) substations. We studied avian abundance, diversity, and nesting density, and investigated the efficacy of avian deterrents at 62 substations in the WFEC system in Oklahoma in 1996-97. Data from the first year of the study were consistent with this history: nesting densities and bird abundance were higher at heavily damaged substations (≥0.3 incidents/ yr) than moderately damaged or undamaged substations. Low-profile substations had more nests, higher bird abundance, and more damage than lattice-design substations. In the second year of the study, we tested 2 bird deterrents: electronic distress calls (noisemakers) and sodium-iodide lights. Neither deterrent was effective at reducing bird nesting or abundance, although noisemakers slightly reduced bird numbers inside the substations relative to the control. We do not recommend noisemakers or sodium iodide light arrays as bird deterrents at electrical substations.

Declines in Abundance and Species Richness of Birds Following a Major Flood on the Upper Mississippi River

-We examined the abundance and species richness of birds breeding in floodplain forests of the Upper Mississippi River from 1992 to 1994 to identify effects of a major flood in 1993 on the bird assemblage. Sixty-five study plots were divided into treatments and controls based on whether they were flooded in 1993. Among 84 species observed on all plots, 41 species decreased in abundance from 1992 to 1994, 13 species increased, and numbers of 5 species were unchanged. Sample sizes were inadequate to evaluate trends for 25 species. Species richness declined over the three-year period. The main effect was significant in 20 of the 36 species tested. Cool, wet conditions may have contributed to poor reproductive success in 1993 and resulted in widespread declines in abundance during the year following the flood. Bird abundance increased on most unflooded plots in 1993, probably because birds were displaced from flooded plots. This pattern was most striking for Neotropical migrants, species preferring habitat edges, lower-canopy nesters, and species that forage in the air. We suggest that periodic major flooding maintains suitable floodplain habitat for Prothonotary Warblers (Protonotaria citrea) in the face of competition with House Wrens (Troglodytes aedon) for nest sites. Received 30 November 1995, accepted 22 January 1996. FLOODPLAINS ARE DYNAMIC HABITATS where periodic flooding creates natural disturbances that set back succession and provide new sites for plant establishment (Peck and Smart 1986, Kupfer and Malanson 1993, Jones et al. 1994, Sparks 1995, Yin and Nelson 1995). Large floodplain forests like those of the Upper Mississippi River are uncommon in northern landscapes (Mitsch and Gosselink 1993) and provide unique habitats that support a high abundance and diversity of birds (Grettenberger 1991, Best et al. 1995, Knutson et al. 1996). Few studies have examined the avifauna of large floodplain forests (Emlen et al. 1986, Decamps et al. 1987, Knutson 1995), and studies of the effects of floods that include pre-flood data are even less common (Knopf and Sedgwick 1987). Direct effects of flooding on birds nesting in floodplains tend to be minor because spring floods usually recede by midto late May. Summer floods are rare and usually of short duration. However, major floods of long duration or high amplitude can create large-scale disturbances that dramatically change vegetative I Present address: U.S. Geological Survey, Biological Resources Division, Upper Mississippi Science Center, 2630 Fanta Reed Road, La Crosse, Wisconsin 54602, USA. E-mail: melinda_knutson@nb?.gov cover (Yin et al. 1994). Bird assemblages may respond to changes in availability of rnest sites and food resources caused by these disturbances. Flooding had a major effect or a riparian bird community in Arizona by elirminating cottonwood and willow nesting habitat (Hunter et al. 1987). Depending upon the severity of the disturbance, some species may in(rease in abundance while others decline. In addition, flooding may maintain suitable habitat for some bird species that are uncommon in drier, less-disturbed habitats. Rainfall during May to August 1993 in the Upper Mississippi River basin reached historically high levels (ca. twice normal), zpproximating 75to 300-year recurrence i ztervals (Wahl et al. 1993, Interagency Floodplain Management Review Committee 1994). River water levels were well above normal throughout the breeding season (Parrett et al. 1993). We examined the effects of this major flood on the abundance and species richness of bird,; breeding in floodplain forests by comparing cLata collected from 1992 to 1994. We conside-ed our data in relation to two hypotheses of how the flood and associated weather patterns aLffected the floodplain bird assemblage. The Flood Disturbance Hypothesis proposes that the pattern of change in abundance will differ between flooded and unflooded plots because structural

Removal of vertebrate-dispersed fruits in vegetation on fertile and infertile soils.

The hypothesis that more plant species with vertebrate-dispersed fruits occur on fertile soils because there is a greater probability of fruit removal from the parent plant was tested at 16 sites around Sydney, Australia. Removal rates from artificial fruit spikes were two and a half times greater on fertile than infertile soil sites, although this was not quite statistically significant. High variability in removal rate between sites was evident irrespective of fertility. Most removal occurred during the day indicating that birds were important consumers, rather than nocturnal mammals. Bird abundance and diversity did not differ between soil types. More frugivorous species were found in plant communities growing on fertile soil. Two models could explain the patterns observed. Firstly, plants with vertebrate-dispersed fruits could be favoured on fertile soils because of a high abundance of frugivorous birds accomplishing seed dispersal. Alternatively, plants with vertebrate-dispersed fruits could be favoured on fertile soil sites for some other reason and frugivorous birds could be attracted to these areas of abundant food. The correlation between soil fertility and the percentage of vertebrate-dispersed fruits was stronger than the correlation between soil fertility and removal rates and suggests that the second model is more likely to be true. Frugivorous birds are unlikely to be responsible for the high percentage of species with vertebrate-dispersed fruits in fertile soil environments.

The Insectivorous Bird as an Adaptive Strategy

Birds and insects are virtually unique among extant forms, sharing only with bats the ability to fly. Having this characteristic in common, it is not surprising that these groups have developed intricate and fundamental ecological interrelationships. Perhaps best known, and to be discussed here, is the preying of birds upon insects. As a group, insectivorous birds display a wide variety of feeding specializations, from hunting upon the wing (swifts, swallows) to excavating deeply in wood (woodpeckers). Roughly 60% of the approximately 8600 species recognized by Mayr & Amadon (50) are partly or largely insectivorous (percentage estimated from 90). Most are relatively small, and some attain high densities (data from breeding-bird and winter bird censuses in Audubon Field Notes). In spite of their small size, their total biomass may make up a high percentage of the avian contribution. Since this diversity and abundance occurs in spite of the severe restrictions imposed by food chain energetics (42, 84), it points out even more clearly the importance of the bird-insect relationship. At least within the macroscopic realm insect diversity on a world-wide scale is unprecedented (49), and in well-studied terrestrial communities the numbers of insect species may reach equally impressive figures (cf. 11). Despite their small size their overall energetic contribution may be substantial (reviewed in 13). Relative to other major taxonomic groups the number of life forms of insects is also impressive (83), and where metamorphosis occurs, it compounds effective diversity. Given their abundance of general life forms and productivity, insects not surprisingly support many predatory species. Flying predators should enjoy a distinct advantage in capturing flying insects and ones in otherwise practically inaccessible locations (tips of vegetation, etc.). This may well represent a partial basis for the high abundance and diversity of birds. In this paper I consider insectivorous birds as adaptive strategies, that is, as adaptations or sets of adaptations to the spatial and temporal patterning of the environment. Adaptive strategies cut across community barriers and are in theory distinct from special adaptations to particular environmental regimes. For example, will a population become polymorphic or re-