Margaret C. Brittingham

Effects of forest patch size on nesting success of wood thrushes

-Declines of many forest-dwelling Neotropical migrants have been attributed, in part, to fragmentation of forest habitat on the breeding grounds in North America. During 1990-1991, we determined reproductive success of Wood Thrushes (Hylocichla mustelina) nesting within contiguous forest habitat (>10,000 ha) and in nine forest fragments ranging in size from 9.2 to 126.5 ha in Berks County, Pennsylvania. We located 171 Wood Thrush nests. Nesting success differed significantly among forest size categories, with 86% of the nests successful in contiguous forest, 72% successful in large fragments (>100 ha), and 43% successful in small fragments (<80 ha). The variable that best predicted nest survival was forest area (R2 = 0.86). Rates of predation differed significantly among forest size categories, and predation was the primary cause of nesting failure. We found 56% of the nests in small fragments were lost to predators as compared to 22% in large fragments and 10% within the contiguous forest. Visitation by mammalian predators to scent posts was significantly different between small and large forest sites (41 vs. 14%, respectively), and relative abundance of avian nest predators was significantly higher in small forest fragments than in the large forest sites (x = 1.04 vs. 0.41 birds per census point). Brown-headed Cowbirds (Molothrus ater) parasitized 9% of the nests. Rates of brood parasitism did not differ significantly among forest size categories and had little influence on nesting success. Our results suggest that reproductive success of Wood Thrushes nesting within contiguous forest is high and that severe reproductive dysfunction as a result of high rates of nest predation is an important consequence of forest fragmentation. Received 6 July 1993, accepted 21 November 1993. MANY FOREST-DWELLING NEOTROPICAL migrant songbirds have undergone population declines in eastern deciduous forests of North America (Robbins et al. 1989a, Askins et al. 1990). Most declining forest species are found less commonly than expected in small areas of forest than in large (area sensitive), and some of these songbirds have decreased in abundance or disappeared completely from small forest patches (Ambuel and Temple 1983, Blake and Karr 1984, Askins et al. 1987). Because forest fragmentation results in smaller forest patches or fragments separated from each other by nonforest habitat, these declines have been attributed, in part, to fragmentation of forest habitat in North America (Robbins 1979, Askins et al. 1990). Numerous hypotheses have been proposed to explain why forest-dwelling Neotropical migrants decrease in abundance or disappear from small forest patches (Askins et al. 1990). As a forest becomes fragmented, the amount of edge habitat increases and the amount of interior de3Present address: Illinois Natural History Survey, 607 E. Peabody Drive, Champaign, Illinois, 61820, USA. creases. Mammalian nest predators such as eastern chipmunks (Tamias striatus) and raccoons (Procyon lotor; Bider 1968, Forsyth and Smith 1973) tend to be more abundant along the forest edge than in the forest interior. Whitcomb et al. (1981) suggested that avian nest predators, such as Blue Jays (Cyanocitta cristata) and American Crows (Corvus brachyrhynchos), show a similar pattern. The Brown-headed Cowbird (Molothus ater), a brood parasite, also is more abundant along edges (Brittingham and Temple 1983). In addition, rates of both nest depredation and brood parasitism are higher near forest edges than within the forest interior (Gates and Gysel 1978, Chasko and Gates 1982, Brittingham and Temple 1983, Temple and Cary 1988). Consequently, one hypothesis is that forest birds decline in number in small forest patches as a result of poor reproductive success due to high rates of predation on eggs and nestlings, and to brood parasitism (Brittingham and Temple 1983, Wilcove 1985, Askins et al. 1990). Experiments with artificial nests indicate that nest predation is more frequent in small forest patches than within extensive areas of forest, thus supporting this hypothesis (Wilcove 1985,

How Well Do Artificial Nests Estimate Success of Real Nests

Artificial nests frequently are used to assess levels and patterns of nest predation, but how well these nests measure rates of predation or trends in predation rates at real nests is unclear. We compared predation rates between 58 active Wood Thrush (Hylocichla mustelina) nests paired with 58 artificial nests designed to resemble Wood Thrush nests. Paired nests were available to the same predator community both spatially and temporally. Rates of nest predation were significantly lower for active Wood Thrush nests (33%) than for artificial nests (64%). Rates of parasitism by Brown-headed Cowbirds (Molothrus ater) also differed between the two groups. Twenty-six percent of active nests and none of the artificial nests were parasitized by cowbirds. During 1993 and 1994, we conducted three artificial nest trials on six study sites per year. Rates of predation were highest in small woodlots and declined with increasing forest patch size consistent with the trend reported for active nests on the same sites. Within sites, rates of predation varied among trials with the amount of variation highest on sites with high predation rates. Our results suggest that although artificial nests should not be used to measure actual rates of nest predation or parasitism, they may be valuable for detecting trends in rates of predation. However, because there are many potential biases associated with the use of artificial nests that may make interpretation of trend data difficult, we recommend using artificial nests primarily in pilot studies or in conjunction with active nests.

STOPOVER HABITATS OF LANDBIRDS DURING FALL: USE OF EDGE-DOMINATED AND EARLY-SUCCESSIONAL FORESTS

Abstract Despite much interest in the conservation of landbirds during migratory stopover periods, relatively few studies have examined spatial and temporal variation in habitat use and identified important habitats for migrating landbirds in North America. We surveyed migrant landbirds in five habitats (mature forest interior, mature forest-agricultural edge, mature suburban forest, mid-successional pole-stage forest, and early successional shrub-saplingstage forest) in central Pennsylvania from late August to early October, 1997–1999. We used abundances of individual species and migrant guilds, species richness, and fruit availability to assess relative habitat quality for fall migrants and measured structural characteristics associated with migrant habitat use. Of 15 species that differed in abundance among habitats, species that breed in mature forest (n = 10) were typically broadly distributed among habitats during stopover, with highest abundance in edge-dominated forests (forest-agricultural edge and suburban forest) and lowest abundance in pole-stage forests. Mature-forest-breeding migrants also regularly used early successional forests, where as many individuals were recorded as in forest interior. Shrub-sapling-breeding species (n = 5) generally were more narrowly distributed among habitats and were most abundant in early successional and edge-dominated forests. We detected among-year differences in relative use of habitats by mature-forest-breeding species, which suggests that the relative quality of stopover habitats may vary from year to year. Fruit availability was highest in shrub-sapling and forest-agricultural edge habitats and was positively associated with abundance of primary frugivores in all three years, indicating that fruit may be driving habitat selection by that guild. Mature-forest-breeding migrants were positively associated with forests that had more understory vegetation and lower percentage of canopy cover (i.e. more tree-fall gaps), which suggests that migrants selected sites with greater vertical and horizontal habitat heterogeneity. Migrating shrub-sapling-breeding species were positively associated with small-diameter stems (0–2.5 cm) and negatively associated with percentage of canopy cover (i.e. characteristics of breeding habitats). Consistently high use of mature edge-dominated and early-successional forests by a wide diversity of landbird species during fall stopover indicates the potential importance of those habitats for migratory landbird conservation.

Ecological risks of shale oil and gas development to wildlife, aquatic resources and their habitats.

Technological advances in hydraulic fracturing and horizontal drilling have led to the exploration and exploitation of shale oil and gas both nationally and internationally. Extensive development of shale resources has occurred within the United States over the past decade, yet full build out is not expected to occur for years. Moreover, countries across the globe have large shale resources and are beginning to explore extraction of these resources. Extraction of shale resources is a multistep process that includes site identification, well pad and infrastructure development, well drilling, high-volume hydraulic fracturing and production; each with its own propensity to affect associated ecosystems. Some potential effects, for example from well pad, road and pipeline development, will likely be similar to other anthropogenic activities like conventional gas drilling, land clearing, exurban and agricultural development and surface mining (e.g., habitat fragmentation and sedimentation). Therefore, we can use the large body of literature available on the ecological effects of these activities to estimate potential effects from shale development on nearby ecosystems. However, other effects, such as accidental release of wastewaters, are novel to the shale gas extraction process making it harder to predict potential outcomes. Here, we review current knowledge of the effects of high-volume hydraulic fracturing coupled with horizontal drilling on terrestrial and aquatic ecosystems in the contiguous United States, an area that includes 20 shale plays many of which have experienced extensive development over the past decade. We conclude that species and habitats most at risk are ones where there is an extensive overlap between a species range or habitat type and one of the shale plays (leading to high vulnerability) coupled with intrinsic characteristics such as limited range, small population size, specialized habitat requirements, and high sensitivity to disturbance. Examples include core forest habitat and forest specialists, sagebrush habitat and specialists, vernal pond inhabitants and stream biota. We suggest five general areas of research and monitoring that could aid in development of effective guidelines and policies to minimize negative impacts and protect vulnerable species and ecosystems: (1) spatial analyses, (2) species-based modeling, (3) vulnerability assessments, (4) ecoregional assessments, and (5) threshold and toxicity evaluations.

A century of hybridization : decreasing genetic distance between American black ducks and mallards

American black ducks (Anas rubripes) and mallards (A. platyrhynchos) are morphologically and behaviorally similar species that were primarily allopatric prior to European colonization of North America. Subsequent sympatry has resulted in hybridization, and recent molecular analyses of mallards and black ducks failed to identify two distinct taxa, either due to horizontal gene flow, homoplasy, or shared ancestry. We analyzed microsatellite markers in modern and museum specimens to determine if the inter-relatedness of mallards and black ducks was an ancestral or recent character. Gst, a measure of genetic differentiation, decreased from 0.146 for mallards and black ducks living before 1940, to 0.008 for birds taken in 1998. This is a significant reduction in genetic differentiation, and represents a breakdown in species integrity most likely due to hybridization. Using modern specimens, we observed that despite a lower incidence of sympatry, northern black ducks are now no more distinct from mallards than their southern conspecifics.

Selection of maternity roosts by big brown bats

Exclusion is the recommended method for removing roosting bats from buildings, but is often difficult to accomplish. A simpler way to limit bat-human conflicts may be to modify new and existing buildings to discourage colonies from initially taking up residence. An understanding of the physical and micro-climatic characteristics of maternity roosts is a prerequisite when modifying buildings to discourage colonies. We investigated factors influencing maternity roost selection in big brown bats (Eptesicusfuscus) by comparing characteristics of bat-occupied buildings with bat-unoccupied buildings at 10 sites. Bat-occupied buildings were significantly older, more likely to have galvanized steel (tin) roofs, more accessible to bats, and taller than randomly selected unoccupied buildings. In paired surveys, occupied attics were significantly more accessible to bats than physically similar unoccupied attics and exhibited significantly higher temperatures and wider temperature gradients. Disturbance levels, light levels, and humidity did not differ between occupied and paired unoccupied attics. To discourage bats from initially establishing a maternity roost within a building, limit all access points. In buildings where this is difficult, attics can be made less suitable as roost sites by reducing attic temperatures during the summer months. Bat boxes intended to house displaced maternity colonies should be designed to provide high daily temperatures and wide temperature gradients.

HABITAT USE AND BEHAVIOR OF MIXED SPECIES LANDBIRD FLOCKS DURING FALL MIGRATION

Abstract Little research has examined the ecology of mixed species flocks of migrant and resident landbirds during migratory periods. We studied habitat use and behavior of mixed species insectivorous landbird flocks during fall migration in central Pennsylvania. From late August to early October, 1998 and 1999, 220 flocks were observed for 30-min periods in six forest habitat types: mature forest interior, mature forest edge, mature forest agricultural edge, mature suburban forest, pole stage forest, and shrub/sapling stage forest. Sixty species were recorded in flocks that contained 2–24 species each (mean = 9.25 ± 0.29 SE). Flocks contained 2–181 individuals (mean = 22.12 ± 1.18 SE). Flocks in the six habitats had 49–61% Nearctic-Neotropical migrant individuals, 5–15% temperate migrants, and 23–37% residents. Abundance and species richness of migratory guilds (Nearctic-Neotropical migrants, temperate migrants, and resident species) within flocks were highest in structurally heterogeneous habitats (especially forest edge habitat) and were lowest in homogeneous pole stage forest. Of nine migrant species whose abundance varied significantly among habitats, six had highest abundance in flocks in forest edge habitat: Blue-headed Vireo (Vireo solitarius), Red-eyed Vireo (V. olivaceus), Blue-gray Gnatcatcher (Polioptila caerulea), Chestnut-sided Warbler (Dendroica pensylvanica), Black-throated Green Warbler (D. virens), and Magnolia Warbler (D. magnolia). Resident parids (Black-capped Chickadee, Poecile atricapillus, and/or Tufted Titmouse, Baeolophus bicolor) occurred within 82% of flocks and were observed leading 68% of these flocks. Movement rate (m/min) of flocks varied among habitats with flocks in edge-dominated habitats (forest edge, forest agricultural edge, and suburban forest) tending to have slower movement rates than in pole stage forest and forest interior, suggesting that food availability may have been greater in edge-dominated habitats. Consistently high species richness and abundance of migrant guilds and individual species strongly suggests that structurally diverse forest edge habitats were selected and provided relatively high quality stopover habitat for landbirds during fall migration.

TERRESTRIAL LIMING BENEFITS BIRDS IN AN ACIDIFIED FOREST IN THE NORTHEAST

Studies in Europe have reported negative effects of acid deposition on forest birds, and research in North America has identified links between forest bird abundance and rates of acid deposition. We examined the bird community in an acidified forest in central Pennsylvania (USA) and evaluated the effects of terrestrial lime application on birds. We used a before-after control-impact (BACI) study design, with one year of observation before (2003) and three years after lime application (2004, 2005, and 2006). Between the 2003 and 2004 field seasons, 4500 kg/ha of dolomitic lime were applied to two of four 100-ha watersheds. Each year, we monitored bird abundance and Ovenbird (Seiurus aurocapilla) eggshell thickness and territory size. Soil and snail abundance data were also collected. The bird community and territory size results indicated that the study area may be providing low-quality habitat for forest birds, perhaps as a result of acid deposition. We found lower forest bird abundances than have been found in less acidified areas of Pennsylvania, and larger Ovenbird territory sizes than have been found in other studies. We found a significant positive relationship between soil calcium and bird abundance, indicating that soil calcium may affect bird abundance. Liming increased soil calcium and pH and led to increased snail and bird abundances. After liming, bird abundance was positively related to snail abundance. No significant changes occurred in Ovenbird territory size or eggshell thickness. Our results suggest that acid deposition could be responsible for reduced bird abundance, and that liming is a potential mitigation technique.

Use of winter bird feeders by black-capped chickadees

Bird feeding is widespread and a frequent component of urban wildlife management; however, no data on how individuals use the resource or what it contributes to their energy needs are availabe. Consequently, we studied the foraging behavior of 348 color-banded black-capped chickadees (Parus atricapillus) at winter bird feeders in Wisconsin, from 1983 to 1985

Wildlife damage to corn in Pennsylvania: Farmer and on-the-ground estimates

Agricultural damage is a concern of farmers and agricultural and wildlife agencies at the state and federal levels. We compared questionnaire and independent on-the-ground sampling results to obtain estimates of wildlife-related damage to the 1995 corn crop in Pennsylvania, USA. We sampled 222 randomly selected cornfields (2.5 ± 0.65 ha; x ± SE) and the respective operators. Pennsylvania farmers reported an average of 33 ± 1.3 years of farming experience, 60% had >75% of their income from farming, and 49% were dairy farmers. They owned 125.4 ± 11.33 ha of which 55.8 ± 4.05 ha were planted in corn. Average corn yield was 7.31 m 3 /ha (84 bu/ac), which was negatively influenced by drought. Average wildlife-related loss was 0.48 m 3 /ha (5.5 bu/ac); white-tailed deer (Odocoileus virginianus) loss was 0.35 m 3 /ha (4.0 bu/ac). For 132 farms, we had the farmers (farm-wide) and the on-the-ground (field-specific) estimates of wildlife damage. The correlation between loss estimates (r = 0.263, P= 0.0013) was low. But no difference (t=1.30, P= 0.196) occurred between the mean estimates of corn loss (x ± SE) reported by farmers for the farm (9.68 ± 0.89%) and the on-the-ground estimate for a field (7.67 ± 1.27%).