Richard H. Yahner

Effects of Forest Fragmentation on Depredation of Artificial Nests

Depredation on artificial ground and arboreal (1.5 m above ground) nests was studied in mature (uncut) forest stands on a ruffed grouse (Bonasa umbellus) management area in central Pennsylvania from May to August 1986. Predation was evaluated in mature stands with zero, 25, and 50% zones of the surrounding forest fragmented by clearcutting. Nest depredation was highest in the 50% zone and least in the zero percent zone; more arboreal nests than ground nests were disturbed. American crows (Corvus brachyrhynchos) were major predators on nests in the 50% zone, and crows and blue jays (Cyanocitta cristata) caused most nest disturbances in the 25% zone. Our results suggest that fragmentation of surrounding mature forest stands may negatively impact avian nesting success, especially when the principal nest predators are corvids. Predation in fragmented forests may have a greater effect on nesting success of birds nesting above ground than on those nesting at ground level. J. WILDL. MANAGE. 52(1):158-161 Forest fragmentation, or the cutting of remaining natural forest into progressively smaller patches (Harris 1980), negatively impacts abundance and distribution of insectivorous passerines in the eastern United States (Whitcomb et al. 1981). Because forest fragmentation results in greater edge (Ranney et al. 1981) and predators select edges for foraging (Gates and Gysel 1978, Chasko and Gates 1982), predation on avian nests could increase with extensive forest fragmentation (Noss 1983). However, with the exception of a study dealing with predation on artificial nests in woodlots of various sizes (Wilcove 1985), the influence of forest fragmentation on nest predation is not well documented. Clearcutting forest to improve ruffed grouse habitat at the Barrens Grouse Habitat Management Study Area (BGMA) in central Pennsylvania provides suitable nesting habitat for a variety of avian species that nest ?2 m from the ground (Yahner 1986b, Yahner and Cypher 1987). The BGMA contains a sector with no forest fragmentation (zero clearcutting) and sectors with 2 intensities of forest fragmentation (25 and 50% clearcutting) that provided an opportunity to examine the relationship between nest depredation and forest fragmentation. Our objective was to compare depredation on artificial ground and arboreal nests placed in mature forest stands surrounded by uncut forest and 2 intensities of forest fragmentation at the BGMA. We thank R. A. Voytko and R. W. Bond for field assistance. Funding for this study was provided by the Max McGraw Wildlife Foundation and the Pennsylvania Agricultural Experiment Station. W. A. Creed, R. G. Eckstein, and M. J. Lacki reviewed the manuscript. This is Journal Series Paper 7565 of the Agricultural Experiment Station, The Pennsylvania State University, University Park.

INFLUENCE OF LANDSCAPE COMPOSITION ON AVIAN COMMUNITY STRUCTURE AND ASSOCIATED MECHANISMS

Many recent studies have addressed the effects of patch size and isolation on bird communities in highly fragmented landscapes, but the importance of landscape composition in more forested landscapes remains poorly understood. The objectives of our study were (1) to determine the effects of two structurally and temporally distinct distur- bance types (agriculture and silviculture) and extent of disturbance (percentage of distur- bance within 1.0 km) on avian community structure within forested landscapes, and (2) to investigate underlying mechanisms responsible for associations between landscape com- position and avian communities. Differences in microhabitat, microclimate, and nesting success among landscapes were examined as possible mechanisms. Breeding bird com- munities in central Pennsylvania (USA) were surveyed twice per year during 1997-1999 at 34 sites within contiguous mature forest, which represented a range of disturbance extent (4-59% nonforest cover within 1.0 km). Each site was in a forested landscape containing predominantly agricultural or silvicultural disturbances (n = 17 each). Our results indicate that type of disturbance within landscapes influenced bird community composition and relative abundance more than extent of disturbance. Compared to forests within landscapes disturbed by silviculture, forests within landscapes disturbed by agriculture, irrespective of the extent of disturbance, had fewer forest-associated species, long-distance migrants, forest-canopy and forest-understory-nesting species, and greater numbers of edge-associated species, including avian nest predators and Brown-headed Cowbirds (Molothrus ater). Few- er species and guilds were associated with the extent of disturbance within a landscape or interactions between disturbance type and extent. Abundances of edge-associated species, residents, and forest-canopy nesters increased with increasing amounts of disturbance within forested landscapes. Local variation in microhabitat and microclimate among landscapes did not explain observed differences in avian community structure. However, nesting suc- cess was greater and numbers of some avian and small mammalian nest predators were lower in stands within forested landscapes disturbed by silviculture than in forested land- scapes disturbed by agriculture. Nesting success was not associated with the extent of a given disturbance type within landscapes. These results demonstrate that, even within for- ested landscapes, the types of disturbance can influence avian community structure and, thus, should be considered in conservation and forest management plans. In particular, agricultural disturbances within forested landscapes seemed to negatively affect bird com- munities in adjacent forests more than silvicultural disturbances. Both species richness and abundance of forest-associated species were greater on sites with higher levels of nesting success. Thus, differences in nesting success resulting from altered interactions between nest predators and nesting birds may be an important underlying mechanism of avian community structure and organization at the landscape scale.

Dynamics of a small mammal community in a fragmented forest.

-Community structure [species richness (S) and numbers of individuals captured] of a small mammal community were studied in July and August, 1987-1989, on a 1166-ha forested area in central Pennsylvania, which was divided into reference and treated sectors of comparable size. The reference sector was uncut, whereas the treated sector contained two areas with 50% and 75% fragmentation, respectively, resulting from forest clear-cutting in winter 1985-1986 and 1986-1987. I examined whether community structure of small mammals differed among years (1987-1989) or among forested areas that varied in extent of fragmentation. In addition, I compared community structure in 1987-1989 to that obtained on the same study site in 1982-1984 when extent of forest fragmentation was considerably less (25% and 50%) on the treated sector. Four-hundred nine individuals of seven species were captured during 4500 trap nights in 1987-1989, including white-footed mice (Peromyscus leucopus) (67.3% of total individuals), southern red-backed voles (Clethrionomys gapperi) (13.3%), and masked shrews (Sorex cinereus) (12.0%). Numbers of individuals captured of all species combined and of the three common species were significantly (P < 0.05) lower in 1988 than in 1987 and 1989, which I attributed to a drought in 1988 that presumably reduced the availability of terrestrial arthropods as a food resource. Although S was similar in 1987-1989 among areas of forest fragmentation and between 1982-1984 and 1987-1989, numbers of P. leucopus and C. gapperi captured were significantly (P < 0.05) higher in 1987-1989, especially in the area with 75% fragmentation. Numbers of P. leucopus were greater in 1987-1989 than in 1982-1984, perhaps because of a greater variety of microenvironments created by clear-cutting and increased availability of mast (acorns of oaks [Quercus] spp.) in 1987-1989. Numbers of C. gapperi likely were higher in 1987-1989 than in 1982-1984 because of moist microenvironments resulting from abundant low vegetation in clear-cut plots on the treated sector. I conclude that forests fragmented into small, even-aged plots can benefit P. leucopus and C. gapperi by increasing abundance of these two

Effects of nest location on depredation of artificial arboreal nests

Depredation on artificial arboreal nests was studied in 1-ha aspen (Populus spp.) plots on a ruffed grouse (Bonasa umbellus) management area in central Pennsylvania from May to August 1985. Predation was evaluated with respect to 2 factors associated with nest placement: height of nest aboveground and plot age. American crows (Corvus brachyrhynchos) were the major nest predator. Fewer low nests (0.5 m aboveground) were disturbed than high nests (1.5 m), and more nests were disturbed in both 8-year-old and mature plots than in 4-year-old plots. Low nests in 4-year-old plots were least susceptible to depredation. Dense shrub growth in 4-year-old plots presumably reduced the foraging efficiency of predators, thereby lowering the probability of disturbing low nests. Young clearcuts provide well-concealed nest sites for avifauna that construct nests near ground level in shrubby vegetation. J. WILDL. MANAGE. 51(1):178-181 Predation is a major factor reducing nesting success in birds (Ricklefs 1969). Avian predators may have a greater impact than large mammalian predators on nests located high in small trees and shrubs than nests positioned close to ground level (Joern and Jackson 1983, Shalaway 1985). The relationship between nest height and susceptibility to disturbance by predators may vary with habitat type. For instance, Caccamise (1977) found that in tidal marshes, the percentage of successful nests decreased as nest height increased, whereas Best and Stauffer (1980) noted that fledging success was greater in higher nests (1-2 m aboveground) compared to that of lower nests (<1 m) in riparian habitats. In general, avian nests, particularly those located near ground level, may be less conspicuous to predators when located in areas with complex vegetative structure (Bowman and Harris 1980, Redmond et al. 1982, Yahner and Wright 1985). An even-aged system of forest clearcutting provides suitable habitat for a variety of songbird species that construct nests near (<2 m) ground level (Yahner 1986b). The objective of this study was to compare depredation on artificial arboreal nests placed at 2 heights above ground level in small aspen plots of 3 age classes (time since clearcutting) and, hence, different vegetative structure. We thank E. A. Cypher and R. L. Schooley for field assistance and L. B. Best, D. P. Scott, and R. D. Shipman for comments on the manuscript. This study was supported by the Pa. Agric. Exp. Stn. and the Max McGraw Wildl. Found. This is J. Ser. Pap. 7417 of the Pa. Agric. Exp. Stn., The Pennsylvania State Univ., University Park.

AVIAN NESTING SUCCESS IN FORESTED LANDSCAPES: INFLUENCE OF LANDSCAPE COMPOSITION, STAND AND NEST-PATCH MICROHABITAT, AND BIOTIC INTERACTIONS

Abstract Although area and isolation effects on avian communities in highly fragmented landscapes are well known, importance of landscape composition in more forested landscapes remains poorly understood. We determined if the type (agriculture and silviculture) and extent (percentage within 1 km radius) of disturbance within forested landscapes influenced avian nesting success, and then examined if differences in stand-level habitat structure, nest-patch microhabitat, distance of nests to habitat edges, brood parasitism rates, and nest-predator abundance were potential underlying mechanisms of observed associations between landscape composition and nesting success. We monitored active songbird nests (n = 341), surveyed Brown-headed Cowbirds (Molothrus ater) and nest predators, and measured stand-level and nest-patch microhabitat from May–July 1998 and 1999. Each of 10 study sites was located within contiguous mature forest in central Pennsylvania and contained either agricultural or silvicultural disturbances (n = 5 each). Sites of the two landscape types had similar ranges of disturbance within 1 km (21–55% for agriculture, 18–51% for silviculture). Daily nest survival for all species combined (94.0 ± 0.55 in agriculture and 96.9 ± 0.87 in silviculture) and midstory-canopy nesters (93.8 ± 0.97 in agriculture and 97.4 ± 0.75 in silviculture) were greater within forested landscapes disturbed by silviculture than by agriculture, but rates did not significantly differ between landscapes for ground nesters (92.2 ± 1.32 in agriculture and 94.6 ± 1.63 in silviculture) or understory nesters (95.4 ± 1.60 in agriculture and 95.0 ± 1.47 in silviculture). Nest survival was not significantly associated with disturbance extent. Rates of brood parasitism were low, with only 11% of nests containing cowbird eggs or young. Neither nest fate nor differences in daily nest survival between the two landscape types were explained by variation in brood parasitism rates, stand-level or nest-patch habitat characteristics, or distance of nests to edges. Instead, the lower nest success within forested landscapes disturbed by agriculture was best explained by greater abundances of some avian and small mammalian predators (American Crow [Corvus brachyrhynchos] and squirrels) in those landscapes in one or both years. Results suggest that landscape composition within forested landscapes significantly influences avian nesting success by altering interactions between nest predators and nesting birds.

Small Mammals in Farmstead Shelterbelts: Habitat Correlates of Seasonal Abundance and Community Structure

Small mammals were studied from September 1978 to November 1980 in 5 Minnesota farmstead shelterbelts. Based on correlations between habitat variables characterizing vegetative features of shelterbelts and numbers of individuals captured in each season, Peromyscus leucopus and Clethrionomys gapperi were considered as woodland species, Microtus pennsylvanicus as a grassland species, and Sorex cinereus and Blarina brevicauda as intermediate in terms of their dependency on shelterbelts. Area and perimeter of shelterbelts were associated with numbers of all species except C. gapperi. Numbers of P. leucopus and C. gapperi tended to be higher in shelterbelts that were isolated from other wooded habitat, whereas numbers of S. cinereus were lower in isolated shelterbelts. Species richness was greater in larger shelterbelts with complex vegetative structure. Species of small mammals residing in shelterbelts were those that typically are not considered as agricultural pests. Management recommendations include maintenance practices that do not reduce stratification of vegetation, leaving woody and man-made debris within shelterbelts, and establishing shelterbelts that are as large as possible within the economic constraints of farming. These recommendations simultaneously would benefit other species of mammals and birds in the intensively farmed regions of the Midwest. J. WILDL. MANAGE. 47(1):74-84 Vegetative complexity and habitat size are major determinants of the abundance of individual mammalian species and the structure of mammalian communities in natural habitats (Dueser and Shugart 1978, Dueser and Brown 1980, Geier and Best 1980). Farmstead shelterbelts are small (generally 1.5 m in depth) that prevented access to traps. Traps were opened for 2 consecutive days each session and were closed between sessions. Rolled oats mixed with small amounts of peanut butter were used as bait. I marked each captured animal with ear tags (#1, National Band and Tag Co., Newport, Ky.) or toe clips for individual recognition. Sex, age, weight, reproductive condition, and trap location were recorded for each capture. Species captured >20 times during the study were included in the analyses; these were Sorex cinereus, Blarina brevicauda, Clethrionomys gapperi, Microtus pennsylvanicus. and Peromyscus leucopus. Fifty-four habitat variables were measured for each shelterbelt during August 1979 and 1980. These included 2 dimensional, 14 proximal land-use, 27 vegetative, and 11 physical variables (Appendix). Vegetative and physical features were based on sampling methods and procedures modified from Dueser and Shugart (1978) using 3 sampling units centered on each trap: a 10-m radius plot, a 1-m2 ring, and 2 perpendicular 20-m2 transects. Species variables were derived in each of the 9 seasons (autumn 1978, winter 1979, etc.) for the 5 species of small mammals. These variables consisted of the total number of individuals (all age-sex classes combined) and the total number of indiJ. Wildl. Manage. 47(1):1983 This content downloaded from 207.46.13.113 on Thu, 06 Oct 2016 04:06:29 UTC All use subject to http://about.jstor.org/terms 76 SMALL MAMMALS IN FARMSTEAD SHELTERBELTS * Yahner viduals per age-sex class (except for Sorex and Blarina) per shelterbelt in each season. In addition, species richness (S), or the total number of different species of small mammals captured, was determined per shelterbelt for the entire study period and for each season. Relationships between the 54 habitat variables and the species variables were examined. As a result of small sample size (N = 5 shelterbelts) and multicollinearity (Chatterjee and Price 1977) among habitat variables, simple correlation analyses were used (Sokal and Rohlf 1969). Both untransformed and log-transformed data were analyzed (after Dueser and Brown 1980). Throughout the text, significant correlations (P < 0.05) were based on product-moment correlation coefficients (r) ?0.878 and df = 3. RESULTS AND DISCUSSION

Effects of Nest-Site Selection on Depredation of Artificial Nests

We compared depredation of artificial arboreal nests between nests placed at nest sites used by birds in the previous season (actual nests) and those located at random nest sites (random nests) on a ruffed grouse (Bonasa umbellus) management area in central Pennsylvania from May to July 1987. Ninety-eight (65%) actual nests and 109 (73%) random nests were disturbed by predators. The number of actual and random nests disturbed did not differ (P = 0.74) from expected. Fate (undisturbed vs. disturbed by predators) of actual and random nests was not related (P > 0.05) to time of nest placement during the study or to plot age, nest height, nesting substrate, and woody stem densities at ground level or at nest height. Because nesting substrate and, hence, degree of nest conspicuousness varies among bird species in nature, we suggest that results obtained from studies of predation on artificial nests be used with caution. J. WILDL. MANAGE. 53(1):21-25 Depredation of artificial avian nests may vary with time of placement, age of plot, height of nests above ground, extent of forest fragmentation, and degree of concealment provided by vegetation surrounding nest sites (Sugden and Beyersbergen 1986, Yahner and Cypher 1987, Yahner and Scott 1988). In artificial nest studies, nests typically are placed at random nest sites selected by the investigator, e.g., in the nearest tree or shrub species (Yahner and Scott 1988). However, whether or not depredation of artificial nests placed at random nest sites differs from that of artificial nests placed at nest sites selected by birds is unknown. If artificial nests at these 2 types of nest sites differ in susceptibility to predation, then caution would be advis d when interpreting results obtained from artificial nests placed at random and extrapolati g to depredation of avian nests in nature. Our objectives were to compare depredation rates between artificial nests placed at random ne t sites to those placed at nest sites used by birds in the preceding breeding season, and determine if depredation of nests at these 2 types of nest sites is related to time of nest placement uring the breeding season or to plot age, nest height, nesting substrate, and woody stem densiti at ground level or at nest height. We thank J. L. Bauer for field assistance and T. E. Morrell for reviewing the manuscript. Funding for this research was provided by the I Present address: Box 175, Spangler, PA 15775. This content downloaded from 207.46.13.33 on Sat, 26 Nov 2016 04:10:49 UTC All use subject to http://about.jstor.org/terms 22 DEPREDATION ON ARTIFICIAL NESTS * Yahner and Voytko J. Wildl. Manage. 53(1):1989 Pennsylvania Agricultural Experiment Station and the Max McGraw Wildlife Foundation. This is Journal Series Paper 7826 of the Agricultural Experiment Station, The Pennsylvania State University, University Park.

Effects of time of day and season on winter bird counts

We examined the effects of time of day and season on counts of wintering birds in a central Pennsylvania forest. Black-capped Chickadees (Parus atricapillus), Tufted Titmice (Parus bicolor), and White-breasted Nuthatches (Sitta carolinensis) were detected more often in the morning (07:00-10:59) than at other times, whereas White-throated Sparrows (Zonotrichia albicollis) were noted more often at midday (11:00-13:59). Numbers of contacts of all species combined were relatively similar between morning and midday but were much lower in the afternoon (_ 14:00). Black-capped Chickadees, Tufted Titmice, White-breasted Nuthatches, Golde -crowned Kinglets (Regulus satrapa), and White-throated Sparrows were observed more often in early winter (before 19 January) than later in the season. We conclude that winter bird counts can be conducted in both morning and midday hours with little or no qualitative loss of data. In addition, counts should be made in early winter to minimize the effects of food shortages later in winter that may influence avian mortality and movements to feeders.

Bird communities associated with harvested hardwood stands containing residual trees.

Retention of residual trees in even-aged harvested stands is an alternative to traditional clearcutting, seed-tree, and shelterwood systems, but little is known about effects of new even-aged retention methods on bird communities. Clearcutting on Pennsylvania state forests recently has been replaced by a new forest-management practice termed even-aged reproduction stands with reservation guidelines (hereafter, EAR stands), in which high densities of trees in multiple crown and size classes (101 live trees/ ha ±28 SE on study sites) of both commercially and non-commercially important tree species are permanently reserved to maintain species and structural diversity. We compared habitat structure and breeding-bird communities between EAR stands (harvested) and reference stands (unharvested) in 2 state forests of Pennsylvania in 1997-98 and related bird abundance within harvested stands to differences in habitat characteristics among EAR stands and the surrounding landscapes. Total abundance of all bird species combined, abundances of early-successional and edge-habitat guilds, and abundances of many early-successional bird species were significantly higher in EAR stands than in reference stands, but abundances of the forest habitat guild and of 8 forest-associated species were lower in EAR stands. Although EAR stands provide suitable habitat for bird species associated with early-successional forests, abundances of species associated with mature forests were lower in EAR stands than in reference stands despite retention of residual trees. However, some species of forest birds (e.g., red-eyed vireos [Vireo olivaceus]), which usually are absent from recent clearcut stands until 12-20 years post-harvest, were often detected in EAR stands. Thus, residual trees in EAR stands provide to forest birds habitat components that are generally lacking in clearcut stands. Because abundances of both forest habitat and forest-canopy nesting guilds declined and abundance of brown-headed cowbirds (Molothrus ater) increased with size of EAR stands (especially when >20 ha), managers should consider limiting the size of EAR stands.

Avian nesting ecology in small even-aged aspen stands

I examined abundance, location, and success of avian nests from June through August, 1985-87, in small (1 ha) even-aged aspen (Populus spp.) stands on a 240-ha study area managed as ruffed grouse (Bonasa umbellus) habitat in central Pennsylvania. Ninety-five nests of 14 species, mainly rufous-sided towhee (Pipilo erythrophthalmus) (28.4%) and gray catbird (Dumetella carolinensis) (24.2%), were located. Observed versus expected numbers of total nests (all species combined) and those of towhees and catbirds differed (P 0.05) of stand age and distance from an edge but was inversely related (P 0.5 m above ground level) were more susceptible than lower nests (≤0.5 m) to predators