Amanda D. Rodewald

NEST PREDATION IN AN URBANIZING LANDSCAPE:THE ROLE OF EXOTIC SHRUBS

Nest predation is considered a primary force shaping avian communities, and landscape-scale features (e.g., amount of fragmentation) are generally recognized as factors mediating nest predation. These same landscape-scale features, however, may pro- mote invasion by exotic plants, which may, in turn, increase risk of nest predation. We examined whether the use of exotic shrubs (Lonicera spp. and Rosa multiflora Thumb.) affected nest predation across 12 riparian forest sites along a rural-urban gradient ( ,1- 47% urban land cover within 1 km). From 2001 to 2003, 188 Northern Cardinal (Cardinalis cardinalis) and American Robin (Turdus migratorius) nests #5 m tall were monitored. Nest substrate, nest height, and distance from the forest edge were recorded for each nest, whereas nest placement and nest patch characteristics were measured only for Northern Cardinal nests (n 5 68). To further assess relative rates of nest predation in native vs. exotic shrubs while controlling for nest height, distance to edge, and land use, we conducted an artificial nest experiment at two rural sites. Artificial nests (n 5 79) were placed at similar heights in honeysuckle, rose, and native nest substrates along a transect 50-75 m from the forest edge. Nest substrate and landscape type alone failed to account for differences in daily mortality rates. Instead, the effect of nest substrate varied with the landscape matrix, such that nests in exotic shrubs in urbanizing landscapes were twice as likely to be dep- redated than nests in native substrates, irrespective of distance from the edge. Artificial nests placed in exotic shrubs in rural landscapes also suffered higher rates of nest failure than artificial nests in native substrates. Daily mortality rates were greater for nests in exotic shrubs, likely due to reduced nest height and larger shrub volume surrounding the nest. Nests in exotic shrubs were 1.5-2 m lower to the ground and within patches containing 6-9 times more exotic shrub volume. These differences may improve search efficiency of mammalian predators, which appear to be the main predators at our study sites. Based on marks present on recovered clay eggs, 68% of the predation events were attributed to mammals. These findings demonstrate that exotic shrubs can reduce nesting success of forest birds and may cause increased nest failure in urbanizing landscapes. This illustrates another way that exotic plants may diminish habitat quality and limit the capacity of urban forests to contribute to wildlife conservation; therefore, restoring the native shrub com- munity may prove beneficial.

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.

Anthropogenic resource subsidies decouple predator—prey relationships

The extent to which resource subsidies affect food web dynamics is poorly understood in anthropogenic landscapes. To better understand how species interactions are influenced by subsidies, we studied breeding birds and nest predators along a rural-to-urban landscape gradient that varied in subsidies provided to generalist predators. We hypothesized that resource subsidies in urban landscapes would decouple predator-prey relationships, as predators switch from natural to anthropogenic foods. From 2004 to 2009, we surveyed nest predators and monitored 2942 nests of five songbird species breeding in 19 mature forest stands in Ohio, USA. Eighteen species were video-recorded depredating nests. Numbers of avian and mammalian nest predators were positively associated with the amount of urban development surrounding forests, with the exception of Brown-headed Cowbirds (Molothrus ater). Although nest survival strongly declined with detections of nest predators in rural landscapes, nest survival and predator numbers were unrelated in urban landscapes. Thus, the strength of interaction between breeding birds and nest predators diminished as landscapes surrounding forested parks became more urbanized. Our work suggests that decoupling of predator-prey relationships can arise when synanthropic predators are heavily subsidized by anthropogenic resources. In this way, human drivers can alter, and completely disarticulate, relationships among species that are well established in more natural systems.

Exotic shrubs as ephemeral ecological traps for nesting birds

Certain exotic plants may increase risk of nest predation, and, in this way, may act as ecological traps. We hypothesized that the greater vulnerability to predation was a consequence of either (1) reduced nest height due to architectural differences among plant species or (2) seasonal changes in the distribution of nests among forest strata. To test this, we examined temporal variation in nest survival of 888 nests of Northern Cardinal (Cardinalis cardinalis) in native substrates and two exotic shrubs (Lonicera maackii and Rosa multiflora) in Ohio, USA, 2001–2006. We evaluated evidence for an ecological trap by monitoring the annual reproductive productivity of 245 breeding pairs of cardinals. Only nests in Rosa experienced relatively constant survival rates across the season, whereas probability of survival increased over the season for nests in other substrates. Interestingly, the relative vulnerability of nests in different substrates varied across the season. Most strikingly, nests in Lonicera in early spring showed the lowest survival rates but exceeded survival rates of nests in native substrates late in the season. Nest height failed to explain seasonal changes in nest survival, as only nests in native plants significantly increased in height as the season progressed. Rather, predation risk seemed to be a function of the proportion of nests within each substrate, as illustrated by the decreased predation in Lonicera as the relative proportion of nests in native substrates increased. The patterns of temporal variation in predation risk that we detected show that impacts of Lonicera are not a function of plant architecture alone and may be related to leaf phenology, changes in nest density, nest site location, and/or nest synchrony. Examination of the reproductive productivity of cardinals showed that pairs that made their first nest attempt in Lonicera fledged 20% fewer cardinal young than birds that began the season using other substrates. Thus, we suggest that exotic plants may represent an ephemeral ecological trap for certain nesting birds, where negative effects persist only during certain periods.

CONSUMER RESOURCE MATCHING IN URBANIZING LANDSCAPES: ARE SYNANTHROPIC SPECIES OVER-MATCHING

Population responses of synanthropic species to urbanization may be explained by the resource-matching rule, which postulates that individuals should distribute themselves according to resource availability. According to the resource-matching rule, urban habitats will contain greater densities if they provide better resources than rural habitats. However, because resource availability is density dependent, individuals in urban areas would ultimately achieve fitness levels comparable to, but no better than, individuals in less urban areas. Some ecologists suggest that synanthropic birds may not conform to the resource-matching rule and may instead overmatch (i.e., overexploit) in urban habitats, ultimately leading to lower fitness despite greater resource levels. Using the Northern Cardinal (Cardinalis cardinalis) as a focal species, we evaluated if Cardinal populations in urban and rural habitats were consistent with predictions of consumer resource matching. During 2003-2006 we documented population density, adult body condition, apparent survival, and annual reproductive productivity of Cardinals in riparian forest stands within urban (n = 8 stands) and rural (n = 6 stands) landscapes in Ohio, USA. Density of Cardinals in urban forests was four times that found in more rural forests. Mark-resight data from 147 males and 125 females over four years indicated that apparent survival rates were similar between urban and rural landscapes (phi = 0.64, SE = 0.039 for males and phi = 0.57, SE = 0.04 for females). Similarly, body condition indices of 168 males, 142 females, and 118 nestlings did not differ significantly between landscapes. Annual reproductive productivity (mean number of fledglings per pair over breeding season) of 294 pairs was comparable for urban (2.4 +/- 0.18 [mean +/- SE] and rural (2.1 +/- 0.18) young birds. Thus, contrary to recent suggestions, we find that high densities of certain synanthropic species in urban landscapes are consistent with expectations of consumer resource matching.

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.

EDGE- AND AREA-SENSITIVITY OF SHRUBLAND BIRDS

Abstract Populations of shrubland birds in eastern North America have consistently declined since the 1960s, but conservation is hampered by an inadequate understanding of the area requirements of most species. We examined the sensitivity of shrubland specialists to (a) the area of shrub stands and (b) proximity to mature-forest edges, and we evaluated whether habitat characteristics, food resources, or productivity of bird populations could have caused the relationships we identified. In 2002–2003, we used constant-effort mist-netting on 6 small (4–8 ha) and 6 large (13–16 ha) regenerating clearcuts that were 4–6 years post-harvest in southern Ohio, USA. We placed 3 nets at 20, 50, and 80 m from the mature-forest edge (n = 9 nets per site), and we sampled vegetation, fruit, and arthropods at each net. Seven of 8 shrubland specialists, particularly blue-winged warbler, prairie warbler, yellow-breasted chat, indigo bunting, and field sparrow, avoided mature-forest edges, with twice as many birds caught 80 m from edges compared to 20 m. Abundances of most species, especially yellow-breasted chats, were positively correlated with area, though the combined area effect was not statistically significant. We found no evidence of reduced avian productivity in small stands. Neither area nor edge was associated with habitat characteristics, fruit abundance, or arthropod biomass. Our results suggest shrubland birds avoid habitat edges. Thus, small or narrow cuts may not provide optimal habitat for this suite of declining species, and managers should consider options to minimize edge and provide larger patches of shrubland habitats in landscape-scale planning efforts.

VEGETATIVE AND FRUIT RESOURCES AS DETERMINANTS OF HABITAT USE BY MATURE-FOREST BIRDS DURING THE POSTBREEDING PERIOD

Abstract Recent studies have demonstrated that many birds of mature forests heavily use early-successional habitat during the postbreeding period. Two frequently invoked hypotheses to explain these shifts are that postbreeding birds select (1) dense cover to reduce risk of predation and (2) abundant fruit resources to facilitate foraging. Using mist nets between 15 June and 16 August in 2002 and 2003, we captured mature-forest birds during the postbreeding period in 12 regenerating hardwood clearcuts (three to seven years old) in southeast Ohio. Vegetation structure and fruit resources were measured at nine net locations within each clearcut. We applied an information-theoretic approach, where we used vegetative and fruit variables as predictors in seven a priori models to evaluate how habitat factors might explain capture rates. In two seasons, we captured 1,089 hatch-year (HY) and 445 after-hatch-year (AHY) postbreeding birds of 32 mature-forest species. In general, models reflecting habitat structure (density of low vegetation, canopy height) best explained variation in capture rates, which were negatively related to the density of low vegetation (<1.5 m) and positively related to canopy height. Extremely dense low vegetation may be disadvantageous if it inhibits movements and provides cryptic locations for ground predators. Instead, habitats with greater vertical structure (e.g., taller vegetation) and relatively less dense low vegetation below may provide better protection from aerial and ground predators. Overall, results suggest that vegetation structure may explain high use of early-successional forests by many birds during the postbreeding period, though fruit may be the most important factor for seasonal frugivores. Recursos Vegetativos y de Frutos como Determinantes del Uso de Hábitat por Aves de Bosque Maduro Durante el Período Posterior a la Reproducción

Nest Predation in Forested Regions: Landscape and Edge Effects

The generality of edge-related nest predation across different types of edges and in landscapes variously impacted by fragmentation has been debated in recent years. I investigated whether land uses within forested areas exerted stronger landscape-level than local-edge effects on avian nesting success. Ten study sites were selected within contiguous mature forest in central Pennsylvania, USA. Landscape composition (percent cover by different disturbances) was quantified within a 314-ha forested landscape surrounding each study site, and from this, I defined 2 forested landscape types (agricultural and silvicultural; n = 5 each) with similar ranges of forest cover within 1 km (45-82%). From May to July 1998 and 1999, 341 songbird nests were monitored to determine nesting success. For each nest, I measured the distance to a habitat edge (1-200+ m) and identified the type of edge (agriculture, timber harvest, unimproved forest road, or residence). At the local-edge scale, nest fate was not related to the distance from edge for the 4 common edge types. However, at the landscape scale, nesting success was up to 2.6 times lower within forested landscapes disturbed by agriculture than within those disturbed by silviculture. Thus, within forested landscapes, agriculture did not produce edge effects per se but did negatively impact forest birds at the landscape scale. These results provide evidence that, rather than edge effects being important within forested landscapes, the types of land uses within the landscape may in part determine predation rates.

Postfledging Survivorship and Habitat Selection Across a Rural-to-Urban Landscape Gradient

ABSTRACT. Despite recent increases in the number of demographic studies of birds in urban environments, the postfledging period remains poorly understood. Because novel ecological factors, including changes in predator abundance and invasive exotic shrubs, are associated with urbanization, we asked (1) how does postfledging survivorship vary across a rural-to-urban landscape gradient and (2) to what extent does Amur Honeysuckle (Lonicera maackii), an invasive exotic shrub, influence patterns of survivorship and habitat selection? During the 2008 and 2009 breeding seasons, we placed radiotransmitters on fledgling Northern Cardinals (Cardinalis cardinalis; n = 45) and Acadian Flycatchers (Empidonax virescens; n = 31) that occupied riparian forest stands embedded within a rural-to-urban landscape gradient in central Ohio, USA. Predation was the primary cause of fledgling mortality for both species, but cumulative survivorship (± SE) for Acadian Flycatchers (0.720 ± 0.097; 22 days) was 1.6 × that of Northern Cardinals (0.440 ± 0.077; 71 days). Survivorship across the entire postfledging period was not associated with urbanization, but during the initial 3 days after fledging, when mortality rates were highest, Northern Cardinal survivorship was positively related to urbanization. Northern Cardinals strongly selected for complex understory vegetation that was positively associated with survivorship, but survival was not related specifically to cover by Amur Honeysuckle. Contrary to assumptions that postfledging survival declines as landscapes urbanize, our results suggest that urban forests may provide suitable habitat for juvenile birds living within metropolitan areas.