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Aim Avian migration strategies balance the costs and benefits of annual movements between breeding and wintering grounds. If similar constraints affect a large numbers of species, geographical concentrations of migration routes, or migration flyways, may result. Here we provide the first population‐level empirical evaluation of the structure and seasonal dynamics of migration flyways for North American terrestrial birds and their association with atmospheric conditions. Location Contiguous USA. Methods We modelled weekly probability of occurrence for 93 migratory species using spatio‐temporal exploratory models and eBird occurrence data for the combined period 2004 to 2011. We used hierarchical cluster analysis to identify species with shared migration routes based on normalized spatio‐temporal representations of autumn migration. We summarized atmospheric conditions within flyways using nocturnal wind velocity and bearing estimated at three isobaric levels (725, 825 and 925 mbar) for the combined period 2008 to 2011. Results We identified three migration flyways: an eastern and western flyway whose paths shifted westwards in the spring, and a central flyway whose core boundaries overlapped with the eastern flyway and whose width was more constricted in the autumn. The seasonal shift of the eastern flyway created potentially longer migration journeys in the spring, but this longer route coincides with a low‐level jet stream that may enhance migration speeds. Atmospheric conditions appeared to have a more limited role in the seasonal dynamics of the western flyway. Main conclusions Migration routes for terrestrial species in North America can be organized into three broadly defined migration flyways: a geographically distinct flyway located west of the 103rd meridian and two interrelated flyways located east of the 103rd meridian. Seasonal shifts in flyway locations reflect the influence of looped migration strategies that for the eastern flyway can be explained by the trade‐off between minimizing total migration distance while maintaining an association with favourable atmospheric conditions.

Abstract. 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.

., et al. 2012. Positive relationships between association strength and phenotypic similarity characterize the assembly of mixed-species bird flocks worldwide. American Naturalist 180: 777-90. AuthorsHari Sridhar, Umesh Srinivasan, Robert A. Askins, Julio Cesar Canales-Delgadillo, Chao-Chieh Chen Submitted February 10, 2012; Accepted August 1, 2012; Electronically published MONTH? xx, 2012 Online enhancement: appendix. Dryad data: http://dx.doi.org/10.5061/dryad.th198.abstract: 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 matrixes 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.

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Abstract. 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 promote 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 (Ͻ1-47% urban land cover within 1 km). From 2001to 2003 and American Robin (Turdus migratorius) nests Յ5 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 ϭ 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 ϭ 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 depredated 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 community may prove beneficial.

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