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 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 (<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 = 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 = 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

Breeding phenology of birds: mechanisms underlying seasonal declines in the risk of nest predation.

Seasonal declines in avian clutch size are well documented, but seasonal variation in other reproductive parameters has received less attention. For example, the probability of complete brood mortality typically explains much of the variation in reproductive success and often varies seasonally, but we know little about the underlying cause of that variation. This oversight is surprising given that nest predation influences many other life-history traits and varies throughout the breeding season in many songbirds. To determine the underlying causes of observed seasonal decreases in risk of nest predation, we modeled nest predation of Dusky Flycatchers (Empidonax oberholseri) in northern California as a function of foliage phenology, energetic demand, developmental stage, conspecific nest density, food availability for nest predators, and nest predator abundance. Seasonal variation in the risk of nest predation was not associated with seasonal changes in energetic demand, conspecific nest density, or predator abundance. Instead, seasonal variation in the risk of nest predation was associated with foliage density (early, but not late, in the breeding season) and seasonal changes in food available to nest predators. Supplemental food provided to nest predators resulted in a numerical response by nest predators, increasing the risk of nest predation at nests that were near supplemental feeders. Our results suggest that seasonal changes in foliage density and factors associated with changes in food availability for nest predators are important drivers of temporal patterns in risk of avian nest predation

Does Balsam Fir (\u3cem\u3eAbies balsamea\u3c/em\u3e) Facilitate the Recruitment of Eastern Hemlock (\u3cem\u3eTsuga canadensis\u3c/em\u3e)?

Eastern hemlock (Tsuga canadensis) was a major component of mesic forests in the Upper Great Lakes region, but presently persists in only a few locations. Many of these stands experience poor regeneration due to herbivory by white-tailed deer (Odocoileus virginianus), suggesting that hemlock will become progressively less common in these stands. We tested the hypothesis that balsam fir (Abies balsamea) facilitates establishment of eastern hemlock at 11 sites in northern Wisconsin. Hemlock saplings are three times as dense and twice as tall when growing within patches of balsam fir compared to growing outside such patches. Hemlock saplings growing outside balsam fir patches are also four times as likely to exhibit deer browsing damage as those growing inside. These results suggest that patches of balsam fir create a physical or visual barrier to deer and thus provide a refuge for hemlock saplings from white-tailed deer browsing. Because balsam fir saplings are much more abundant than hemlock in northern Wisconsin forests and establish on a wider range of sites, foresters could use patches of balsam fir to facilitate local hemlock establishment and so promote restoration of this important forest type

Seasonal variation in the daily probability of nest predation.

<p>Estimates of the daily probability of nest predation in Dusky Flycatchers (solid line) with 95% upper and lower confidence limits (dashed lines) generated from the best-supported model in step one of our modeling approach (Nest predation = Date+Date²; <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0065909#pone-0065909-t001" target="_blank">Table 1</a>).</p

Model selection results examining the potential relationship between the risk of nest predation and date during the breeding season for Dusky Flycatchers (<i>n</i> = 167) from 2006 to 2008, Lake Tahoe, California.

<p>Effective sample size used to calculate AIC_c =2882.</p

Model selection results examining the effect of providing supplemental food for nest predators on the risk of nest predation for Dusky Flycatchers (<i>n = </i>76) from 2007 to 2008, Lake Tahoe, California.

<p>Effective sample size used to calculate AIC_c = 1446.</p

Daily probability of nest predation at supplemented and unsupplemented nest sites.

<p>Estimates of daily probability of nest predation for Dusky Flycatchers (heavy lines) with 95% upper and lower confidence limits (fine lines) generated from the best-supported model from our supplemental food experiment (Nest predation  =  Food+Date+Date²+Food*Date²; <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0065909#pone-0065909-t004" target="_blank">Table 4</a>). Daily nest predation was higher in areas near feeders (dashed line) compared to areas further from feeders (solid line), but the effect dissipated later in the season.</p

Model selection results examining the effect of foliage phenology (Foliage), minimum energetic demand (Temp), developmental stage (Stage), and conspecific nest density (Density) on the risk of nest predation (1-Survival) for Dusky Flycatchers (<i>n = </i>167) from 2006 to 2008, Lake Tahoe, California.

<p>Effective sample size used to calculate AIC_c = 2882.</p