Invasive shrubs and songbird nesting success: effects of climate variability and predator abundance

Previous studies have demonstrated that songbirds often use exotic plants as nesting substrates and may suffer elevated predation rates relative to nests placed in native plants. Veeries (Catharus fuscescens) frequently build nests in an exotic shrub, Japanese barberry (Berberis thunbergii), in forests of southeastern New York State, USA. We monitored Veery nesting success over a six-year period and supplemented these observations with experimental nests to determine whether predation rates differed between nests placed on the ground, in barberry shrubs, or in alternative native substrates. Using experimental nests, we found that nests raised above the ground in either barberry or native plants survived significantly longer than ground nests, but predation did not differ between the two former sites. Similarly, Veery nests on the ground suffered higher predation rates than nests in either raised native substrates or barberry; however, despite a substantially lower daily mortality rate in barberry, the difference was not significant. The lack of significance is apparently related to temporal variability in substrate or habitat preference by Veeries and strong temporal variation in the assemblage of nest predators. Barberry offered relatively greater refuge from nest predators during outbreaks of rodent populations. Years with higher spring precipitation reduced the use of mesic drainages (where barberry is abundant) for nest sites, and concomitantly fewer nests were placed in barberry. Although the differences in nest predation rates are relatively small, we estimated that nesting in barberry vs. on the ground can cause an ~10% increase in annual fecundity. Thus, strong differences in the density of barberry or chronic changes in climate can have long-term impacts on songbird populations.