Songbird Nest Survival Is Invariant to Early-successional Restoration Treatments in a Large River Floodplain

—We monitored songbird nest survival in two reforesting, ,50-ha former cropland sites along the Missouri River in central Missouri from 2001 to 2003. Sites were partitioned into three experimental units, each receiving one of three tree planting treatments. Nest densities varied among restoration treatments for four of five species, but overall nest survival rates did not. Nest survival varied with day-of-year and with incidence of brood parasitism by Brown-headed Cowbirds (Molothrus ater). Nest survival was higher early and late in the season, and parasitized nests experienced lower nest survival, despite few complete losses directly attributable to parasitism. Probability of parasitism was inversely related to distance to the nearest tree, and was much lower than in old field study sites in the same region. High cowbird parasitism frequencies are usually associated with landscapes low in forest cover, yet these sites in an agriculturally-dominated bottomland landscape experienced low (,0.8–24%) cowbird parasitism. The assumed negative relationship between landscape-level forest cover and cowbird parasitism needs further study in habitats other than forest. Received 1 September 2008. Accepted 26 November 2009. Studies of nest predation have noted variation in nesting success among habitat types within the same species or suites of species (McCoy et al. 1999, Lloyd and Martin 2005), or variation in nesting success among regions or fragment sizes (Donovan et al. 1995, Robinson et al. 1995). These differences could be attributed to predators, which are known to vary across regions (Thompson et al. 1999, Pietz and Granfors 2000). Predator identities may vary across habitat types within the same landscape or region (Chalfoun et al. 2002), although predator differences may not be reflected in differences in nesting success (Thompson and Burhans 2003). Nest predation is considered the largest cause of nest loss (Ricklefs 1969, Martin 1992), and Brown-headed Cowbird (Molothrus ater) brood parasitism, which also affects nest success, may also vary by region, landscape, or habitat type (Robinson et al. 1995, Burhans 1997, Burhans and Thompson 2006). Much of our present understanding about the interactions of habitat and landscape features affecting cowbird parasitism comes from studies in upland forested habitats (Brittingham and Temple 1983, Robinson et al. 1995, Donovan et al. 2000, Thompson et al. 2000). We monitored songbird nesting success at two sites having three contiguous 16.2-ha habitats of former cropland along the Missouri River. The habitats varied in densities of planted oaks (Quercus spp.) and in presence of a homogenous herbaceous cover crop. The treatments were sufficiently large to attract reasonable numbers of species of nesting birds across the different habitat types. Habitats were physically adjacent to one another, so that any differences in predation within the site should have been attributable to variation in habitat type only, and not to factors such as region, landscape context, or fragmentation. Our specific goals were to examine: (1) whether nesting success varied among the planted habitat types, and (2) whether frequencies of cowbird parasitism varied among habitat types.