Spatial heterogeneity in predator activity: Effects on nest-site selection, reproductive success and information gathering by two songbirds

Spatial heterogeneity in predator activity can create small-scale spatial refugia for prey to occupy to reduce mortality. As songbird nest mortality is primarily due to predation, selection should favor traits that allow birds to assess and use spatial refugia to increase reproductive success. I examined if nest-site selection and success of shrub- and ground-nesting veeries and ovenbirds (Seiurus aurocapilla) were related to small-scale nest predator activity (10s of meters), primarily white-footed mice (Peromyscus leucopus) and eastern chipmunks (Tamias striatus). Predator activity was estimated over different temporal scales in two studies using different approaches. First, track plates estimated predator activity (rodents and mesopredators) surrounding nest sites (15-m radius) over a seven-day period. Second, from 1998 – 2012 on six 2.25 ha trapping grids mouse and chipmunk spatial activity and abundance (grid-level) was estimated over the songbird breeding season (May – July). Results from both track plate and capture data found veery nests were were not spatially associated with areas of lower predator activity. Additionally, total rodent abundance, not local rodent activity, was substantially the better predictor of veery nest success. Likewise, ovenbirds did not select nest sites with lower predator activity. However, ovenbird nest success was best predicted by chipmunk activity within a 900 m2 area (15 × 15 m) surrounding their nests. These findings indicate that despite putative vulnerability some songbirds may respond differently to common nest predators suggesting that they differ in vulnerability or adopt different behavioral strategies for managing nest-predation risk. Furthermore, nest predation in sympatrically nesting songbirds may be predicted by different measures of predation risk such as abundance or local activity.
Information reliably associated with nest failure, such as predator activity, can be used to adjust breeding decisions leading to higher reproductive success. Predator vocalizations may provide a source of information for songbirds to assess spatial heterogeneity in risk that enables them to make appropriate nest-site and territory placement decisions. To determine whether songbirds eavesdrop on a common nest predator, eastern chipmunks, I conducted a playback experiment to create spatial heterogeneity in perceived predation risk. Playback plots broadcast chipmunk vocalizations, frog calls (procedural control) or no playback (silent control). Point counts were conducted from plot centers to compare bird activity among treatments and I measured the distance ovenbirds and veeries placed their nests from playback stations. Ground–nesting birds significantly reduced their activities up to 30 m from plot centers in response to playbacks of chipmunk calls suggesting an adjustment of territory placement or a reduction of overt behaviors (e.g., singing frequency). In contrast, less vulnerable canopy-nesting species showed no effect across experimental plots. Correspondingly, veeries and ovenbirds nested significantly further from chipmunk playback stations relative to control stations. Interestingly, the magnitude of this response was more than twice as high in ovenbirds than in veeries. Findings indicate that some breeding passerines may eavesdrop on predator communication, providing an explanation for how some birds assess spatial heterogeneity in predation risk to make breeding-site decisions. Thus, heterospecific eavesdropping may be a common feature of predator–prey interactions that allows birds to avoid nest predators in space providing greater stability to predator–prey dynamics.