Linking snake behavior to nest predation in a Midwestern bird community

Abstract

Nest predators can adversely affect the viability of songbird populations, and their impact is exacerbated in fragmented habitats. Despite substantial research on this predator-prey interaction, however, almost all of the focus has been on the birds rather than their nest predators, thereby limiting our understanding of the factors that bring predators and nests into contact. We used radiotelemetry to document the activity of two snake species (rat snakes, Elaphe obsoleta; racers, Coluber constrictor) known to prey on nests in Midwestern bird communities and simultaneously monitored 300 songbird nests and tested the hypothesis that predation risk should increase for nests when snakes were more active and in edge habitat preferred by both snake species. Predation risk increased when rat snakes were more active, for all nests combined and for two of the six bird species for which we had sufficient nests to allow separate analyses. This result is consistent with rat snakes being more important nest predators than racers. We found no evidence, however, that nests closer to forest edges were at greater risk. These results are generally consistent with the one previous study that investigated rat snakes and nest predation simultaneously. The seemingly paradoxical failure to find higher predation risk in the snakes' preferred habitat (i.e., edge) might be explained by the snakes using edges at least in part for non-foraging activities. We propose that higher nest predation in fragmented habitats (at least that attributable to snakes) results indirectly from edges promoting larger snake populations, rather than from edges directly increasing the risk of nest predation by snakes. If so, the notion of edges per se functioning as ecological "traps" merits further study.

Power lines, roads, and avian nest survival: effects on predator identity and predation intensity

“…We interpret this as a consequence of the relative importance of each predator to overall nest predation. Similar with results from other studies of nest predators (Thompson et al 1999; Weatherhead et al 2010), rat snakes were more important nest predators than racers. In fact, rat snakes were the locally dominant nest predator at our site, accounting for 28% (38 of 137 predation events) of all filmed predation.…”

“…Both rat snakes and racers are important nest predators in our study region (Thompson and Ribic 2012; DeGregorio et al In Press), the activity and habitat use of which have been linked to nest predation risk (Sperry et al 2008, 2010; Klug et al 2010; Weatherhead et al 2010). Snakes were captured opportunistically by hand throughout the nesting season.…”

Crop type influences edge effects on the reproduction of songbirds in sagebrush habitat near agriculture

“…This discrepancy between snake abundance and nest predation rates suggests snakes use vineyard edge habitat for activities other than foraging. Two similar studies also concluded that snakes were not foraging in edge habitat where they were most abundant because they did not cause higher predation rates of songbird nests (Sperry et al 2009, Weatherhead et al 2010. There is anecdotal evidence that snakes in this region are traveling through vineyard edge habitat to the adjacent vineyards because they are attracted to high rodent abundance and irrigated areas (C. Bishop, personal communication), while snakes are likely residing and foraging in orchard edge habitat.…”

Indirect effect of climate change: Shifts in ratsnake behavior alter intensity and timing of avian nest predation

“…This work predicts both an expansion of seasonal snake activity in response to warmer climates, with snakes becoming active earlier in the spring and continuing later in the fall, and a shift in diel patterns, with snakes in warmer climates switching from diurnal to nocturnal activity at the warmest time of the year. Because snake activity and behavior can be directly linked to songbird nest predation risk (Sperry et al, 2008;Klug et al, 2010;Weatherhead et al, 2010;Cox et al, 2013;DeGregorio et al, 2014b), this shift in seasonal and nocturnal activity is likely to alter the predator-prey interactions between snakes and birds, most likely to the detriment of bird populations.…”

“…Nests initiated early in the nesting season often contain more and larger eggs than later nests, indicating that birds invest more in early-season nests (Perrins, 1970;Daan et al, 1990;Nager and Noordwijk, 1995). In areas where snakes are primary nest predators, nest survival rates can vary with snake activity, with high nest survival early in the season before snakes are active and declining as snakes become active during the hotter months (Sperry et al, 2008;Weatherhead et al, 2010). If climate warming allows snakes to become active earlier in the nesting season, nests initiated early in the season may no longer be safe and overall reproductive output for birds may decline.…”