Snakes as predators of bird nests: now and in a warming world

Exploring the causes and consequences of variation in species’ reproductive success is central to many research questions in ecology and evolutionary biology. For most species of birds, nest predation is the primary cause of nest failure and snakes have often been implicated as important nest predators. However, in other studies snakes are absent or infrequent predators. Here, I reviewed available nest camera studies from North America to better understand how the role of snakes as nest predators varies geographically and by snake species. I then conducted focal studies of two snake species (ratsnake [Elaphe obsoleta] and black racer [Coluber constrictor]) to better understand: 1) what factors influence nest predation rates by these two snakes, 2) which sensory mechanisms snakes use to locate prey, 3) how snake activity patterns vary with temperature, and finally, 4) how nest predation by snakes is predicted to change in a warming world. To address my first research question I reviewed 53 North American nest camera studies to identify geographic and species-specific patterns in nest predation by snakes. Snakes accounted for 26% (range: 0-90%) of recorded predation events and the ratsnake was the most frequent predator accounting for 65% of all recorded nest predation by snakes. Overall, snakes were more frequent predators at lower latitudes and in open macrohabitats. At a local scale, a single site in South Carolina, I found ratsnakes were the dominant snake predator of bird nests (28% of recorded nest predation) followed by corn snakes (E. guttata: 15%), black racers (12%), and coachwhips (Masticophis flagellum: 3%). In addition to monitoring 206 bird nests with video cameras to determine their fates, I simultaneously tracked 33 ratsnakes and 16 black racers using radiotelemetry. An examination of snake habitat use and avian nest survival revealed that racers and coachwhips were frequent predators of nests located near powerlines, due to their use of the shrubland habitat maintained below powerlines. Because racers and coachwhips were relatively infrequent predators of nests, however, daily nest survival rate was not influenced by distance to powerlines. Ratsnakes, the locally dominant nest predator, frequently preyed on nests near roads and distance to roads was the best predictor of daily nest survival of monitored nests. Radiotelemetry showed that ratsnakes were often near roads due to the associated forest edges, which ratsnakes are known to use for thermoregulatory purposes. Relatively little is known regarding how snakes locate nests. Because many snakes are active both during the day and night, I investigated how ratsnakes and racers locate prey, how temperature affects their daily activity patterns, and if the foraging mode of each species constrains when they are active. Because studies in Texas have reported a high rate of ratsnakes capturing adult birds on nests at night, I was particularly interested in understanding why ratsnakes switch to nocturnal activity, how they locate prey in the dark, and why racers appear unable to switch to nocturnal activity. In laboratory trials, I found that ratsnakes were active when temperature was optimal, regardless of whether it was day or night, suggesting they have a genuinely plastic ability to switch between diurnal and nocturnal activity. Consistent with this flexibility, ratsnakes were successful at detecting prey in both low and high light using visual or chemical cues, and were most successful when visual and chemical cues were coupled. Racers were almost always active during the day and when temperatures were not optimal, they simply reduced their activity. Given that these results suggest an expansion of nocturnal ratsnake activity when temperatures are optimal, I next investigated how climate warming might alter ratsnake activity and patterns of songbird nest predation. To test if climate warming will alter nest predation patterns by ratsnakes, I used a spatially-explicit agent-based model to evaluate how the timing (both daily and seasonal) of nest predation would change with a warmer climate. Overall, daily nest predation by ratsnakes was predicted to increase 7% with a 2o C increase in temperature. Even modest increases in ambient temperature (0.5o C) caused nocturnal predation by ratsnakes to increase by 30%, particularly in the early spring (200% increase in nocturnal nest predation in March) when nocturnal snake activity is currently limited. Increased temperatures were also predicted to cause nest predation to increase substantially in forest and forest edge habitats due to the thermal heterogeneity of forests buffering snakes against potentially lethal environmental temperatures. If ratsnakes become more concentrated in small forest patches and edges, nest survival in these patches may fall below a sustainable level. Conversely, as temperatures increase, ratsnakes will be less likely to prey on nests in open habitats such as shrublands, which may provide refuges for some nesting birds