Enhancing or essential? Aquatic insect availability influences the diet and reprodutive success of a riparian songbird

Background/Question/Methods

Fluxes of nutrients and organisms across the aquatic-terrestrial interface are characteristic of riparian food webs.  Emergent aquatic insects are a key vector of aquatic resources to terrestrial consumers, and their availability may affect consumer diet, density, and behavior.  We studied how tree swallows (Tachycineta bicolor) responded to varying levels of aquatic insect prey, an important resource subsidy.  Nest boxes for swallows were arrayed at distances ranging from 25 to 425 meters away from water at several sites along a watershed in California’s Sacramento Valley.  Patterns of occupancy, diet, and reproductive success were tracked by monitoring nesting birds and their young.  We hypothesized that birds nesting closer to a river would be more likely to fledge nestlings than birds located more distantly from a river and its associated insect subsidies. 

Results/Conclusions

Proximity to water impacted the availability of emergent aquatic insects and the diet of tree swallows.  Using compound-specific isotope analysis, we found that riverside birds had stronger aquatic signatures in their feather amino acids.  However, aquatic insects were a component of the diet of birds nesting as far as 425 meters from water, the result of both incursion of insects into uplands and directed foraging trips by swallows.  The links between nesting location and reproductive success are complex, as a bird’s nesting habitat had the ability to ameliorate lack of aquatic insects to varying degrees.  Proximity to water and emergent aquatic insects appears to be most critical during times of severe terrestrial prey declines (e.g., late-season storms).  Aquatic subsidies may help buffer upland, generalist consumers from declines in local prey, thereby stabilizing food intake and promoting higher reproductive success.