Aquatic insects subsidize a riparian songbird and alter key resource threshholds
In riparian zones, both aquatic and terrestrial habitats may generate resources used by the same consumers. However, the magnitude and phenology of resource production often differs between these realms, which may require consumers to respond to unique dynamics in resource availability. We examined prey use and reproductive success—which is strongly resource-dependent—in the Tree Swallow (Tachycineta bicolor), a common riparian songbird which feeds on both terrestrial insects and emergent aquatic insects.
We studied these birds using an array of nestboxes placed in summer-dry California grasslands at a range of distances from a perennial creek. Nesting phenology and success were tracked with frequent nest visits, and diet was studied with fecal dissection and compound-specific stable isotope analysis of nestling feathers. We hypothesized that birds nesting closer to water would have higher resource availability in the late-spring nesting period, a time when terrestrial insect production declines due to moisture limitation but when aquatic insect emergence remains abundant. This was predicted to lead to higher reproductive success for birds nesting near water and to differences in diet and reproductive timing based on location in the landscape.
Compound-specific isotopic analysis of nestling Tree Swallow feathers showed both a land-plant based and algae-based amino acid isotopic “fingerprint,” suggesting consumption of both terrestrial and aquatic insect prey. Even birds nesting >400m from water showed this signature, and the presence of aquatic insect parts in their feces (e.g., odonate fragments) confirmed that aquatic insect use occurred despite considerable distance from the natal creek of aquatic insects.
We did not detect strong effects of distance from water on reproductive success; site effects were more important. Because aquatic insects were heavily concentrated near water and widely used by birds, this suggests that highly-mobile Tree Swallows compensated for low terrestrial prey availability by directed foraging towards regions with abundant aquatic prey.
By tracking the relative contributions of aquatic and terrestrial insects to the flying-insect prey pool exploited by swallows, we suggest that aquatic insects may be important to reaching critical prey density thresholds that make reproduction feasible for swallows. Prior studies suggest that the timing of breeding is resource dependent, so examining the additive effects of the timing/magnitude of aquatic and terrestrial insect production is key to understanding breeding decisions in these and perhaps other riparian birds.