Droughts have the potential to negatively impact the breeding of insectivorous songbirds by diminishing populations of their prey. Where drought-plagued habitats abut perennial surface waters, however, emergent aquatic insects have the potential to serve as a “drought-proof” subsidy to breeding birds, compensating for shortfalls in moisture-limited terrestrial insect productivity. A key mechanism underlying the proposed stabilization of terrestrial food webs via aquatic subsidies is the ability of birds to exhibit prey-switching behavior. That is, a bird’s prey choice must relate to prey abundance, but whether and when this happens can be difficult to document. A long-term monitored nestbox trail in California’s Central Valley provided a chance to assess whether songbird diet varied in a manner consistent with the hypothesis of drought-induced prey-switching for four species: ash-throated flycatchers (Myiarchus cinerescens), house wrens (Troglodytes aedon), tree swallows (Tachycineta bicolor), and western bluebirds (Sialia mexicana). Our study spanned 2008-2014, a period that included years of widespread and exceptional drought. We predicted greater reliance on insects emerging from a perennial river in years in which uplands were affected by drought, thereby stabilizing consumer populations.
Across all study years, >95% of nestboxes were used for at least one nesting attempt, without significant differences between years. Additionally, the body condition of nestlings (age-controlled body mass) in drought years was not usually significantly different between normal and wet years. The diets of study species were determined using compound-specific stable isotope analysis of amino acids (CSIA-AA) of nestling feathers, with partitioning of diet between aquatic and terrestrial sources using Bayesian mixing models. The average amount of emergent aquatic insect prey use varied by species, with flycatchers, wrens, and bluebirds consuming aquatic prey across all years, though it did not represent the majority of their diets. Tree swallows consumed far more aquatic prey, which constituted the majority of the diet (range: 36% - 89%). Tree swallow diet also varied significantly across years, with a weak positive correlation between amount of aquatic insect prey consumed and settling/breeding season precipitation (R2 = 0.36). This pattern of steady nest occupancy rates and stable nestling condition in drought years coupled with dietary changes suggests that, at least for tree swallows, the concept of drought-induced prey-switching remains a viable hypothesis to explain reproductive success. As theory suggests, habitat/food web coupling may stabilize generalist consumers.