Supplemental feeding of insect larvae increases mass of nestling Eastern Bluebirds, but not nestling Black-capped Chickadees

Globally, billions of U.S. dollars are spent annually to provide food for wild birds. The variety of foods offered continues to diversify, yet the consequences for wildlife of this widespread human activity are relatively unexplored, as are the relative benefits of different types of supplemental food. To investigate the potential implications of supplemental feeding for reproductive success, we experimentally examined the effects of providing supplemental insect prey on nestling mass for two species of birds across two nesting seasons (2016–2017) in Tompkins County, New York. Nesting adults in the experimental group were provided with 10 grams of insect larvae (mealworms, Tenebrio molitor) per day from the day eggs hatched until nestlings fledged. Results were mixed, with nestling Eastern Bluebirds (Sialia sialis) having 5.2% greater mass at fledging in supplemented nests, on average, than those in nests not receiving supplemental food. In contrast, no difference in mass was detected between treatment and control groups of nestling Black-capped Chickadees (Poecile atricapillus). Models also supported variation in bluebird mass depending on weather conditions, with greater mass at higher mean temperatures and during periods with less precipitation. Although people who feed wild birds are often motivated by a desire to contribute to conservation and help birds, the consequences of supplementing birds vary, and feeding may not have the desired effects across taxa. People seeking to improve the condition of nestlings should not assume that feeding is universally beneficial and should also consider making habitat improvements, keeping cats indoors, and limiting access of predators to nest boxes by installing predator guards. Implementing a suite of conservation behaviors may be more effective at promoting reproductive success than feeding alone.