- The majority of households in the U.S., Great Britain, and Australia feed wild birds, and 53 million people in the U.S. spent more than $5 billion on bird food and feeding accessories in 2016. Given the massive scale over which it occurs, purposeful feeding of wild birds has the potential for widespread ecological consequences. Research has demonstrated that supplemental feeding can exert wide-ranging effects on wild bird populations, yet we have a poor understanding of how recreational bird feeding influences the behavior and physiology of individual, free-living wild birds. This is especially true of small-scale supplemental feeding during the non-breeding season practiced by private citizens who feed wild birds recreationally.
In this study, we used radio frequency identification (RFID) and passive integrated transponder (PIT) tags to quantify fine-scale variation in supplemental feeder use by wild birds during the non-breeding season (October 2016 through March 2017) to assess how environmental conditions are associated with feeder use in a Mediterranean climate. We obtained >500,000 observations of 120 individual Black-capped Chickadees (Poecile atricapillus) at supplemental feeders to investigate (1) the relationship between daily feeder use and environmental variables (minimum daily ambient temperature, precipitation, wind speed, and photoperiod), (2) the relationship between hourly feeder visitation by chickadees and time since sunrise during the shortest days of the year, and (3) how individual birds varied their use of supplemental bird feeders in response to an experimental increase in the energetic cost of flight.
We found that photoperiod, combined with whether feeder visits occurred before or after winter solstice were the main associates of feeder visitation rates in our study system. Despite the gradual increase in daylight hours available for foraging after the winter solstice, feeder use decreased during late winter and spring. The circadian pattern of feeder visitation by birds in our study was not a bimodal pattern as predicted by theory; rather, feeder visitation increased steadily until late morning, leveled off for approximately three hours around mid-day, then gradually decreased before an abrupt decline at sunset. Although individual birds were relatively consistent in their daily rate of feeder visitation, variation in daily visitation rates among individual birds was two orders of magnitude greater than the variation in daily visitation rates within individuals.
To assess how chickadees responded to supplemental food under challenging environmental conditions, we handicapped individuals by clipping primary flight feathers to increase energetic costs of flight, and evaluated subsequent feeder use. We found that the handicapped birds in both light (4 primary wing feathers clipped) and heavy (8 primary wing feathers clipped) feather-clipping treatments reduced feeder visitation rates for approximately 10 days relative to control birds, after which handicapped birds returned to levels of feeder visitation similar to control birds. In addition, birds reduced the average number of feeders visited by approximately one-half a feeder following the handicapping treatment. A reduction in use of supplemental feeders may have been an adaptive behavioral response whereby birds avoided feeders, perhaps to reduce energy expenditure rates to compensate for handicapping and/or to limit exposure to predators during a period of higher susceptibility to predation due to handicapping. Our results reject the hypothesis that birds are dependent on supplemental bird feeders in the winter. This study not only advances our understanding of how chickadees use supplemental feeders in a Mediterranean climate during the winter, but also how chickadees respond to an increase in the energetic cost of flight.