April 27, 2015
Thanks to Facundo Fernandez-Duque for this excellent student blog post about Carolina Wrens and their love of feeders in winter. If you see one visiting you feeder in winter, you might be helping it more than you think.
Carolina Wrens (Thryothorus ludovicianus) are small, enthusiastic insectivores that people hear more often than see. Their diet consists mostly of insects with only about five percent being seeds and vegetable matter. Nonetheless, they visit feeders; Carolina Wrens are tiny, active birds that are constantly in search of food to satisfy their high metabolic rate. When winters are rough and snowfalls abundant, these birds may struggle to survive. Thus, their northern range seems to be limited by harsh winter. However, is that because of the cold, or because there isn’t enough food in winter?
One possibility is that its range is limited by temperature. Northern, non-migratory birds must face tough winters every year. Prolonged periods of sub-freezing temperatures, high competition, and shortages of food all lower the probability of these birds to make it through winter. However, is it possible that their winter survival rate can be positively affected by supplemental food?
In 2011, a study done in Michigan sought out to find the answer. Researches set up an experiment to determine whether supplemental food played an important role in the winter survival of Carolina Wrens in their Northern range. They set up sites in three different types of habitat to test this: city parks, residential areas, and rural areas. The temperature was also taken into account and recorded hourly. Even though they found that the city parks had the greatest density and highest overall temperature, the wrens tended to abandon the site completely if there were no feeders available. Their study suggests that Carolina Wrens are directly limited by food supply, and that bird feeders play a crucial role when other food is in short supply.
The snow on the ground can make it hard for these birds to find food, thus feeders become an important dietary supplement when snowfall is heavy. Although sunflower seeds might attract more colorful birds, Carolina Wrens generally prefer suet and peanuts. One peanut alone can provide more than a third of their daily metabolic need! Furthermore, Carolina Wrens aren’t the only species that may be shifting its range according to human influence. The Northern Cardinal (Cardinalis cardinalis) and Tufted Titmouse (Baeolophus bicolor) have both shown similar patterns to those of the Carolina Wren. They may also visit our feeders to help get them through those tough winters!
Job, J. and Bednekoff, P. A. (2011), Wrens on the edge: feeders predict Carolina wren Thryothorus ludovicianus abundance at the northern edge of their range. Journal of Avian Biology, 42: 16–21. doi: 10.1111/j.1600-048X.2010.05242.x