Bathing Behavior of the American Robin
They may bathe more than most birds, about twice daily, to remove excessive oil from their feathers. Fluffy feathers provide proper insulation against bitter winds. Too much oil on the feathers can cause the feathers clump and lower the body temperature to dangerous levels.
Bathing also helps keep ectoparasites off of Robins. Ectoparasites or external hitchhikers include microscopic feather mites, flat hippoboscid flies that bite, or visible ticks.
Water is a powerful attractor and will increase the number and variety of birds coming to your yard. I have a running stream that the birds like to use. However as the winter weather blows through mid-Michigan the birds will start looking for any open water. Having heated birdbaths or adding a heater to the bath you already have set up would be ideal for helping birds during the winter.