Seen a huge flock of robins lately? This is a late-winter phenomenon that occurs as robins begin their seasonal migration north. Between late-February and the end of March, it’s not unusual to see hundreds of American robins congregating in trees and shrubs, looking for food and shelter as they wait for temperatures to moderate. Evergreens like American holly and eastern red cedar provide some of the best habitat structure this time of year, because the trees’ dense branches protect birds from harsh winds. These evergreen thickets also provide much needed berries for food.
We have populations of robins that stay in Virginia year round, but in mid-to-late February, we see an influx of other robins that are passing through on their way to more northern states. Robins’ seasonal movements are said to be tied to a 37-degree “isotherm.” An isotherm is a line on a map where the average temperature is the same at various points across the line. As robins move from southern states into more northern ones, they stop and hunker down when they reach the limits of the isotherm. Yesterday in Richmond (February 18) the temperature climbed to between 35 and 39 degrees — and the migratory robins are in essence waiting here in Virginia for the line to “move” further north, so they can continue their journey. You can learn more about these seasonal movements at Journey North’s “Robin Migration Study” website.
In the meantime, try sharing a few goodies from your cupboards like raisins and dried apple pieces for the robins to snack on. If your yard doesn’t have much winter cover for birds to stick around, now would be a good time to plant native trees and shrubs for future seasons. In addition to trees like American holly (Ilex opaca) and eastern red cedar (Juniperus virginiana), there are several other native shrubs with high wildlife value that provide persistent fruit in winter for many birds and mammals, such as inkberry (Ilex glabra), smooth winterberry (Ilex laevigata) and winterberry holly (I. verticillata).
— Carol A. Heiser, Habitat Education Coordinator