Last week, robins returned to the South River watershed. They are on fields, in highway medians, and in lawns.
Perhaps you've seen one, making a fast dash north from its wintering grounds?
No, not Batman's sidekick Robin. The American robin.
Referred to as a "harbinger of Spring" in the Mid-Atlantic states, the American Robin has a highly variable migration pattern that is largely based upon balancing caloric needs and food availability.
The robin is a tough, adaptable bird, which is one reason we can see so many in an area like the South River watershed. By songbird standards, the robin is a "big bird" (actually, North America's largest thrush), and it's comfortable in agricultural, suburban, and forest habitats. It can eat almost anything you'd call "bird food."
So why do they migrate south at all?
Like many ducks and shorebirds in the Atlantic flyway, "our" Robins only migrate south when they feel they have no other choice. What forces that choice? Songbirds like the American robin use between 40 percent and 80 percent of their (winter) calorie intake simply to maintain body temperature.
Once a few hard frosts have hit their habitat, their favorite food (soft-bodied insects, worms, and arthropods) become inactive and harder to find. Robins then shift their diet to berries and seeds, which have higher carbohydrates, but less protein than live food, and also require significantly more calories to eat and digest than live, soft food items like earthworms, grubs, and millipedes.
As the available seed supply starts to thin, the number of robins in the South River watershed start to thin out—even though a few may stay through the winter.
It's the robin's migration back north that captivates people. An old farmer's tale is that robins migrate north when the night and day temperatures average 36 degrees—and observations generally bear this out.
However, a little closer inspection tells us that it's not the slightly warmer air itself, but what that particular temperature does to the robin's favorite food item—earthworms.
In the fall months, earthworms migrate downward through the soil to avoid freezing temperatures. They seem conscious of where the frost line lies, and can often hibernate in large groups right below that important depth.
However, as the spring returns and the soil temperature bounces from 34 to 36 degrees near the surface, the earthworm's internal organs begin functioning again, including their respiration (breathing) apparatus in their skin.
Unfortunately for the earthworms, around this time, snow and ice begin to melt and spring rains begin, all of which fill soil pores with water instead of air. The newly active earthworms have no choice but to climb to the surface to breathe, where new flocks of Robins are patiently waiting for them (like in the image below).
So much for "bird brains"—the American robin has it all figured out.