Flocks of Robins and Winter Territoriality

Where I live in Colorado, and in fact across much of North America, the American robin (Turdus migratorius) is a common sight, especially during the spring and summer.  You can often find this bird hopping across the ground, rooting around in the ground for worms.  However, this morning, on my walk across campus, I noticed a flock of about thirty or so robins in a random tree, and couldn't remember a time when I had seen so many robins in one place, not even close.  What was going on?

Just like for a robin, a little digging proved fruitful....or should I say wormful?  "The Great Backyard Bird Count" (GBBC) revealed that during the winter, the territoriality of the American robin decreases dramatically.  Two graphs on the website, which I have included below, help us to understand what is going on.  The first graph, titled "Percent Submissions With Positive Sightings," charts the number of people who see robins.  As you can see, the peak months for robin sightings are between the months of April and July, with a peak in July.  March, as well as August through October, are also pretty good times to see robins, but during the winter months between November and February, robins are not seen as often.

The second graph, titled "Average Number of Birds for Positive Sightings" charts how many robins people typically see together: i.e., if they report a robin sighting, they also report how many robins there were together.  As you can see, April through September are definitely the worst months to see multiple robins together, with October through February being the best time, and a large peak in November.

Graphs are tough: what does this all mean in English?  Essentially, during the breeding season (spring and summer), robins maintain strict territories, which causes them to spread out, with a more even distribution.  This means that you would be more likely to see a robin on a walk through the park, but not several robins in the same place.  During the winter, presumably because their breeding territories are simply too small for them to get enough food, and since it is no longer the breeding season, they abandon their territories, allowing different robins to go wherever they wish.  For whatever reason, probably something to do with mutual protection, they also form flocks.  The forming of flocks also causes the number of individuals per sighting to increase.

Another study conducted by the GBBC (read the full write-up HERE) pertained to snow depth and frequency of robin sightings.  As you can see in the graph below, the likelihood of spotting an American robin with even just a little bit of snow drops quite dramatically.  Similar results were reported for the red-winged blackbird, while birds such as the white-throated sparrow and northern flicker responded to increased snow depth as well, but not quite as dramatically.

For the robin, a bird that typically feeds on the ground, even just a little bit of snow cover might prove to be an issue.  However, other ground-feeding birds, such as the dark-eyed junco, didn't seem to mind the snow anywhere near as much as the robin.  GBBC suggested that other factors must affect the distribution of these birds, such as "weather, diet, and the species' normal range and altitudinal limits."

A subspecies of the dark-eyed junco called the gray-headed junco.  I took this picture of the stuffed animal when we were in Ouray over the summer.

A picture of a dark-eyed junco that I took in my backyard.

The GBBC devotes a paragraph to acknowledge the uncertainties in these results, a humility that I find refreshing and, sadly, not too common on many Internet sources these days.  GBBC looks at the graph below, plotting the snow depth against the likelihood of seeing a house finch, and notes the steady decline, as opposed to the sharp drop seen in the robin graph, or the lack of real pattern seen in the junco graph.  GBBC reports that this would be a pattern not expected if it was access to the ground (or lack thereof) that was causing these birds to move.  Instead, one would expect the sharp decline of the robin graph.  Other factors, such as temperature, could be what is causing birds like the house finch or the house sparrow to move.

A male house sparrow.

A female house finch, silhouetted against the backdrop of the evening sky.

Works Cited:

"Snow Depth Survey." The Great Backyard Bird Count. http://www.birdsource.org/gbbcX/science-stories/past-stories/snow-depth-survey (accessed January 23, 2014).

Stokes, Donald, and Lillian Stokes. The Stokes Field Guide to the Birds of North America. New York: Little, Brown and Company, 2010. (accessed January 23, 2014).

"Winter Robins." The Great Backyard Bird Count. http://www.birdsource.org/gbbcX/science-stories/past-stories/is-that-winter-flock-of-robins-in-your-yard-unusual/ (accessed January 23, 2014).