Red-winged Blackbirds flocking in a tree. Photo by Robert Dryja

PEEC Amateur Naturalist: The Flocking of Birds

Red-winged Blackbirds have been gathering in flocks throughout the winter.

The birds appear to have preferred trees, gathering in the same tree or adjacent trees for several days. They provide a choir of singing in the morning and so cannot be missed.  There has been one flock in the Western area near the hospital. Another flock has been at the end of the road on North Mesa.

There is a pattern to their flocking behavior when they gather in a tree. The birds typically sit with a distance of two to three feet from each other. You do not see the birds sitting closer to one another. There is only an occasional bird that is sitting several feet from the main group of birds. If you look closely at the photo below, you will see that the birds are gathered together behind several readily available branches. They apparently would become too spread out if they used all of the branches. If two birds appear close to one another, one bird is actually behind and a little to the side of the other bird.

Interestingly, this same pattern for flocking also occurs when the birds are flying. The birds may suddenly erupt from a tree as a group and all fly in the same direction. Most of the birds fly with about two to three feet of space among them and they somehow keep this distance even when they change direction.

There is no apparent lead bird when a flock is flying. How do 20 or 30 birds know how to position themselves to create this kind of behavior? How can one Red-winged Blackbird simultaneously monitor another 20 or 30 Red-winged Blackbirds and also adjust its flying at the same time? How can the other Red-wing Blackbirds in the flock do this and not scatter in all directions?

An adult male Red-winged Blackbird. Photo by David Yeamans

Immature male Red-winged Blackbirds have little wing color. Photo by David Yeamans 

It turns out that flocking is not difficult to achieve based on a form of mathematics concerned with making models of complex behavior. If each bird follows the same three rules, then flocking behavior emerges. The three rules are:

• Steer yourself to avoid crowding your local flock mates. Do not be concerned about birds that are not adjacent to you.
   
• Also steer yourself to move toward the same distance between you and your adjacent flock mates. Again, do not be concerned about birds that are not adjacent.

Apparently a distance of two to three feet is about right to meet these two rules if you are a Red-winged Blackbird.

• Finally, steer toward the same direction as that of your adjacent flock mates. As before, do not be concerned about birds that are not adjacent.

A basic software program called “Boids” shows what happens when you put these three rules into action. You can watch flocking behavior at the following website: http://www.red3d.com/cwr/boids/. 

The Netlogo website additionally allows you to manipulate the three rules as well as the number of birds in a flock. What happens with 30 birds in a flock? What about 1,000 birds? What happens when you change the closeness of adjacent birds? What happens when the direction changes by a large amount rather than a small amount? You can explore these questions at the Netlogo web address: http://ccl.northwestern.edu/netlogo/models/run.cgi?Flocking.783.569

The Netlogo website will require some practice to use since you can manipulate all three rules individually for their effect on flocking behavior. To start, you click the “setup” button and then click the “go” button. These two buttons are turned on or off by repeatedly clicking them. You then can move the sliders that control the rules and population size of the birds.

There is always something that may change flocking behavior. A Red-tailed Hawk may be one of them. You would anticipate that a flock of Red-winged Blackbirds would scatter from a tree if a hawk flew toward it.

Red-tailed Hawk. Photo by David Yeamans

The assumption is that the hawk is actively hunting for food. Surprisingly, a hawk instead may circle by a flock or even just roost in a nearby tree. It may show no interest in Red-winged Blackbirds other than to stay nearby. The Red-winged Blackbirds in turn may appear to ignore the hawk and continue their same flocking behavior.

So what is going on? Is there a new set of rules that guide the behavior between hawks and Red-winged Blackbirds, just as there are rules that guide flocking behavior?