Winter is a great time to observe flocks of birds. Large birds, like geese for example, often fly in specific geometric formations (the “V’s”) in order for individuals in the flock to take advantage of both reduced wind resistance and also the up-wash of air that a front-flying bird generates both behind it and to its sides. Birds flying in a “V” formation beat their wings less often than a bird flying alone and also have lower heart rates. Therefore, they are able to glide more and use substantially less energy to power their flying.
Smaller birds, like blackbirds, starlings, and grackles, do not generate large enough air vortices to assist those other birds flying near them. So, there is no energetic advantage to flying in a “V.” These birds tend to fly in large, shape-changing flocks that can number from many thousands to several million individuals (one winter “blackbird” flock in the Great Dismal Swamp on the Virginia and North Carolina border was estimated to contain fifteen million birds!). Watching these swirling and flowing flocks can be a surreal experience! The coordinated twisting and turning movements that they display suggests an immense, living being rather than simply a mob-like mass.
Deborah and I have seen many of these “blackbird” flocks during our commuting drives back and forth from campus and our home. They rise up from corn and soybean fields or out of dense shrub fields or woodlots and move as a undulating mass over the roadways. Our car has been pelted by feces from these birds (the woodlot birds were eating wild grapes!). Thank goodness there have been frequent rain showers to clean off the car!
The emergent patterns of these small bird flock movements are generated by the expression of three very simple rules of dynamics (the work of Craig Reynolds was instrumental in bringing these internal control forces to light (see http://www.red3d.com/cwr/boids/)). The first rule is Separation: each individual in the flock tries to maintain an optimal distance from its neighbors. The second rule is Alignment: each individual steers toward the average direction their neighbor is going. The third rule is Cohesion: each individual tries to maintain a standard density of individuals in space.
These three rules are seen not only in flocks of birds but also in schools of fish, herds of mammals, and even in crowds of people! Complex group movements result as each individual in the group tries to keep their spacing, alignment, and crowd density constant.
Flocks (or schools or herds) may seem to be a huge disadvantage for a prey species seeking not to be noticed by a predator. Any predator could see or hear or smell a flock or a herd of thousands to millions of individuals! There are, though, many consequences of being in a large crowd that might lessen the impact of predation on an individual, and these benefits have undoubtedly been sorted out via natural selection and evolution to generate optimal sizes and timings of formation of these flocks, herds, or schools.
The most obvious benefit is “safety in numbers.” There are many eyes (and noses) “watching” for predators. A group of great size should always have someone watching each point in space around the flock/herd/school. Also, there are so many other individuals around for a potential predator to take! This simple risk reduction by large available numbers of prey of equal or even greater quality may be sufficient to offset the increased risk from excessive visibility . This has been referred to as the “dilution effect” in which risk decreases when it is shared over an increasing number of individuals. The famous William Hamilton (the evolutionary biologist from England and colleague of Richard Dawkins whom I talked about in a previous blog) described this dilution effect back in 1971 and used it to coin the term the “selfish herd.” He noted that the survival of individuals increases when they are in a group even though each member of that group is acting in their own self-interests (and are fervently hoping that the guy next to him becomes the predator’s mid-day snack!).
The flocks (or schools or herds) may also confuse predators via their mass movements or obstruct their access to specific individuals by their spacing and density. Some animals take turns rotating from the edges of their flock/herd/school into the more protected center. Many species also keep the more vulnerable individuals of their group (especially young individuals) inside these protected centers and the larger, more robust adults on the more vulnerable edges.
Predators (like many car drivers weaving around under a large, surging flock of birds) may also be confused and maybe even more than a bit intimidated by a huge, moving mass of even the most potentially vulnerable prey species.
Birds form winter flocks for other reasons, too. Finding food is major task in the winter and being in a large group of fellow food seekers makes the probability of finding food much higher. And, even though whatever food is found is then shared by many “beaks,” the net gain of food opportunities more than offsets the loss due to smaller individual portions!
Winter flocks of red-wing blackbirds, brown headed cowbirds, common grackles, and European starlings gather together in huge night roosts. The marshes of the coastal regions of mid-Atlantic states (New Jersey, Delaware, Maryland, Virginia, and North Carolina) have especially large blackbird flocks. These roosts disperse during the day into smaller cohorts that may fly as far as fifty miles seeking food.
These large blackbird flocks are also seen in Kansas shortly after the birds’ breeding season has ended. The flocks steadily increase in size with coming winter and transition from roosting in deciduous trees in the later summer and early fall to coniferous trees in the late fall and winter. They feed in the harvested grain fields of the Kansas plains during the day and hide from the prairie winds and the deep cold in the evergreen branches at night. In March the species separate and mating pairs form to get about the serious work of reproduction.
Looking out my back window, I see the falling snow and a mixed flock of chickadees, titmice, downy woodpeckers, and juncos hopping around the leaf piles and poking into the compost pile. When one finds some food item hidden in the leaf litter several others cluster around him and make short work of whatever it is. They then immediately set off looking for more. The cost of resource sharing is more than offset by the benefit of increased resource discovery. Ah. Winter!