Why Birds Form Flocks

How Flocks Help (and Sometimes Hurt) Birds

A flock of seagulls on a beach

Steve Jurvetson / Flickr / CC by 2.0

Why do birds of a feather flock together? Understanding why birds flock can help birders better understand bird behavior and how a flock of birds can win the fight for survival that all birds face.

About Bird Flocks

Many bird species are gregarious and form flocks for different reasons. Flocks may be different sizes, occur in different seasons, and may even be composed of different species that can work well together in a group. It always has both benefits and problems for the birds that make it up.

Fun Fact

Flocks are so prevalent in some bird species that these groups of birds have special names, such as a raft of ducks, a charm of finches, a horde of ravens, or any other of the names for flocks of birds.

Advantages of Bird Flocks

Birds do not do anything that does not help their survival in some way. There are many advantages to flocking, including:

  • Foraging: Birds often form flocks while foraging, which allows many birds to take advantage of the same food supplies. Feeding in a group also gives more birds the opportunity to use food sources that one bird has already located. Foraging flocks are often comprised of mixed species that may feed on similar foods but in non-competing ways. For example, chickadees that glean insects from leaves are often found flocking with nuthatches that glean the same insects from bark.
  • Protection: A larger group of birds has a better chance of spotting a predator or another potential threat than a single bird has. Furthermore, a group of birds may be able to confuse or overwhelm a predator through mobbing or agile flight. Staying in a flock also presents a predator with more possible targets, which lowers the total danger for any single bird.
  • Mating: Some bird species, most notably game birds, form mating flocks in areas called leks where males show off their breeding plumage and courtship behavior in an attempt to attract a mate. By performing in a flock, these birds make themselves more visible to a greater number of females, increasing their chances of successful mating. Flamingos, birds-of-paradise, manakins, and several other types of birds also use leks or flocking behavior as part of breeding courtship.
  • Raising Families: Different types of birds form communal flocks on nesting grounds called rookeries. In a rookery, each nest is individually tended by parent birds caring for their young, but the nests may be very close to one another. This allows the full group of birds to take advantage of flock benefits against predators to care for their vulnerable chicks. Birds that do not use rookeries may still form family flocks, and juvenile birds from a first brood may help contribute to raising their late-season siblings.
  • Aerodynamics: When birds fly in flocks, they often arrange themselves in specific shapes or formations. Those formations take advantage of the changing wind patterns based on the number of birds in the flock and how each bird's wings create different currents. This allows flying birds to use the surrounding air in the most energy-efficient way. This can increase the distance birds can fly without rest, which is particularly crucial during migration.
  • Warmth: In winter, bird flocks can share the benefit of communal warmth to survive severely cold temperatures. Many small birds, such as chickadees and bluebirds, will share the same tiny roost space to keep warm, often in bird roost boxes, hollow trees, or other similar spaces that can help them conserve heat. Large flocks may congregate in a single tree to share their body heat as well.
Starling flock
David Tipling / Getty Images

Disadvantages of Bird Flocks

While there are many advantages to flocking behavior, birds also face several risks when they assemble in large groups. The problems with flocks include:

  • Visibility: The more birds there are in a flock, the more noise and motion it makes, and the more visible it can be to predators. Predators may stalk flocks searching for the weakest members, and several predators can be attracted to the same flock, causing a constant threat to the birds.
  • Competition: Larger flocks need greater amounts of food and have more competition for mates, making it more difficult for each bird in the flock to find enough food or a suitable mate. Weaker, slower members of the flock may suffer if supplies are limited and they cannot compete as well to get the resources they need to survive.
  • Disease: When many birds congregate closely together, the risk of spreading diseases increases dramatically. Many avian diseases are spread through either direct contact or fecal matter, and a larger flock has more potential for a disease to ravage an entire local population of a particular bird species. This can often be seen in backyard flocks with house finch eye disease or avian pox.

Despite the risks, the advantages of flocking are great enough that many different types of birds assemble in small, medium, and large groups for different reasons. From a roaming band of foraging finches to a stupendous migrating group of geese, flocks of birds can be an amazing sight for birders to enjoy. That enjoyment can be even more satisfying when birders recognize how important those flocks are for birds' survival and just how clever group cooperation can be.