What Is Coloniality?

Colonial birds nest in the same place at

the same time, and coloniality has been a

successful evolutionary strategy for

many bird species. One estimate is that

1 in 8 bird species worldwide nest

colonially; other estimates are higher.

Colony sites take many forms: mud

nests plastered on vertical surfaces;

burrows riddling a seaside cliff, a stretch

of depressions in a sandy beach, or bulky

stick nests forming a woodland rookery.

Colonies also vary in size; some have

thousands, sometimes millions, of birds

packed together within inches of one

another, while others are made up of

nests scattered across a broad area or

include just a few pairs of birds.

However, in all colonies, the members

have a social bond, interacting and

affecting eachother’s behavior. In fact,

though some species that nest colonially

may also be found nesting alone, many

require the presence of their own kind to

nest and reproduce.

Birds of a Feather

Nesting Together

IMBD Celebrates Colonial Birds

International

Migratory Bird

Day (IMBD),

held annually

on the second

Saturday in

May, is an

invitation to

celebrate and

support

migratory bird

conservation.

IMBD Information

web - http://birds.fws.gov/imbd

phone - 703/358-2318

IMBD Materials

web - http://www.BirdDay.org

phone - 1-866/334-3330

February 2004

Snowy Egrets credit: Robert Savannah

Which Species Are Colonial?

The following is a list of North American

bird families in which most species are

colonial nesters. Note that most are

marine or coastal waterbirds or birds

that feed on airborne insects.

Albatrosses

Shearwaters and Petrels

Storm-Petrels

Tropicbirds

Boobies and Gannets

Pelicans

Cormorants

Anhingas

Frigatebirds

Herons and Egrets

Ibises and Spoonbills

Storks

Flamingos

Gulls and Terns

Auks, Murres, Puffins

Swifts

Swallows and Martins

Colonial-nesting species are also found in

a number of other North American bird

families. Some grebes, waterfowl, birds

of prey, and blackbirds will nest

colonially. Colonial-nesting birds found

outside of North America include

penguins, parrots, bee-eaters, and

African weavers.
Costs and Benefits of Coloniality

Exactly why coloniality evolved or persists has not been resolved, and it

may be that the behavior arose for different reasons in different groups of

birds. Better known are the costs and benefits experienced by colonial

birds, although additional research is also needed to fully understand

these. Also, the balance of costs and benefits varies over time as

environmental conditions change.

Benefits

Birds in colonies may enjoy greater safety from predators, because when a

group of birds is vigilant, predators are more likely to be seen or seen

sooner.

Also, some predators are deterred by the ability of colonial birds to

simultaneously attack intruders.

Individuals may also enjoy simple "safety in numbers," meaning that a

predator can rarely kill all the individuals in a colony.

Colonial birds may also benefit from information exchange. Individuals

may learn about spotty and scattered food supplies from observing their

neighbors, either coming or going from the colony, or while out foraging.

Also, the presence of a colony can signal a suitable nesting site for young

birds returning to breed for the first time.

Costs

Colonies, which are louder and more visible than solitary nests, may

actually attract predators and increase predation.

The high densities of birds in colonies may foster higher rates of disease or

abundances of parasites.

Colony members may experience increased competition for nest materials

and may deplete food sources near the colony, requiring more effort to find

food.

Birds in colonies can be very aggressive to one another, with neighboring

adults harming eggs or chicks.

“Misdirected" parental care can occur; that is, a bird may inadvertently

raise its neighbors young. This happens if chicks wander or if birds lay

their eggs in others nests. [Of course, this could be viewed as a cost or a

benefit depending on whose side you're on!]

Coloniality and Conservation

Though usually considered in terms of

the individual, the costs and benefits of

coloniality can also be viewed at a species

level. Coloniality may increase

population risks by concentrating birds

in a limited area. In other words, single

events or incidents can affect the nesting

success of a large number of birds. Also,

because many colonial species require

that individuals be present in the same

place at the same time before breeding

can take place, the restoration or

formation of new colonies can be difficult.

There have always been natural threats

to colonies such as storms and

predators, but human activities have

brought many new threats to colonies.

The introduction of exotic predators and

vegetation to breeding areas,

disturbance of colonies by human

activity, and outright loss of breeding

habitat threaten many species of colonial

birds.

Fortunately, colonial birds and their

colonies are often highly visible and

impressive, and thus can garner positive

public attention and support in terms of

protection and study.

Swallow

credit: Alan Brooks