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
