Merriam-Webster’s online dictionary includes “distinguishable localized population” as a key definition of “colony.” It then gives groups of birds and termites as examples.
But birds and termites have very different kinds of colonies — and there’s actually a third kind, too.
In some colonies, the members must live together to survive; in other colonies, living together is a social or reproductive advantage. But another way to think about how colonies are different is by considering the relationships between colony members.
Bird colonies, like human colonies, are social/reproductive groups with members that are all one species. A flock of double-crested cormorants roosting and nesting together in a group of waterside trees, are an example. (The acidic dropping of the cormorants in one such colony on Qochyax Island, just outside Sunset Bay, killed the tall trees there.)
A “seabird colony” may also refer to a group of different species of seabirds living together, such as the common murres, tufted puffins, and cormorants living on many of Oregon’s offshore rocks. The “colony” is pretty loosely defined in those instances, since the different species tend to nest in different places on the rock: common murres on top, tufted puffins burrowing into grassy slopes, pelagic cormorants on the top, Brant’s cormorants on the cliff sides.
Most insect colonies, however, are formed by the members of a single, giant family. Many species of ants, termites, bees, and wasps form colonies that are headed by one fertile queen. In most of those family colonies, nearly all the members outside of the queen are sisters. These social insects usually have clearly defined roles; a solo member of the species that’s not a founding female or male (queen or drone) won’t survive long on its own. In fact, certain insect colonies work so closely together that some consider them “super organisms.”
The third kind of colony is on a still finer scale: many invertebrate colonies are formed by clones of a single founding individual, making the individual members genetically identical. Sometimes individuals in those colonies are linked to each other, sharing resources or connecting body parts; sometimes individuals break off from the colony to live individually or start their own colonies.
Most soft patches of aggregating anemones (sea jelly relatives) that develop on intertidal rocks are formed by a single individual that grew, then stretched to split in two — over and over again. The result is a tight group of identical clones.
The bumpy, cylindrical pyrosomes that washed up on our coastal beaches recently are also colonies of cloned individuals. (An individual in such a colony is usually called a “zooid.”) Pyrosomes, however, are tunicates, organisms that start out in life with a primitive backbone. Undisturbed, some species of pyrosomes clone and clone and clone to produce tubes of very tiny zooids that can reach up to 60 feet long. While the individuals of those pyrosome colonies are quite small, individual zooids of some other colonial tunicates (“salps”) can be much larger — up to 12 inches long.
In some groups of colonial animals, the colony is one stage of their life cycle, with the other stage(s) featuring free-living individuals.
Microscopic organisms often form colonies of identical or nearly-identical members, too: each visible spot of bacteria or mold is a colony derived from one individual or spore.
For some species, forming a colony is simply the effect of reproducing without dispersal — the young just don’t leave. But for others, there are significant advantages for living together, ranging from finding mates or raising offspring more easily to making a larger/safer group to gathering and sharing resources.
In the end, not all colonies are created equal.