Introduction

Aggregation behavior can be found in almost every animal taxa, from the rudimentary assemblages of planktonic larvae reacting to photo- or geokineses to the highly developed social orders of terrestrial mammals. Somewhere in the middle of this cline lie the dense aggregations of birds, insects, and fish – perhaps directed by individually based attractions/repulsions to extraneous source material, as well as by the development of simple social order. Visually, these groups may appear, at least superficially, similar. They are dense, structured congregations with distinct edges, often moving as a group from place to place. It is tempting to imagine that the forces regulating the evolution and maintenance of these congregations were/are similar as well. However, practically, we know that species are subject to very different selective pressures.

Seeing is believing

An aggregation of anything against a background of sameness captures our eye. Congregations of creatures that routinely swarm and cluster or crowd together capture our imagination and generate new descriptive, often florid, collective terms for groups of living things; descriptors that are species-specific and etymologically precise (see Lipton's “Exaltation of Larks,” 1991). A swarm of bees, a host of sparrows, and a smack of jellyfish generate crisp images in our mind's eye, while a cloud of goats, a gaggle of flies, and a pod of parrots only generate confusion. There is no collective term in the English language for this wealth of collective adjectives (Lipton 1991), other than terms of “venery” (from the Latin venari, to hunt game), words that initially described aggregations of game animals, clustered conveniently for the huntsman.

Some of these terms denote protean behavioral displays that are visually compulsive. However, when our congregations of creatures are behaviorally coordinated in space and time, synchronously moving and wheeling and twisting before us in three-dimensional space, as in a school of smelt or a flock of phalaropes, they subvert our visual ability to focus on an individual animal and, somehow, suddenly the sum of the parts becomes a cohesive whole. Those of us who are terminally entranced with the three-dimensional, hypnotic beauty of synchronized flocks of birds and schools of fish quite simply cannot be cured. We know there is order within these three-dimensional displays, but it is not immediately obvious how to quantify it.