One of the many reasons I like birds so much is that they are a lot like us. By and large, birds are conspicuous and active in the day, communicate through vocally or through visual displays and body language, see in color, often raise their young in nuclear families, and, on the whole, are very social. It is the diversity and complexity of bird societies that attract my attention the most.
One of the ways that birds surpass us and most other animals is in the propensity of different species to join forces to form mixed aggregations, roosts, and flocks. I recently returned from the lowland forests of Costa Rica, where my encounters with the myriad flocks made me feel like each morning walk was a kaleidoscope of colorful and not-so-colorful tropical birds. But you don’t have to travel to distant lands to know the wonders of these diverse societies. They are nearby and everywhere else.
Mixed species groups of birds vary by why species occur together, how strong the bond is between birds, and to what degree birds change their behavior to associate and communicate with each other. Based on these attributes, mixed species groupings can be arranged on a ladder of complexity and evolutionary sophistication.
At the lowest rung, birds can be found in what is often termed “mere” aggregations. These multi-species congregations are found at a particularly attractive food resource (fruiting tree, bird feeder etc.) primarily because in response to the food and not because they birds are particularly fond of each other. Because the groups occur without any obvious social bond, they receive the rather judgmental adjective “mere”. Despite their lack of true underlying social bonds, aggregations can be both spectacular (fruiting trees in the tropics can easily host 40 or 50 species) and fascinating to watch.
One of the most intriguing and well studied examples of a food based aggregation are formed around the swarms of army ants that march through the tropical forest understory by the hundreds of thousands cleaning out ant nests and flushing and consuming all insects, lizards, and frogs in their way. Contrary to what is often depicted in nature films from the tropics, a walk through a rainforest often involves hours of dead quiet, except for the drone of cicadas and the distant roar of Howler Monkeys.
For an astute bird, the somber silence is occasionally broken by the snarls and growls of birds in hot competition to obtain perches over the front of an antswarm allowing them to snatch up fleeing insects escaping the mindless pursuit of the ants below them. Certain species of birds, deemed “professional” ant followers, spend their entire lives finding and following swarms.
Bicolored, ocellated, and spotted antbirds, pictured from left to right, often follow army ant swarms.
© Christian Ziegler/STRI
These species dominate the “A” zone where most of the action is. Within this cadre of antbirds, ground cuckoos, and woodcreepers, the larger the bird, the better their seating assignment at this ant-generated feast. Many other birds including migratory species of thrushes and warblers, follow the swarm opportunistically, fending off attacks from the dominants and sneaking opportunities to snap up a fleeing roach or cricket.
Army ants are often the focus of tales of horror and the occasional disaster film. Not only are they pretty harmless, but locals often welcome them to their houses to clean out the wasp nests. They can bite, however, so watch your feet! With some care, a few hours following the ants should reward the tropical birder with a real show. Unfortunately, we don’t have army ants in Maryland, but whenever you see a flock of birds following a tractor as it plows through a field, you are observing a fairly similar phenomenon.
What we do have in Maryland are true mixed species flocks. Mixed species flocks at their highest manifestation of interspecific sociality are characteristic of forests and woodlands. While several species can be found together in fields and along shoreline, more often then not, they are comprised of several single species subflocks that often go their own way when push comes to shove. The truly integrated mixed societies occur in forests.
Black-capped chickadee, tufted titmouse, and white-breasted nuthatch, pictured from left to right, often flock together.
Most birders know that the action in the fall and winter woods is focused on the small bands of birds that form around a small group of chickadees and titmice. These flocks are comprised of birds that often spend months traveling together and attract other itinerant visitors to the woods. This mixed society is their social world and their entry to this world does not depend on an attractive food source or any other external reward.
In these flocks, different species play different social roles. The most important role is that of the “nucleus” species which, much like the dust in a rain drop, accumulates others as it moves. For most of the Temperate Zone, the tits are the nucleus, themselves occurring in clans or groups of paired individuals that travel across a home-range that will be divided into territories in the early spring. Noisy and adept at locating and communicating the presence of a predator, the chickadee core attracts a number of more solitary species that use their communication system to survive the winter. Migrants unfamiliar with the local terrain can join and learn the ropes from the year-long residents.
As you might imagine, these flocks get much more complex in the tropics. For one thing, the different levels of the forest are often host to different types of flocks with different nuclear and joining species. More interesting that this, the nucleus often consists of pairs of many species of birds which defend a multispecific territory. Where I worked in Panama, the nucleus of the understory flocks comprised three species of antwrens (small warbler sized insectivores). In the Amazon, as many as 11 species of birds traverse the same paths and defend the same boundaries against their same-species competitors in elaborate boundary displays.
Unlike the Maryland flocks, led by a generalist foraging in all nooks and crannies of the woods, the tropical flocks have a team of ecological specialist – the sum of which are examining all of hiding places where predators could lurk. But in tropical flocks there is often one species that rises to supreme importance in its ability to call the flock together and signal and incoming predator attack.
These “sentinel species" include shrike tanagers in the canopy and antshrikes in the understory of Amazonian forests. These species get a special bonus for their participation in this multi-taxa society. They are major kelptoparasites–waiting for smaller birds to capture a large katydid or roach and swooping to take them away. They are particularly successful when their victims are providing large prey to their mates or young. What makes their role in the flock particularly insidious is that from time to time they give what appears to be false alarm calls to cause other birds to drop their food in fear.
As we all know from personal experience, sociality has its advantages and disadvantages. The advantages usually come in the form of information: information about good habitat, feeding opportunities, and most importantly, the threat from a bird-eating hawk. The main disadvantage is the competition that comes when more birds are searching for a few insects in a winter woods in Maryland or a tropical forest. Joining other species enhances the advantages, as different species are monitoring different places and minimizes the costs, as different species forage on different prey.
Whatever they do for birds, mixed flocks are where the action is for birding in our nearby woods and forests throughout the world. Every flock is a new adventure.
This article first appeared in the Naturalist News, a publication of the Audubon Naturalist Society.