Window on Nature
By Lowell & Kaye Christie, F47246
This time of year, you’ll notice that birds frequently feed and move about in mixed flocks. Why would they do that? What is the advantage in having more mouths at the table? Actually, there are several payoffs for such behavior.
For one thing, feeding with companions increases the number of eyes and ears available to detect predators. All it takes is a single bird sensing (hearing, seeing, etc.) danger for the entire group to be warned. They don’t even need to “speak” the same language. Bird calls, songs, and squawks vary, but it’s the high volume and the shrill voice that alert other birds to flee.
Behavior plays a role, too. When a bird suddenly becomes rigid, its change in posture is easily read by birds nearby. And yes, the behavior carries a message of whether to act now or look around for more information.
When the predator turns out to be a falcon, for example, the distraction produced by the sudden motion of birds fleeing for their lives can save the lives of all. And although it may sound heartless, even if the falcon does snatch one bird, at least the rest go free. Predators don’t eat cafeteria-style, snatching a leg here and a wing there. They grab the first bird of manageable size and keep going, leaving the rest to live another day.
There is a downside, however, when foraging groups get too large. They attract more attention. More voices and moving bodies are easier for predators to spot. On balance, studies have shown that the individuals in a flock spend far more time eating and far less time watching for predators. The optimal size for a flock of birds varies with the size and the eating habits of the birds. If the group is too small, the benefits are negligible; but if the group is too large, it might outpace the availability of food.
Defense against predators is just one of the reasons some birds prefer to feed in groups. Consider how quail coveys increase in size over the summer. At the onset of the breeding season, quail involved in mating rituals don’t congregate. It only takes two to bond, mate, and build a nest “” not a group. When quail chicks hatch, they are able to walk, run, and follow within hours. During this phase, an adult quail nearly always stands sentinel “” calling frequently to keep the family together and, more importantly, to watch for danger. When the sentinel’s call turns frantic and the bird begins flitting around, the family runs for cover. All through this season, the coveys consist only of quail.
By late summer, quail form groups of a dozen, two dozen, or more birds. (Quail commonly “double clutch.” That is, they raise two families each year, so the group is mostly made of that species.) But that doesn’t keep other birds from being nearby, if not actually joining the group. During feeding, the quail may be accompanied by a couple of robins, a thrasher, and perhaps several towhees and finches hovering in the bushes nearby. They don’t join the covey, but they appear to be attracted by the larger group.
Even in mixed flocks, many eyes come in handy when locating food. Obviously, not all species in the group eat the same kind of food, but a good feeding area contains countless insects and seeds of various sizes and flavors. More birds can dine without producing too much competition.
Now we come to commensal feeding, where one bird inadvertently provides food for a bird of another species at no cost or benefit to itself. The presence of two or more ground-feeding birds provides a good example. An insect-eating bird that scratches the grass to turn up bugs may well “kick” up the seeds relished by another species. It doesn’t bother the original insect-seeking bird, who just keeps digging around until it finds enough of its preferred morsels.
In North America, mixed-species flocks are seen primarily during the non-breeding season, whereas in tropical lands, mixed-species flocks are the norm year-round. There, groups come together, feed together, and leave together “” most likely because there’s so much food available.
In your own backyard, you may spot a special type of commensal feeding “” species feeding together that compensate for the physical limitations of one of the group. Nearsightedness is quite common among ground feeders that make a living from scratching for food right under their feet. Keeping their beaks to the ground, however, increases their vulnerability to predators. Therefore, they benefit from being around birds with far better distant vision. Nearsighted woodpeckers use their feet to obtain food, but in their case, their feet are clinging to a tree. A foraging woodpecker flits around, using its beak to lever up little sections of tree bark to find insects hiding below. Farsighted titmice and chickadees often can be found chattering in nearby branches, hoping to catch prey that escape the woodpecker’s attention. Unlike woodpeckers, the beaks of these small birds aren’t strong enough for them to pry up bark.
As their name implies, sapsuckers, a type of woodpecker, feed on tree sap. They drill a series of small holes in a tree trunk just deep enough for some of the sap to seep out. When this bird finishes eating and moves away, the tree can’t turn off the sap flow “” but it doesn’t go to waste. A few hungry chickadees, titmice, or kinglets are usually waiting for their turn.
We frequently have noticed mixed species of water birds feeding in marshy areas, ponds, and in the shallow water of lakes and riverbanks. Egrets and herons keep a close eye on each other, knowing that if one of them brings up an edible morsel, there’s a decent chance that more food is nearby.
We’re lucky enough to have a pond less than a block from our house, and we often stroll over to watch the ducks. (It helps us stand the wait until our next trip in the coach.) The pond has a resident great blue heron, and various other species of herons and egrets fly in after their breeding and nesting seasons. These birds certainly aren’t groupies “” they nest communally, but the rest of the year they fly solo. However, they do keep a close eye on one another from a respectable distance. That way, when one sees another one scoring a meal, it gradually edges over to (hopefully) catch a fish or a frog of its own. These birds know instinctively that, while there may be no free lunch, in situations like these there’s a definite advantage to watching the competition.
It must be nearing the time for us to pack up the coach and hit the road “” we’ve been watching wildlife over at the pond at least once a day, all week long.