The coldest spell of the winter had broken, and the eight remaining black-capped chickadees flitted out of the dense spruce forest and back into the urban jungle where they had spent their fall. They were four fewer than three months ago. One had perished in the talons of a screech-owl in an early dawn, and another had broken its neck on a window pane when frightened by an invisible predator. The other two died less violently, simply falling off their perches into frozen white graves on an especially bitterly cold night.
Fresh sunflower seed awaited them at the feeder on a bright sunny morning. The chickadees flew back and forth, grabbing a seed and returning to a favorite perch to open and eat it.
Unlike finches, they seldom flocked together at the feeder, instead visiting it one by one. It was not first-come, first-serve, either. A hierarchy existed. The three males were dominant over the five females. And within each of the sexes, the subordinate young birds gave way to the adults.
The first two hours of feeding passed quickly, when suddenly a seeet call pierced the air. It was the chickadees' high-frequency alarm call. The birds froze. Their gray, black, and white bodies seemed to dissolve into the landscape. As fortune would have it, none was on the feeder at that precise moment.
They watched the dark shadow of death glide about six feet high over the sparkling, untrampled snow in the back portion of the yard and then beyond the cedar hedge out of sight.
The danger had passed.
But the sharp-shinned hawk did not move on. Silently and smartly, it reversed course, heading back toward the feeder from the far end of the hedge.
Meanwhile, a second seeet call was issued and the chickadees were just about to resume their frenzied feeding activities when the sharpshin suddenly reappeared from behind them. With a few powerful flaps, it closed its talons around the calling chickadee before it could utter another sound. It virtually died of fright. While the hawk settled on a branch to pluck and eat its feathery brunch, the remaining chickadees disappeared, putting as much distance as possible between themselves and the grisly scene. Their flock had one less female, one less juvenile, one less subordinate.
This was no random happening. The hawk had deliberately hunted and killed the subordinate chickadee that had called out. It had not only been the most exposed and vulnerable, but it had also revealed itself first.
While flocking together generally reduces the risk of predation, each and every bird may not be equally at risk. Whether a given individual falls victim to a predator could be a function of the victim's location and conspicuousness within the flock. If it is relegated to the more exposed positions where it is expected to scan for predators, and if it draws attention to itself by being the first to issue alarm calls, death in the claws of a predator may be all the more likely. Once a predator has come and gone, presumably out of sight, a subordinate could be expected to issue an all-clear call, also in the form of a seeet or the more typical chick-a-dee. Thus, it would be the first to break the freeze. However, if the predator has not truly vacated the area as in our little fictional hawk story above, then the subordinate could be placing itself at risk.
These behaviors were tested in a study by Liana Zanette and Laurene Ratcliffe of Queen's University in Kingston, Ontario, and published in the July 1994 issue of Animal Behaviour. They presented recorded playbacks of red-shouldered hawk calls (and purple finch calls as a control) as well as a taxidermic model of a sharp-shinned hawk to captive chickadees. None of the birds were hungrier than one another, all of the captive birds were equally likely to spot the "hawk" first, and none of the flock members were related, ruling out the possibility of helping out one's relatives.
While they found that all members of the flock were equally prone to seeet, the subdominant chickadees consistently gave the chick-a-dee call before the dominant birds, bringing attention to themselves. Also, the subdominants were the first to break the freeze once the predator was supposedly out of sight.
Thus, it does not pay to be a subordinate bird in a hierarchical flock. One is forced to take greater risks and suffer a higher mortality as a result. And such is the life of the tiny black-capped chickadee in the dead of winter.