by Whit Gibbons

April 16, 2006

Last week a colleague looking out my office window onto a patio where I spread sunflower seeds noted that squirrels occasionally ventured into the open area but that birds were nearly always around at the time. Birds are absent about as often as they are present, whereas squirrels never seem to be present unless birds are. Such a phenomenon could be from random chance of when we happened to be watching. But based on a behavioral ecology study conducted on birds and mammals eating together, a meaningful biological pattern could be involved.

Behavioral ecology is a field that attempts to identify what animals do and explain why from a biological standpoint. Many behavioral patterns are obvious; for example, mothers of many animal species actively protect their young. Honeybees defend their hive and prairie dogs whistle to alert their close kin that a predator is near. But in many instances the behavior of individuals or groups can be protective of or otherwise beneficial to members of a completely different species. Sometimes both species may benefit from a relationship. The little birds that ride unmolested on the backs of rhinoceroses rid the big animals of insect pests and warn them of approaching danger, while picking up a meal in exchange. Some relationships are one-way streets, with only one species profiting, although the other may be unaffected. For example, Spanish moss provides no obvious benefit or harm to oak trees whose limbs are used as a means of support for the uninvited guest.

Another relationship that benefits one species but not necessarily the other was discovered in a study conducted by David Young, a University of Georgia graduate student. The study confirmed and helped explain an earlier observation made by Ronald Pulliam of an unusual group behavior between sparrows and rats. Anyone with a bird feeder is aware that sparrows often feed in groups. Behavioral ecologists attribute this to a survival tactic in that an approaching predator, such as a hawk by air or a fox by land, is more likely to be seen if several pairs of eyes are on guard. Thus, when one bird flies in fright, the entire flock is alerted and can scatter to avoid the danger.

Cotton rats are fat, brown balls of coarse fur that inhabit the southern tier of states and are considered an ideal meal by many predators. Cotton rats know that they are considered "good eating" and are highly cautious about venturing into open areas where they might be caught. Nonetheless, if a board covered with seeds is placed in a field, cotton rats will eventually move onto the open area to get the free meal--usually on one condition. A flock of feeding sparrows must already be on the board.

In the study, video cameras were used to record the response of sparrows and cotton rats to the presence of large plywood squares that had been sprinkled with seeds. Shortly after the food was put out, sparrows began to come and go. And so did the cotton rats. However, the rats would wait along the margins until the sparrows arrived. The rats and sparrows ignored one another while they were eating, but if the sparrows flew away, the rats would scurry off the board.

The rats were taking advantage of the sparrows as sentinels and were too wary to venture into the open without benefit of the warning system. Confirmation that the cotton rats were aware of the potential danger of feeding in the open was made on the very few occasions when a rat was present with no sparrows around. When feeding on the board with sparrows, a rat had a food intake per minute (as measured on the videotape) ten times that of one that dined alone. The reason? Solitary cotton rats spend most of their time scanning the skies for hawks and have little time to enjoy the free meal. Whether the sparrows derive any benefit from having cotton rats at their dinner table is still to be determined.

Whatever the case with sparrows and cotton rats, the outdoor world has many intriguing mysteries awaiting our discovery.

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