Human Scent Trails And Predators

by Eirik A.T. Blom

The study of nesting birds raises the question of the effect of observer presence on breeding success (see numerous BWD Skimmer notes). One long held theory is that human scent left in the vicinity of nests attracts potential predators, and studies have been conducted in an effort to learn if human presence increases predation. The results of the studies have been somewhat contradictory and may depend on the habitat and the nature of the predators involved. A recent study of nesting in shortgrass habitats suggests that human scent is not an attractant ("Do Mammalian Nest Predators Follow Human Scent Trails in the Shortgrass Prairie?" 1999. Susan K. Skagen, Thomas R. Stanley, and M. Beth Dillion. Wilson Bulletin 111:415-420). The authors placed 100 artificial ground nests in a shortgrass prairie in Colorado. In half the cases they made no effort to mask their scent when putting the nests in place, and in half the instances did cover their scent and used cow manure to disguise the trail to the nest. Of the 100 nests, 49 were disturbed by predators, 24 of the nests not treated and 25 of those in which the scent was masked. The lack of any difference in predation rates suggests that potential nest predators were not following human scent to locate the nests. One possibility is that the area is rarely visited by humans and potential predators may not have leaned to associate human scent with possible prey. A study of nest predation in a woodland, where raccoons were a major threat to nests, found that human scent may be an attractant to that species. Studies in grasslands that suggest human presence may be an attractant have identified another possible cause-vegetation trampling. Frequent visits to nests almost invariably results in trampling of surrounding vegetation and such disturbance may serve as a clue to predators. In this case, single visits are thought to have eliminated the possibility of vegetation disturbance. The authors recommend that studies of human visitation to nests and their effect on success need additional study, but that the results may depend on frequency of visitation, vegetation structure, and the type of nest predators found at the site.

Printed with permission from Bird Watcher's Digest "BWD Skimmer April 2000 issue [Volume 8, Number 2]"