Song sparrow chicks. Photo by Tobyotter.Predators have an obvious impact on their prey: eating them. But if the threat of predators prompts prey species to change their behavior, those behavioral changes can also affect prey population dynamics [$a]—and thereby, potentially, the prey's evolution—even if the predators never actually catch any prey.
This is the effect documented in a short, sharp study just published in Science, in which Liana Y. Zanette and her coauthors show that song sparrows raise fewer chicks if they simply think that there are predators nearby [$a].
The team's experimental design was simple but probably pretty work-intensive. Over the course of one summer on several small islands off the coast of British Columbia, they watched song sparrows choose mates and build nests. Once nests were established, the team surrounded them with anti-predator defenses: netting and electrified fences. They confirmed that these measures kept predators out with regular video surveillance. And then they turned on the loudspeakers.
At some nests, the team broadcast looped recordings of calls made by song sparrow predators—raccoons, crows and ravens, hawks, owls, and cowbirds. At control nests, the broadcast was instead a playlist of similar-sounding calls made by non-predators, including seals, geese, hummingbirds, and loons. The team then monitored the nests, recording the behavior of the mated pair at each nest, and the ultimate success of the eggs they laid.
An adult song sparrow, looking watchful. Photo by kenschneiderusa.
The results are pretty unambiguous. Pairs of song sparrows that heard predator calls laid fewer eggs than pairs that heard non-predators. Of the eggs laid by pairs who heard predator calls, fewer hatched, and of those hatched chicks, fewer survived fledge. Just the continuous, threat of predators—predators that were never visible—reduced the number of chicks the sparrows fledged.
The reasons for the reduced offspring are apparent from other behavioral observations. Birds in the predator-call treatment were perpetually on high alert, as measured by "flight initiation distance," the distance up to which a researcher could approach the nest before the birds took flight. Sparrows in the non-predator treatment let researchers get about 120 meters from the nest before taking off; sparrows in the predator treatment wouldn't tolerate humans within twice that distance. In the predator treatment, sparrows spent less time sitting on their eggs, and visited to feed their chicks less frequently. Not surprisingly, chicks in the predator treatment also gained less weight than chicks in the non-predator treatment.
And, in what may be the most poignant data set I've ever seen in print, the team also measured the skin temperature of chicks in each nest 10 minutes after the parents had left. Chicks in the predator-call treatment were measurably, and significantly, colder.
So the simple fear of predators is enough to prompt free-living song sparrows to lay fewer eggs, and raise fewer of the eggs they do lay to fledging. However, the absolute difference in offspring between sparrow pairs in the predator and non-predator treatments—40%—probably reflects the maximum effect we might expect to see in natural populations.
That's because left to themselves, sparrows probably seek nesting spots with less predator activity. Here, all the sparrows had established nests in what, presumably, were the best spots they could find—but for half of them, the new neighborhood suddenly seemed to become a lot less safe shortly after they settled in. What Zanette et al. document is very much a behavioral, short-term response, and it's one that many prey animals may be able to mitigate, or avoid altogether, with other behavioral responses. It's hard to say how exactly it reflects the impact that fear of predators might have in sparrow populations unmolested by ornithologists.
Nevertheless, this result does suggest that for many prey animals, the fear of predators can, itself, be something to fear. ◼
Creel, S., & Christianson, D. (2008). Relationships between direct predation and risk effects. Trends in Ecology & Evolution, 23 (4), 194-201 DOI: 10.1016/j.tree.2007.12.004
Martin, T. (2011). The cost of fear. Science, 334 (6061), 1353-4 DOI: 10.1126/science.1216109
Zanette, L., White, A., Allen, M., & Clinchy, M. (2011). Perceived predation risk reduces the number of offspring songbirds produce per year. Science, 334 (6061), 1398-1401 DOI: 10.1126/science.1210908