Well, at least, the only thing THESE birds have to fear is fear itself.
This is one of those studies that, when you lay it out, seems really...simple. Clear layout. Clear results. But it challenges a lot of the things that we once assumed about predator:prey relationships. Most particularly, it overturns the idea that the only thing making prey die from predation is the predators themselves.
This seems really simple, right? Fox eats bunny, lots of foxes mean the bunny population declines. Just foxes being AROUND bunnies (but eating, say, SmartOnes meals or something), well the bunnies wouldn't get eaten and the prey population would stay the same or even increase. Right?
Well...wrong. It turns out that sometimes what the prey population has to fear, is FEAR of predation itself.
Zanette et al. "Perceived Predation Risk Reduces the Number of Offspring Songbirds Produce per Year" Science, 2011.
(this is a full video by the authors, showing the results of their finding! Wish more people could do this!)
The issue with trying to figure out if FEAR of predation (as opposed to actual predation) is causing changes in songbird offspring is this: getting rid of the predation. Obviously you want to work in the wild if you possibly can, but how do you stop the circle of life? Well in this case, you surround YOUR circle of life with electric fences and large amounts of bird netting.
The authors picked a spot among the Gulf Islands off British Columbia. There they can watch songbirds nesting, and they can also fence off the areas well enough to make sure nothing can get in. They also put a video camera up over each nest, to make sure they knew exactly what was happening to every single egg.
So once they had fenced off nests (12 for each conditions) they set up some loudspeakers. One group got innocuous native animal sounds: geese honking, loons, seals barking), the other group got PREDATOR associated sounds (raccoons, hawks, cowbirds, etc).
This all made me wonder what a raccoon SOUNDS like. Now I know:
They started the predator or normal noises, played continuously, just before the lady sparrows were due to drop their eggs. They checked over the next two months (during which sparrows will produce two clutches of eggs), to see how the birds did.
The results are pretty striking. You can see on the left that the birds with predator sounds laid 40% fewer eggs. The eggs they had didn't hatch as often, and the babies didn't survive to adulthood as often either (possibly due to being less able to control their own body heat when mom was away). Survival rates in general were lower.
There's no doubt these sparrow moms were stressed. They weren't actually being predated, but the sounds of the predators gave them a higher sense of predation RISK. The stress of that was definitely enough to decrease the prey population on its own, even without actual predation.
And this could be very important for things like how we look at predator:prey relationships, as well as how we CONSERVE certain species. After all, just protecting the animal may not be enough, if the sounds of predators (or other things) have it so stressed it can't get it on! It means we're going to have to consider FEAR as an extra variable when we're looking at predators and prey. You can't just keep the predators out, you may have to keep the fear out, too.
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