Rats are among the most widely introduced invasive species in the world. Throughout history, they have hitched rides on boats, wagons, and trains, invading nearly every corner of the globe. Birds are a common prey for rats, and have been hit especially hard by the population increase and range expansion of these rodents; many avian species have become endangered, or even gone extinct, as a result of increased predation. Reintroducing birds is also a challenge, since resident rats quickly learn that the new species is a profitable source of food.
But now, two Australian scientists have identified a clever method that may limit predation by rats and increase bird survival. Their research, detailed in this week’s PNAS, offers a potential way to improve reintroduction success in areas where rodents are a problem.
Omnivorous predators such as rats often find new food sources by smell. They quickly investigate new scents, and if these odors are related to a recurring food reward—for example, if following a particular scent leads repeatedly to a nest full of plump baby birds—the scent and the reward become coupled in the rat’s mind. The researchers took this basic idea and reversed it: if they could decouple the cue from the reward, would the rats learn to ignore the cue? In other words, if rats never learned that the scent of a nest proffered a tasty treat, would bird survival increase?
To determine whether this idea would work, the researchers conducted trials with wild rats in Sydney, Australia. They introduced domestic quail—a species that the rats didn’t already know as food—in two treatment groups. In the first treatment, the control group, quail nests with both real and artificial eggs were simply introduced into the rats’ habitat. Each day ten grams of “quail odor,” collected via feathers and feces from a nearby quail farm, was added to each nest to approximate realistic prey cues. Based on the number of eggs that were taken or damaged, the researchers could determine the rate of predation by rats (tooth marks in the artificial eggs helped researchers determine what species was responsible for the predation). This treatment illustrated how the reintroduction of a bird species might actually play out.
The second treatment was somewhat different; for seven days before the nests were introduced, the researchers applied quail odor across the habitat. Then, on the eighth day, the nests were put out, and predation was monitored as in the control group.
Incredibly, “survival” of the eggs was 62 percent higher in the second treatment group, where the rats were pre-exposed to the odor, than in the control group. Rats that were already familiar with the quail scent when the eggs were introduced raided nests significantly less often than did rats that were introduced to the odor and the nests simultaneously. This effect remained for seven days after the nests were added.
The scientists behind the study believe that if a novel odor is not reinforced with a reward immediately, when the rats’ motivation to investigate the new scent is strongest, the odor cue fades into the “olfactory background.” Once this occurs, it may be more difficult for the rats to learn that the odor is rewarding, even if there is later prey associated with it. By that point, familiar food may be more appealing to the predators.
The research has considerable implications for conservation, since decoupling a cue from its reward, at least initially, may result in more successful bird introductions. Since the study only lasted seven days, it’s impossible to know what could happen weeks, months, or years down the road; in time, rats may learn to link the cue with the prey. However, if reintroduced birds can be protected at their most vulnerable period, even for a short amount of time, the species may receive the bump it needs to thrive in its new habitat.