Their journey didn’t start there, but they’ve since made themselves at home in many U.K. woodlands. Meet the gray squirrel — a textbook example of the disruptive ecological and economic impacts a non-native species can have. Introduced in the 19th century and now numbering over 2.5 million, gray squirrels in the U.K. have contributed to population decline in native red squirrels and, by damaging trees, are estimated to cause at least US$1.4 billion in economic losses over the next 40 years.

Scientists are studying oral contraceptives as a large-scale way to control wild animal populations and preserve biodiversity, but they’re at least six years away from developing and registering an oral contraceptive for gray squirrels in the United Kingdom. In the meantime, researchers can study squirrel feeding behavior to determine how bait delivery systems could most effectively deliver contraceptives once they become available.

To have the greatest impact on slowing population growth, enough females would need to consume a dosage that makes them infertile throughout peak breeding periods. Scientists are still working to determine precisely what an effective dosage would be and how long the contraceptive effects would last. Delivery systems should also be as targeted as possible to minimize impacts on other species. In this study, a group of researchers looked at how different factors influenced both gray squirrels’ likelihood of visiting baited feeders and the amount of bait they consumed.

Field Trials And Data Analysis

The researchers conducted six trials across five similar U.K. woods over three seasons: two in winter 2017/2018, two in summer 2022, and two in spring 2023. The researchers used one wood for two trials since a new squirrel population had recolonized it following the removal of the previous population.

In each wood, the researchers first trapped squirrels and tagged them with small electronic implants, recording key physical and biological characteristics of each individual. Then, they set out hoppers — bait feeders designed for squirrels — evenly throughout the wood. The researchers pre-baited hoppers with hazelnut paste to get squirrels comfortable eating from them before baiting them daily with a mixture of hazelnut paste and a fluorescent dye that stood in for an oral contraceptive.

In winter and summer, the researchers placed hoppers at a density of three per hectare (about 108,000 square feet) and baited them for four days. In spring, baiting lasted eight days, with the researchers using the four extra days to test whether squirrel feeding behavior would be similar with fewer hoppers. For the first four days of the spring trial, they set out two hoppers per hectare, then increased one woodland to three per hectare for the second half. At the end of each trial, the authors trapped and removed the inhabiting squirrel population.

The hoppers logged squirrel visits, identifying individuals through their tags and tracking how long they fed. The researchers used time spent visiting hoppers as a proxy for bait consumed. Using these data, the researchers built statistical models that predicted how factors including sex, season, and the number of squirrels per hopper influenced a squirrel’s likelihood of visiting a hopper and the amount of bait they consumed.

Key Findings

• More squirrels took the bait than didn’t. Of the 271 squirrels trapped across all woods, 56% tested positive for the fluorescent dye, confirming they’d eaten the bait. However, depending on the contraceptive in question, this proportion could still fall short. Previous research has estimated a threshold of 75% contraceptive efficacy for effective gray squirrel management, so further tweaks to the delivery system may be needed.
• Squirrels returned to hoppers multiple times to feed. The fact that squirrels visited hoppers multiple times within the short baiting periods is a good sign, as certain oral contraceptives may need the animal to consume multiple doses to become infertile.
• Spring yielded the highest success. The researchers’ models predicted spring as the only season where female squirrels would be more likely than males to both visit hoppers and consume more bait. They also found that both male and female squirrels would have higher visitation and bait consumption in spring, followed by winter, with the lowest usage occurring in summer. This finding is encouraging, as early December and May are key times to target gray squirrels before their peak breeding periods.
• Two hoppers per hectare worked just as well as three. The spring trial suggested no difference between a density of three versus two hoppers per hectare on likelihood of visiting or amount of bait consumed. However, the authors’ model also predicted that if more squirrels had to compete for hoppers, it would reduce visitation and bait consumption. For future bait delivery campaigns, the authors recommend placing just enough hoppers so there are fewer than three squirrels for every hopper.
• The hoppers effectively targeted gray squirrels. During the whole study, no other animals took bait from the hoppers. This suggests that using a thick, sticky hazelnut paste and weighted doors that encouraged eating in place successfully limited access for nontarget species.

Limitations

Having limited resources, the researchers were only able to study two woods per season, with each season being in a different year. This means there could’ve been other factors that varied from year to year that influenced how accurately the model predicted each season’s potential for bait delivery. The authors also couldn’t assess the impact of age or breeding status on bait consumption. They call for more research to confirm their findings and investigate these other factors.

Finally, as previously noted, the bait didn’t contain an actual contraceptive. The dye’s taste could’ve deterred some squirrels from taking the bait, and the taste of an actual contraceptive could have a similar or different effect.

Stopping The Spread: A Responsible Path Forward

This study’s success in targeting gray squirrels before peak breeding seasons is encouraging. Advocates should recognize oral contraceptives as a humane approach to wild animal population control, with promising potential to be both more practical and cost-effective compared to injectable contraceptives or lethal methods. As the authors note, scientists could also use oral bait delivery systems to deliver vaccines for disease control. Supporting relevant research, funding, and policy initiatives will be important for bringing this minimally disruptive method into common practice.

Education and awareness campaigns can also teach the general public about ways they can help prevent the movement of non-native species. While humans have played a major role in the establishment of invasive animal populations, we also have the power to stop their spread.