Both in Estonia and across Europe, cat populations are continuing to grow. A bachelor's thesis recently defended at Tallinn University (TLÜ) focused on the question of how freely roaming domestic cats impact wildlife.

Thesis supervisor Christel Rose Bachmann, a board member at the Estonian Theriological Society (ETS), noted that no systematic research has yet been done in Estonia on cats' impact on the natural environment. Even so, billions of animals around the world are known to be killed by cats each year — including birds, rodents, amphibians and even reptiles.

"I personally like cats very much, but the problem is that cat owners often don't know what their cats are up to outside," Bachmann said during an appearance on Vikerraadio's "Ökoskoop." That lack of awareness is what prompted this bachelor's thesis to collect data on 160 cats, with owners reporting the prey their cats brought home.

Earlier studies have shown that cats don't always bring their prey home; in fact, only about a quarter of their kills make it back to their owners. This has been studied by equipping cats with cameras, allowing owners to see what their cats were actually up to.

The recent bachelor's thesis revealed that the majority of cats' prey in Estonia consisted of rodents — mainly short-tailed field voles, which are an important food source for many protected birds of prey, including lesser spotted eagles and earless owls.

Birds came in second, including young songbirds, as well as several species under II and III category protection. Cats also caught lizards, common vipers, grass snakes, amphibians and even bats — all of which are strictly protected species in Estonia.

Full stomach not enough

According to Bachmann, one of the biggest issues is poor awareness among cat owners. There is a common belief that if a cat is fed at home, it won't hunt in the wild — but that isn't true.

"Cats have a natural hunting instinct," she explained. "They move, play and catch prey even when their stomachs are full." So even a full food bowl doesn't necessarily mean a cat will remain inactive outdoors.

Several approaches can help reduce the threat cats pose to other species, although their effectiveness varies. One option is cat collars with bells, the sound of which could warn other species of nearby danger. However, according to the thesis supervisor, traditional bells aren't the best solution — especially when it comes to young birds, which don't recognize the sounds as danger signals.

Another option is colorful collars, which can help more with birds, but not with rodents, since most small mammals are colorblind.

More effective are physical barriers: cat balconies, outdoor enclosures — sometimes known as "catios" — and enriched outdoor environments where cats can satisfy their movement and hunting needs through play. If trained from an early age, you can even walk cats with a leash and harness.

People often think that cats are independent and only need food and a litter box," Bachmann noted. "But if they aren't given attention, that can lead to problems arising, including stress, aggression and marking. And owners may even end up abandoning the cat."

The study also showed that it makes no difference whether a cat lives in the city or the countryside — if allowed to roam freely, its amount of prey increases just the same. Unfortunately, this also subjects cats themselves to a variety of dangers, from cars to foxes, jackals and diseases carried by wild animals.

One of the greatest dangers is rabies, which can also be spread by bats. If a cat becomes infected, it can pose a risk to humans as well.

Legal and owner responsibilities

Bachmann noted that regulations governing cat ownership in Estonia are still lacking, with mandatory registration and microchipping still only required in a handful of local governments.

Around 93,000 cats are listed in Estonia's centralized pet register — likely representing only a fraction of their actual number.

The total number of stray or free-roaming cats in the country is difficult to estimate, but according to her, they may number in the hundreds of thousands. If each of them catches dozens or even hundreds of animals per year, Estonia could be losing upwards of five million wild animals to cats each year.

"We're not against cats," Bachmann emphasized. "Cats are wonderful companions. They bring joy, and they can be kept [as pets] in a nature-friendly way. But for that to happen, owners must consciously take responsibility [for their cats]. Let's not forget — every cat is someone's responsibility."

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