Evaluation of the Impacts of Free-Ranging Domestic Cats on Global

Bird Populations and Ecosystems

Yuxian Zhao

School of Life Science, Central China Normal University, Wuhan, China

Keywords: Ecological Impact, Domestic Cats, Bird Populations, Management Strategies, Public Awareness.

Abstract: Free-ranging domestic cats (Felis catus) are owned and unowned cats that can range freely. These cats appear

for a variety of reasons like deliberate grazing by owners or lack of management. Garden birds are an

important part of nature, often involved in seed dispersal and pest control. It is now known that free-ranging

domestic cats have caused a huge reduction in population and species abundance of garden birds. This article

discusses the impacts of free-ranging domestic cats on garden birds and evaluate and suggests the existing

relative policies. It outlines the scale of bird fatalities attributed to both owned and unowned cats, which are

estimated to kill billions of birds annually across various regions including the US, Australia, and China. The

distinction in impact between rural and urban settings is highlighted, with rural cats often used for pest control,

while urban cats are primarily companions. Governments in different countries have different strategies for

dealing with free-ranging domestic cat problems. Common treatment methods include TNR and euthanasia

and hunting, but there are a variety of problems to be solved in implementation. Many governments even lack

current policies to control free-ranging domestic cats or protect garden birds.

1 INTRODUCTION

Anthropogenic activities pose a serious threat to

ecosystems, significantly reducing biodiversity in

areas inhabited by humans. Domestic cats, cherished

for their companionship and pest control capabilities,

notably against rodents, have been integrated into

human societies globally. House cats (Felis catus)

were domesticated by human over 9000 years ago.

According to molecular biological identification of

DNA, the population gene pool of domestic cats was

mainly derived from the Middle East and Egypt.

These felines were most widely spread in the

Classical period, as a pest control agent, symbol and

companion animal (Ottoni et al., 2017). As obligate

carnivores, domestic cats show play and hunting

behavior more frequently after they are weaned.

When deprived of humans interaction and left to

wandering outdoors, they are more likely to fulfill

their needs on local wildlife. Notably, even hungry

cats are more likely to choose live prey than to feast

on palatable food, but then return to what was offered.

This can cause domestic cats to exhibit excessive

killing (Cecchetti et al., 2021). Historically, the

practice of allowing cats to range freely, combined

with some owners' negligence and intentionally

abandoned, has resulted in less controlled

management of domestic cats.

Garden birds, encompassing various species that

visit gardens for food and shelter, are a common sight

in many parts of the world. These birds are not only

prevalent in gardens but also has a high coincidence

degree with the densely populated areas of humans.

Their adaptation to the artificial environment is

exactly successful. At the same time, these avian

species are an important part of urban ecological

environment, and intertwining interdependent with

other organisms through the food chain to maintain

the balance of ecological development. Specifically,

birds can help control the population of pests and

reduce the amount of pesticides used, thus reducing

pollution to the environment. Beyond pest control,

birds are vital agents in plant reproduction and

growth. In the process of feeding, birds stick pollen

on their bodies to complete the process of pollination.

At the same time, they also disperse seeds by

excreting them, promoting plant reproduction and

distribution.

Zhao, Y.

Evaluation of the Impacts of Free-Ranging Domestic Cats on Global Bird Populations and Ecosystems.

DOI: 10.5220/0013846500004914

Paper published under CC license (CC BY-NC-ND 4.0)

In Proceedings of the 2nd International Conference on Renewable Energy and Ecosystem (ICREE 2024), pages 87-91

ISBN: 978-989-758-776-4

With the development of social and economic and

the process of urbanization, free-ranging domestic

cats have become one of the most destructive invasive

species at present, especially to the local garden bird

population. Research indicates that these cats are

responsible for the deaths of approximately 1.3-4.0

billion birds annually on the earth (Loss, Will and

Marra, 2013). In semi-wild, human-modified

environments, free-ranging domestic cats lack natural

predators and can seek to be provided by their owners

when resources are scarce. Thus, they are far more

ecologically competitive than local wild animals

(Lepczyk et al., 2023). By catamount nature,

domestic cats tend to prey on birds, reptiles and

mammals, which cover 90 percent of diet.

Additionally, though the owners often provide

sufficient energy, free-ranging domestic cats actively

hunt many wild animals for entertainment and

curiosity, resulting in even more killings. Unowned

free-range domestic cats may breed outside,

expanding the population and leading environmental

effects. The difference in the impacts of free-ranging

domestic cats in urban and rural areas may be due to

the different purposes of their owners. Domestic cats

in rural areas are mainly kept to protect crops and

stored food. Keeping domestic cats free can help

remove pests faster and more efficiently. In urban

areas, domestic cats are mainly regarded as family

companions of their owners. Thus, fewer domestic

cats spend time outdoors.

At present, for unowned domestic cats,

governments address the issue through initiatives

such as trap-neuter-return (TNR) activities, adoption

efforts, and culling measures to control the

population. When it comes to domesticated cats, the

situation is more complicated due to their legal status

as property of their owners. So many of the methods

used for unowned cats are inappropriate. This article

discussed the impacts of free-ranging domestic cats

on garden birds, reviewed the policies in place to

manage these cats, and provided recommendations

for mitigating their negative impacts.

2 IMPACTS OF CATS ON BIRDS

Free-ranging domestic cats have had an impact on

garden birds all over the world. There is a difference

in the extent of the impact between unowned and

owned cats, with unowned cats generally killing more

wild birds. There are also differences in the

destructive magnitude of free-ranging cats in

different environments. On the mainland, free-

ranging cats often reduce some bird populations or

affect bird ranges without directly causing the species

to distinct. On islands, due to space constraints, the

introduction of domestic cats is more fatal to the local

ecosystem. Predation by cats has led to extinctions of

many island species, some of which are even endemic

to the islands. In the red list of International Union for

Conservation of Nature (IUCN), free-ranging

domestic cats on islands have caused 33 extinctions

of modern bird.

2.1 Magnitude of Bird Mortality

Caused by Cats on Mainlands

Loss et al. have done a research based on cat

predation data the United States, Europe, Australia

and New Zealand (Loss, Will and Marra, 2013). In

the three estimates of bird mortality, they mainly

focus on interpretating on the data of US and Europe.

Cats in the United States kill between 1.3 and 4.0

billion birds each year (median=2.4 billion), 69

percent of which are caused by unowned cats.

Unowned free-ranging cats hunt at three times the

predation rate of owned cats. The article shows that

the mortality rate of birds caused by cats is much

higher than previously estimated in the United States,

and it is higher than any other human cause of death

(like strikes and poisoning). Most of the birds of cat

predation are native species. According to the data,

less than a third of the bird species that encounter

predation are nonnative. This could be strong

evidence of the impact of free-ranging domestic cats

on native species diversity. In addition, it was found

that there was a certain technical difficulty in the

investigation of unowned domestic cats, resulting in

a decrease in the accuracy of the predation data.

Counting the number of unowned cats is prohibitively

expensive. Moreover, due to the lack of widespread

public knowledge, human subsidize a large part of the

maintenance of the population of unowned free-

ranging domestic cats. There are more than 300

colonies of human-maintained cats in Washington,

D.C. alone. The results suggest that the presence of

cats has led to population declines in certain species

in some areas. But the specific population changes

and the spatio-temporal variations in the presence of

the most affected species make it difficult to

determine the accurate data.

In Europe, Roman et al. analyzed data on cat

predation from ringing programmes in France and

Belgium (Pavisse, Vangeluwe and Clergeau, 2019).

ICREE 2024 - International Conference on Renewable Energy and Ecosystem

They examined bird species killed by domestic cats

between 2000 and 2015. Passerines were found to be

the species most affected by cat predation, a common

native ground-feeding bird. The behavior of birds

living on the ground is related to the rate of cat

predation. Between 2000 and 2015, the number of

birds killed by predation or attack by cats increased

by more than 50 percent, matching the trend of local

cat population growth.

The proportion of birds in cats diet increases in

hot and arid areas. In Australia, feral cats kill 272

million birds a year, 99% of which are native species.

Annual bird mortality is related to precipitation. The

drought period is 161 million birds per year, which is

significantly lower than the tide period with 757

million birds per year. On average, feral cats kill 356

million birds every two years. The researchers also

found that in more urbanized areas, owned-cats killed

44 million birds a year and owned-cats killed 61

million birds a year. That's more than 1 million birds

killed by cats every day. The proportion of birds in

the diet of feral cats is higher than that of mammals

(Woinarski et al., 2017).

Li et al. estimated total annual predation of owned

and unowned domestic cats in China by

questionnaires (Li et al., 2021). According to their

research, domestic cats in China prey on a substantial

number of wildlife, with about 1.213-3.298 billion

wild animals killed by free-range domestic cats each

year. Between 30 and 40 percent of cat-owning

households allow domestic cats to roam outdoors.

These cats bring home more than 3,000 species of

prey. Each urban household has an average of 0.14

free-ranging domestic cats. In rural areas, each

household has an average of 0.36 free-range domestic

cats. It is estimated that urban free-ranging domestic

cats prey on an average of 1.41 birds per year while

rural free-ranging domestic cats hunt an average of

2.17 birds per year. Due to the limitations of the

questionnaire method, wildlife mortality caused by

free-ranging domestic cats is likely to be greatly

underestimated.

2.2 Extinctions on Islands

Cats have been introduced to 179,000 islands around

the world. As a highly invasive species, cats cause

more than ten times as much damage to island

ecosystems as they do to mainland ecosystems

(Woinarski et al., 2017). The population size of

species on islands is often out of proportion to that on

land, and is therefore more affected by cats. In the

Pacific, the impact of birds is mainly on the

passerines and petrels. Medina et al. found that feral

cats were responsible for 13.9% of bird, mammal and

reptile extinctions recorded by the IUCN Red List

(Medina et al., 2011). After a Meta-analysis, they

demonstrated that cats have a negative impact on bird

populations on the island (effect size: 0.5484).

3 STRATEGIES AND

EVOLUTION

Despite these enormous harmful effects of free-

ranging domestic cats on garden birds, effective

control measures are still lacking. The commonly

used TNR method considers more about the rights

and interests of cats, in fact, has no obvious effect on

protecting the ecosystem, and is costly and difficult to

operate exactly. It is due to a lack of basic public

awareness of the threats of free-ranging cats. The

prevailing view is that other disturbing human

activities (such as habitat destruction and building

collisions) have much greater impact on garden birds

than free-ranging cats do, which might only be a

minor factor. However, according to the survey data,

the harm caused by domestic cats is very considerable

and the number is statistically greater than the death

of birds caused by building collisions, the second

harmful impacts of human on birds.

3.1 Current Methods

On islands, particularly those with significant

biodiversity threats from introduced species such as

domestic cats, eradication programs have removed

these animals from at least 48 islands globally. The

programs often employ trapping and hunting,

sometimes assisted by dogs, alongside poisoning and

the introduction of viral diseases such as feline

panleukopaenia (Nogales et al., 2004). The success of

the eradication efforts underscores the importance of

combining multiple methods to effectively manage

cat populations in sensitive habitats.

On the contrary, the controlling management of

unowned and owned cats in urban and remote

communities tends to focus more on humane

strategies which consider the welfare of both the cats

and the human community. A systematic review

identified three primary methods suitable for remote

Indigenous communities in Australia: Surgical

Sterilisation (SS), Trap-Neuter-Release (TNR), and

Trap-Remove (TR). These methods aim to reduce the

Evaluation of the Impacts of Free-Ranging Domestic Cats on Global Bird Populations and Ecosystems

number of free-ranging cats through sterilization

while also addressing the welfare of social cats

through adoption or euthanasia (Brooke et al., 2020).

The TNR method, involving trapping, neutering, and

releasing cats back into their environment, has been

highlighted as an effective strategy for controlling

stray cat populations in urban communities. The

approach not only addresses the immediate issue of

cat overpopulation but also considers the welfare of

the cats by preventing them from being euthanized

(Tao et al., 2020).

In Australia, Indigenous tracking experts have

been employed to hunt feral cats specially. This

method has shown promise in reducing cat

populations at specific sites, such as the Kiwirrkurra

Indigenous Protected Area (Kiwirrkurra IPA), where

130 cats were removed over five years. The hunting

activities were conducted efficiently, with teams of

four hunters capable of catching up to four cats per

day. The approach not only targets individual

problem cats but also contributes to the protection of

threatened species by maintaining a presence of

hunters in the landscape, which may have co-benefits

like fine-scale fire management (Paltridge et al.,

2020).

In continental Europe, a study demonstrated that

colorful collar-covers and bells can effectively reduce

wildlife predation by domestic cats. This simple

intervention aims to make cats more visible to their

prey, thereby reducing the likelihood of successful

predation. The effectiveness of this method suggests

that non-lethal interventions can play a crucial role in

protecting birds and other wildlife from cats

(Simmons and Seymour, 2022). Additionally, the

various obligations relating to the protection of Birds

in the European Union Birds and Habitats Directives

(Nature Directives) have significant effect on the

management of free-ranging domestic cats. In the

directives, cat population must be controlled when

unowned cats pose a threat to local species. For EU

member States, owned domestic cats are prevented

and prohibited from roaming freely outdoors

(Trouwborst and Somsen, 2020).

3.2 Evaluations

It is not difficult to see from the existing various

methods that there are still many problems in control

management of free-ranging domestic cats around the

world.

First, the existing regulations are inherently

flawed. Currently the most commonly used and

popular TNR means have a long cycle and are

difficult to operate. The process of capturing and

neutering only one cat requires the long-term

cooperation of multiple professionals, consuming a

lot of time, money and human resources. Moreover,

the threat free-ranging cats pose to local wildlife, such

as birds, remains even after their release in TNR.

Therefore, this approach is costing and slow and

cannot achieve the purpose of protecting the

ecosystem immediately.

Secondly, some provisions are not effectively

complied with because of implementation

difficulties. The EU Nature Directives, for example,

make it difficult for governments to determine how

much of a threat cats pose to local wildlife, so control

management of cat populations may not be timely.

Also, the public does not have a correct

understanding of the damage of free-ranging

domestic cats to local wildlife such as birds.

Statistically, a significant number of people believe

that cats are members of local ecosystems and

therefore should not be interfered with in their

population growth and their predation on wildlife (Li

et al., 2021). It is important for the popularization of

common sense among the public. In areas severely

affected by free-ranging cats, the government should

include the learning of relevant facts in the

compulsory education stage to form a better social

atmosphere and help control the population of cats

roaming outdoors.

Finally, some countries still lack any effective

methods related to controlling cats. Domestic cats

have been introduced to many areas for many years,

but few researchers have investigated the damage cats

can do to ecosystems. Even in China, where many

investigations have already been conducted,

regulations for controlling free-ranging cat

populations are still limited.

4 CONCLUSION

The ecological repercussions of free-ranging

domestic cats are profound, especially concerning the

decimation of garden bird populations and the

broader impact on biodiversity. This review has

highlighted that both owned and unowned cats

contribute to a significant loss of wildlife, with

billions of birds and other small mammals falling

prey annually worldwide.

Free-ranging domestic cats include both owned

and unowned cats, which both have a huge impact on

ICREE 2024 - International Conference on Renewable Energy and Ecosystem

garden birds. In continental areas, cats generally

reduce populations of many bird species through

predation. Cats kill an average of 2.4 billion birds in

the United States each year, 69 percent of which are

caused by ownerless cats. In Australia, cats kill an

average of 2.72 billion birds a year. In China, the

figure is 2.57 billion. The data is universally lower

than it really is. In island areas, due to geographical

peculiarities, the impact of free-ranging cats often

leads to more serious consequences, namely the

extinctions of some native species. Bird species that

are active on the ground are more likely to be killed

by cats and their populations are more affected.

At present, there are some methods and policies

for controlling free-ranging domestic cats around the

world, such as sterilization and hunting, but they are

all flawed. These approaches always have low

popularity, are generally difficult to implement, and

are hindered by the public.

This paper argues that the best means to control

the harmful impacts of cats to wild animals such as

birds is capture and euthanasia. Public awareness is

also important because it not only benefits

government efforts, but also reduces the free-ranging

domestic cat population in the first place.

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Evaluation of the Impacts of Free-Ranging Domestic Cats on Global Bird Populations and Ecosystems