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Wildlife impacts of free-roaming cats: Estimates vs. evidence

Much of the controversy surrounding the management of free-roaming outdoor cats is related to their impact on wildlife, in particular on songbirds. Although recent research has fueled this controversy, many of the central arguments have been made for more than 100 years.


Wildlife impacts of outdoor cats in the United States:

a brief history


Early 1900s

In The Domestic Cat: Bird Killer, Mouser and Destroyer of Wild Life; Means of Utilizing and Controlling It, published in 1916, Massachusetts State Ornithologist Edward Howe Forbush describes the cat’s threat to wildlife in dramatic fashion: “No animal that it can reach and master is safe from its ravenous clutches” [1]. Forbush explains that — even then — the debate over how best to manage free-roaming cats had “reached an acute stage,” hampered by “many loose and ill-considered statements” and “assertions for and against the cat… [having] absolutely no foundation in fact” [1].


Late 1900s

In the mid-1990s, University of Wisconsin researchers published estimates suggesting that rural cats alone were killing up to 219 million birds annually in the state [2]. Despite weaknesses associated with the underlying methods used to reach this estimate [3,4], the number attracted national attention and was subsequently used to lobby the Wisconsin Conservation Congress, which soon approved a measure that would have allowed the hunting of “feral cats.” The Congress later reversed course, dropping the measure [5].


2000 to present

More recently, researchers from the Smithsonian Migratory Bird Center and U.S. Fish and Wildlife Service (USFWS) estimated that free-roaming cats in the contiguous U.S. kill 1.3–4.0 billion birds and 6.3–22.3 billion mammals annually [6]. Again, the estimates attracted national attention. And, again, questions were raised about their validity and what, if anything, such estimates say about the population-level impact of cats on wildlife [7–10].

Still, these unprecedented mortality estimates were used by some to argue for the removal of “all free-ranging cats from the landscape by any means necessary” [11].

“there are few, if any studies apart from island ones that actually demonstrate that cats have reduced bird populations"

An earlier review of dozens of predation studies, published in 2000, found that “there are few, if any studies apart from island ones that actually demonstrate that cats have reduced bird populations” [12]. A review of the scientific literature currently available suggests that little has changed today: evidence of population-level impacts from anywhere but small islands remains elusive.


Research into the hunting behavior of cats:

weighing the evidence


Wildlife impacts generally fall into one of three categories: predation, indirect “fear effects,” or disease transmission, examples of which are used to illustrate key points in the following sections.


The focus on cat predation

Predation is the most commonly cited form of wildlife impact, in part because it is more visible than indirect mortalities or those associated with disease transmission. And, as explained above, it’s also received considerable attention from researchers, especially as it relates to possible impacts on birds.


It’s important to remember that even large mortality estimates must be compared with population estimates — and at a geographic scale small enough to be meaningful from a management perspective. Estimates of the number of birds killed annually across the contiguous Unites States, for example, might make for good headlines, but these numbers — often published without context — provide little guidance for those interested in effectively managing bird (or cat) populations [6].

Birds killed by cats are, on average, significantly less healthy than birds killed in collisions with windows or cars.

It’s also important to distinguish between mortality estimates and population impacts. It’s well understood that predators — including cats — tend to prey on vulnerable individuals (e.g., sick, weak, young) considered the “doomed surplus,” resulting in little or no impact on the population of a species [13,14]. In fact, two peer-reviewed studies have shown that birds killed by cats are, on average, significantly less healthy than birds killed in collisions with windows or cars [15,16].


It’s often claimed that “even well-fed cats hunt,” an observation that can be traced to an experiment conducted in the 1970s. As it turns out, though, the cats involved in this study were neither well fed nor actually hunting [17].


While it’s no doubt true that some cats who are regularly fed by humans will continue to hunt, the question is to what extent (and with what impact)? Researchers have found that “poorly fed” pet cats were much more likely to prey on wildlife [18], as were cats living more than two miles from human habitation [19]. Such findings are important in light of the fact that the domestic cat is a commensal species: cats are found virtually everywhere people are found.


The estimated impact of cats on bird populations

Studies suggesting that free-roaming cats have a negative impact on various species (especially those considered native) are common. However, relatively few stand up to careful scrutiny. Indeed, many are contradicted by empirical evidence.


The mortality estimates sometimes attributed to free-roaming cats [6], for example, cannot be reconciled with the best population estimates available [20], or with the population trends documented by the annual North American Breeding Bird Survey [21,22]. In some cases, these mortality estimates actually exceed the total number of birds estimated to populate an area. In addition, such estimates leave no accounting for other well-documented causes of bird mortality, such as pesticide use, oil spills, habitat loss, window strikes, or other anthropogenic causes.

The mortality estimates sometimes attributed to free-roaming cats cannot be reconciled with the best population estimates available.

Similarly, a 2013 U.K study published in the Journal of Applied Ecology reported that “the brief presence of a domestic cat at [blackbird] nest sites reduces subsequent provisioning rates and induces lethal trait-mediated indirect effects” [23].


In other words, the mere presence of cats creates a “fear effect” with negative consequences for birds. However, a review of data provided by the British Trust for Ornithology reveals an upward trend in both blackbird populations and nest success rate across the U.K. [24]. Meanwhile, cat ownership has doubled over roughly the same period [25] and, unlike in the U.S., the majority of U.K cat owners (74–87%) allow their cats to go outdoors regularly [26,27]. The purported claim of a “negative impact” is therefore difficult to reconcile in light of the empirical evidence — nevertheless, other studies have repeated the assertion [28–30], leading to considerable confusion.


The perceived spread of disease from cats to wildlife

Free-roaming cats have also been implicated in the spread of infectious diseases to wildlife. Various media accounts, for example, suggest that free-roaming cats are responsible for the decline of marine mammal populations because of their ability to contaminate the environment with the Toxoplasma gondii parasite, which can then contaminate rainwater runoff [31,32].


However, peer-reviewed studies have demonstrated that the majority of California sea otter mortalities often attributed to cats [31] were actually the result of a strain of T. gondii found almost exclusively in wild felids and only rarely in domestic cats [33,34]. Indeed, research from the U.S. Geological Survey concluded that “spillover from wildlife, not pets” is likely responsible for infection in California sea otters [35]. And the agency’s most recent census found “the otter population is likely at its highest level in at least 100 years” [36,37].


Similarly, free-roaming cats in Hawaii have been blamed for declining numbers of the endangered Hawaiian monk seal. However, according to the National Oceanic and Atmospheric Administration, T. gondii infection resulted in the deaths of only eight seals between 2001 and 2015 — just 4.4% of known mortalities during that period — and was suspected in another three seal deaths [38]. And contrary to what was otherwise being conveyed about the decline of this species, a 2016 report from the agency notes that “there have been an increasing number of seal sightings and births in the main Hawaiian Islands” since 1990 [39].

Cats living in close proximity to humans are much less likely to be exposed to the T. gondii parasite.

On one hand, this is surprising since these are the most densely populated of the Hawaiian Islands (by both humans and cats) and therefore nearby waters are presumed to be more contaminated. On the other hand, these findings correspond with research showing that cats living in close proximity to humans are much less likely to be exposed to the T. gondii parasite than “solitary, feral domestic cats living in undeveloped landscapes” [40].


Context (and habitat loss) matters

Although it’s true that cats can have significant negative impacts on island species (especially seabirds) [41,42], it’s important to distinguish these cases from mainland contexts, and especially in urban areas where most cats are found [43]. Moreover, the complete eradication of cats from islands has at times caused more significant harm to native wildlife than was originally caused by the cats themselves [44,45].


Published research and mainstream media accounts often focus on areas where free-roaming cats come into conflict with protected native wildlife species [46–49]. Although this attention is understandable, it’s important to recognize that such situations attract attention precisely because they are exceptional.

“habitat loss is by far the greatest cause of bird population declines" -2014 State of the Birds report

Across the country, species of concern are under increasing pressure as a result of habitat loss, development, environmental contamination, and many other anthropogenic factors. Indeed, the 2014 State of the Birds report, published by the North American Bird Conservation Initiative, notes that “habitat loss is by far the greatest cause of bird population declines” [50]. Targeting free-roaming cats (another anthropogenic factor) not only raises ethical questions [51] but can also distract from efforts to mitigate these other, more concerning, factors.


The need for evidence-based decision-making in community cat control


There is general agreement that free-roaming cats can pose a significant risk to wildlife populations; however, the credible evidence is quite clear that this risk is limited to very specific contexts (e.g., small islands) and even then is likely only one part of a larger story. Sweeping claims that lack necessary context (e.g., conflating island and mainland environments) confuse the issue and impede productive conversation about how best to manage free-roaming cat populations.

Related resources



References

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