I. The Prohibition

Section 703 of the Migratory Bird Treaty Act, 16 U.S.C. § 703(a), provides that “it shall be unlawful at any time, by any means or in any manner, to pursue, hunt, take, capture, kill, attempt to take, capture, or kill” any migratory bird protected by the Act.¹ The statute was enacted in 1918 to implement the treaty between the United States and Canada, and it now protects 1,093 species of birds, including eagles, hawks, waterfowl, shorebirds, seabirds, wading birds, and songbirds.² The protection extends to live and dead birds alike, and to any bird parts, including feathers, eggs, and nests.³

The four words that do the heavy lifting are “by any means or in any manner.” They are the broadest words Congress had available. They do not say “by hunting.” They do not say “by shooting.” They say by any means. The means is not limited to the means a person can personally operate. A means can have whiskers.

II. The Penalty Schedule

Section 707 of the Act, 16 U.S.C. § 707(a), provides that any person who violates the Act “shall be deemed guilty of a misdemeanor and upon conviction thereof shall be fined not more than $15,000 or be imprisoned not more than six months, or both.”⁴ The fine is assessed per violation. A violation is a dead bird.

The courts apply a strict liability standard to misdemeanor violations of the Act, which means the government need not prove that the defendant intended to kill anything.⁵ The Fifth Circuit, in its opinion in United States v. CITGO Petroleum Corp., notes that “the act imposes strict liability on violators, punishable by a maximum $15,000 fine and six months imprisonment.”⁶ Intent is not an element. Knowledge is not an element. The bird is dead, the fine is available, and the defendant’s state of mind is not part of the inquiry.

III. The Offenders

There are approximately 114 million cats living in the contiguous United States, of which roughly 84 million are pets and the remainder are feral.⁷ As many as 70 percent of pet cats spend some time roaming outside and hunting.⁸ Other estimates place the national cat population between 60 and 90 million, with approximately half of all cat owners permitting their animals outdoors.⁹ The American Bird Conservancy has estimated that 60 to 100 million free-ranging feral cats live in the country in addition to the owned population.¹⁰

The offenders are, in other words, everywhere, and most of them are permitted outdoors by the persons who own them. Researchers at the University of Georgia attached miniature cameras to 60 outdoor house cats and found that about 30 percent of them successfully captured and killed prey, averaging one kill for every 17 hours outdoors, or roughly 2.1 kills per week.¹¹ The cameras also revealed that cats bring home only 23 percent of what they kill; nearly half the time they leave the prey at the capture site, and 28 percent of the time they eat it and never bring it home.¹² The doorstep census, the traditional measure of feline predation, captures less than a quarter of the take. The compliance picture is worse than it looks.

IV. The Body Count

A systematic review published in Nature Communications in 2013 by scientists from the Smithsonian Conservation Biology Institute and the U.S. Fish and Wildlife Service estimated that free-ranging domestic cats in the contiguous United States kill between 1.3 and 4.0 billion birds every year, with a median estimate of 2.4 billion, alongside 6.3 to 22.3 billion mammals.¹³ Approximately 69 percent of the bird mortality is caused by un-owned cats, whose per-capita predation rates average three times those of owned cats.¹⁴ The authors conclude that free-ranging cats “are likely the single greatest source of anthropogenic mortality for US birds and mammals.”¹⁵

Two findings bear on the question of mitigation. First, feeding the cats has no measurable effect on the predation rate; setting out food for feral cats does not reduce the killing.¹⁶ Second, well-fed colony cats hunt anyway; the predation is not driven by hunger.¹⁷ The killing is, to a substantial degree, recreational. This does not matter under the statute, which does not ask why. It matters only to the question of whether the operation could be made compliant by feeding the offenders more. It cannot.

V. The Principal

The cat cannot be indicted. This is the point on which the entire defense of the American cat rests, and it is precisely the point that the law has already answered.

Section 2 of Title 18, 18 U.S.C. § 2(a), provides: “Whoever commits an offense against the United States or aids, abets, counsels, commands, induces or procures its commission, is punishable as a principal.”¹⁸ Subsection (b) adds that whoever willfully causes an act to be done which, if directly performed by him, would be an offense, is punishable as a principal. The legislative history, preserved in the notes to the section, states the rule in terms that could hardly be more on point: Congress intended to punish as a principal “one who puts in motion or assists in the illegal enterprise or causes the commission of an indispensable element of the offense by an innocent agent or instrumentality, … even though he intentionally refrained from the direct act constituting the completed offense.”¹⁹

Consider the person who feeds the cat. He purchases the food, fills the bowl, and maintains the animal at hunting weight. He shelters the cat, vaccinates the cat, and names the cat. He opens the door each morning and the cat proceeds, by its own locomotion, to the commission of between one and several federal misdemeanors per week. He has put the instrumentality in motion. He has refrained from the direct act. The statute was written for exactly this arrangement. It does not require that the instrumentality be human. It requires only that it be innocent, which the cat plainly is, having no concept of federal law whatsoever.

The statute says “by any means or in any manner.” A cat is a means.

VI. The Irrelevance of Motive

It will be objected that the cat does not intend to violate the Migratory Bird Treaty Act, and that the feeder does not intend for the cat to violate it either. This objection misunderstands the penalty schedule. The misdemeanor provision is a strict liability offense. The government need not prove intent to kill; intentional violations are a separate matter, graded as felonies.²⁰ The feeder’s good intentions, the cat’s sporting instincts, and the bird’s failure to check the list of protected species are all legally immaterial. The statute asks one question: is the bird dead. The cameras answer it 2.1 times per week per hunting cat.

The feeder sometimes protests that he did not know the cat was killing birds, because the cat brings home only 23 percent of its kills. Ignorance of the body count is not a defense under a strict liability statute. It is, at most, an argument about the size of the operation the feeder was running without his knowledge, which is not the direction in which a defense wants to go.

VII. The Island Precedent

The record contains a precedent that illustrates the capacity of a single instrumentality. In 1894, the assistant lighthouse keeper David Lyall arrived on Stephens Island, New Zealand, with a cat named Tibbles. Tibbles brought Lyall a bird Lyall had never seen: a small, flightless wren. Lyall sent the skin to the ornithologist Walter Buller, who identified it as a previously undescribed species, the Stephens Island wren, Traversia lyalli. By 1899 the bird was never again found alive. Fifteen specimens survive in museum collections.²¹ A single cat, or at most a small number of cats, is popularly credited with the extinction of a species that science had known for approximately one year.

The pattern scales. Domestic cats have been implicated in the global extinction of at least 63 species, 40 of them birds, which is 26 percent of all known contemporary extinctions in these species groups, and they currently endanger at least 367 more species at risk of extinction.²² A University of Nebraska study found feral cats responsible for the extinction of 33 bird species worldwide.²³ The Stephens Island wren is not an anomaly. It is the business model, documented at planetary scale.

VIII. The Enforcement Record

The government has not been shy about enforcing the Migratory Bird Treaty Act against other instrumentalities. In August 2009, ExxonMobil pleaded guilty in federal court in Denver to violating the Act in five states, agreeing to pay $600,000 in fines and community service payments.²⁴ The charges stemmed from the deaths of approximately 85 protected birds, including waterfowl, hawks, and owls, at company drilling and production facilities between 2004 and 2009. Most of the birds died after exposure to hydrocarbons in uncovered natural gas well reserve pits and wastewater storage facilities.²⁵ The company did not personally drown 85 birds. Its tanks did. The company pleaded guilty anyway, because the tanks were its instrumentality and the statute says “by any means.”

The ExxonMobil prosecution was, in the words of contemporary reporting, the latest of hundreds of cases federal officials had brought against energy companies over two decades for violations of the Act.²⁶ In July 2009, the Oregon-based utility PacifiCorp paid $1.4 million in fines and restitution for killing 232 eagles in Wyoming, electrocuted by the company’s poorly designed power lines.²⁷ The power lines were the instrumentality. The company paid.

The number of enforcement actions brought against cats, or against the persons who feed, house, and deploy them, is zero. The government fines the tank and ignores the cat. The tank killed 85 birds in five years. The cat kills 2.4 billion birds every year, and the government has never filed a single charge. The disparity is not a close call. It is nine orders of magnitude.

IX. The Arithmetic

The penalty schedule is $15,000 per violation, and a violation is a dead bird. At the median estimate of 2.4 billion birds per year, the maximum annual misdemeanor exposure is $36 trillion. At the upper bound of 4.0 billion birds, it is $60 trillion. At the lower bound of 1.3 billion, it is $19.5 trillion. The gross domestic product of the United States is approximately $30 trillion. The median estimate therefore represents unassessed misdemeanor liability equal to roughly 1.2 times the output of the entire American economy, accruing again each year, while the upper bound is twice the economy.

The daily figure is $98.6 billion, assuming the median. The imprisonment exposure, at six months per violation, is 1.2 billion years, to be served consecutively in theory and, one assumes, concurrently in practice.

For reference, the government’s own realized price, set in the ExxonMobil plea, was $600,000 for 85 birds, or approximately $7,059 per bird. At that rate, the annual tab for the median cat take is $16.9 trillion. This is the discounted figure, reflecting the government’s demonstrated willingness to settle. It is still more than half the economy.

X. The Defense

The defense must be stated, because the genre requires it and the law requires it first.

Section 707 punishes “any person, association, partnership, or corporation” who violates the Act.²⁸ A cat is not a person, an association, a partnership, or a corporation. Aiding and abetting requires a principal offense, and a principal offense requires a principal capable of committing it. The cat’s non-personhood, which Section V treats as the reason the feeder is liable, is equally the reason no court would accept the charge: you cannot aid and abet a non-person.

There is, moreover, a genuine and unresolved dispute about whether the Act reaches incidental take at all. The Fifth Circuit, in the CITGO case, held that the Act covers only deliberate acts directed at birds, vacating convictions based on birds that died in uncovered refinery tanks.²⁹ The Tenth Circuit, in United States v. Apollo Energies, upheld convictions for birds found dead in oilfield heater treaters.³⁰ The Interior Department has changed its position twice: a 2017 Solicitor’s opinion held that incidental take was prohibited; a 2021 opinion held that it was not; a second 2021 opinion revoked the first 2021 opinion and returned the Department to prohibiting incidental take while exercising enforcement discretion.³¹ The current position of the Fish and Wildlife Service is that incidental take is prohibited but that enforcement will be guided by discretion and prioritization.³²

It should also be noted that the Act does not protect every bird the cat kills. Introduced species, including the house sparrow, the European starling, the rock dove, and the Eurasian collared-dove, are not protected, and neither are nonmigratory upland game birds.³³ Some fraction of the 2.4 billion is therefore outside the statute. The remainder, consisting of the protected native species that make up the great majority of the take, is not.

Finally, no prosecutor would bring this case, no jury would convict on it, and the Department of Justice has finite resources and a published practice of directing them at industrial actors. Enforcement discretion is real, it is lawful, and it is the reason the American cat sleeps indoors at night, on the bed, having committed no prosecutable offense that day.

This is the defense, and it is correct about the law.

XI. Conclusion

The defense is correct about the law, and the law is beside the point, because the statute does not ask who the offender is. It asks by what means the bird died. The words are “by any means or in any manner,” and they were chosen to be inexhaustible.

ExxonMobil did not personally drown 85 birds in five states. Its reserve pits did, and the company pleaded guilty and paid $600,000, because the pits were the instrumentality it owned, operated, and failed to cover. The feeder does not personally kill 2.4 billion birds. His cat does, and he feeds the means, shelters the means, vaccinates the means, names the means, and opens the door for the means each morning. The means is innocent. The statute anticipated the innocent instrumentality and provided for it at 18 U.S.C. § 2: the one who puts it in motion is punishable as a principal.

The government has assessed $7,059 per bird against the tank. It has assessed $0 per bird against the cat. The tank killed 85. The cat kills 2,400,000,000. The maximum annual exposure is $36 trillion, the discounted figure is $16.9 trillion, and the enforcement record is zero. Somewhere in America, at this moment, a person is opening a door for a federal instrumentality, and the instrumentality is walking out into the yard, where the birds are.

Ergo.