I. The Statutory Framework

The Migratory Bird Treaty Act of 1918, codified at 16 U.S.C. §§ 703–712, implements the United States’ treaty obligations under four international agreements for the protection of migratory birds: with Great Britain (on behalf of Canada, 1916), Mexico (1936), Japan (1972), and Russia (1976).¹ It is the oldest and most comprehensive federal statute devoted exclusively to avian conservation.

Section 703(a) provides, in language that has not been meaningfully narrowed since 1918:

Unless and except as permitted by regulations made as hereinafter provided in this subchapter, it shall be unlawful at any time, by any means or in any manner, to pursue, hunt, take, capture, kill, or attempt to take, capture, or kill… any migratory bird… included in the terms of the conventions.²

The implementing regulations at 50 CFR § 10.12 define “take” to mean “to pursue, hunt, shoot, wound, kill, trap, capture, or collect, or attempt to pursue, hunt, shoot, wound, kill, trap, capture, or collect.”³ The statute protects more than 1,000 species of migratory birds as listed at 50 CFR § 10.13.⁴

Three features of the statute merit attention. First, the prohibition applies “at any time.” There is no seasonal exemption. There is no daytime-only rule. The prohibition is continuous. Second, the prohibition covers killing “by any means or in any manner.” Congress did not limit the statute to intentional hunting. It did not limit it to firearms, poisons, or traps of conventional design. It used the broadest possible language to encompass any mechanism by which a protected bird might die at human hands. Third, the statute imposes strict liability, at least in the Second and Tenth Circuits. As the Second Circuit held in United States v. FMC Corp., “it is not necessary to prove that a defendant violated the Migratory Bird Treaty Act with specific intent or guilty knowledge.”⁵

A violation of Section 703 is a misdemeanor punishable by a fine of up to $15,000 per offense, imprisonment for up to six months, or both.⁶ The phrase “per offense” means per bird. Each dead bird is a separate violation. This is settled law and has been applied in every prosecution brought under the statute.

The question is not whether the MBTA prohibits the killing of migratory birds by means other than hunting. It does. The Department of Justice has said so repeatedly, in court filings, press releases, and federal criminal convictions. The question is why the largest single source of non-predatory bird mortality in the United States has never been the subject of a single enforcement action under a statute that was enacted 108 years ago for exactly this purpose.

II. The Kill Data

In 2014, researchers Scott R. Loss, Tom Will, and Peter P. Marra of the Smithsonian Conservation Biology Institute and the U.S. Fish and Wildlife Service published a systematic meta-analysis of bird-building collision mortality in the United States. The study, published in The Condor: Ornithological Applications, synthesized data from 23 studies and estimated that between 365 million and 988 million birds are killed annually by building collisions in the United States, with a median estimate of 599 million.⁷

This figure makes building collisions the second-largest source of direct human-caused bird mortality in the country, surpassed only by free-ranging domestic cats, which kill an estimated 1.3 to 4 billion birds per year.⁸ Building collisions kill more birds than motor vehicles (89 to 340 million per year), power lines (8 to 57 million), communication towers (6.6 million), and wind turbines (approximately 234,000 per year in 2012).⁹

The U.S. Geological Survey, which maintains the national Bird Banding Laboratory and receives reports of banded birds killed by window collisions, states on its public-facing website that “up to two billion birds are killed each year in the United States due to collisions with glass.”¹⁰

The distribution of mortality across building types is instructive. Loss et al. found that approximately 44 percent of bird-window collision deaths occur at residences, 56 percent at low-rise commercial buildings of one to eleven stories, and less than 1 percent at high-rise buildings of twelve or more stories.¹¹ The most prolific killer is not the glass curtain wall of a Manhattan skyscraper. It is the picture window in a suburban living room.

This is not a new finding. Daniel Klem Jr. of Muhlenberg College published the first systematic estimate of bird-window collision mortality in 1990, calculating between 97.6 million and 975.6 million deaths per year in the United States.¹² The range has been refined. The order of magnitude has not changed. The scientific community has known for 36 years that windows kill hundreds of millions of birds annually. The enforcement community has known for exactly as long and has done exactly nothing.

III. The Mechanism

The mechanism by which windows kill birds is not ambiguous, disputed, or poorly understood. It is straightforward physics and ornithology.

Birds cannot perceive glass as a solid barrier. Transparent glass presents what the avian visual system interprets as unobstructed flight path: habitat, sky, or open space visible through the pane. Reflective glass presents what the visual system interprets as actual habitat: trees, shrubs, and sky reflected on the outer surface. In both cases, the bird perceives the window as something it is not. It accelerates toward the perceived environment. It strikes the glass at full flight speed. The impact produces blunt force trauma to the skull and cervical spine. The bird dies on impact or within hours from traumatic brain injury, cervical fracture, or internal hemorrhage.¹³

This is not a design deficiency of the bird. It is a design deficiency of the glass. Avian visual perception evolved over approximately 150 million years in an environment that contained no transparent solid barriers.¹⁴ Glass has been in widespread architectural use for approximately 200 years. Natural selection has not had time to produce a heritable response to a material that did not exist for 99.999987 percent of avian evolutionary history. The bird is operating exactly as designed. The glass is operating exactly as designed. The interaction between the two designs is fatal, and the glass arrived second.

Peer-reviewed research has demonstrated that collisions occur across all seasons, with peaks during spring and fall migration when billions of migratory birds transit the continent.¹⁵ Collisions occur at all times of day but are concentrated in the morning and midday hours when light conditions maximize reflectivity and transparency.¹⁶ Nocturnal migrants are particularly susceptible to illuminated buildings, which disrupt their celestial navigation and attract them into collision zones.¹⁷

The mechanism is not a mystery. It has been described in scientific literature for decades. It has been communicated to the glass industry, the construction industry, and the federal government. No party has claimed ignorance. No party has claimed the science is uncertain. The birds keep dying at the same rate because the windows keep being installed at the same rate, and no statute, regulation, or enforcement action has ever required them to be installed differently.

IV. The Enforcement Record

The Department of Justice has demonstrated, through federal criminal prosecutions, that the MBTA applies to the incidental killing of migratory birds by commercial infrastructure.

In November 2013, Duke Energy Renewables Inc. pleaded guilty to violating the MBTA in connection with the deaths of golden eagles and other protected migratory birds at two wind energy projects comprising 176 wind turbines on private land in Converse County, Wyoming. The Department of Justice described the case as “the first criminal conviction under the Migratory Bird Treaty Act for unlawful avian takings at wind projects.”¹⁸

In December 2014, PacifiCorp Energy was sentenced in the United States District Court for the District of Wyoming after pleading guilty to two misdemeanor violations of the MBTA arising from 38 golden eagle deaths and 336 other protected bird deaths at two of its commercial wind farms in Wyoming. The total number of birds for which the company was prosecuted was 374. The company was sentenced to five years’ probation and ordered to pay $2.5 million in fines, restitution, and community service payments.¹⁹

Three hundred and seventy-four birds. That is the number that triggered a federal criminal prosecution, a guilty plea, a $2.5 million financial penalty, and five years of court-supervised compliance monitoring. The government brought the full weight of the Migratory Bird Treaty Act against a utility company that killed 374 birds over the course of several years of wind farm operations.

In the same year that PacifiCorp was sentenced, approximately 599 million birds died by striking windows in the United States. The ratio of window kills to prosecuted wind farm kills is 599,000,000 to 374. That is approximately 1,601,604 to 1. For every bird death the Department of Justice prosecuted, windows killed 1.6 million birds that nobody prosecuted.

V. The “Any Means” Problem

The MBTA prohibits killing migratory birds “by any means or in any manner.” The Department of Justice has argued, and federal courts have accepted, that this language encompasses the incidental killing of birds by industrial infrastructure. Wind turbine blades are a “means.” Open oil waste pits are a “means.”²⁰ Unshielded power lines are a “means.”²¹

A window is a means.

A window is a physical object installed by a human being in a human-built structure. It is composed of manufactured materials—soda-lime glass, aluminum or vinyl frames, sealants, and hardware—produced by commercial enterprises and sold at retail. It is installed pursuant to a building permit issued by a government authority. It is inspected for compliance with building codes, energy standards, and fire safety requirements. It is maintained by the building owner. It is, by every conceivable definition, a human artifact deployed in the built environment.

When a bird strikes a window and dies, the death is caused by the window. Remove the window and the death does not occur. The window is not a contributing factor. It is not an environmental cofactor. It is the proximate, direct, and sole mechanical cause of the fatal impact. The bird’s inability to perceive glass is a constant. The presence of the glass is the variable. The variable was introduced by a person. The person installed a means of killing migratory birds “in a manner” that the statute prohibits.

The Department of Justice has never argued otherwise. It has simply never applied this reasoning to windows. The legal theory that convicted PacifiCorp of killing 374 birds applies with identical force to the owner of a suburban ranch house whose bay window kills two warblers per migration season. The statute draws no distinction between a 100-meter wind turbine and a 3-foot-by-5-foot pane of residential glass. Both are means. Both are manners. Both kill protected birds. One is prosecuted. The other is not.

VI. The Building Owner as “Person”

The MBTA applies to “any person.” Under 50 CFR § 10.12, “person” means “any individual, firm, corporation, association, partnership, club, or private body, any one or all, as the context requires.”²²

According to the 2020 United States Census, there were 140,498,736 housing units in the United States.²³ The 2024 American Community Survey estimates the current figure at 146,740,964.²⁴ The Energy Information Administration’s 2018 Commercial Buildings Energy Consumption Survey estimated 5.9 million commercial buildings, virtually all of which have windows.²⁵

Each of these buildings is owned or operated by a “person” within the meaning of the statute. Each of these buildings has windows. Each of these windows is a documented mechanism of migratory bird mortality. The owners of these buildings have deployed, maintained, and continuously operated a documented means of killing protected wildlife without obtaining a permit, submitting an incidental take plan, or filing a single report with the U.S. Fish and Wildlife Service.

If the PacifiCorp enforcement standard were applied uniformly—prosecution triggered by documented bird deaths at an identified facility operated by an identified person—the potential defendant pool would include every homeowner, landlord, commercial tenant, property management company, real estate investment trust, government building administrator, school superintendent, hospital director, and house of worship in the United States. It would include the U.S. Fish and Wildlife Service itself, which operates offices with windows.

VII. The Incidental Take Doctrine

For more than four decades, the Fish and Wildlife Service maintained the position that the MBTA’s take prohibition applies to the incidental killing of migratory birds—that is, bird deaths that occur as a byproduct of otherwise lawful activity, rather than as the result of intentional hunting or capture.²⁶

Federal courts have split on the question. The Second Circuit, in United States v. FMC Corp. (1978), held that a pesticide manufacturer was liable under the MBTA for bird deaths caused by wastewater discharged into a pond, even though the company did not intend to kill birds.²⁷ The Tenth Circuit, in United States v. Apollo Energies (2010), upheld the application of the MBTA to incidental bird kills at oil and gas operations.²⁸ The District Court for the District of Colorado, in United States v. Moon Lake Electric Ass’n (1999), held that an electric utility could be prosecuted under the MBTA for eagle electrocutions on uninsulated power lines, reasoning that the statute requires only “cause in fact” between the defendant’s conduct and the bird’s death.²⁹

The Fifth, Eighth, and Ninth Circuits have taken narrower positions, generally limiting the MBTA to conduct directed at birds.³⁰ This circuit split has produced an enforcement landscape in which a wind farm operator in Wyoming can be prosecuted for the same bird deaths that a wind farm operator in Oregon cannot.

The policy pendulum has swung accordingly. In January 2017, the Department of the Interior issued Solicitor’s Opinion M-37050, concluding that the MBTA does not prohibit incidental take.³¹ In January 2021, the Interior Department revoked M-37050 and restored the prior interpretation.³² As of July 2026, the Fish and Wildlife Service maintains that incidental take is prohibited under the MBTA, subject to the development of a general permit framework that has been proposed but not finalized.

Under the current interpretation, every window in America that kills a migratory bird does so in violation of the MBTA. The Fish and Wildlife Service’s own position compels this conclusion. The agency has simply declined to enforce it. The distance between the agency’s stated legal interpretation and its actual enforcement posture is approximately 599 million birds per year.

VIII. The Manufacturer’s Knowledge

The glass industry has known about bird-window collisions for decades. This is not a contested point. The peer-reviewed literature documenting the problem dates to at least 1979, when Daniel Klem Jr. published his doctoral research on bird-glass collisions at Southern Illinois University.¹³ By 1990, Klem had published an estimate of nearly one billion annual bird deaths that was available in the Journal of Field Ornithology and widely cited in conservation literature.¹²

The glass industry has not merely been aware of the problem. It has commercialized solutions. Multiple manufacturers now produce bird-safe glass products incorporating ultraviolet-reflective coatings, fritted patterns, and acid-etched designs that are visible to birds but minimally visible to humans. Research has demonstrated that these treatments reduce collision rates by approximately 50 to 90 percent.³⁴ The technology exists. It is commercially available. It is not prohibitively expensive. It is simply not required.

Under the CPSC’s product safety framework, a manufacturer that knows its product causes injury and knows that a feasible remedy exists is expected to report the hazard and implement corrective action. Under the MBTA, knowledge is not even required—the statute imposes strict liability. The glass industry has both knowledge and available remedies and faces neither a reporting obligation nor an enforcement risk. It continues to ship approximately 680 million square feet of flat glass annually in the United States for architectural applications,³⁵ virtually none of which incorporates bird-safe design features unless a local jurisdiction requires them.

One city in the United States has enacted a comprehensive bird-safe building law. New York City’s Local Law 15 of 2020 requires that new construction and major alterations use bird-friendly materials on at least 90 percent of the exterior glazed area below 75 feet.³⁶ There are approximately 19,502 incorporated places in the United States.³⁷ The compliance rate across American municipalities is 0.005 percent.

IX. The Population Consequence

In September 2019, a team of researchers led by Kenneth V. Rosenberg published a landmark study in Science documenting a net loss of approximately 2.9 billion birds—29 percent of the total population—from North America since 1970.³⁸ The study, based on continent-wide population surveys and weather-radar analysis of nocturnal migration, concluded that “the crisis of North American bird populations extends beyond threatened species to include common and widespread species.”

Building collisions are identified in the conservation literature as a significant contributor to this decline, alongside habitat loss, agricultural intensification, and predation by domestic cats.³⁹ Loss et al. (2014) identified several species disproportionately vulnerable to building collisions that are simultaneously listed as Birds of Conservation Concern due to their declining populations, including Golden-winged Warbler (Vermivora chrysoptera), Painted Bunting (Passerina ciris), Canada Warbler (Cardellina canadensis), Wood Thrush (Hylocichla mustelina), and Kentucky Warbler (Geothlypis formosa).⁷

When the DOJ prosecuted PacifiCorp, it emphasized the deaths of golden eagles—an iconic species with special protections under the Bald and Golden Eagle Protection Act. When building collisions kill Painted Buntings and Wood Thrushes—species in documented population decline, protected under the same MBTA—the agency that maintains the list of declining species and the agency that enforces the statute protecting them are the same agency: the U.S. Fish and Wildlife Service. It has issued the conservation concern. It has not issued the citation.

X. The Aggregate Penalty Exposure

Under 16 U.S.C. § 707(a), each MBTA misdemeanor violation carries a maximum fine of $15,000.⁶ Each bird killed constitutes a separate violation. The federal courts have applied this per-bird calculus in every prosecution brought under the statute.

At the median estimate of 599 million bird deaths per year, the annual penalty exposure for building-related MBTA violations is:

599,000,000 birds × $15,000 per violation = $8,985,000,000,000.

Eight trillion, nine hundred eighty-five billion dollars. Per year.

For context, the gross domestic product of the United States in 2025 was approximately $29.2 trillion.⁴⁰ The annual penalty exposure for window-killed birds is approximately 30.8 percent of GDP. The total assessed value of all residential real estate in the United States is approximately $49.5 trillion.⁴¹ Two years of maximum MBTA penalties for window kills would exceed the assessed value of every home in America.

The U.S. Fish and Wildlife Service’s Office of Law Enforcement employs approximately 250 special agents and 154 wildlife inspectors.⁴² If each agent devoted their entire annual caseload to window-related bird kills, each agent would be responsible for approximately 2,396,000 dead birds per year, or approximately 6,564 per day. At the statutory maximum fine of $15,000 per bird, each agent would need to process $35,940,000,000 in penalties per year. This exceeds the Fish and Wildlife Service’s total annual budget by a factor of approximately 19,000.⁴³

The penalty framework was designed for a world in which protected birds are killed by identifiable actors in countable numbers. It was not designed for a world in which the second-largest source of anthropogenic bird mortality is a standard architectural component installed in every building in the country. The framework is not wrong. It is simply operating in a jurisdiction it was never intended to govern, which is all of them.

XI. The Comparative Enforcement Analysis

The selective enforcement of the MBTA against industrial bird kills while exempting building-related bird kills produces some instructive comparisons.

Wind turbines in the United States killed approximately 234,000 birds in 2012, according to the Loss et al. estimate.⁹ The DOJ prosecuted two companies for a combined total of approximately 374 of those deaths. Buildings killed approximately 599 million birds in the same period. The ratio of building kills to wind turbine kills is approximately 2,560 to 1. The ratio of building kills to prosecuted wind turbine kills is approximately 1,601,604 to 1.

Communication towers killed approximately 6.6 million birds per year as of the same study period. The FWS has engaged with the communication tower industry on voluntary guidelines for lighting modifications to reduce bird attraction. Buildings killed 91 times more birds than communication towers and have been the subject of no federal guidance whatsoever.

Oil waste pits, which were the subject of the FMC Corp. prosecution in 1978 and have been a focus of MBTA enforcement in oil-producing states for decades, kill an estimated 500,000 to 1 million birds per year.⁴⁴ Buildings kill 599 to 1,198 times more. The FWS has issued guidelines for covering oil waste pits to prevent bird drowning. It has not issued guidelines for covering windows to prevent bird collision, despite the fact that the kill ratio is three orders of magnitude higher.

The enforcement pattern is consistent: the DOJ and FWS pursue MBTA enforcement against industries that kill birds in the thousands or tens of thousands, while the largest source of direct human-caused bird mortality after cats operates entirely outside the enforcement perimeter. The statute does not create this distinction. The statute says “any means.” The enforcement apparatus says “some means.”

XII. Conclusion

The Migratory Bird Treaty Act of 1918 prohibits the killing of migratory birds by any means or in any manner. It imposes strict liability. It applies to any person. It covers more than 1,000 species. It has been in continuous force for 108 years.

Windows kill between 365 million and 988 million of those protected birds every year. This is not an estimate contested by industry. It is a peer-reviewed finding published by researchers at the Smithsonian Institution and the U.S. Fish and Wildlife Service itself. The mechanism is understood. The mortality data are robust. The species killed include those the Fish and Wildlife Service has specifically identified as populations of conservation concern.

The Department of Justice has prosecuted energy companies for killing hundreds of birds under the same statute. The legal theory is established. The precedent is set. The per-bird penalty has been calculated and applied. The only element that changes between a wind farm prosecution and a window prosecution is the number of dead birds, which is larger, and the number of defendants, which is everyone.

There are 146 million buildings with windows in the United States. Each one is a documented source of migratory bird mortality. Each one is operated by a person within the meaning of the statute. Each dead bird is a separate misdemeanor violation carrying a maximum fine of $15,000. The aggregate annual penalty exposure exceeds $8.9 trillion, or approximately 31 percent of gross domestic product. The Fish and Wildlife Service has 250 special agents to enforce it. No enforcement action has ever been initiated. No notice of violation has been sent. No building owner has been informed that the picture window through which they watch the birds in their yard is, under the statute that protects those birds, the most prolific instrument of their destruction.

The statute does not say “by any means except glass.” It does not say “in any manner except architecture.” It does not contain a residential exemption, a commercial exemption, a low-rise exemption, or an exemption for means of killing that are too common to prosecute. It says what it has said since 1918: it shall be unlawful at any time, by any means or in any manner, to kill any migratory bird. Six hundred million birds per year have taken the statute at its word. The enforcement apparatus has not.

Ergo.