The Cornell Lab of Ornithology’s BirdCast program opened its fall Lights Out period on August 15. It runs through November 30 across the United States and Canada, and covers the stretch when night-migrating birds move in the largest numbers.
What the date actually marks
August 15 is not a forecast. It is the start of the window in which BirdCast asks households and building managers to turn off non-essential lights from dusk to dawn. The spring equivalent runs March 1 to June 15.
BirdCast builds its nightly migration forecasts from 23 years of weather and radar data, and triggers an alert when a night’s predicted volume falls into its “high” category. Those alerts are the practical part. They tell you which specific nights your outdoor lighting matters most.
Most collisions happen in daylight, not at night
This is the part that surprises people who have only ever heard the lights-out message. American Bird Conservancy states plainly that most collisions take place during the day, caused by habitat reflected in or seen through glass rather than by artificial light after dark.
Light still matters. ABC describes urban glow pulling migrants into built-up areas, and a “beacon effect” from intense light sources that can trap or disorient birds overhead. But switching off a porch light while leaving the glass untreated addresses only half the problem.
The buildings involved are not the skyscrapers
Citing 2014 Smithsonian research, ABC attributes roughly 253 million US bird deaths a year to buildings one to three stories tall — 44 percent of the total by their accounting — with another 339 million at low-rise buildings of four to eleven stories. High-rises of eleven floors and up account for 508,000 in total.
ABC puts the overall annual US toll between 365 million and one billion, median estimate 599 million, and says it believes the true figure sits closer to a billion or higher. Around 300 species have been documented in window strikes across the US and Canada, including hawks, grouse, kingfishers, hummingbirds and woodpeckers.
Treat the outside of the glass, and watch the spacing
BirdCast’s Treat Glass guidance, supplied by the Bird Collision Prevention Alliance, comes down to two rules. Patterns go on the outside surface, because that is the face carrying the reflection. And markers must sit no more than two inches apart for adhered products, or three to four inches for hung treatments such as cords.
ABC adds a second test: whatever you use has to be visible to a bird from at least ten feet away, so it has time to change course. ABC also names external insect screens as one of the best home fixes, because they nearly eliminate reflections and cushion the impact if a bird does hit. Spacing and placement are where home attempts tend to come apart rather than product choice, which is the ground our own guide to which glass treatments hold up across a full season covers in detail.
Which windows to do first
You do not have to treat the whole house. BirdCast ranks the priorities: glass that has already caused a known collision, glass near feeders, water features, gardens, trees or shrubs, and glass that creates a “passage effect” where a bird can see straight through to habitat on the other side. Courtyard glass and panes at the end of walls or rock outcrops follow.
Renters are not stuck. The removable options — tempera paint, paint markers, paracord curtains — are on BirdCast’s own list of things that work. ABC notes that effective treatments can cover as little as one percent of the window area, so this is not a choice between birds and your view.
Applying anything to the interior is the common shortcut, and ABC is direct about it: outer-surface reflections will hide an inside pattern for part or all of the day. If outside access is genuinely impossible, ABC suggests taping a sticky note to the inside and checking it from outside every hour or two starting in early morning. If you can still see it most of the time, a bird probably can too.
Why the estimates keep moving upward
A 2024 study in PLOS ONE by researchers at ABC, Fordham University, NYC Bird Alliance and Stony Brook University followed more than 3,000 birds injured in building collisions and brought to rehabilitators. Only 40 percent survived, even under experienced care.
“Birds who appear stunned or fly away after a collision are far more likely to die than previously thought,” said lead author Ar Kornreich of Fordham University. The injuries the team recorded — bruising, eye ulcers, fractures — are invisible to someone watching a bird pick itself up and go.
That changes how you should read your own windows. ABC cites a study in which, out of 29 window collisions, only two birds died on the spot and left a carcass at the foot of the glass. Scavengers take much of the rest, which is why a quiet window is not proof of a safe one.
If you keep birds indoors as well
Glass is a hazard on both sides of the wall. A companion bird out of its cage meets the same invisible barrier, which is worth a pass before free-flight time — the seven-point check before your bird uses a window perch handles the glass and sun side, and the five-minute routine before any window opens handles the escape side.
One distinction is worth making before you start counting strikes. A wild bird that returns to tap or lunge at the same pane every morning is usually not colliding at all. That is territorial behavior aimed at its own reflection, and it calls for a different response than a collision does.
Where to get more
BirdCast’s Lights Out and Treat Glass pages carry the full guidance, and its local migration alerts are free to subscribe to — those are what tell you which nights in your area are the heavy ones. ABC’s glass and birds FAQ answers the product questions and links its rated-product database.
The window that needs attention first is almost certainly one you already know about, because something has already hit it. That is the one to start with this week, while the migration period is still in its opening weeks rather than its peak.