Collisions with glass kill birds in very large numbers, and the deaths are spread thinly across ordinary buildings rather than concentrated anywhere obvious. That distribution is why the scale went unrecognised for so long.
Why birds cannot see glass
Birds have excellent vision but no evolutionary experience of transparent solid surfaces, and nothing in a natural environment behaves the way a window does.
Clear glass shows what lies beyond it, so a bird sees vegetation or open sky and flies toward it at full speed.
Reflective glass is worse, because it presents a mirror image of the habitat behind the bird, which appears in every respect to be somewhere it could fly.
Where the deaths actually occur
Tall illuminated towers attract attention because collisions there happen in visible clusters, particularly during migration nights with low cloud.
The larger total comes from low-rise buildings, including houses, because there are so many of them and they sit at the height where birds ordinarily fly.
Detection is poor at these sites. Scavengers remove carcasses quickly, and a single dead bird beneath a window is rarely recorded anywhere.
Which birds are affected
Migrants passing through unfamiliar areas are heavily represented, since resident birds learn the local hazards and adjust their flight paths.
Species that forage in vegetation are more exposed than open-country birds, because they are the ones drawn toward reflected foliage.
The birds killed are not a random sample of the population, so the effect falls disproportionately on species already declining for other reasons.
What lighting contributes
Many songbirds migrate at night and orient partly by celestial cues, which artificial light disrupts.
Birds drawn toward lit buildings circle and become trapped in illuminated areas, depleting energy reserves and increasing the chance of striking glass.
Programmes that switch off decorative lighting during migration periods reduce these incidents markedly, and they cost less than any physical modification.
What actually stops collisions
Markings must be on the outer surface of the glass to be visible under reflection, and internal blinds or decals behind the pane do very little.
Effective patterns break up the reflection at a spacing tight enough that a bird does not perceive a gap large enough to fly through.
Fritted, etched and patterned glass achieves this at the design stage, which is far cheaper than retrofitting, and is why the issue is increasingly handled through building standards.