Advocates are calling for more awareness of the danger that glass buildings pose to birds, following a mass bird death in Canada’s capital.

More than 30 bohemian waxwings died outside Ottawa City Hall on April 2 after flying into the elevated glass walkway that connects the old building to the new.

The walkway had featured hawk decals to dissuade the small birds from flying in that direction, but Safe Wings Ottawa–a local program dedicated to reducing bird deaths from window collisions–says such precautions don’t cut it. (The walkway has since been covered with brown crafting paper as a temporary solution.)

“Birds do not understand glass as we do,” Safe Wings Ottawa states on their website. “Where we see reflections of trees, they simply see trees. Where we see a solid surface that separates indoors from out, they see open space…. Mirrored glass is especially dangerous, but regular glass is also highly reflective depending on light conditions.”

The unfortunate event was likely exacerbated by the nearby crab apple trees, whose fermented berries can intoxicate hungry birds around this time of year.

At least 100 million birds die each year in North America as a result of window collisions, with most of those happening within 5 storeys of the ground. To help prevent collisions, ensure your windows are visible to birds by applying visual markers in a dense pattern on the exterior surface of the glass.

Related: Mapping bird/building collisions in Toronto