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In a single day, more than 1,000 migrating birds collided with buildings within a small area in downtown Philadelphia, killing a particularly large number of Parula, Magnolia, Black-and-white, and Black-throated Blue Warblers, Common Yellowthroats, and Ovenbirds as well as smaller numbers of many other species. Birds collide with many types of buildings on a regular basis, and nearly half of the buildings involved are less than three stories tall. But, this October day was extraordinarily disastrous.

Before dawn on October 2, ���Ϲ��� embarked on his daily surveillance – in search of dead and injured birds that had collided with buildings – in a 3.5 square block section of downtown Philadelphia. Birds were already everywhere, lying injured and dead on the ground, with many of the injured barely able to stand or move. Within the first two hours, 150 dead birds were collected and bagged, destined to be added to the scientific research collection of Philadelphia’s Academy of Natural Sciences.

���Ϲ��� Pennsylvania’s program manager for urban conservation, Keith Russell, joined Maciejewski in collecting the dead birds and moving the injured but they struggled to keep up as the number of harmed birds continued to rise throughout the morning. Russell and Maciejewski moved as many injured birds as they could to safety in a large garden to prevent them from being swept up or vacuumed by cleaning crews.

“Finding hundreds of dead and injured birds on a single morning was a gruesome and overwhelming experience. But one that has motivated those of us who witnessed it to push even harder for new measures that will reduce hazards for migratory birds in the Philadelphia region,” says Russell. “We’ve worked diligently for many years to mitigate threats birds face in our city through outreach, education, and monitoring programs as part of the network. This is a reminder that there’s still a lot more work to do.”

With some relief, warblers and other migrants that had landed safely were observed flitting through plant areas, composed mostly of non-native vegetation in search of anything that they could eat. As birds struck in higher floors of tall buildings, many fell on inaccessible rooftops while others fell on to the street and were crushed by cars. It’s hard to truly determine just how many collided with buildings that day.

This mass collision event was unlike anything Russell or Maciejewski had witnessed in this area in recent study years. However, similar events have been documented in cities throughout the U.S. for over a century, including a . While specific circumstances associated with these events vary, most have been associated with some combination of a low cloud ceiling, fog, and rain overnight, which can cause nocturnal migrants to become confused and fly towards brightly lit buildings or other structures leading to deadly collisions.

While mass collision events of this size do not occur with great frequency, nocturnal migrants collide with buildings and other manmade structures on a regular basis, in all areas where they encounter glass and/or lights. Fueled mainly by windows, these less overwhelming incidents can occur 24 hours a day. Collectively, these events are taking a toll on the populations of many species that are particularly susceptible to collisions like the Black-throated Blue Warbler, of which dozens were found on October 2.

It is impossible to know how many birds are killed by collisions with buildings worldwide, but we do know the scale is massive. According to the report from Science, up to 1 billion birds collide with buildings each year in the U.S. This research also indicates that North American bird populations have dropped by nearly 30 percent since 1970 and collisions have clearly contributed to that decline.

“These bird collisions are preventable. ���Ϲ��� works with municipal leaders, building managers, businesses, designers, and homeowners alike across the country to implement solutions in building guidelines and protocols to reduce them,” says John Rowden, PhD, senior director of bird-friendly communities at the ���Ϲ���. “We’re also working at the state and federal levels to advocate for broader legislation to encourage more bird-friendly building practices, such as the Bird-Safe Buildings Act.”  

We continue to increase our understanding of bird migration patterns, using the latest science and technology, to inform our conservation efforts and more effectively mitigate human-built hazards to migrating birds. This analysis is particularly beneficial in identifying the key places to focus efforts within migration corridors, such as Philadelphia and the Delaware River Watershed along the Atlantic Flyway.

Through the Migratory Bird Initiative, ���Ϲ��� is partnering with organizations like the USGS Bird Banding Lab, Smithsonian, Georgetown University, the Cornell Lab of Ornithology, and Movebank to create a Migratory Bird Conservation Platform. This platform will help conservationists better understand when and where migratory birds will be and how to drive actions that protect them along their journey.  

"Migratory bird conservation is only as good as the migration science that informs actions on the ground,” says Jill Deppe, PhD, senior director of the Migratory Bird Initiative. “By gathering the latest and best tracking and connectivity data for North American birds, the Migratory Bird Conservation Platform will enable decision makers to more effectively protect birds and the places they need.” 

Coupling our advances in migration science with the many Lights Out Programs, public awareness, and engagement with decision makers at all levels, we can give birds a better chance of survival on their journeys between their breeding and wintering grounds.

Russell also offered a resource of to prevent future window collisions, including dimming or changing the color or direction of outdoor lights and obscuring or obstructing reflective windows on your home or building.

Even in tragedy, there is hope and opportunity for a better future. It has renewed a conversation around how these tragedies are avoidable; and it provides the opportunity to discuss what building managers, developers, and anyone at home can do to protect birds.