Raccoon caught scavenging an American Crow carcass during a carcass persistence study | Birds Connect Seattle
by Joshua Morris, Urban Conservation Manager
Have you ever wondered where birds go when they die?
It’s a reasonable question. We live our lives alongside thousands of birds that inevitably go the way of all flesh. Especially in cities, where they run a gauntlet of human-related hazards—pollution, pesticides, traffic…just the windows on our buildings are estimated to kill more than a billion birds per year in the US.
Brown Creeper after a suspected window collision | Birds Connect Seattle
So why do we not notice the dead birds that must be all around us?
One important reason is the amazing and efficient community of scavengers we also live alongside. In cities, where French fries grow on sidewalks and take-out containers flow from trash cans, scavenging as a feeding strategy can be particularly effective. Some of our most familiar urban wildlife are scavengers: raccoons, squirrels, crows, gulls, and rats, for example.
The diets of our urban scavengers are supplemented by the large number of birds that fatally collide with windows. Anecdotally we’ve heard of crows that wait around glassy buildings or that seemingly drive songbirds into windows as part of their hunting strategy. But how important of a food source could window collisions be?
I did a back-of-the-envelope calculation to estimate the possible contribution of bird-window collisions to the diets of our local slugs, snails, crows, and raccoons: assuming a conservative average body mass of 20 grams per bird-window collisions victim (about the size of a Song Sparrow) and 100,000 fatal events in Seattle per year (also a conservative estimate based on early results from the Seattle Bird Collision Monitoring Project), that’s a protein infusion of 2.2 tons, enough to satisfy 100 crows for an entire year (assuming they’d be content with 50 grams per day).
Birds Connect Seattle studies carcass persistence as part of the Seattle Bird Collision Monitoring Project. We do this because estimating the number of fatal collisions that occur around the buildings we study requires us to estimate the probability that we find a carcass before a scavenger does.
To study carcass persistence, we place a trial carcass, usually one we salvaged from through our collision monitoring project, around a building and observe how long it remains in place. Last spring we started using camera traps in our carcass persistence studies. Not only does this save us the trouble of checking each trial carcass multiple times per day, it also gives us greater precision in our estimates of persistence and allows us to identify which scavengers are taking advantage of the collision buffet.
Here is some of the footage we’ve captured so far:
This research provides a glimpse into the world of urban scavengers that we don’t often get to see. After a bit more data collection, we’ll be able to test if scavenging rates differ between times of day or season. This will help improve our estimates on collision mortality locally and help us build our case for why we should all take preventing bird-window collisions seriously as a conservation concern.
About Joshua Morris
Josh is Birds Connect Seattle’s Urban Conservation Manager. His work focuses on reducing urban hazards to birds, protecting and enhancing urban habitat, and engaging communities in conservation right in their own neighborhoods. Josh leads the Bird-safe Seattle project focused on understanding and preventing bird-window collisions.
Ethics statement: Carcasses salvaged via the Seattle Bird Collision Monitoring Project and used in carcass persistence studies are permitted under US Fish and Wildlife Service Salvage Permit MBPER0031885 and Washington State Scientific Collection Permit MORRIS 23-312.
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