Hermit Thrush / Anne Freudenthal / Seattle Bird Collision Monitors

“What did you not like about participating as a collision monitor?”


I posed this question to the first cohort of Seattle Bird Collision Monitors at the end of the project’s pilot season, which ran from September 1-October 31, 2021. Some noted the weather, others the schedule or the datasheets. But the most frequent response?


“Not finding dead birds.”

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Strange thing, it may seem, to dislike not finding dead birds. But I felt it, too. After eight weeks of scouring sidewalks, I hoped for the satisfaction of finding something, as a rarified few others in our group had. I had to remind myself that nothing is actually an interesting finding, that zero is an important data point.

While most of us ended the season with an unbroken tally of naughts on our datasheets, collectively we did, in fact, find carcasses during our surveys. Four to be exact. And we found or were alerted to five additional specimens within the study area but outside of our structured monitoring surveys. Still, after 61 days of daily surveys and more than 140 search hours during peak fall migration, those numbers felt low.

What do we make of that? Are collisions just infrequent in Capitol Hill? Or did we not detect them? Our efforts so far aren’t robust enough to draw conclusions regarding collision frequency, but we did gain some insight into the challenges of detecting collisions. Read on to learn more about what we did, what we found, and how you can get involved.

Seattle Bird Collision Monitoring Project: pilot season by the numbers

Monitoring period Sept. 1-Oct. 31, 2021
Number of monitoring days 61
Number of volunteer monitors 28
Number of study buildings 8
Number monitoring shifts completed 133
Number of building surveys completed 521
Total search hours 140+
Total project hours 600+
Number of survey carcass detections 4
Number of incidental carcass detections/reports from the study area 5

Where did we look?

We conducted structured collision monitoring surveys for dead and injured birds around eight study buildings in the Capitol Hill/First Hill neighborhoods. I’ve outlined the study buildings in black and shaded them gray on the map below. They are a mix of commercial, multi-family residential, and institutional buildings with a combined search perimeter length greater than a mile. We selected these buildings based on architectural features known to increase or reduce collision risks, some sustainability certifications (e.g., LEED or Living Building), and to allow a comfortable walking route between them.

What did we find, where?

We found four carcasses during structured surveys, and found or were alerted to an additional five specimens within the study area outside of structured surveys. Eight of nine were fatalities. Five of the nine were detected on the Seattle University campus. One was found at the Seattle Academy, one at the Bullitt Center, one at Hugo House, and one at Cal Anderson Park.

Specimens detected during structured surveys:

A. Unidentified. Remains. Law Building, south façade. Sept 4, 2021. 11:30 am. Cause of death: unknown.

B. Rock Dove. Fresh carcass. Bullitt Center, northwest façade. Sept. 28, 2021. 11:20 am. Cause of death: suspected vehicle strike.

C. Unidentified. Remains. Science Building. North façade. Oct. 2, 2021. 2:54 pm. Cause of Death: unknown.

D. Hermit Thrush. Fresh carcass. Science Building. October 30, 2021. 2:45 pm. Cause of death: suspected window collision.

Specimens found or reported outside of structured surveys:

A. Mourning Dove. Injured (wing). Seattle University, parking lot. Sept. 1, 2021. 12:00 pm. Cause of injury: unknown.

B. Rock Dove. Fresh carcass. Cal Anderson Park. Sept. 23, 2021. 12:30 am. Cause of death: raptor predation.

C. Song Sparrow. Fresh carcass. Hugo House, north façade. Sept. 23, 2021. 12:30 pm. Cause of death: suspected window collision.

D. Anna’s Hummingbird. Fresh carcass. Science Building, north facade. October 7, 2021. Afternoon. Cause of death: suspected window collision.

E. Dark-eyed Junco. Fresh carcass. Seattle Academy of Arts and Sciences. October 27, 2021. Afternoon. Cause of death: suspected window collision.

Locations where specimens were found are shown on the map below. Red circles represent specimens we found during structured surveys, yellow are those we found or were alerted to outside of structured surveys. Zoom in to see common names.

What did we miss?

Despite our efforts, I suspect we missed a lot. Volunteers repeated surveys at each building approximately every 24 hours (sometimes repeating as quickly as two hours later, sometimes as late as 30 hours). Birds that strike windows during the intervals between surveys are not guaranteed to remain in place until monitors return. Scavengers like crows, gulls, racoons, and rodents may consume or carry off carcasses, and people may remove and dispose of them. And what carcasses remain when surveyors return are not guaranteed to spotted by surveyors. They may be hidden, camoflauged, inaccessbile, or a surveyor’s attention might be on the other side of the sidewalk. That means the specimens we found represent just those that were not removed by people or scavengers between survey periods and that we managed to find as imperfect detectors. The graphic below illustrates the point assuming 50% of collision fatalities are scavenged or removed between surveys and that moniters are able to detect 50% of carcasses that are present (note: those figures are are derived from the results of our carcass persistence study, see below). If these assumptions hold, we may have missed 75% of the total number of collision carcasses in the area during our pilot season. And it’s possible we may have missed more.

How do we know how quickly carcasses are removed? Or how likely surveyors are to detect carcasses?

To understand how long carcasses tend to remain in the environment, we ran a “carcass persistence” study. We used 15 bird carcasses that individuals had brought to Seattle Audubon, usually after colliding with windows, and that we had stored in our freezer.  These were our “trial carcasses.” Trial carcasses ranged in size and species, from a penny weight Rufous Hummingbird to a quarter pound Steller’s Jay. We then distributed them in our study area. We tried to simulate “natural” collision positions by placing thawed trial carcasses within six feet of buildings under large areas of glass. After placing the carcasses, we returned three times each day for a week to check the status (present/absent) and condition of the carcasses (intact, partially scavenged, decomposing, or gone without a trace). We shared the location, photos, and other details of trial carcasses with collision monitoring volunteers. We asked them to report trial carcasses if they spotted them during surveys and to please not disturb them.

At the end of a week, two thirds of trial carcasses had been scavenged or removed. The remainder began to decay in place. Median persistence time was about 26 hours. Just under half (47%) of carcasses were removed in 18 or fewer hours and 13% were removed in less than three hours. Other studies have found that body mass is an important predictor of persistence time, with larger carcasses tending to persist longer as there are fewer large scavengers to take interest in them. While our dataset is small and the results are not statistically significant, a quick analysis suggests that we may see the opposite effect in an intensely urban environment like Capitol Hill. Large carcasses are more conspicuous and more likely to attract attention of building mangers, maintenance crews, or pedestrians who find them unacceptable and remove them quickly.

When trial carcasses were present, collision monitors detected them approximately 51% of the time. I also struggled to re-find carcasses I had placed and that hadn’t moved. It was eye opening to discover that even knowing where and what to look for, we were still challenged to detect carcasses.

Why? Are dead birds really that hard to see? Compared to live birds, I’d now say they are the hardest birds to see. Live birds give themselves away with movement and noise. They make themselves look bigger by puffing feathers, spreading wings, or raising crests. Their plumage is preened and vibrant. Dead birds are silent, small, and still. Their weathered plumage tricks the eye to seeing a leaf, woodchip, or doesn’t register at all.

This was an important finding for us. In subsequent seasons, we will prioritize developing volunteers’ “dead bird eyes” by using real carcasses during field training sessions. I believe this will help improve detection rates. For the pilot season, we used toys to train volunteers in the protocol and data collection. It was convenient, but ultimately did not help us construct useful search images of real dead and injured birds. Take a look at the two photos below. The top is from a field training session on August 31st. I bet you can spot the bright green, bird-shaped object. But can you spot the Chestnut-backed Chickadee in the next photo? We had a hard time with it.

We will improve the materials we train volunteers with in subsequent seasons. Goodbye, bird-shaped objects! Hello, real birds.

Can you spot the Chestnut-backed Chickadee in the photo? Despite being relatively “out in the open,” it was incredibly difficult to detect in the field. 

So what’s up for the next season?

Next season is going to be even better. We’re taking what we learned from the pilot season and making improvements to the program, including:

  • removing some buildings from the study and adding others outside of Capitol/First Hill;
  • upgrading field training experience and materials;
  • clarifying search methods;
  • improving data collection and data management; and
  • potentially reducing the number of monitoring days in a season by 25%.

This research is powered by volunteers and you can get involved! If you’re interested in learning more about the program and potentially participating in surveys in April 2022, add your name to the interest form and we’ll be in touch in 2022!

Big thanks to all the Seattle Bird Collision Monitors who made the pilot season a success and a good time!

Thank you Adrienne Dorf, Amy Candiotti, Anders Chen, Andy Siegel, Anne Freudenthal, Barbara Mandula, Elaine Chuang, Elizabeth Bacher, Elizabeth Cameron, Hye-In Mary Shim, Jack Pauw, Jessica Schiffman,Karissa Bosshart, Kyle Elfman, Madison Mayfield, Maria Kahn, Megan Elfman, Michelle Flowers, Michelle Hope, Nancy Schutt, Olga Levaniouk, Rachel Lodge, Steven Gary, Tracey Marsh, Wendy Walker, and Yoon Lee!


Carcasses collected through or used in the Seattle Bird Collision Monitoring are done so under Federal Salvage Permit and Washington State Scientific Collection Permit BETTENCOURT 21-152 (exp. 03/21/22).

Joshua Morris

Josh is Seattle Audubon’s Urban Conservation Manager and oversaw the first season of the Seattle Bird Collision Monitoring Project. Josh’s work focuses on reducing urban hazards to birds, protecting and enhancing urban habitat, and engaging communities in conservation right in their own neighborhoods.