I was out of town from 21–30 June and no surveys were run during that time. On June 30th, however, I heard from Dawn Brown and Corey Riding that there were three casualties at the southwestern alcove of the Noble Research Center: a badly decayed Northern Parula (adult male), a female Ruby-throated Hummingbird, and a female (with brood patch!) Indigo Bunting. It’s possible that the bunting came in on the 30th, but the others were clearly killed prior to that date. (Photos by Dawn Brown.) This is officially the first Northern Parula found on the project.
May 15th was another odd one, and I’ll be glad when this pulse of window-killed migrants is passed.
On my morning survey, I found a SY male Painted Bunting at the southwestern alcove, and right in front of a treated pane. (The bird off to the left is May 12th’s Indigo Bunting.)
That’s bad enough. The building cost a Painted Bunting and the ABC bird tape apparently did not steer it away from danger.
Then I heard from Dawn Brown later in the day (~3:45 in the afternoon) that she had found and collected a Painted Bunting at the same location. When I got there moments later, the Indigo Bunting was gone (so it was removed sometime during the day on the 15th), and Dawn handed me a bag with this bird inside:
Ugh – a second dead Painted Bunting. This one was more difficult to sex but also clearly an SY bird. Note the beak damage on both individuals.
Today was one of those “just when I think I have this figured out” days.
As I was rounding the west perimeter of the Noble Research Center between the southwest and northwest alcoves, some feathers caught my eye up against the brick side of the building. This is the first time (in nearly 8 years) I found a bird at this spot and it was also pretty clearly one new to the study: a bright orange and black Baltimore Oriole, or at least a nice pile of feather remnants from what had lately been an adult male (ASY) Baltimore Oriole.
Though for consistency’s sake I’ll record that spot on the building as the location of collision, I in fact don’t know where the bird hit. All I know is that a predator (and very likely a cat based on the neatly sheared primaries) appears to have eaten said oriole at that spot.
Around the corner and into the northwest alcove, I found the remnants of a scavenged adult Mourning Dove. Here again was a bird in a very odd location. Strangely enough, the bird was in the exact location (beneath an ornamental buttonbush) where collaborator and OSU PhD student Corey Riding had the week before left a Cedar Waxwing carcass for a scavenging trial. Corey, however, had left neither a dove, an oriole, nor anything else at that spot since the waxwing. Puzzling for sure . . .
Finally, there was another bird at the end of the alcove in front of one of the untreated panes. Here was another oddity – a House Wren.
No new casualties today, but I noticed immediately that the Mourning Dove carcass had been removed. Closer inspection revealed it to have been scavenged from its original location with remains scattered near the base of the building about 5 m away.
So what is scavenging rate all about, anyway?
The idea is that our detection of dead birds (or anything else) is imperfect. We can collect data and report that, for example, 50 birds died at a building. That estimate can only be a minimum, however. Our raw counts underestimate the true number of casualties because our detection cannot be > 100% but it can be far lower than 100%. Birds can collide but manage to flutter away and die outside of our search area. Some might be difficult to see against the substrate on which they land. Most important, some will be removed before we get there to find them. Cats, rats, opossums, raccoons, crows, etc. tend to be abundant in urban/suburban areas where most window collision research takes place and they can often remove a carcass before the investigator arrives onsite to conduct a survey.
For example, assume that the removal rate (whether by scavengers, human maintenance crews, etc.) is 25%. This means that, at best, the investigator is only predicted to encounter 75% of the casualties. That raw count of 50 dead birds? The detection-corrected number is actually closer to 50/0.75 = 67 dead birds.
Does that matter, though? I struggle to attach relevance to what the removal rate is for any given study. Is there some magic number of casualties that is a threshold for conservation action? Are there people for whom 50 dead birds wouldn’t register as important but 67 would? For comparing mortality rates among sites where removal rate might vary we assume that it is important to determine a separate removal rate for each site, but is it? Imagine 50 dead birds at our site with high removal of 25% compared to 50 dead birds at a site with low removal rate of 5%. That’d be 67 compared to 50/0.95 = 53. So? Would we really be concerned about 67 dead birds at one building but not 53 at another?
My final concern is the false sense of security that we’ve determined “the” removal rate. These rates are widely variable across space and time. We’re kidding ourselves to think that we’re improving our estimates of collision mortality by adjusting raw counts with a detection probability that is itself a moving target.
In my study, I’ve conducted approximately 86 removal trials over the past several years. On average, a carcass lasts about 10.5 days on the ground before it is removed. On average, I conduct a survey every 1.5 days. That gives me 10.5/1.5 = 7.0 opportunities to find a dead bird before it is removed. Ergo, removal rate is hardly noticeable in my study. Whatsmore, scavenging and removal are not the same thing. It is often the case – as with today’s Mourning Dove – that the carcass is scavenged but evidence remains. The Mourning Dove died on April 5th and was scavenged on the 15th. That’s 10 days. The remaining bones and feathers, however, might still be here weeks from now. On multiple occasions, I have found evidence of scavenging in the 24 hrs since my previous survey. For example, I check one morning and find feathers that weren’t there the day before. I refer to these as “day 0” removals, but the feathers are still there to provide evidence of the casualty for days and weeks after the event. The longest I have had feathers or other remains in evidence is > 90 days.
So I see scavenging and removal rates – and detection rates in general – as red herrings in our monitoring of collision mortality. Unless part of a well-controlled design to compare, for example, mortality from two facades of the same building, there’s not much to gain from collecting data to estimate such rates. There are, however, potential costs. Many avocational birders and conservationists collect data on collisions opportunistically, and their presumed lack of rigor in methods limits the use of their data for serious analysis. I maintain that those data are perhaps far more useful than we might presume because of an ill-defined obsession with calculation of detection as a study’s ticket to the club of legitimacy.