I found an immature Mourning Dove at the northwestern alcove this morning.
I found a pre-scavenged Mourning Dove at the main north entrance today. This is another great example of the difference between scavenging and removal.
The carcass was scavenged even before I found it. The whole point of working to determine scavenging rate is a matter of detectability, i.e., that our raw counts will always underestimate mortality because some carcasses are scavenged before they can be found. But scavenging isn’t the issue per se, removal is. If the carcass is scavenged but not completely removed, then it is still detectable. Therefore, the act of scavenging was irrelevant to my ability to detect the carcass, and thus the event.
We can do some field trials with known specimens and determine that our observers detect, for example, 95% of the carcasses in their search area. We can also do removal trials by setting out specimens and determining what proportion of them are removed in a set period of time. For example, let’s say 25%.
If we do some window-collision monitoring and find 10 dead birds at a building, we can modify our estimate according to our imperfect detection rate: 10.00/0.95 = 10.53. That’s the detection-adjusted estimate of mortality. The removal rate of 0.25 suggests that another 2.5 carcasses were removed before they could be detected (or at least before 95% of them could be detected). Removal rate bias then bumps our estimate from the raw count of 10 to an adjusted count of 12.5. Factoring in the detection rate on that estimate increases our adjusted mortality to 13.16 from the raw count of 10 carcasses we actually found.
This matters naught if our objective is to highlight the total number of casualties. It’s is not the case that public outcry to help solve the problem of window collision mortality with be louder for 13.16 casualties than it is for 10. For comparisons among studies, however, it is important to have this information presented and standardized. If, for example, two sites are compared according to their respective landscaping or lighting influence on mortality, that analysis would be corrupted if there was an unaccounted stark difference in removal rate between the two sites. So it is important to quantify rates of detection and removal in monitoring so that our efforts can be of greatest use.
In this long-term monitoring project, I have approached removal rate differently. I leave some carcasses in place to determine for how long they are detectable. Some are removed before I ever find them; some are immediately scavenged but not removed so I can detect them for weeks after the event. Some are never removed and their feathers and bones can still be detected months afterwards. On average, carcasses in my my study last about 10 days on the ground, and I conduct my surveys every 1–2 days. This means that, on average, I have 5–10 opportunities to detect a carcass before it is removed.
That’s pretty good.
No new casualties this morning, but the Mourning Dove was scavenged. Just this pile of feathers remains.
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.
A powerful cold front with rotating bands of heavy rain moved into the Plains this weekend, leaving Stillwater waterlogged with nearly 6″ of rain. Snow accumulated in the Panhandle and, here in central Oklahoma, icy winds from the South kept temps from rising much higher than the low 50s F.
Several of us got to experience this weird watery weather all day long, as we braved those elements to participate in the Audubon Birdathon Big Day. Bands (squalls, really) of cold rain and occasional sleet swept through every 30 minutes or so throughout the day. It was cold, it was wet, and the Cimarron filled its banks and then some. Shorebirding was excellent, but the birds were in flooded fields as there certainly wasn’t any exposed mudflat for foraging.
We ended the day with a quite respectable 126 species, but it was a tough day for many of these birds. There were a few grim reminders that hard spring weather can rapidly turn deadly:
I’d been following this Mourning Dove that had nested in last year’s robin’s nest near the main north entrance of the Noble Research Center:
That sturdy adobe nest blew out of its tree in the wee morning hours of 4/29, and mama Mourning Dove wasn’t quite ready to give up on her eggs in the afternoon.
We found 14 (!) Soras at one site on the 30th, and a few days later I found this unfortunate one that died in a window collision at Eagle Heights Baptist Church:
The saddest case, however, had to be this one: Carolina Chickadees in one of my nest boxes were trying to fledge during that horrible weather on the 30th. A few of them made it – or at least made it outside the box and were promptly snatched up by my local Cooper’s Hawks – but at least one did not. A few days later, I checked the box to find these contents – a dead nestling and one of its parents, presumably the mother. Near as I can tell, the adult was in the box brooding the youngster and they both succumbed to the elements (or were killed but not retrieved by the Coops). I’ll never know the real cause of death, but either directly or indirectly, the rain squalls of Apr. 30th seem to have played a role.
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.
Regular readers (are there any?) will note my absence over the past couple of weeks. I was on the road, in part to deliver a presentation on this research to the joint annual meeting of the Wilson Ornithological Society and the Association of Field Ornithologists. I was very lucky to have Dr. Scott Loss keep an eye on the Noble Research Center for me. He reported something new: a pair of trapped Scissor-tailed Flycatchers on June 1st. This is a new species for the study, despite the abundance of scissortails here in Stillwater. (Both were shooed away from the building and flew off strongly.) Scott also found a recently fledged Mourning Dove dead at the northwestern alcove on June 2nd.
On my return, I found a scavenged carcass of an adult Mourning Dove at the north entrance on Wed., June 11. I have conservatively recorded that bird as having died on the previous day (6/10), and its remains were still in evidence this morning. That’s it, you are now up to date with Avian Window Kills.
Yesterday’s Mourning Dove had vanished without a trace today, so it lasted 1 day. I actually suspect that it was human intervention that removed it because predators so often leave feathers behind when dealing with Mourning Doves. This is one of those cases in which I would’ve missed the event completely had I not been here within 24 hrs of the collision.
I guess I spoke too soon yesterday: There was a beautiful, adult Mourning Dove dead along the south wall of the NRC this morning. Mourning Doves nest on and near the Noble Research Center, and I had a live bird very near this location yesterday.
I have a photo but the bird sustained a rather gruesome injury so I’ll not post it. I left the bird in place (after moving it about 1m out of the middle of the sidewalk) to conduct a removal trial.
It’d been a few days since I had checked the Noble Research Center, and we’d had a few storms during that time. June 1st was actually just a plain, old rainy day – very unusual. Given the unusual weather, I figured there might be something waiting limply at the base of a window. I did not, however, anticipate that it would be a resident bird: an adult Mourning Dove:
Given the state of decomposition on this bird, I will assume that it died at least one day ago. It might have been longer, but I will consider for my analysis that it died on June 2nd.
Next, I was surprised by a window-killed Black-and-white Warbler. This was a bedraggled and somewhat decomposed 2nd year female, and for my records I will also consider June 2nd as the date of death. This is at least the second time that I’ve found a Black-and-white Warbler in June. What to make of these birds? It’s well past the normal spring migration for this species (March and April here in Oklahoma), and late enough that this movement could be post-breeding dispersal.
Here is a range map for Black-and-white Warbler:
While they breed far to the north in Canada, they don’t even range into Alaska. I wouldn’t be too surprised to find something that breeds way north like a Wilson’s Warbler at this time of year, given that some of its breeding range might just now be productive for breeding. So it doesn’t make sense – at least to me – that there should be migrants streaming north this late in the season.
Check this out though – frequency of Black-and-whites in eBird checklists:
Well there it is. You see two distinct peaks of the species appearing on checklists that obviously indicate spring and fall migration, respectively. But where we might expect a steep drop in reports at the end of May, there’s actually a little bit of a bump in early June. Now that could just be people finding them on territory where they breed, but it could also reflect movements of Black-and-whites that cause them to be detected in other places as well. Given that we have found fully fledged Black-and-whites as early as 28 April, I’m suspicious that there is a bit of post-breeding dispersal that I might be experiencing with these window-killed birds in June. Anyway, here’s the bird: