19 September 2017 – no casualties

No new casualties this morning, but the Mourning Dove was scavenged.  Just this pile of feathers remains.

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16 September 2017 – Mourning Dove

I found this hatch-year Mourning Dove in front of the main north entrance today.

24 August 2017 – Mourning Warbler and Chipping Sparrow

On the heels of an impressive southbound flight last night,

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. . . I found two casualties this morning.

There was a HY Chipping Sparrow in the northwest alcove.  The bird had evidently been stepped on or perhaps run over by a maintenance vehicle. I left it in place for a removal trial.

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Also, near the main north entrance (actually at a west-facing facade in the corner) was a Mourning Warbler. The bold eyering and long undertail coverts looked tantalyzingly like a Connecticut Warbler. It was, however, an AHY-U Mourning Warbler. The bird was 13.5g and bulging with fat (3).

 

This was the 10th Mourning Warbler on the project.

 

 

 

 

 

 

 

 

 

10 August 2017 – no casualties

Both hummingbird carcasses still in evidence. The whole bird from the southeastern alcove has been decapitated, presumably by the ants.

The tail from the southwestern alcove made things a bit more interesting by being gone.  I wasn’t too surprised by that because we had storms roll through overnight that I assumed would have blown that little bit of feathers away.  So I started looking around just to see if I could figure out in which crack in the bricks it ended up.  I couldn’t find it, but my more intensive searching did turn up these tidbits:

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Aha!  So it looks like yesterday’s tail was not necessarily from a hummingbird that had been scavenged.  It looks more like a lawnmower got it. It also seems to have been a HY male, Ruby-throated Hummingbird.

9 August 2017 – 1.33 hummingbirds

Hummingbirds have been takin’ it in the shorts of late . . .

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But what’s with the fractional hummingbird reference above?  Well, “scavenging” and “removal” are not necessarily the same thing. Often I find the remains of a bird that has been scavenged, but there’s enough left behind that I am able to detect that a casualty has occurred. Those remnants – let’s say a pile of flight feathers left behind by a scavenging cat – can last in place for many weeks.  Each time I survey for casualties I’m greeted at that spot with the information that a bird died there (or at least somewhere nearby) even though all the edible bits are long gone. We should be clear that scavenging isn’t the important rate to help us better estimate detection of window-killed birds, removal is.  It’s only when every piece of that carcass is gone that I’m unable to detect the evidence of a casualty.

We see this played out all the time with hummingbirds.  For some reason, the ants seem to get to the hummingbirds more quickly than they get to other birds.  (I have some hypotheses about this, e.g., that hummingbirds are more often flying in daylight and might have lain in place longer than the night-flying warbler and sparrow migrants.) The result is that hummingbird carcasses are rarely in pristine shape when I find them, but they are still there.

Check out hummingbird #1 this morning from the southeast alcove.  First, look how obvious it is to detect the carcass even from ~20m away.  Up close, note that a good bit of her face has already been carted away, one ant-mandible-sized piece at a time.

 

 

This next one (or 0.33 of one) from the southwest alcove will put your observational skills to the test:

See it yet?

 

 

Oh, that explains it!

 

 

In this case, there’s a pre-scavenged hummingbird (likely also AHY-F Ruby-throated) that is represented by its tail only. But it’s still detectable and largely identifiable.

(UPDATE 10 AUGUST 2017: Today I found a head and wing near where I found the tail on Aug. 9. First, this looks much more likely as a HY-M than an AHY-F.  Also, literal scavenging seems less likely.  I suspect more that the carcass was chopped up by a lawnmower.)

16 April 2017 – Mourning Dove scavenged

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.