27 August 2019 – Mourning Dove

I found an immature Mourning Dove at the northwestern alcove this morning.

31 July 2018: end of season wrap-up

Well, here we go. Today marks the end of my 9th year conducting spring/summer monitoring for window-killed birds at the Noble Research Center. Tomorrow I begin year 10. Ten years of near daily monitoring of window-killed birds. Here’s a quick 9-year wrap-up:

  • 40: average minimum casualties annually
  • 360: total casualties (minimum)
  • 64: species confirmed as fatalities
  • 10: average number of days for birds to be removed/scavenged

 

Top ten (eleven) species most commonly encountered as casualties at this site:

  • Lincoln’s Sparrow (45)
  • Ruby-throated Hummingbird (29)
  • Painted Bunting (24)
  • Indigo Bunting (20)    *tie*    Grasshopper Sparrow (20)
  • Mourning Dove (17)
  • Clay-colored Sparrow (16)
  • Nashville Warbler (14)
  • Common Yellowthroat (11)    *tie*   Mourning Warbler (11)  *tie*  Song Sparrow (11)

 

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9 July 2018 – Mourning Dove remains, and another note on scavenging and removal

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.

 

16 September 2017 – Mourning Dove

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

Spring/Summer 2017 was busy

As I’m about to head out for a conference this week, spring and summer monitoring comes to a close.  I’ll begin August 2017 the 9th consecutive year of (mostly) daily monitoring for window casualties at the Noble Research Center on the campus of Oklahoma State University in Stillwater, Oklahoma, USA.

It’s been a busy spring.

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Beginning Mar 1st, here’s what has turned up at the Noble Research Center.

Dead Birds

  1. Indigo Bunting – 5
  2. Painted Bunting – 5
  3. Ruby-throated Hummingbird – 3
  4. Lincoln’s Sparrow – 2
  5. Mourning Dove – 2
  6. Nashville Warbler – 2
  7. Orange-crowned Warbler – 2
  8. Baltimore Oriole – 1
  9. Chipping Sparrow – 1
  10. Eastern Meadowlark – 1
  11. House Wren – 1
  12. Northern Parula – 1
  13. Tennessee Warbler – 1
  14. Yellow-billed Cuckoo – 1

That’s 28 individuals of 14 species, and damn, that is disheartening.

On the plus side, my commitment to checking almost every day has put me in position to save a few birds by getting them safely away from the building and taking them someplace secure to rest and recuperate for a bit. I can’t guarantee that all 6 of these survived the ordeal, but they seemed to be in good shape when I last saw them:

  1. Northern Cardinal
  2. Common Yellowthroat
  3. Mourning Dove
  4. Song Sparrow
  5. Yellow Warbler
  6. Carolina Wren

 

 

6 May 2017 – Baltimore Oriole, Mourning Dove, and House Wren

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 . . .

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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.

 

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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.

13 April 2017 – Lincoln’s Sparrow

The Mourning Dove was still there this morning, but it has been disturbed a bit and is now on its back.

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New this morning was an unfortunate Lincoln’s Sparrow at the main north entrance to the NRC. As is so often puzzling, this was a bird that had to have been traveling south to hit the glass there even though the net movement of Lincoln’s Sparrows in April in Oklahoma is north.

This bird had 0 fat, was of indeterminate sex, and looks to be a SY.  Note trauma to the bill tip indicating the point of collision.

5 April 2017 – Mourning Dove

This poor waterlogged Mourning Dove is the 13th MODO casualty and, unofficially, the 274th victim of collision at the Noble Research Center.  I left it in place for the first scavenging trial of 2017.

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7 August 2014 – trapped Grasshopper Sparrow and local Mourning Dove

Screen shot 2014-08-07 at 10.37.11 AMI’m quite conservative when it comes to Mourning Doves around the Noble Research Center. This is one locally breeding species that can sometimes be found just hanging out near the building but not really trapped by it as our migrants often are. Our recent fledglings (“local” birds in bird-banding parlance) seem sometimes to seek out these protected places until they become stronger fliers and when i find them thus I almost never document them as “trapped.”
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Almost never. Today, I found one of these local Mourning Doves that flew up and away from the north entrance and right into a window in the southeast alcove. Then it flew back toward the north entrance and banged into the windows there again. In all, I watched hit windows (low-speed crashes, but still) three times before it headed off a bit further from the building and out of what I considered immediate danger. Given its behavior, I’m counting it as trapped. There was a second local bird as well, but at no point did it fly into a window or seem in any way impeded by the structure of the building so I did not count that one as a trapped bird.

A more obvious trapped migrant was the Grasshopper Sparrow I found at the north entrance. It took a bit to get that one out of harm’s way too, but ultimately I watched it take off and fly strongly to a spot about 300m north of the NRC from where I hope it can get its bearings and be on its merry way.
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8 June 2012 – Bewick’s Wren

Today both Mourning Doves remain apparent.  There was also a juvenile Bewick’s Wren that met its untimely end at an NRC window this morning.

 

Like the recent Tufted Titmouse and Red-bellied Woodpecker, this is a resident species that I have never encountered on campus in life so I was surprised to find one in death.  The bird was confusing to age, as it appeared to have a recovering brood patch.  The loose texture to its feathers, accumulated fat, lack of feather wear, and especially its pale yellow gape at the corners of its bill confirm that it is a young bird hatched this year.  Sadly, it will not reach the destination of its dispersal.

6 April 2012 – possibly stunned baby Mourning Dove

I found a fledgling Mourning Dove on the ground in a corner of the NRC this morning, and managed to herd it out away from that spot.  The bird was alert and able to fly and didn’t look to me to be suffering any effects from a window collision, if indeed it had one at all.

24 August 2011 – I killed a Mourning Dove

Ugh.  This wouldn’t be much of a headline for sporting types as dove season is about to kick off, but there’s a bit of sad irony here in my case.

During the unprecedented heat of the summer of 2011 in Oklahoma, I have diligently turned on the water mister in my garden hoses each evening to provide some welcome relief for all my backyard birds and soften the ground for species like robins and mockingbirds. Yesterday, as I approached one of those hoses in anticipation of setting up the mister, I flushed a Mourning Dove from the grass that flew about 20′ directly into the relatively small and screened window of my bedroom.  It didn’t sound like a killing blow and the bird flew off apparently unharmed.  Just a minute or two later, however, I found the poor bird writhing on the ground, where it quickly expired.

A quandary, this. Have my efforts to keep my backyard a little moister than the rest of Stillwater actually saved any birds this summer?  I know now that those efforts led to the death of at least one.  It’s the same with feeding birds in winter. I usually have at least one junco each winter meet its end at one of my windows, but those juncos wouldn’t be hanging around in the first place were it not for the abundant seed I put out for them.  If that seed wasn’t there, would those juncos be spending the winter in some field or woodland instead where the probability of hitting a window would be nil? I know that my bird feeding operation leads to the death of several birds each year at the the talons of our local Cooper’s Hawks, but I just consider that I’m feeding them too.  The occasional window collisions seem a bit more . . . pointless.

When it comes to wildlife management – and make no mistake, attracting birds to our backyards is wildlife management – there are consequences to every action.  The loss of one Mourning Dove is ecologically meaningless in the grand scheme of ecosystem function in suburban Stillwater, except that it was enough to make me think.  I doubt I’ll change much about my bird feeding operation or my decision to make water available, but I might be a bit more careful from now on as I walk through my yard, and maybe I’ll walk closer to the house to flush any birds that might be hidden from my view away from my windows.  Just something to think about.

18 May 2011 – no casualties, but two visitors

We had a line of storms come through last night around 2:00 am that I thought might have the chance to drop out some migrants.  As if on cue, there was something unusual at the NRC this morning: a Marsh Wren.  The bird was very much alive (perhaps a bit stunned), and I herded it away from the building toward a line of shrubs that would provide cover if it needed to stick around for a while.

Also, huddled at the base of one of the windows was a recently fledged Mourning Dove.  This bird too seemed to be in fine shape.

Six-month summary: July–December 2010

Here is just a quick summary of casualties at the Noble Research Center from July through December 2010:


Very fat Mourning Warbler that never made it to the wintering grounds.

I detected 25 individuals of at least 16 species among the casualties. The complete list:

grasshopper sparrow – 4
ruby-throated hummingbird – 2
mourning warbler – 2
song sparrow – 2
Lincoln’s sparrow – 2
unidentified passerine (1 warbler, 1 sparrow) – 2
black-and-white warbler – 1
Carolina wren – 1
mourning dove (juv) – 1
least flycatcher – 1
common yellowthroat – 1
black-throated green warbler – 1
brown thrasher – 1
house wren – 1
red-breasted nuthatch – 1
white-throated sparrow – 1
field sparrow – 1

Because I was able to get to the NRC earlier each day during autumn than practical in 2009, I encountered more individuals that were stunned and “trapped” by the building for some time period without obvious mortal injury. Most of these birds are presumed to have eventually moved on, but it is quite likely that the house wren and one of the Lincoln’s sparrows on the “stunned” list were unsuccessful in their respective bids to escape from the confusion of the NRC, and are listed above. The bat represents the first mammalian “capture” by the NRC:

Lincoln’s sparrow – 5 (4 in one flock)
house wren – 1
common yellowthroat – 1
Nashville warbler – 1
grasshopper sparrow – 1
dark-eyed junco – 1


It’s dark when migrants like this Lincoln’s sparrow drop out of the sky and try to find a good spot in which to rest for the day. I’m beginning to think that most collisions are occurring in that last hour before sunrise.

A Year in the Death – Aug. 2009–Aug. 2010

On August 20th, I completed a full year of regular surveys for window-killed birds at the Noble Research Center. Here are some vital statistics for August 20 2009–August 20 2010:

Effort.
I conducted (occasionally with help from assistants like Danielle Benson) 153 surveys over the full year of monitoring. This equates to an average of 2.39 days between surveys.

Coverage was irregular throughout the year. For example I did no monitoring in January 2010. For this reason, analysis of casualties by season must be viewed through the lens of dissimilarities in sampling effort among the seasons. I defined sampling effort as the number of surveys per days in a given season. I defined seasons as follows: Dec. 1–Mar. 20 (110 days) = “winter,” Mar. 21–May 20 (61 d) = “spring,” May 21–Aug. 19 (91 d) = “summer,” and Aug. 20–Nov. 30 (102 d) = “autumn.” The 11 winter surveys provided an effort of 0.10. This approximates 10 days on average between surveys. Effort indices were 0.87 for spring (53 surveys in 61 days), 0.44 for summer, and 0.48 for autumn. Thus effort was most consistent during spring with near daily surveys.

Data.
I recorded window strike mortality for 38 individuals of 22 different species over the year of monitoring. This rate of collision mortality places the NRC on par with other high-mortality buildings referenced in Klem 1990 and O’Connell 2001.

Of the 38 casualties, 5 were “local” (i.e., recently-fledged offspring of local breeders), 5 were “hatch year” birds (i.e., “immature”, or birds < 1 yr. old), and 28 were adults or of undetermined age.

Of the the 38 casualties, 8 were identifiable as male, 5 as female, and 25 were of undetermined sex.

I encountered more casualties during spring and fall migration periods than in winter or summer (though effort may have played a role in this result):

As in O’Connell 2001, Neotropical migrants in passage comprised the greatest percentage of individuals among all casualties. The birds dying at the NRC are not local residents that commonly occur in the OSU campus. These are transient individuals traveling long distances that just happen to meet their end here. (Note that the number of resident individuals among the casualties is inflated by the fact that 5 of the 7 casualties were recently fledged mourning doves and northern cardinals.)

Non-lethal collisions.
On at least four occasions, I encountered live birds that appeared to be trapped near a window but were not injured from a collision. These individuals are not included in the collision data, but they may have been had I not been there to flush them away from the windows and encourage them to move along:
9/22/09: Grasshopper Sparrow and suspected Swainson’s Thrush
10/19/09: Grasshopper Sparrow
11/2/09: Two Dark-eyed Juncos
6/21/10: Carolina Wren

Scavenging rate.
The scavenging rate proved to be unpredictable over the year. For example, some carcasses left in place remained visible for several weeks and were untouched during that time. Others were identifiable only from feathers left behind of a carcass that, based on the timing of my most recent survey, had been scavenged just a few hours after the bird’s unfortunate collision. Further confounding the interpretation of scavenging rates, some carcasses were scavenged but readily identifiable feathers of the carcass were left behind and still in evidence long after scavenging. For example, the Yellow-billed Cuckoo that I found on 6 June 2010 was scavenged on 15 June. As of 3 September 2010, that bird’s primaries are still readily apparent at the location where I first found it on June 10th, approximately 86 days after the bird’s death.


Scavenged Swainson’s Thrush.

I found evidence of 15 scavenged carcasses over the year. Thus, out of 38 total casualties, 39% were ultimately scavenged. (Of course, I collected the majority of the carcasses I found, most of which were fresh and in excellent condition and were photographed for entries in this blog. Presumably, a high percentage of these would ultimately have been scavenged.) Two carcasses were scavenged on day 0 and three were scavenged after just one day in situ. Four were in evidence for at least 30 days; the average number of days a carcass was apparent in place was 18.6 – nearly 8 times the length of time between consecutive surveys. Thus, the regular, frequent surveys provided ample opportunity to discover carcasses before they were removed or no longer visible. In addition, the most frequent condition of freshly scavenged carcasses of warbler-sized birds was a pile of remiges cleanly sheared off near the base of the feathers. Thus, even small birds were usually left in place after scavenging; this increased the probability that I would find the carcass even if it had been scavenged. Nonetheless, it is likely that at least some individuals were scavenged and removed from the site before I could document the casualty, so mortality rates calculated from my surveys must be viewed as underestimates of actual mortality.

Total data.
The following table lists all the species found as window collision casualties at the Noble Research Center, 8/20 2009–8/20 2010:

species number
Common Yellowthroat 5
Mourning Dove 5
Lincoln’s Sparrow 4
Black-and-White Warbler 3
Grasshopper Sparrow 3
Painted Bunting 2
Mourning Warbler 1
Gray Catbird 1
Wilson’s Warbler 1
Canada Warbler 1
Indigo Bunting 1
Orange-crowned Warbler 1
Song Sparrow 1
Nashville Warbler 1
Dark-eyed Junco 1
Sprague’s Pipit 1
Swainson’s Thrush 1
Orchard Oriole 1
Northern Cardinal 1
Yellow-billed Cuckoo 1
Ruby-throated Hummingbird 1
Carolina Wren 1

O’Connell2001

2 August 2010 – immature Mourning Dove (scavenged)

I found more scavenged remains this morning. This time it was a young Mourning Dove. I guess the apparent spate of scavenging illustrates how weeks can go by with no evidence of scavengers (removal rate effectively = zero) and then – bam! – some cat takes up residence near the building and starts patrolling for easy meals.

17 May 2010 – another baby Mourning Dove

There were two hanging around in the bushes near the NRC yesterday, accompanied by their nervous parents. One flushed ahead of me, slowly and weakly flying like juvenile birds often do, and plopped into one of the windows ~75m away. It was unclear to me if these birds had already hit the windows and damaged themselves, or if they were just new at this whole “flying” thing. Either way, I was concerned that they were quite vulnerable to predators.

This morning I found the scavenged remains of at least one of the babies. My observations yesterday make me suspicious that this could have simply been a predator catching one of the slow little squabs – no window necessary. Had I not observed them yesterday, however, I would certainly have attributed the pile of feathers to a scavenged window-kill, so that’s what I’m doing.

Perhaps the most interesting thing happening this week is that we apparently have a predator hanging around the NRC and snatching some easy meals.

27 September 2009 – 1 Mourning Dove

OK, so I guess some resident birds get whacked at the NRC too. This morning is was a still-warm, juvenile Mourning Dove that lay limp on the sidewalk. I decided to leave the specimen as a test of how long the carcass would remain visible, given a known time of death. In my estimation, that time of death was about 7:00 am. I moved the bird away from the entranceway to a spot about 20′ from where it hit, and about 1′ from the building.