22 May 2016 – 2 Swainson’s Thrushes

Northwest alcove.  The only thing sadder than the sight of these two birds is the realization that they were probably traveling together.

15 May 2016 – Swainson’s Thrush

Whilst I was traveling this weekend, Corey Riding took over monitoring at the NRC for me.  On Sunday the 15th, Corey found this Swainson’s Thrush in the northwest alcove. For those keeping score, it’s 20 August 2009–30 April 2016: 2 Swainson’s Thrushes, and 1 May 2016–15 May 2016: 4 Swainson’s Thrushes.

Screen Shot 2016-05-17 at 3.58.48 PM

Screen Shot 2016-05-17 at 4.19.03 PM

 

10 May 2016 – Swainson’s Thrush

I found this ASY male Swainson’s Thrush this morning in the southwest alcove.

 

Note the “booted” tarsus.  On thrushes the tarsus is smooth, i.e., without a lot of obvious scaling. It’s sort of like the leading edge of the tarsus is wrapped in one huge scale.

Screen Shot 2016-05-10 at 3.35.21 PM

The Painted Bunting lasted 2 days at the main north entrance; it has been removed.

3 May 2016 – another Swainson’s Thrush and a stunned Summer Tanager

Update: I got a call about a “cardinal” that had struck the southeast alcove window at the NRC around 1:00 pm today.  The bird was in fact a gorgeous ASY male SUMMER TANAGER (and me without my camera).  I was a bit concerned that it was on the ground (a well-meaning woman was offering it some water) and that it let me grab it pretty easily. The bird was pretty feisty, however, and when I took it to the opposite side of the building to see if it could perch on its own among the row of oaks there, it took off and strongly flew up into an adjacent tree.  It’s still dusting off the cobwebs as they say, but when I last saw it the bird was perched strongly about 20′ up in a sturdy red oak.

I’ll count this one among my stunned/trapped victims, and I’ve amended the map below accordingly.


In the last few days, the number of Swainson’s Thrushes killed by window collision at the Noble Research Center has doubled from 2 to 4. This one was an after second-year bird with fat = 1.

It’s really odd how predictably unpredictable window collisions can be.  In this case, one of the most abundant migrants through our area has only rarely fallen victim to the building I monitor – despite it being a fairly common window-kill in spring at other Stillwater buildings. I’m in my 6th year of near daily monitoring for casualties at the NRC, and during that time I’ve documented Swainson’s Thrush . . .

Is it just happenstance that two Swainson’s Thrushes are killed within a few days of each other in 2016 when the previous two records were 5 years apart?  Do the now 3 birds from 2015–2016 indicate that something has changed compared to previous years of monitoring? Do the two birds at the end of April/beginning of May in 2016 indicate that the primary movement  of Swainson’s Thrush is a week earlier than typical? My sample is, of course, much to small to help answer such questions, but it is questions such as these that keep me going day after day and year after year . . .

Screen Shot 2016-05-03 at 2.28.19 PM

1 May 2016 – Swainson’s Thrush

The longest stretch without a casualty in 6 years was broken this weekend, with the unfortunate Swainson’s Thrush below the first confirmed window kill at the Noble Research Center since 19 November, 2015.

Screen Shot 2016-05-01 at 12.23.31 PM

If you were in need of evidence to convince you that it is healthy individuals that succumb to window collisions, check out the fat deposits clearly visible in the furcular hollow and on the belly of this bird.  This bird was in the prime of life.

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

11 May 2010 – two scavenged

This morning on my route to check for casualties at the NRC, I found a tuft of mostly contour feathers that had been cleanly sheared from their owner: At some point in the past day or two (I didn’t check yesterday), a bird hit a window and was scavenged before I could claim it. Not far away (same side of the building, ~ 20m away), I found a primary feather from a different bird that had also been scavenged! So we had two birds hit, and both were scavenged, in the past couple of days.

I am quite confident that the tuft of contour feathers came from a PAINTED BUNTING. The edges of the feathers are green, not olive like we might see in any number of species, but clearly green. Other than parrots (of which we have no naturalized species in our area), it’s really just Painted Bunting that could have left feathers like these.

The primary feather has a warm, brown edge to it – almost rusty. Given its size and color, likely candidates include Great Crested Flycatcher or one of the Catharus thrushes, of which Swainson’s is moving through in numbers these days.

Feathers of birds scavenged at the NRC, 11 May 2010.

I had pretty well convinced myself that the primary feather came from a Swainson’s Thrush, when on 12 May I discovered a window-killed Swainson’s Thrush at another building on campus, the Kerr Residence Hall. Looks like the right size, shape, and color to me:

Swainson's Thrush window-kill from Kerr Residence Hall, 12 May 2010.

I found a third Swainson’s Thrush window kill at Ag Hall on Friday, 14 May. So that’s THREE Swainson’s Thrush casualties on the OSU campus in four days, all found in one rather small corner of campus.

Now there are a lot of Swainson’s Thrushes in the world, and they’ve got a broad breeding distribution across the great boreal forest of the Nearctic. But this is what I’m talking about when I say that long distance migrants face greater risk from window collision mortality that short-distance migrants and residents. In less than a week, one guy found three window-killed Swainson’s Thrushes in an area of about 6 ha in one unremarkable little corner of Oklahoma. How many were possibly killed overall this week? Considering the broad front of migration this species is using this week, is the number dozens, hundreds, thousands?