27 September 2010 – no casualties; hummingbird scavenged

The hummingbird that I found on 21 September has been scavenged. A single rectrix remains. That’s interesting in itself: A tiny hummingbird falls victim to a window collision, ants attack the body quickly, but it lasts 6 days before being removed. Even after being scavenged, however, a single feather alerts me to the event. The identifiable cuckoo feathers from June remain in place; I wonder how long this lone hummingbird feather will be in evidence.

25 September 2010 – Brown Thrasher

The Brown Thrasher is one of the most common and conspicuous migrants here in central Oklahoma, so I suppose it was inevitable to find one at the NRC some day. The bright yellow iris reveals this bird to be an adult (AHY). It was still warm when I found it.

I scored the fat on this bird a “1”, so it was a migrant (they breed here somewhat sparingly). What I don’t know is if this is one of our local breeders making its way south, or a bird from say, Alberta, passing through. It’s sad either way.

Hummingbird and cuckoo still in evidence.

14 September 2010 – Mourning Warbler

Yet another Mourning Warbler has met its end on one of the west alcoves of the Noble Research Center. This one is a hatch year bird of undetermined sex (though the faint orangey wash to the yellow on the upper breast has me thinking it’s a male). This bird was also in prime condition for migration: fat score = 3.

I can’t tell if the damage to its right foot was caused by one catastrophic bump against the glass or if this was one of the poor birds who struggled against the building for hours before exhausting itself. The latter scenario is unlikely given the amount of fat the bird still had.

12 September 2010 – Black-throated Green Warbler

I added a new species to the list of collision mortalities at the NRC this morning, and another bird I’ve yet to find in Oklahoma while out birding: Black-throated Green Warbler.

This bird was an adult male and in very bright plumage, with a fat score = 1.

For all I know, this bird spent the spring and summer delivering his “zee-zee-zee-zee-zoo-zee” from within the dark canopies of mature trees in the northern forests, only to be snuffed out in an instant at a modern building of stone, steel, and glass in a small city in the prairie. It ain’t right.

10 September 2010 – no casualties, but . . .

I did find a trapped immature Mourning Warbler in one of the west side alcoves of the NRC. The bird was on the ground when I found it. It wasn’t panting, but did look tired. My disturbance cause it to flutter against the glass before turning around and heading back out the way it came in. It took 2 or 3 tries of me shooing it away from the building before it finally flew almost completely vertically up and out over the roofline.

It’s possible this bird would have survived on its own so I won’t count it as a casualty. But it certainly was vulnerable to predators, and it might have a tough time today finding a safe place to rest and refuel.

Is bird-safe glass on the way?

An astute reader directed my attention to an 8/28 article by Anne Eisenberg in the New York Times online Business Day section. The article describes the use of a new glass product developed in Germany, Ornilux, that holds promise for reducing window collisions because it is visible as a barrier to birds – its visible pattering, however, is largely invisible to humans, so the product could become popular for sustainable designs of homes and businesses.

7 Sep. 2010 – Least Flycatcher and Common Yellowthroat

I found two unlucky young migrants this morning, a Common Yellowthroat and a Least Flycatcher. Both were hatch year birds with fat scores of 1. The yellowthroat was a male; the sex of the flycatcher is unknown.

While the yellowthroat was easy to identify, the still-warm Empidonax flycatcher presented a challenge. I used Sibley’s Guide to Birds and Kaufman’s Advanced Birding to nail it down.

The bird is small (total length ~ 132 mm or 5.33″) and it has a broad yellow-orange bill with a diffuse dusky tip. It has complete eyerings that do not flare behind the eye. The primary projection is relatively short (0.18 the length of the wing chord). The belly and undertail coverts are a creamy yellowish-white – this area is separated from the grayish-white throat by the olive-gray breast band. All of these features are congruous with Least Flycatcher. The buffy wing bars and tertial edges confirm it as an immature bird. Because this is a new species for the NRC monitoring program, here’s the range map from the Cornell Lab or Ornithology’s All About Birds:

3 September 2010 – two warblers

Last night (Thursday), our recent intense heat and humidity (108 heat index on Sep. 1) broke in a major way with a powerful gust front and storm system. A straight line wind speed of 87 mph was recorded in Oklahoma City. Overnight the temperature dropped considerably, and temperatures were in the upper 50s with a north breeze this morning. That weather seems to have finally brought down some migrants from the North.

I found two birds this morning, and both represented identification challenges. The first was an immature Mourning Warbler:

This bird’s throat was actually pretty grayish-white, rather than the obvious yellow one might expect on this species. Thus, I considered very carefully whether it might be a MacGillivray’s Warbler. But the broken eye-ring on this bird was barely broken, extending closer to the “corners” of the eye than typical for MacGillivray’s. Also, the undertail coverts extended very near to the tail tip, as expected for Mourning. Thus despeite the lack of a clear yellow throat, I’m confident calling this bird a Mourning Warbler.

I’m also confident in describing it as a fat score = 3. The bird’s fat completely filled the furcular hollow and extended over a large portion of the breast. The fat can be seen in the following photo right through the bird’s relatively transparent skin. The breast skin of a bird without a fat deposit would look sort of brick red in color, kind of like dark meat on a chicken. On this bird, the off-white color of subcutaneous fat is obvious:

The next bird was represented only by a few remiges:

These were smaller than the primaries on the Mourning Warbler, and rather dark gray with just a slight edge of olive green. The size, color, and lack of pattern indicate “warbler” to me, but it will have to be listed as “unknown.”

In this case, both birds hit the building after it stopped raining last night (~ 9 pm), judging from the excellent condition of the feathers. One was scavenged in that time – but still obvious to me – and one was untouched. The specimens were found about 7m from each other on the north side of the building.

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