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.

22 September 2009 – no casualties, but . . . .

Two stunned birds by the windows this morning.

We had a powerful cold front go through yesterday, such that it was only 60 degrees (F) this morning, with clouds, sprinkles, and a northwest wind. It’s been months since we’ve seen weather like this.

So, dark and dreary as it was this morning, I didn’t notice the first bird until it flew. All I can say is that it was a songbird, about 6″ long, with an olive-brown back and pale yellowish breast. The bird flew in front of me, hit a window at slow speed, turned around, almost hit another window, and finally made its way out of the canyon of windows on the west side of the NRC.

The second bird, a Grasshopper Sparrow, looked a lot more alert and was flying normally. It was clear that this one had also hit a window, however. When I first found it, it was on the ground in the open and pointed at a window about 6″ away. As I drew to within about 20 feet, it flew back away from the window and into a little bench area with some ornamental trees and shrubs. There is perched beneath a shrub, and eventually let me get to within about 15 feet. It otherwise looked bright and alert, so I left it there.

These two birds illustrate very well how a non-lethal window collision can make birds vulnerable to predation.

Do I miss any birds on my counts?

Of course.

Any raw count of individual birds, bats, etc. that die from colliding with some structure is a conservative estimate of the actual mortality at that structure. Here’s why:

1) I could just plain not see the carcass during my check. Maybe it’s hidden by some vegetation, someone has kicked it under a bush, or the animal landed outside my search zone, say 10m from the building as opposed to the 2-3m zone I’m checking. Thanks to some of Dan Klem’s excellent work, we also have an idea that about 50% of the birds that strike windows but manage to fly away only temporarily stunned will also die, and potentially at quite a distance from where the collision took place. In fact, many bird deaths attributed to cats may actually be made possible by stunned birds that have hit windows.

2) Some predator has scavenged a carcass before I get there to record it. Cats, raccoons, opossums, skunks – any of these common predators would be happy to snatch a dead bird lying conspicuously on a sidewalk near a building.

3) Some human – perhaps someone doing a study like mine – has “scavenged” a carcass before I get there to record it. Groundskeeping crews do occasionally remove dead birds from sidewalks as well.

All of this means that if I count 50 dead birds around a building over the course of an observation period, that 50 is some fraction of the real total number of birds that died from colliding with that building during that period. Okay, but is 50 a large fraction or a small one?

I could calculate observer detection rate by having a partner place dead birds around the building and then going through my normal routine to determine how many I miss. Let’s say that I have a high detection rate of 95%, which is likely for my building because I have easy visual access to the entire perimeter. I would then modify my estimate of 50 by adding 5% (2.5) to my raw count. OK, so now we think that 52.5 birds died.

We can also test removal rate by putting out carcasses, chicken wings, etc. and seeing how many are removed over a set period of time. Let’s say that 25% are removed. We now add another 25% (13.125) to the estimate and conclude that 66 individuals died at the location, even though only 50 were actually counted.

But there are some problems with making these estimates. The first is that they greatly increase the complexity of gathering the data.

I’ve actually been a long-time opponent of Herculean efforts to quantify “removal rate” of carcasses around things. We invest a huge amount of time, money, and effort to do things like improve our accountability in detection a few percentage points. The real story is not whether 50 or 66 birds died by colliding with some structure, it’s that a lot of birds died by colliding with the structure. That’s the message that needs to get out; estimating that there were 16 more casualties than actually counted isn’t going to change anyone’s mind about the severity of the problem. What’s more, in cases in which detectability is low and estimating things like removal rates bumps up the estimate a great deal (say, 10 carcasses found but 100 estimated deaths), the public can become dismissive of the problem because they’re skeptical of the means used to derive the estimate.

The bigger problem, however, lies in attaching a static rate of removal to a raw count. Determining “the” scavenging rate is not only extremely labor and time-intensive, it’s silly. Scavenging rates likely vary over time at a location. It only takes one cat or crow or raccoon to learn that dead birds appear at the base of some building every night and the scavenging rate can change dramatically over a short time frame. Especially given that we study scavenging rate by placing food around the base of buildings, we need to be sensitive to how our own actions as researchers could be teaching scavengers about a reliable food source.

The solution? I advocate a healthy dose of common sense. If you check your building daily, you’re a lot less likely to have carcasses removed before you get there. If you leave carcasses in place after you identify them, you can get a sense for the removal rate by determining how long the carcass lasts. Right now, I’ve had three carcasses present at the NRC – all still plainly visible to me – that have been in place for more than 1 week. This tells me that scavenging rate is probably quite low.

Lastly, recognize that the number is not the most important part of the story when it comes to collision mortality. I’m far more interested in the species and life history composition among the dead birds I find. For example, abundant residents like starlings and House Sparrows are almost never counted among the casualties at a site like the NRC. In contrast, the great majority of victims are long distance migrants. So far this fall I’ve counted 7 victims of which 6 were Neotropical migrants – the 7th was a migrant too, a Grasshopper Sparrow. This is where I think the most interesting questions lie regarding collision mortality. The risk of collision varies by species, and this may mean that some species are disproportionately affected by window collisions.

16 September 2009 – two unlucky warblers

I hadn’t planned to check the NRC this morning, but I received an email from a colleague with an office in the building that an unusual warbler had met its maker near one of the west entrances. The warbler was indeed unusual, and one I have yet to see in the Sooner State: Canada Warbler.

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Photo via cameraphone by Jesse Burton. Looks to be an AHY female.

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Unfortunately, in the roughly 10 minutes that elapsed between Jesse taking the photo and me getting to the location, someone had swept the area and removed the bird! I did some dumpster diving to try and find it, but to no avail. I am hopeful that whoever collected it did so because they recognized it, and the specimen will eventually make its way to our collections.

Although I missed the Canada Warbler, I did find another Common Yellowthroat. This one looked to be a HY female, fat = 2.

Video Snapshot

12 September 2009

I was a way for a couple of days, and unable to check the NRC until Sep. 12th. Casualties were:

1 Gray Catbird – badly decomposed
1 Grasshopper Sparrow – ditto
1 Wilson’s Warbler
wils_pusi_AllAm_map

Fall migration 2009 underway – Mourning Warbler

I have checked the Noble Research Center on the following dates (Aug. 20, 22, 25, 28, and Sep. 3) with the following results: no casualties yet. Today (Sep. 7), I found the first casualty of the 2009 season: a hatch year male Mourning Warbler (fat = 2).

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The range map indicates migration through Oklahoma en route to wintering grounds in northern South America:

opor_phil_AllAm_map

The Noble Research Center, OSU

This blog focuses on window strike mortality of birds at a single building on the campus of Oklahoma State University in Stillwater, OK. The Noble Research Center (NRC) is a large, irregularly shaped building that presents expanses of glass on its north and south sides, as well as recessed alcoves on the east and west through which vegetation inside the building can be seen from the outside. Thus, all surfaces of the building present a hazard to migrating birds.

Preliminary examinations of this building reveal essentially zero window killed birds during summer and winter and no resident birds among the casualties. Instead, mortality peaks for several weeks during spring and fall migration, and the casualties are almost all migrant birds in passage.

I surveyed this building approximately daily from mid-April to mid-May in 2009, and was surprised by the species composition and number of individuals found. For example, I found dead Yellow and Nashville warblers, Grasshopper and Clay-colored sparrows, and Indigo Buntings. These are all common migrants in the area, but the NRC is located in an urban setting, with very little vegetation suitable for any of these species in the immediate vicinity. The dead represent birds migrating through at night, and perhaps mistaking the vegetation reflected in the windows as appropriate places to stop after a night of migratory flight. The most surprising find of the Spring 2009 window kills was a Cassin’s Sparrow – a species that normally occurs in the state about 3 hours west of Stillwater.

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Clay-colored Sparrow