Eared Grebe Surveys on Mono Lake
By Sean Boyd, Pacific Wildlife Research Centre, Canadian Wildlife Service; and the California Department of Fish and Game
Photographic protocol to count grebes: Colonial Waterbirds 21 (2): 236-241, 1998... "Estimating the abundance of eared grebes on Mono Lake, CA, by aerial photography"
All the above counts can be increased by 15-20% to account for submerged birds (see +15% column).
This is a fairly large increase over the previous year, mid-October
Ron VanBenthuysen of Cal DFG conducted the photo count and he more than doubled the number of photos taken over last year. Although not really needed this year because the birds were fairly evenly distributed, the larger sample size will help reduce the variance in years when grebes are highly clumped. For this data set, I generated total estimates using every 2nd, 3rd, and 4th photos and found that, while the totals varied by up to 50,000 birds over the baseline number above, the %SE increased to ca. 7%, 8.5% and 10%, respectively.
Also, using Ronís PhotoLink setup I was able to correct density values for altitude variability, and this made a difference of about 70,000 birds over the uncorrected total.
I think the protocol we now have established for Mono Lake is a good one. Thanks to Ron, Esther, Melanie, Dan and others at Cal DFG for making the arrangements and conducting the survey. Also thanks to Greg Reis for supplying data to calculate the surface area of Mono Lake.
Digital photos were used for the first time this year by Cal DFG which is definitely the way to go. We are in the process of developing an even better protocol for next year to improve coverage and reduce variance.
Thanks to Ron VanBenthuysen, Dan Yparraguire, and Melanie Weaver of Cal DFG for setting up and conducting the aerial survey and Peter Vorster, Gregory Reis and Geoff McQuilkin for supplying info on how to calculate lake surface area.
Cal State Dept Fish and Game have graciously agreed to do the counts. After a couple of attempts they were successful at doing a good digital survey in fall 2004 (see some correspondence below). The agreement is that Sean will count the birds on the pictures, calculate a mean density and then figure out the total number of grebes on the lake. Below is some discussion of the protocols:
1. Unfortunately, the camera we used in 2004 is different from the camera we used last time. It's much better but 105mm is the longest focal length we have for it right now. I agree that a higher altitude with a longer focal length would be better. I think we need to get a new lens to do that though.
2. There were still diving/splashing birds on some photos so it's possible that the dual engine aircraft is still a problem at 1200 ft (I used a single engine aircraft). So, I suggest next year to do the surveys at 1500 ft altitude above water level and use a longer lens (but, again, try to achieve a scale result of 150 m from top to bottom).
3. I noticed sun reflection on some photos, making it difficult to count birds in some cases. This can occur if there is a slight wind on the water and the survey is conducted when the sun is high. I found that doing the survey between 0900-1100 h is best to avoid this situation (between 0830-1030 h is even better)... the sun is lower and there is usually less wind. So, if possible, it would be best to do the survey earlier if your schedule allows.
4. You took about 350 pics whereas I usually took 450 - 500 photos during each survey and found that this kept the variance down to a minimum. Of course, the more photos the better for both accuracy and variance so you might want to increase the number of pics taken over water to at least 500.
If it's not a problem then I suggest flying more transects and taking more photos per transect (I flew 16 total transects over the water; 8 E-W transects going from south to north followed by another 8 going north to south but offset so that the transects evenly covered the entire lake) so that you end up with about 500 photos total taken over the water.
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