Rosyfinch Ramblings
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February 2008
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How Many Rosy-Finches?
Filed under: General, Birding & Outdoors, NM & SW US, Sandia Crest
Posted by: Ken @ 4:32 pm

Waving Flag-- Rosies have returnedUpdate on the Rosy-Finches of Sandia Crest, New Mexico.  The flag is waving–
flocks of Rosy-Finches are still visiting the feeders at Sandia Crest House.

Flocks of up to 150 rosy-finches are still being reported at and around the feeders. So far this winter, 427 rosies have been newly banded, and about 400 more previously banded birds have been recaptured. About 90 percent of the recaptured birds are from the same season; some, even on the same day as first banded. However, each year, increasing numbers of trapped birds bear bands from previous winters. This winter, to date, there have been 40 such recaptures:

Recaptures BLRF BCRF GCRF Totals
Winter 2004/2005   5   5
Winter 2005/2006 7 4 1 12
Winter 2006/2007 16 1 6 23
Totals 23 10 7 40
BLRF = Black; BCRF = Brown-capped; GCRF = Gray-crowned Rosy-finches, respectively

As each winter season progresses, the percentage of recapture of same-season banded rosy-finches at Sandia Crest increases. An isolated population of birds could be compared to an unknown number of beans in a jar that could be shaken up uniformly. If we mark a known number of the beans as “banded,” we then may withdraw a random handful, count the total and the percentage that are “banded,” and extrapolate to determine the total number of beans in the jar. The larger the sample we examine, the greater our certainty about the total number of beans in the jar. Simple?

Yes indeed, for beans in a jar, that is. Now let’s try it on the rosy-finches. Let’s say that on two consecutive weekends in February the banders found that 25 of 36 and 21 of 32 were newly recaptured birds banded this winter. This comes to, roughly, 2 of every 3 birds trapped. Assuming that the researchers know that the 450 birds they banded this winter are out there, the total flock would be (if everything else is constant), about 1/3 more than the number banded, or about 600 birds. Now, lets get real…

Steve Cox, who, with Nancy Cox, leads the banding team at Sandia Crest, related a formula that can be used by banders to estimate flock/population size. It is a bit more complex:

N/M = n/R, then N = (M)(n)/R

Where N is the total population size to be estimated

M is the sample  that was captured, marked, and released

n is the number captured the second day

and R is the number of marked individuals that were recaptured on the second day.

Steve goes on to say,

“What the capture/recapture equation is saying is that the ratio of the total population to the total number marked on the first day is equal to the ratio of the total number caught on the second day to the number of marked individuals that were recaptured.
“(Example: M = 32 birds caught, marked & released on the first day, n = 40 birds caught on the second day, R = 20 recaptured, marked birds. Therefore N/32 = 40/20 and N = (32)(40)/20 or 640)

“You have to assume that the population is stable during the two events.”

Ah, but there’s the rub. Steve adds:

“However, this year I think the birds are not stable and the flock size is fluctuating week to week.  Added to this problem we have not had what I would call 2 consecutive days that are relatively the same (weather wise).  I believe that the weather is a very important  factor.  If it is really cold and windy the birds come to the feeders more often. On warmer days the birds are likely to be feeding somewhere else as well.  I have always assumed that they were somewhere else along the ridgeline.  Based on how much they fly, and the high fluctuation of birds, I now wonder if some might not be traveling to other close by mountain tops.  Another reason to get a few birds with transmitters on them.”

Transmitters— What a great idea! They could also solve the mystery of where these birds spend their nights. Steve and Nancy are working on that right now, and you may have the opportunity to assist them in getting the transmitters. Stay tuned.

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