Update 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:
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: