I want to clarify an earlier post today regarding Golden-winged Warblers
(GWWA) which contained a link to a brief, but very interesting, BBC
piece on temporarynesting areaabandonment to avoid tornadic activity.
Gordon Andersson mentioned "an apparent 50% mortality [unrelated to the
tornadoes] of the 20 study birds [carrying geolocators] in one year."
This is not quite correct.
The researchers resighted 10 of the 20 geolocator-bearing study birds a
year after the attachment of geolocators and color bands. One bird
already had dropped the geolocator somewhere, so if he is excluded from
analysis, 9 of the 19 geolocator birds--47%--were resighted. This means
the SURVIVAL RATE of that group was a MINIMUM of 47%, but could be as
much as 100%, because there is no way to know what happened to the 10
birds that were not resighted. Geolocators are quite different from
radio-transmitters or satellite-tracked devices that can be remotely
relocated by signal receivers. Geolocators do not transmit radio
signals, so birds carrying them must be resighted to determine if they
survived. (The birds must be recaptured to download the geolocator's
data.) If a geolocator-carrying bird isn't seen in the area being
searched it isn't correct to assume that it died. It may simply be
nesting a few miles away, but beyond the perimeter of the study area.
With geolocators one can't simply hop in a plane and fly over a large
area and listen for the appropriate ping on a receiver to refind more
study birds.
What the study actually showed was a 47% RETURN RATE of the
geolocator-carrying birds to their previous nesting area, rather thana
one-year survival rate. All things considered, being able to resight
47% of the study population a year later is pretty darn impressive.
Importantly, as this was a pilot study to assess the feasibility of
using newer, lighter-weight, geolocators on GWWAs, there also was a
control group. Of those GWWAs that were color-banded only (no
geolocators), 42% were resighted a year later. Though it's probably not
a statistically significant difference, the geolocator birds actually
showed a slightly higher return rate than the control birds. A return
rate around 50% is quite normal for small passerine species.
Finally, I want to thank Gordon, and urge him to continue his ongoing
valuable service of using MOU-net to post links to interesting avian
publications. There are a lot of important research findings being
published every week, and Gordon does a better job than most of us
keeping up with them. We all benefit from his efforts, and he probably
doesn't often get thanked for doing it.
Disclaimer: I am related to the lead author of this study. And I'm
probably being overly nit-picky here. But I'm fascinated with the
knowledge that might come from increased use of lightweight tracking
devices like these. It's important to understandthat they're now so
light as to be safely worn by some of our smallest birds during
round-trip migrations.
---Ron Refsnider
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