(Apologies for cross-posting)

In response to some of your queries about the RFID technology mentioned in
the last post: yes passive (no power source) radio tags have been around
for a while.   For an example of a field biology application of the RFID
technology that is available now, see Nature 376 p 649 (Aug. 24 1995).
These tags are made by TIRIS (to contact the manufacturers - Mary Ellen
Walden 508 236 3174 at TIRIS - a branch of TI, or perhaps more profitably
but more expensively a company that specializes in TIRIS tag application to
animals: Allflex 214 456 3686 - Glen Fischer/Lloyd Tate).  These ~$10 tags
can be read from a modest distance - .3 to 1.2m depending on the size of
the tag and the reading conditions.  PIT tags (again ~$10 each) which
veterinarians routinely inject into animals (eg the Trovan tags marketed by
Infopet) must be read from a somewhat shorter distance (typically .3m or
so).  The new tags that are coming (in a few years) can be made to be read
from greater distances, and are less sensitive to tag orientation.  Also
100's (possibly 1000's) will be readable at the same time (current tags can
only be read one at a time and the presence of more than one tag may make
it impossible to read any of the tags).  Finally the new-tech tags can be
made with built-in memory/programming, and they and the readers will be so
cheap that you can imagine putting the tags in just about anything.

Best wishes,

Mark

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Mark Stowe c/o Jon Reiskind
Department of Zoology
University of Florida
Gainesville, FL 32611 USA
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phone: 904 373 3202, 904 392 1187
fax: 904 392 3704