I've been in touch by email with an architect in New York City, partner in a large firm there, who has an interest in the Vikings stadium under construction because she believes strongly in bird-safe buildings. She gave me links to web pages that contained information new to me, which doesn't mean it's new.

One of the links was to the Minneapolis daily business newspaper "Finance and Commerce." In late April it carried an excellent article by Frank Jossi that answers a couple of my questions. I also received information from the Minnesota Sports Facilities Authority.

I learned that 60 percent of the stadium roof will be made of a partly transparent building material known at ETFE (ethylene tetrafluoroethylene, a fluorine-based polymer). It comes in very thin sheets that will be used to form what you might call pillows, inflated by air pressure (but not subject to the puncture failure and collapse we saw at the old stadium).

Jossi explained that the stadium will have glass 20-feet-wide at the top of its exterior walls, just beneath the ETFE roof. And there is the 200-foot-wide wall of glass on the stadium's west side that is causing concern among birders. Maybe the 20-foot surround is regarded as a problem, too; I've not heard it mentioned in particular. But it is glass, and it does reflect.

The figure commonly used for the amount of glass on the stadium's exterior is 200,000 square feet. Actually, and this is the question the authority did answer, the amount is 180,000, not that that makes much difference.

There will be some pattern to the ETFE roofing material. It lets sunlight pass; that could cause a serious heat problem, a greenhouse effect. The material will contain two dot-matrix patterns, Jossi wrote, to block some sunlight and heat. Birders wish such patterns would be incorporated in the glass, to lessen reflection.