I scan slides in large quantities -- art, not birds, but I don't think the
requirements for a good scanned image are any different. In either case you
often need to blow up a detail from the image. For 35mm slides, as Don
Baccus suggests, only a dedicated slide scanner produces good results,
partly because you can't get consistently good scans without the flexibility
to treat different slides differently, in scanning settings as well as
post-scan tweaking. I use high-end equipment at the university, but at home
I have an Olympus ES-10P, which cost c.$450 (as opposed to $1,250-$2,500 for
"professional" quality slide scanners), and compares quite well on quality.
The principal difference is actually in speed and handling rather than
quality -- the Olympus is pretty slow, and you can't leave it to work
through a batch of slides.

James West