A big thank you to everyone who replied to my enquiry over eggshell identification, especially to those who gently pointed out that the shells had to be avian as reptile shells are leathery, not hard (you can tell that I'm out of my comfort zone here, can't you).
 
Because of the age of our eggshells (Roman-period, you'll remember), we've decided to look at DNA testing as one option.  McMaster University in Hamilton, Ontario have a 'McMaster Ancient DNA Centre' that specialises in archaeological and palaeolontological DNA identification.  Perhaps even more relevantly to our specific query, McMaster's biology department recently (2005) published a paper on DNA extraction from external avian eggshells.  Between these two areas of expertise, McMaster seem well-placed to help us, and we're currently discussing with them some of the technical issues to see whether this is feasible.  We haven't reached the point of finalising costs, so I'm not in a position to share this with you yet; sorry.  I'll try and keep everyone updated as to whether this works or not given the number of people who asked me to let them know if we do find a way of identifying eggshell.
 
I recognise that DNA testing may not always be the best way forward for historical archaeology (to be honest, we're not sure that it's the best way forward for us yet, either).  Another option recommended by several people on two continents might be to simply ask a natural history museum.  Keith Doms tells me that the three largest comparative eggshell collections in the US are held by the Smithsonian, the New York Natural History Museum, and the Delaware Natural History Museum, and these might be a more straightforward option in many cases.
 
Thanks again to everyone for their comments and suggestions,
 
Alasdair Brooks