I wouldn't say highly. Mildly, sure. And according to a few articles, pine is almost as bad.
Every blogger says, "avoid cedar, cedar is evil." but when I looked for some scientific articles to back it up, I found maybe five studies on poultry litter? Most cost $50, so I didn't read them. What I did find was the original study that found cedar shavings are bad. And it studied pine as well. Pine and cedar were both found to be toxic. Further reading (studying humans in the lumber industry) suggested that nearly all softwoods are bad. Yes, that article's old. (EDT: again, not paying $50 for a recent article. I will break into a university library first.)
Aspen is the recommended wood bedding, but it's noted to be less absorbent than pine is. It's also the recommended bedding among rat keepers. (Unless they're using paper pulp or walnut hulls, which also seem to be popular).
Sources:
http://www.ratfanclub.org/litters.html
https://www.ncbi.nlm.nih.gov/pubmed/2926083
https://pdfs.semanticscholar.org/6481/e4e10fae4909c0ae483907273d0e4213ca12.pdf
