Cedar contains phenols, which can cause not only respiratory difficulties but also liver damage in birds, small animals, and even humans (such as the workers in mills that process cedar). Pine also contains phenols, although in significantly lower quantities such that it isn't as big a concern (I still get a bit of respiratory irritation if I have to stick my head in a barrel of pine shavings too long though). It's really much more of a concern in confined areas where the birds have nowhere to go to get away from the phenols. Not only are large logs less likely to have as large a surface area to release the phenols as a pile of shavings/chips, but being outside there is nothing to keep them right where the birds will be breathing them. It really shouldn't be a problem outside, especially if the logs aren't fragrant enough for you to tell in an instant from one whiff that they are cedar.