Mixed models fixed effects

Dear All,

This may sound like a dumb question but I am trying to use a mixed model to 
determine the predictors of bat activity along hedges within 8 sites. So my 
response is continuous (bat passes) my predictors fixed effects are 
continuous (height metres), width (metres) etc and the random effect is 
site  - can you tell me if the fixed effects can be continuous as all the 
examples I have read show them as categorical, but this is not covered in 
any documents I can find.

Help!

Emma