modeling nested predictor in MCMCglmm - a nice explanation

http://finzi.psych.upenn.edu/R-sig-mixed-models/2010q3/004437.html 

If every nest has a unique identifier then MCMCglmm should give the  
same answer (up to Monte Carlo error) for random ~ pairID + nest and  
random ~ pairID + pairID:nest,