How to manage a GLMM rank deficient Some background into my study, I'm looking at the abundance of birds across sites in 5 different vegetation states, using a GLMM.  The birds belong to 5 different functional groups and I'd also like to explore whether the representation of different groups differs with state. I've written the following model:
Abundance_Model <-glmer(Abundance~State + State*Group + (1|Site_Number)+ (1|Date), Birds, family=poisson())
summary (Abundance_Model)

However when I run it, I get the warning fixed-effect model matrix is rank deficient so dropping 1 column / coefficient and one of the groups is absent from the output. There are no NAs present in the data. Any suggestions as to why it is rank deficient and what to do about it?

I believe that should be fine. It's just that your columns ( namely group membership) are redundant, like having a column for is_male and column is_female.
The way to read the output of the group coefficients is difference from base (missing) group
