My model is:
model1 <- glmer(Avg_egg_mass ~ Treatment + Alt_cat + Treatment:Alt_cat + (1|Nest) + (1|Site)+ (1|Year), na.action=na.omit, family = poisson (link=log), data = dframe1)
where I am relating average egg mass (continous variable) to a four group treatment (set to factor) and three categories of altitude (also set to factor).
It doesn't seem to work and I get this error message
Error: grouping factors must have > 1 sampled level.