Fitting a mixed negative binomial model

  This is a little bit tricky (nonlinear, mixed, count data ...) Off the 
top of my head, without even looking at the documentation, I think your 
best bet for this problem would be to use the weights statement to allow 
the variance to be proportional to the mean (and add a normal error term 
for individuals) -- this would be close to equivalent to the log-Poisson 
model used by Elston et al. (Parasitology 2001, 122, 563-569, "Analysis 
of aggregation, a worked example: numbers of ticks on red grouse 
chicks"), and might do what you want.