在计算广义线性混合模型时出现报错，请问是什么意思？怎么解决？
> Cand.models[[2]]<-glmer.nb(activity ~wind+(1|time/birdID),data=data)
boundary (singular) fit: see ?isSingular
boundary (singular) fit: see ?isSingular
Error in pwrssUpdate(pp, resp, tol = tolPwrss, GQmat = GQmat, compDev = compDev,  :
  pwrssUpdate did not converge in (maxit) iterations