mixed model

Hi Barbara,
I guess you want to fit a linear mixed model, nlme is specific for non-linear models.

Did you try this code, which assumes a normally distributed response:

model1<-lme(mfreq~avergTemp+altitude, random=~1|country)
summary(model1)

or this code for poisson response:

library(MASS)
model2<-glmmPQL(mfreq~avergTemp+altitude, random=~1|country, family=poisson)
summary(model2)

library(lme4)
lmer(mfreq~avergTemp+altitude+(1|country), family=poisson, method="Laplace")

but first you should check whether country is a factor!
summary(country)

Cheers, Tom