GAMM : how to use a smoother for some levels of a variable, and a linear effect for other levels?

Hi,

I was reading the book on "Mixed Effects Models and Extensions in
Ecology with R" by Zuur et al.
In Section 6.2, an example is discussed where a gamm-model is fitted,
with a smoother for time, which differs for each value of ID (4
different bird species). In earlier versions of R, the following code
was used

BM2<-gamm(Birds~Rain+ID+
       s(Time,by=as.numeric(ID=="Stilt.Oahu"))+
       s(Time,by=as.numeric(ID=="Stilt.Maui"))+
       s(Time,by=as.numeric(ID=="Coot.Oahu"))+
       s(Time,by=as.numeric(ID=="Coot.Maui")),
     correlation=corAR1(form=~Time |ID ),
     weights=varIdent(form=~1|ID))

However, in the current version of R, this does not work anymore, and
should be changed into

BM2<-gamm(Birds~Rain+ID+
       s(Time,by=ID),
     correlation=corAR1(form=~Time |ID ),
     weights=varIdent(form=~1|ID))

It turns out that 2 of the 4 smoothers have estimated degrees of freedom
of 1, so a linear effect would be sufficient.
Now my question is how I need to change the code in order to have a time
smoother for ID=Coot.Oahu and ID=Coot.Maui, and a linear time effect for
ID=Stilt.Oahu and ID=Stilt.Maui. With the "old" R-code, this seems
trivial, but I don't have any idea how to do it in the newest R-version
(interactions with a dummy variable do not work in gamm).

Thanks,
Ivy

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