age models where age at first encounter is not one

[pauldonald]

Dear all

Please forgive me if this is a dumb question. I am trying to model the survival rate of Sociable Lapwing chicks on the steppes of Kazakhstan. We fitted chicks with individual colour ring combinations and followed them to fledging. Often we ringed chicks the day they hatched, but on many occasions came across chicks of older ages (we can age them pretty well on biometrics). As I understand age models, the age at first encounter is assumed to be 1, but how do I specify the model to include chicks whose age at first encounter was more than one?

Any help very gratefully received.

Thanks

Paul

[Jochen]

Dear Paul,
there are 2 options, I think.
The simple is to create a group for each ringing age, and adjust the PIMs for each group such that it starts with the appropriate parameter in the first interval.
A bit more complex would be to use ringing age as covariate and try what is described in chapter 17 under "DSR varies with nest age". But beware, I don't know if this works correctly with "recaptures only" data.
Best
Jochen

[pauldonald]

Thanks very much indeed Jochen, this is very helpful. I am not sure whether any Mark models other than nest survival models will recognise the 'Age Day1' covariate as relating specifically to age. I think that if I chuck a variable called AgeDay1 into a CJS model, it will just be treated in the same way as any other covariate. Does anyone know any different?

The data I have are exactly the same as the case of estimating annual survival from mark-resighting in a species that can be aged accurately for many years after hatching (though in practice there are few if any such species) - how in such a case could one fit an age model? Is it possible to fit a "time since hatching" model, in the same way as a "time since marking" model?

I'll think more about your suggestion of creating separate PIMs for each age group.

Thanks again and best wishes

Paul