model selection question

[ceosborne]

Hi,

I've been using Presence to model marsh bird occupancy and detection. I'm in the report writing phase and I came across,

Darrah, A. J., and D. G. Krementz. 2009. Distribution and habitat use of king rails in the Illinois and Upper Mississippi River Valleys. Journal of Wildlife Management 73: 1380 - 1386.

They use Presence for their methods and the paper has been very helpful to me; however, I noticed for their models (Tables 1 and 2) they keep either occupancy or detection constant when determining the effect of habitat covariates on the other.
psi(.), p(% emergent veg)
psi(% emergent veg), p(.)

I did this too but I also would apply it to both, e.g.
psi(% emer. veg), p(% emer. veg)

When a covariate is applied to both parameters and included in the project file it affects the AIC values of the models where one is held constant (as opposed to if I don't include the model with it applied to both). The c-hat values for all are around 1.5

Is my method valid or does one parameter always need to held constant? Excuse my naivete, I'm just unsure after seeing this paper.

Thanks for your time.
cheers,
Carrie

[jhines]

Adding this model will affect the delta-AIC, and model weights of other models, but not the AIC's.

It's OK to model both psi and p as functions of the same covariates.

Jim