Two species model and SIF delta

[colson]

I am interested in investigating the SIF in the detection of two species with a single season model. We had three surveys and we want to test if the detection of each species varies by time. It would seem to me that if the detection parameter for one species increases over the three surveys while the second species is constant, than the value of delta may also vary by time.

My question is, would it be possible to investigate their interaction by varying delta by time as well?

I have ran a model with delta(t), and for the delta(t2) I get a parameter estimate and SE of 0.201745 (-1.#IND00), which led me to think that delta should remain constant.

However, I then set the matrix with a separate delta(t1)and delta(t2=t3) and the model seemed to work. Is this a valid method?

Thanks,
Colson