The co-variates approach is not the classical and easiest way to deal with Trap-Dependance.
Two main approach exist:
1) The first one is described in
Pradel, 1993, Flexibility in survival analysis from recapture data: handling trap-dependence. Marked individuals in the study of bird population. P29-37
You have to transform your data set according to Pradel (93)
before any analysis and use two age classes to model capture.
The transformation of the data set can be done automatically
by U-CARE -> TRANSFORM DATA -> Split for trap-dep analysis
2) The second one is described in
Gimenez O., R. Choquet, and J.-D. Lebreton (2003). "Parameter redundancy in multistate capture-recapture models". Biometrical Journal 45:704–722
You have to consider two states, previously captured vs not previously
captured. Additional comments how how to implement this approach are given here
http://www.cefe.cnrs.fr/BIOM/pdf/Choque ... models.pdf
Sincerely,
Rémi