AN ERROR IN ESTIMATION OF MIGRATION ANIMALS NUMBER UNDER THE REMOVAL OF SMALL MAMMALS WITH TRAP LINES

The number of residental animals in daily samples is diminishing regul arly during prolonged catching. It might diminish according to exponen tial law if: (1) the probability to be captured for each animal living on the sampling territory does not alter in the course of trapping; ( 2) all animals have the same possibility to be captured. The number of captured non-residental (migrating) animals stays the same during the whole period of trapping, thus increasing the number of animal captur ed at the end of trapping vs. one expected according exponential law. It is not correct however to consider the presence of migrants to be t he only reason of the deviation from exponential decrease, because it is observed also in residental animals. (1) Home ranges of some animal s overlap the trap line only by their edge. The possibility to capture such animals is 2-3 times lower than animals with central position of home ranges on the trap line. (2) The removal of peculiar demografic groups is not equal. Adult Females are captured with highest intensity ; young animals, on the opposite, stay uncaptured longer. It may be re gistered as increasing of proportion of young and subadultus animals p er an adult female with each day of removal. (3) Reproduction is inten sive in summer. Young animals (up to age of 20 days) does not visit tr aps. Some of them become active in the middle and the end of trapping period when a great share of older animals (including their parents) i s already captured. Because of these reasons disturbed exponential rem oval exists even in residential animals, and we can not differ the rea l proportion of migrants even in cases when the disturbance of exponen tial decrease per se is proved properly.