Conditions of the survival of population in Nicholson's models with delay

The Nicholson's model with delay describing dynamics of the magnitude of population is considered. Properties of the model solutions are analyzed, equal apartness of solutions from zero is proved and a lower estimation of decisions as function of the model parameters is found. A two-band model constructed on the basis of Nicholson's model is analyzed. The lower estimations of solutions with various parameters combinations are found.