On today’s, the Gemara analyzes the expression that appears in the Torah by way of introducing this mitzva – “If a bird’s nest chance to be before thee in the way…” – and concludes that even though there is a positive commandment to send the mother bird away, nevertheless there is no obligation to go searching for a nest in order to fulfill this mitzva. The mitzva applies only if the person chanced upon this situation.
Based on this Gemara, Rabbi Yair Chaim Bachrach argues in his Havot Yair that if someone chances across a mother bird nesting on her eggs or on her young, he would be obligated to chase away the mother and take the offspring, even if he has no use for them. This is based on the reasoning that although the Torah limits the obligation in this mitzva to the extent that a person who does not come across a nest is free of the obligation to search for a nest, it does not free a person who finds such a nest from the obligation to fulfill the positive commandment associated with this mitzva.
The Ḥazon Ish disagrees with this reasoning. His argument is that our Gemara is simply deriving from the biblical passage that it is not necessary to go searching high and low for a nest to insure that this mitzva will be fulfilled. Without this passage we would have assumed that every person is obligated to perform this mitzva at least once in his life and would need to go searching for the opportunity to fulfill this commandment. Nevertheless, the mitzva is obligatory only on someone who actually wants the eggs or the chicks; someone who has no interest in them would not be obligated to perform this commandment, even if he were to chance across a nesting mother bird.