Practical Question about Evolution, Population Genetics and Speciation
Infertile descendants indeed sound like a disadvantage. Individual must invest a fair amount of resources into making an offspring. Making sterile offspring is just a waste of resources as those offspring aren't going to pass on any gene further to any grandchildren.

If there is an increase of the tredecdssini females, then male tredecim that sing quite late are going to mate with many tredecdssini leading to the production of sterile offspring. There is therefore selection against the allele 'earlybird' leading to an increase in frequency of the 'EARLYBIRD' allele.