Sentence:  The males of the two species have very similar mating songs but those of the females differ. Although females could be induced to utter their mating song by males of the other species, the males were never lured by the call of females other than of their own species, so that copulation leading to <WSD>bastardization</WSD> did not occur (Strübing, 1965). 
Meaning of the word marked with <WSD> tags:  The creation of offspring from two different species; cross-breeding.