@seadogg
//Can you imagine any self respecting willow warbler with that superb and complex call going for something that calls "chiff-chaff" all day inn the breeding season. //
No (grin)
Some people like music with long and complex guitar solos while others prefer chug-along-a-Status-Quo
But it's not about that. The fact that they look alike (to the casual glance) means they are forced to sing their separate ways, to avoid confusion and wasting energy pursuing the, ahem, wrong lady.
Furthermore, as plain as it is, if the Chiffchaff's song is delivered anything less than perfectly (a 'variant' song being akin to a pub-band cover version of a classic), it's chances of breeding success are reduced.
A research paper I found online seemed to suggest that tagged birds were seen to breed successfully in their 2nd or 3rd year after failing the first time round because it took them time to 'train' their song, by listening to, and mimicking, the adults.