The scientists were studying a group of Great Tits in the suburbs of Oslo.
Apparently birds with access to feeders often delay their song by up to 20 minutes, often beginning only after the sun had risen.
The researchers believe the changes seen in the behaviour of the territorial males is caused by an increase in the number of rival males and predators into areas which have feeding stations.
The change in behaviour by delaying or skipping the start of the dawn chorus may have a detrimental effect on how many chicks males sire. I've certainly read that urban birds have smaller clutch sizes.
According to Dr Amrhein.
Dawn singing in the great tit is thought to serve as a paternity guard,
So males that delay singing may inadvertently allow other males to come in and mate with their partner.
The team concludes that the feeding of wild birds during the winter months is vital for their survival, but Dr Amrhein suggests stopping short of the breeding season, at the end of March.