By Emma Brennand
Earth News reporter
Early to feed, late to sing?
Feeding wild garden birds during the breeding season may delay the start of the dawn chorus sung by some species, say researchers.
Their study claims to have found a link between supplementary feeding and the observed changes in songbird behaviour.
The scientists made the discovery studying populations of great tits living in the suburbs of Oslo, Norway.
Birds with access to feeders delayed their song by up to 20 minutes, often beginning only after the sun had risen.
Details are published in the journal Animal Behaviour.
This delaying effect really came as a surprise to us and to many other researchers
Dr Valentin Amrhein, University of Basel
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 dawn chorus is a particularly sensitive time of singing," explains Dr Valentin Amrhein from the Zoological Institute, University of Basel, Switzerland who led the multi-institute team of researchers.
Earlier studies carried out by the team found that in the six weeks before egg laying male great tits usually start singing well before sunrise.
"Previous studies have shown that the starting time of singing at dawn reflects past territorial intrusions by rival males and is thought to reliably reflect male quality."
More impressive males are thought to sing early to attract more partners and mate with females other than their usual mate.
"If only the best and most energetic males can afford to start singing early, supplementary feeding should lead to an earlier start of singing, Dr Amrhein explained to BBC News.
However, the team found that after two weeks of artificially feeding two populations of great tit, supplemented males actually started dawn singing later than control males.
"This delaying effect really came as a surprise to us and to many other researchers, because it was the opposite of what we have seen from studies using short-term food supply."
Why birds fed supplements sing later was not fully explored by the research team.
However, Dr Amrhein believes it is likely that territorial males simply become distracted by rival males and other bird species feeding in their territory, encouraged by the bird food.
The males' behaviour may also be explained by the quality of the supplementary food or the presence of predators at the feeding station.
This change in behaviour by delaying or even skipping the start of the dawn chorus may have a detrimental effect on how many chicks males sire.
"Dawn singing in the great tit is thought to serve as a paternity guard," says Dr Amrhein.
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.
In the UK feeding birds is a popular past time with over 48% of households providing additional food to wild birds in the form of peanuts and seeds.
This equates to an estimated weight of 60,000 tonnes of supplementary bird food each year.
Many studies have shown that providing food for birds during the winter months can improve their survival when natural food resources are scarce.
However, much less is known about the impact of artificial feeding on bird species during the breeding season.
A study published last year by a team from the University of Birmingham found a relationship between artificially-fed birds and changes in reproduction.
That team, led by ornithologist Timothy Harrison, found that food supplementation during the breeding season advanced the onset of laying and shortened incubation periods in blue and great tits.
Among artificially-fed birds, clutch sizes also declined significantly when more food was available.