By Matt Walker
Editor, Earth News
Good for the birds?
Putting food out for wild birds in spring and summer may hinder their reproduction in some circumstances.
A study by the University of Birmingham, UK has found tits given supplementary food in a woodland during spring and summer have smaller broods.
Bird feeding in the UK is hugely popular, and research has shown it aids bird's survival in the winter.
But scientists say the new study highlights how little we know about the impacts on birds of artificial feeding.
"We do not recommend that people should stop feeding birds in their garden," says Dr Timothy Harrison, an ornithologist now at the British Trust for Ornithology (BTO) in Thetford, Norfolk, UK.
"Indeed, with less natural food in urbans areas, such feeding could be essential. But given that millions of people feed their birds in their gardens, we still know remarkably little about the effects.
"Our study revealed some unexpected findings."
Dr Harrison conducted the study with Dr Jim Reynolds and Professor Graham Martin and colleagues of the Centre of Ornithology at the University of Birmingham, Dr Dan Chamberlain of the BTO and Dr Stuart Bearhop of the University of Exeter, among others.
In a broad-leaved woodland in Worcestershire, they provided food supplements to blue tits (Cyanistes caeruleus) and great tits (Parus major).
Breeding great tit
They split birds nesting in 288 nest boxes into three groups.
Two groups of 96 nest boxes were fed a supplement of commercially available peanut cake, an energy rich food made of 50% ground peanuts and 50% beef tallow.
As a control, the researchers did not feed a third group of 96 nest boxes.
Peanut cake was provided from early March, about four to five weeks before blue and great tits begin laying, to the end of July, six weeks or more after their chicks fledge.
Over three years, the researchers rotated the treatment regimes, so each group of birds were fed peanut cake two years out of three.
Feeding the birds extra food had a significant impact on their breeding, compared to the controls.
"In both blue and great tits food supplementation advanced the onset of laying and shortened incubation periods," Dr Harrison told the BBC.
"These effects can be explained by increased food availability, and can advance the timing of fledging which improves the survival prospects of the fledgling blue and great tits."
"However, we did not predict reduced productivity as a consequence of food supplementation. We expected supplemented birds to lay more eggs and to have larger broods, but the opposite occurred."
Among artificially-fed birds, clutch sizes declined significantly when more food was available, the researchers report in the journal Oecologia.
Hatching success also fell among blue tits, while it didn't change among great tits.
In both species, resulting brood sizes also declined significantly, with food-supplemented birds hatching half a chick fewer on average per nest.
The study is the first to show that supplementary feeding can reduce hatching success and brood sizes in passerine birds.
A blue tit emerges
Crucially, two species were similarly affected, suggesting that the impact may be wider.
The cause is unclear.
It might be the extra food changed the diet of the birds at a crucial stage before breeding, creating a knock-on effect.
Or territorial birds may have fought more over well provisioned sites, harming their breeding success.
However, the researchers are keen to stress that people should continue to put out food for birds.
Studies show that between 34-75% of UK households provide food for birds at some point during the year, providing around 50-60,000 tonnes of food.
"There is strong evidence that food supplementation can have positive effects," says Dr Harrison.
"For example, numerous studies have shown that feeding during winter can enhance the survival prospects of birds. However, the influence of provision across the breeding season is still relatively unknown."
Another important qualification is that due to the difficulties of acquiring an urban study site, the research took place in woodland, not in a city or town where most people feed birds.
Blue and great tits are most productive in broad leaf woodland, due to the abundance of natural foods such as caterpillars.
Because such food is scarcer in towns and cities, supplementary feeding may be more beneficial.
However, it remains unclear if feeding in city gardens really benefits either species.