One of those who conducted the survey Dr Timothy Harrison(now with the BTO) said
We do not recommend that people should stop feeding birds in their garden
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.
The study was carried out in a woodland in Worcestershire 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
The researchers did not feed a the third group.
Over a three year period researchers rotated the so that each group of birds were fed peanut cake two years out of three.
According to Dr Harrison:
In both blue and great tits food supplementation advanced the onset of laying and shortened incubation periods.
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.
Of course the study is not conclusive.
Because of the amount of food fed to birds in towns the research took place in woodland. Blue and great tits are most productive in broad leaf woodland, becuase of the abundance of natural foods (e.g. Caterpillars). This food is scarcer in urban areas so supplementary feeding may be beneficial but there is no evidence for this.