Birds ate up to 20 per cent more of conventionally-grown wheat than organic. Dr Ailsa McKenzie, lead researcher based at Newcastle University's School of Biology, said: "Our results suggest that the current dogma that organic food is preferred to conventional food may not always be true. Birds instinctively chose the non-organic food.
"Protein is an essential nutrient in the diet of all birds and mammals and getting enough of it, especially in winter, can be hard.
"We showed that when given free choice, wild birds opted for the conventional food over organic and the most likely explanation is its higher protein content.
"This study is only looking at one aspect of the organic food debate - it does not take into account the long-term health implications of using chemical fertilisers and pesticides. But it does raise questions about the nutritional benefits of organic food and what consumers are being led to believe."
The three-year study involved feeding stations in 30 gardens. Organic and non-organic wheat seeds of the same variety were placed in adjacent bird feeders. The rate at which birds ate the different seeds was monitored for six weeks.
Half way through the experiment the feeders were swapped around. The experiment was also repeated in a second winter with different wheat samples.