British researchers found that birds such as robins and house sparrows "instinctively" preferred non-organic seeds to the more naturally grown varieties as it appeared to provide them with greater nutritional value through the cold months.
When offered both varieties of wheat seed, they were able to discern between the two and ate up to 20 per cent more of the conventional grown variety than the organic.
That worked out at about 70 grams more a week per garden.
Even when the grain in the feeders were switched around, the birds soon were able to spot the difference and again settled on the more intensively farmed seeds.
Dr Ailsa McKenzie, lead researcher based at Newcastle University's School of Biology, said the findings are likely to add to the debate over the relative merits of consuming organic food.
"Our results suggest that the current dogma that organic food is preferred to conventional food may not always be true," she said.
"We have long lost our instinctive ability to know which food is the most nutritious for us but birds have not. They instinctively chose the non-organic food."
To carry out the three-year study the Newcastle team set up feeding stations in more than 30 gardens across the North of England.
Organic and non-organic wheat seeds (both of the same variety) were placed in adjacent bird feeders and then the rate at which the birds ate the different seeds was monitored over a six-week period.
Half way through the experiment the feeders were swapped around.
The experiment was repeated in a second winter with different wheat samples.
The birds showed a strong preference for the conventional seed, eating significantly more of this than the organic.
When the feeder positions were switched, the birds learnt the new position of the conventional seed and continued to select it in preference to the organic.
Analysis of the wheat found the conventionally grown seeds to have an average 10 per cent higher protein content than the organic seeds.
This the researchers said was common in most foods as chemical fertiliser was much more efficient than natural fertilisers such as manure.
"Protein is an essential nutrient in the diet of all birds and mammals and getting enough of it – especially in winter – can be hard," said Dr McKenzie.
"We showed that when given free choice, wild birds opt for the conventional food over the 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, or the often negative environmental impact of conventional farming; for example, other work has shown that pesticides can strongly reduce availability of seeds for birds.
"But it does raise questions about the nutritional benefits of organic food and what consumers are being led to believe."
Global demand for organic produce is increasing by £4 billion annually – the organic market now accounts for between two and three per cent of food purchased in Europe and the US.
One key reason why consumers buy organic food is because they consider it to be better for human and animal health.
While this may be the case, these are not necessarily the only factors governing food choice in animals and birds.
Other differences between the samples – such as mycotoxin levels, grain size, energy content or pesticide residues – could not explain the preferences shown by the birds.
The garden bird work was confirmed by laboratory studies on canaries, also showing a significant preference for conventional over organically grown seeds.
Dr McKenzie said: "Conventionally grown crops tend to contain significantly higher levels of protein than those grown organically due to the application of inorganic nitrogen fertilisers in conventional farming systems.
"This makes our findings potentially applicable across many food types and suggests the issues surrounding organic food are not as cut and dried as some might think."
The findings are published in the Journal of the Science of Food and Agriculture.
A spokesman for the Soil Association said: "The UK Government’s own advisors found that bird life is up to 50% greater on organic farms showing that most birds do choose organic. Animals like chimpanzees and even rats have been shown to prefer organic food. This study has absolutely no bearing on whether organic food is better for human health or not."
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