A new article written by Ailsa J McKenzie and Mark J Whittingham of Newcastle was just published in Journal of the Science of Food and Agriculture:
But the conclusion was universal: after some time that the birds needed to learn the strategy, the birds would significantly more often select the conventional wheat - treated by various synthetic and chemical products during its production - instead of organic food - where the synthetic compounds are completely avoided.
It's very likely that the birds try to maximize the protein content and after some time, they just learn how to do so and what is good for them. Fertilization adds about 10% to the protein content.
The research indicates that the birds act much more rationally than many people - and whole governments - that routinely select the food that is nutritionally inferior as well as more expensive. My sources point out that if the result of the research were the opposite one, we would read thousands of articles claiming that the birds reject chemistry.
But because the conclusion of the research is the inconvenient one, you won't read about this paper almost anywhere.
Thanks to Jaromír Petr and klimaskeptik.cz. See also Science Daily and Ars Technica