Is switching from meat to more grains better for birds? I strongly doubt that. Converting fields to planting fields is going to kill a lot more small mammals, birds, insects and so on?
This raises a number of problems:
- the poo and urine from the farm animals is way, way too much to distribute on the fields around these mega-farms, meaning it has to be transported across Europe. The worst offenders here are the Dutch, which have so much manure that it's impossible to spread out all of it in the country, and plans to close down farms have led to protests and riots [3].
- These farms themselves, as well as the slaughterhouses, are a blight on their neighborhoods - from personal experience, it's bad enough to live next to half a dozen cows, but large farms? These can be smelled many kilometres away.
- all these farm animals have to be fed insane amounts of food - depending on which study you believe and which animal you look at, 1kg of animal meat requires anything from 3 to 20kg of plant-based food (most often, grain).
The last point is where the meat is at: say, you cut down the demand for meat in half. For Germany, that would mean going from 7.6 million tons a year to 3.8 million tons - by the leverage effect anything between 12 to 38 million tons of grain production can be avoided, or to put it bluntly: out of the 43 million tons of grain produced in Germany in 2022 [5], you can avoid around 25%. Obviously the numbers are a bit off due to imported food and livestock, but the estimate is that, yes, in fact about half the grain produced in the country is food for livestock [6].
Even assuming that humans will eat a bit more grain to compensate for the calories they would have gotten by meat, it's still safe to assume that you could reduce total grain growth by at least 10% - enough to provide strips of greenery surrounding fields and to make them otherwise more friendly for animals. The reduction of manure production would lead to higher water quality in rivers and more aquatic life, and the space occupied by large stables can also be returned to nature for local environments.
[1] https://www.sueddeutsche.de/wirtschaft/gefluegelmaester-in-d...
[2] https://www.boell.de/de/2016/01/13/mecklenburg-vorpommern-wo...
[3] https://www.ndr.de/nachrichten/niedersachsen/Nitrat-im-Grund...
[4] https://www.bmel-statistik.de/ernaehrung-fischerei/versorgun...
[5] https://www.destatis.de/DE/Themen/Branchen-Unternehmen/Landw...
[6] https://www.bmel-statistik.de/ernaehrung-fischerei/versorgun...