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Hi Bruce
The evidence shows that weedy stubbles support more birds and that areas with more stubbles through the winter have lower rates of declines of birds such as skylarks and yellowhammers, so weed seeds seem to be a critical factor. There is also evidence that stubbles which do not receive pre-harvest desiccants or post-harvest herbicides provide more seeds and support more birds.
Although growing weeds may get chopped by the combine, I suspect that they are not killed by harvesting, especially those with a sprawling growth-form. They will then have a flush of growth after the combine has removed the competition from the crop.
Important weed groups for birds include polygonums, chickweed, fathen, charlock and annual meadow grass, which all produce plenty of seeds. The cover provided by stubbles may be an added advantage, but clean stubbles can be as empty of seed-eating birds as winter wheat crops.
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