I tread carefully over a carpet of field pansies and scarlet pimpernel. A host of bees and other pollinators buzz around me. Overhead fly linnets and a family of corn buntings. The nearby grassy margin is alive with grasshoppers and moths. And I am reminded once more of the value of arable weeds to the threatened farmland birds that do so well in the fields I study just south of Cambridge.

Weeds provide food for the birds in the form of leaves, shoots and seeds. They are host plants for the invertebrates that grey partridge chicks and other young birds depend on in their early days. As I describe in A haven for farmland birds, the intensification of farming and in particular the increased use of pesticides are among the main reasons for the shocking declines of farmland birds.

During the spring and summer of 2023, three of the fields – owned by Cambridgeshire County Council and farmed by Peter Wombwell – were planted with beans. As May progressed, a variety of weeds grew between the bean plants, including on the right fumitory, a plant whose flowers provide nectar for pollinators, whose leaves are eaten by a range of invertebrates, and whose seeds provide food for finches and buntings.

As the beans ripened in late July, grasses and flowering plants came into their own:

By early August I was recording good numbers of butterflies, notably small heaths and gatekeepers, but also the stunning brown argus butterflies (right, a female), which already have a small colony elsewhere on the site. Might they be establishing a colony here as well?

Once the crop was harvested around August 17th, the potential benefits to wildlife became even clearer. As well as the pansies and scarlet pimpernel, there was a riot of other flowering plants, including poppies, thistles, yarrow, and mayweed. Many of the flowers were going to seed, attracting finches and buntings. One grassy corner had been left uncut, providing potential cover for the grey partridge that have hopefully been able to raise their young here.

The fields were cultivated a week after harvest, bringing the weed bonanza to an end, but they had nonetheless provided valuable additional food through a crucial time of the year. I checked with the landowner whether herbicide had been applied and although the farmer did spray early, the weather and the type of herbicide used combined to reduce the impact on weeds. I also checked whether the weeds may have had a negative impact on the crop. In practice, yield was down by a quarter to a third, but this was principally due to the frosty weather which knocked back some of the bean plants and left the crop a bit more gappy than usual (which also benefited the weeds and butterflies).

One of the most interesting things about the area of land I study is that such a good level of biodiversity is able to coexist with intensive and profitable agriculture. Several factors contribute to this, of which the variety of habitats (particularly margins) is key, but reduced pesticide use like this is clearly also important.

And on September 11th I recorded a group of 17 grey partridge by one of the fields – a remarkable number which suggested that the young birds had received a good diet in the crucial early weeks.