This year 15 per cent of arable land has not been planted with crops in an effort to cut surplus production.
The set-aside land has attracted large numbers of ground nesting birds including pheasants, partridges and lapwings as well as skylarks. They prefer it to planted land because the dense growth of crops is difficult to nest in.
The new rules allow farmers to plough this set-aside land in May in order to remove weeds before they produce seeds. The farmers want to prevent weeds becoming established because next autumn this land will be put back into production and other fields will be set aside to take their place.
But the May ploughing-in of weeds happened before many of the chicks left their nests. The ornithology trust says the fate of the young skylarks is particularly tragic because this much- loved species is in decline in Britain.
The Ministry of Agriculture is likely to change the rules governing the set-aside scheme next year in order to make them more 'bird friendly'.
The changes will probably allow weed control to be carried out later or earlier in the year, outside of the main bird nesting season.