AbstractBirds are important biodiversity indicators because they depend on a range of invertebrates and plants for food. Farmland bird populations have declined dramatically, especially in Europe, in the latter half of the 20th century due to intensification in agricultural practice. It is important to assess the potential impacts of new technologies, such as genetically modified (GM) crops, on biodiversity given the large amount of funding spent on schemes designed to aid wildlife on farmland. There is not, to my knowledge, any published evidence of direct effects of GM crops on birds. However, there was considerable evidence of potential indirect effects of GM crops on farmland birds available from the recent UK Farm-scale Evaluation (FSE) trials (see Chapter 2, this volume). Results suggested that three out of four varieties of genetically modified herbicide-tolerant (GMHT) crops (spring-sown and winter-sown oilseed rape and sugarbeet) will support between two and three orders of magnitude lower weed abundances than conventionally managed crop varieties. These results were caused by differences in the type of pesticides sprayed on GMHT and conventional crops. For one crop (maize), there were more weeds on the GMHT crop than the conventional variety. Weeds provide key food resources for birds both directly, via seeds, and indirectly, via the invertebrate populations that they support. The declines in weed seed resources reported on the three GMHT crops suggest they have the potential to markedly reduce food resources for farmland birds, although the magnitude of how these changes in food could affect population levels is not currently known. If expensive schemes designed to enhance biodiversity on farmland are not to act in opposition to the environmental effects of GMHT crops then we need new ways of implementing GMHT crops to reduce their effects on weeds.