Flocking behaviour has been studied for decades around the world, but there is little knowledge of this subject in desert bird communities. We investigate winter flock structure in open woodlands of the central Monte desert, Argentina, during two consecutive years. Of the 22 bird species that were recorded in flocks, most were granivorous. There were differences in flock density between years, probably due to fluctuation in seed abundance. More than half of the observed flocks were composed of different species (i.e. mixed-species flocks). We detected only two regular species (found in more than 75% of flocks): Rufous-collared Sparrow Zonotrichia capensis and Ringed Warbling Finch Poospiza torquata, both granivorous birds. Ringed Warbling Finch and Cinnamon Warbling Finch Poospiza ornata co-occurred at a higher frequency than expected by chance, and formed subgroups within flocks. Many-colored Chaco Finch Saltatricula multicolor, Greater Wagtail-tyrant Stigmatura budytoides, Brown Cacholote Pseudoseisura lophotes and Short-billed Canastero Asthenes baeri were found to form family groups, which may be related to delayed natal dispersal. The observed flocking propensity of granivorous birds and the small number of species per flock were similar to that found in other desert bird communities. We discuss some differences and similarities between granivorous and insectivorous flocking patterns, and propose hypotheses on the plausible causes. Our results provide a basis for a better understanding of the flocking behaviour patterns of birds in arid environments.