Worthen's Sparrow (Spizella wortheni) is one of the most endangered species in Mexico. Its survival depends on appropriate conservation strategies and reliable information about the species. Our study on winter groups of Worthen's Sparrow flocking with other grassland bird species provides preliminary ecological and behavioral information on this species, and its relation to other migratory and resident bird species in La Perforadora Valley, Coahuila, Mexico. We observed flocking associations of Worthen's Sparrow with 16 other species. Ochiai's Association Coefficient indicated Worthen's Sparrow often associated with Vesper Sparrow (Pooecetes gramineus) (0.60), Black-throated Sparrow (Amphispiza bilineata) (0.33), Horned Lark (Eremophila alpestris) (0.27), and Western Bluebird (Sialia mexicana) (0.22). Worthen's Sparrow occurred more frequently in mixed than in monospecific flocks in winter. It foraged more frequently on the ground but also searched in shrub stems, branches, and foliage from perches, and at times hawked for flying insects from perches. Most perches were in the shrub stratum, but fences also were used. Worthen's Sparrow in mixed-species flocks spent more time in foraging activities than those in monospecific flocks.