We recorded the species composition of mixed bird flocks, during 366 h of fieldwork in seven localities along an altitudinal gradient on the Central Andes of Colombia. We observed 164 species in 212 mixed bird flocks. Species composition varied along the altitudinal gradient and 27 species could have a cohesive role in the flock, but almost all of them were different among localities. The number of species per mixed flock varied from two to 30. The mean number of species per flock also varied among localities, and the differences among the whole set of species joining mixed flocks in each locality seem to be related with both differences in altitude and geographic distance.