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Selective seed consumption by harvester ants may be affected by several seed attributes, amongst which seed size and environmental availability play a prominent role. In the present study, we considered the effects of seed size and seed availability on the coexistence and diet preference of two Messor species (Messor wasmanni and Messor minor). M. wasmanni colonies collected the most abundant resource in the environment, grass seeds (Poaceae), according to their availability whilst M. minor showed a nonselective process. In addition, the two ant species showed a different seed size preference, with M. wasmanni adopting a selective strategy and M. minor a generalist strategy. However, competition for foraging resources between the two ant species (assessed by null model algorithms) seems not to affect their foraging behaviour. The lack of a competitive structure within the ant assemblage suggests, in fact, that resources are not a limiting factor for species coexistence.