Fluff-thieving birds sabotage seed dispersal.

Characterizing many species interactions as mutualisms can be misleading because some members of the interaction derive greater fitness benefits at the expense of other members. We provide detailed natural history data on a suspected bird-plant mutualism in South Africa where many species of birds use fluffy seed material to construct their nests, potentially dispersing seeds for the plant. We focus on a common bird, , which invests heavily in gathering material. Prinias spent 5 of their median 6-day nest construction period adding seed material to their nests and frequently travelled outside their territory boundary to gather material. Yet, prinias gathered primarily fluff and actively avoided gathering seeds. The average prinia nest contained only 6.6 seeds, but contained fluff from 579 seeds. These data suggest that prinias provide limited dispersal benefits to plants. By contrast, the large amounts of fluff in prinia nests, and the effort that prinias invest in gathering it, suggest that prinias benefit from constructing their nests with material. We end by outlining hypotheses for possible fitness benefits that material could provide prinias and other birds.