Cotton facilitates long‐distance seed dispersal by functioning as nest material for birds

Cotton ( Cossypium ) fibres, which grow naturally in bolls around the seeds of cotton plants, have been used for centuries to produce fabric. The presumed natural function of cotton is that these lightweight and fluffy fibres may support wind dispersal of the seeds inside. However, cotton fibres are also frequently found in bird nests and we therefore argue that cotton fibres have also, or instead, evolved to encourage ‘caliochory': seed dispersal by animals that carry seeds to their nesting locations. To test this idea, we studied cotton collecting behaviour and use in nest building in two mousebird species (family Coliidae) in Eswatini. We revealed that cotton seeds are still present in large numbers inside the fibres that mousebirds used in their nests, and when the nests degrade after the breeding season, the seeds end up on the ground. By colour‐marking individual cotton seeds on their original vines, we found that seeds within a nest could originate from multiple parent plants located considerable distances from the nest (up to 1020 m). Moreover, video recordings revealed cases where mousebirds purposely collected cotton from other nests, potentially spreading the seeds even further away from the parent plant. Together, these findings show that cotton, by functioning as nest material, facilitates seed dispersal to favourable environments over barriers and ranges that might exceed distances possible by wind. We argue that the next step in the study of ‘caliochory' is to assess the ultimate benefits that both animal and plants obtain from seed dispersal through nest material, in order to determine whether cotton fibers have evolved specifically to facilitate seed dispersal.