INFLUENCE OF SEED PROCESSING BY FRUGIVOROUS BIRDS ON GERMINATION SUCCESS OF 3 NORTH-AMERICAN SHRUBS

Coevolutionary models of the interactions between fruiting plants and avian seed dispersers have been influenced by the assumption that regu rgitation and defecation of seeds have diffferent effects on seed coat s, and consequently seed germination. We evaluated how the manner of s eed processing affects seed germination by feeding fruits of three bir d-dispersed shrubs, spicebush (Lindera benzoin), chokecherry (Prunus v irginiana), and arrowwood (Viburnum dentatum), to captive cedar waxwin gs (Bombycilla cedrorum) and thrushes (Turdus migratorius, Hylocichla mustelina, Catharus guttatus and C. minimus).Cedar waxwings defecate a ll seeds, whereas thrushes regurgitate most seeds. For all three shrub species and all five bird species, there were no differences in germi nation success between seeds manually cleaned of pulp, and cleaned, bi rd-passed seeds, regardless of whether seeds were regurgitated or defe cated. However, seeds of Lindera, and Prunus that were defecated by ce dar waxwings and planted with feces, mimicking the depositional enviro nment of defecated seeds in nature, suffered reduced germination relat ive to cleaned seeds. Thus, our results do not suggest that whether se eds are regurgitated or defecated is an important component of dispers al quality because of direct effects on the seed coat. Instead, seed p rocessing modes differed in their effects on seed germination because of the feces associated with defecated seeds. Removal of fruit pulp fr om seeds by frugivores, a fundamental consequence of dispersal in anim al guts, was critical for germination, especially for seeds within the lipid-rich fruits of Lindera and Viburnum. This suggests that for som e fruiting plants, frugivores procide an essential service by freeing seeds from fruit pulp, in addition to their role in seed dispersal.