Massive and effective acorn dispersal into agroforestry systems by an overlooked vector, the Eurasian magpie (Pica pica)

Oak regeneration and the expansion of forested sites in Eurasia rely on acorn dispersal by animals, especially the Eurasian jay (Garrulus glandarius). However, in open agroforestry systems where jays are absent, such as old fields far from acorn sources, oak recruitment still occurs. We hypothesize that the Eurasian magpie (Pica pica), an abundant corvid in this system, substitutes the jay in its seed dispersal function. By ringing 169 magpies, video recording >7500 acorn removal events with trail cameras, and radiotagging 337 acorns, we quantified that (1) magpies cached 41–56% of the annual acorn production of Quercus ilex trees in single caches on the ground; (2) breeding pairs, and especially males, were the main acorn dispersers; (3) each breeding magpie cached 169–1372 acorns in 6 weeks; and (4) the effectiveness of dispersal (percentage of cached acorns resulting in seedlings) was 0.6–2.4%, which (5) yielded a high density of emerged seedlings (56–439 seedlings/ha). We evidence that magpie could be a key species in the regeneration of oak agroforestry mosaics because they massively and effectively dispersed acorns. However, in our particular study site, effectiveness was low probably due to herbivory and summer drought stress (i.e., a context limitation rather than an intrinsic limitation of the disperser). As the distributions of magpies and oaks overlap widely in Eurasia, effective acorn dispersal by magpies could have a significant role in largescale oak forest recovery in strongly fragmented landscapes.

This study was funded by the Spanish Ministerio de Ciencia y Tecnología (CGL2014-53308-P), the REMEDINAL network of the Madrid Autonomous Government (S2013/MAE-2719 and S2018/EMT-4338), and the University of Alcalá (CCG2014/BIO-02, UAH-GP2019- 6). LMB was supported by an FPI fellowship from the Ministerio de Ciencia y Tecnología (BES-2015-075276). ABL acknowledges Juan de la Cierva postdoctoral fellowship and project RTI2018-096187-J-100 from Ministerio de Ciencia, Innovación y Universidades.