The seeds are spread by birds and mammals. Squirrels, opossums, quail, and turkey are known to eat the seeds in the native environment. But Magnolia grandiflora produces phenolic antimicrobial chemicals, coumarins and sesquiterpene lactones, which discourage predation and grazing :)
The specie sis commonly planted here around Lyons.
"Cones" (infrutescences) are usually shed in late autumn. The red seeds are born on a fleshy stalk which is not sweet (tasted). I have not observed any animal feeding on these, here in Lyons.
At the begin of July, here in Toledo (Spain) we have flowers in Magnolia grandigolia. Now the cones are madurating the seeds (https://app.box.com/s/8dfsfc58irttrg6wbqyzdfmniidbmzzh).