The oak forests of eastern North America moved north after the last ice age in the beaks of blue jays.
When the last glaciers retreated about 10,000 years ago, the trees of eastern North America had to migrate north to recolonize the deglaciated land. Most tree seeds (maples, pines) are wind-dispersed and can travel widely.
Acorns are heavy. They fall straight down. By every reasonable model, oaks should have moved north at a rate of a few meters per year.
The fossil pollen record shows they moved closer to 350 meters per year. Faster than wind-dispersed species. Almost impossibly fast for a heavy-seeded tree.
The 1989 Johnson and Webb paper in the Journal of Biogeography identified the mechanism. Blue jays carry acorns much farther than squirrels, preferentially bury them in disturbed open ground at the right depth for germination, and forget enough of them to keep the species moving. A single 50-jay flock in Virginia was documented caching 133,000 acorns in one autumn.
The eastern oak forest, the largest deciduous forest ecosystem in North America, exists in its current form in part because of a bird most people consider a nuisance at the feeder.