The Callery pear (perhaps you know it as a Bradford pear) came to the US as a gift from Asia early in the last century, and after commercial cultivars were introduced by the USDA in the early 1960's, it was quickly adopted all over America as the perfect landscape tree. Fast growing, nicely shaped, lovely white blossoms in early spring, masses of green leaves in summer and spectacular fall color; there was almost no concern about its invasive nature since the cultivars did not produce viable seeds.
You met the European starling (a gift from Europe) in an earlier post. It loves the fruit of the Bradford pear, and the pear produces fruit in incredible abundance. Unfortunately, over time, the various cultivars of the Callery pear have become so commonly planted they've managed to cross-pollinate and bear fruit with viable seeds. The root stock used for grafting various cultivars is often from a different seed-bearing genotype as well. When the roots then produce flower-bearing shoots, they can fertilize the blossoms of the grafted cultivar, yielding fruit with fertile seeds.
When the non-native starling feasts on the thousands of frost-softened fruit of the non-native Callery pear, the now-viable seeds are often deposited on fertile ground, creating thickets of pears which out-compete and stifle many native plants. Starlings flock to these fruit-filled thickets, ...
and the "gifts" just keep on giving.