Amazon executive William Stasior has reportedly left the company in favor of Apple. Stasior is expected to oversee Siri following the recent departure of Siri co-founder Adam Cheyer last month. Siri CEO Dag Kittlaus checked out in October 2011.

Stasior was in charge of A9, an independent subsidiary of Amazon that runs the company's search efforts across all Amazon sites globally. Before taking over A9 in February 2006, he led product and engineering for the Search and Navigation team.

The executive joined the prominent e-retailer in 2003 after spending time at Oracle, Netcentives and Alta Vista. He is an MIT PhD graduate and has even spent some time teaching classes at his alma mater.

As the new head of Siri, most expect Stasior to help bolster the service's search capabilities and transform it from a simple personal assistant into a full-fledged voice-activated search engine. He is expected to join Siri co-founder Tom Gruber who is believed to still be working for Apple.

If you recall, Apple CEO Tim Cook pointed out during his interview at the All Things D conference earlier this year that they've only scratched the surface of what Siri is capable of. The chief said there are a lot of people working on the service and there is plenty more that it can accomplish that has yet to be seen.

Siri was first introduced on last year's iPhone 4S. The product still remains in beta status according to Apple.