Mike Young and Tom Doak bring up a good point. More and more bunkers are being crafted with excavators. Especially with the increasing popularity of 'mini-hoes'. Bunkers are a product of the equipment available to build them with and to a trained eye, the era of construction can be determined ie, slip-scoop - early 20th century, straight blade D-6 post war-70's, 6-way blade 80's-90's, and now track-hoes and rubber-track bobcats.
Having done it on a dozer and with mini-hoes, I much perfer the later. Even a small dozer has limitations due to it's size and the length of the blade. Often you have to work from the outside - in, because there is not enough room to manuver in a small bunker. Also, you can get a much better concave cavity face with a hoe than a dozer. Sorry if this to too much information but it has a lot to do with the final product.