I'm talking about a completely missing foot like this guy:
|Adult Ring-billed. Chicago, IL; 14 October 2011.|
A quick google search of "gull missing foot" will provide tens and tens of hits that describe this unfortunate phenomenon. The culprit? Most times it's fishing line. Like other species that spend a significant amount of time near water, the risks of getting tangled up are prevalent, but even more so with gulls because they tend to gravitate towards fishermen. Most that survive this traumatizing experience seem to eventually manage just fine.
Here are a few YouTube videos that demonstrate this ongoing clash between gulls and fishing line:
Another explanation for missing limbs stems from adults biting newly fledged chicks. It's very common for adult gulls to attack the young of their neighbors, especially if one has wandered out of its designated "space". Here's a great video showing how these "spaces" are established and maintained: