Jet lag is the feeling of disorientation when flying through different times zones. Symptoms include fatigue, loss of concentration, and headaches. People also may have difficulty sleeping at night or getting back on their normal schedule. People are on what is called a circadian rhythm and this is a twenty four hour period. When people travel across a different time zone their circadian rhythm is thrown out of sync. It can take some time to recover and get back on a normal rhythm or adapt to a new one.
Birds do not experience jet lag because they travel slowly. They do not travel as fast as airplanes and they also have no concept of time zones.
Jet lag is caused by our body clocks getting thrown off by having the sun go up and down at the "wrong" time.
I think that birds do not suffer from this for the same reason that humans did not suffer from "ship lag" or "camel lag" or anything else like that in the years before airplanes. This is because neither birds nor pre-airplane humans could travel far enough in a short time to throw their body clocks off. Instead, the time changed slowly for them and their bodies could adjust.
In international travel, especially by fast means of transportation such as by jet planes, people often travel from one place to another with big difference in their time zones within short span of few hours. This does not allow the biological clock of the person to adjust to the new time zone. As a result the body function are out of their normal rhythms, and efficiency falls, and people feel tired. This effect effect of travelling across different time zones in a short time is called jet-lag.
Migrating bird don't experience jet lag, has nothing to do with their being birds rather than humans. Birds do not experience jet lag because they do not fly as fast as the planes that carry people. This birds do not need to adjust with the time zones that fast.