Friday, January 28, 2011 - Bosque Birdwatchers RV Park, San Antonio NM

Movement in a column is always the fastest way to move when there are obstacles, and there are always obstacles on the ground. If the group has to swim through some reeds, or go around an angry heron, it's easiest to just follow the leader. Swimming in a spread out formation requires teamwork which most animals aren't willing to commit to.

In addition, moving behind the leader allows you to draft, reducing the amount of drag you face, but this is only relevant if you are pretty close.

Source:Yahoo!7 Answers

The Mallard is a rare example of both Allen's Rule and Bergmann's Rule in birds. Bergmann's Rule, which states that polar forms tend to be larger than related ones from warmer climates, has numerous examples in birds. Allen's Rule says that appendages like ears tend to be smaller in polar forms to minimize heat loss, and larger in tropical and desert equivalents to facilitate heat diffusion, and that the polar taxa are stockier overall. Examples of this rule in birds are rare, as they lack external ears. However, the bill of ducks is very well supplied with blood vessels and is vulnerable to cold.

Source: Wikipedia