多了 used as "to have" A carrier pigeon brings a letter. In the sentence ”鸽子的腿上多了一封信”, I do not understand the use of 多了, unless it is used here as a verb meaning &quot;to have&quot;, as in &quot;the carrier pigeon had on it's leg a letter&quot;. However, I haven't found any grammar or discussion about 多 or 多了 that  points to it being used with such meaning.
多 = more, much, extra
多了一封信 = to have an extra letter

Another example:

我们组多了一个人。
There is an extra person in our group.

In this case, the sentence can be translated to:

There is an extra letter on the carrier pigeon's leg.

Note: This sentence doesn't make sense as a single sentence. The pigeon has to be mentioned somewhere in the previous text.