多了 used as "to have"
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.