Although it doesn't show airline miles,I just use google map. I right click on one spot and pick directions to here and I look at the right and it shows the distance. Everyone knows that pigeons don't fly straight as well. They even use roads to navigate as well.
There is also a measure tool in google earth that allows you to measure "as the crow (or pigeon) flies"... I've got all my release locations stored in there as well for my new birds, so that I can tell where and how far they've been before.