The next day I heard singing in a tree adjacent to that field and it was clearly a mimicking song. I was sure the Mockingbird was the source so it was to my surprise that I found out it was a Brown Thrasher that was singing! This is a "cousin" to the Mockingbird and although I have seen a few, I have never heard one sing.
That same day, as I progressed down my usual path towards a small grove of trees, I heard another mimicking song. Ok, so was this the Mockingbird or the Thrasher? Neither! It was another "cousin" - the Catbird. They all belong to the family "Mimidae".
Here are the three cousins:
Northern Mockingbird
Brown Thrasher
Below is a 40 second video of this Thrasher singing:
Gray Catbird
Below is a 40 second video of the Catbird singing (he is well hidden in the bushes but you'll see a glimpse of movement towards the end):