Use a Copy Data activity to copy the entire collection from Cosmos DB to Blob Storage without any partitioning.

Use a Data Flow activity to read the collection, partition it by date of birth, and then write each partition to Blob Storage.

Use a Lookup activity to retrieve the distinct dates of birth, followed by a ForEach activity to iterate over the dates and a Copy Data activity to copy each partition to Blob Storage.

Use a Web activity to trigger an external script that partitions the collection before copying it to Blob Storage.