"the bird"
It is an additional static source designed to provide backup or possible un-interuppted data to compare with the static ports on the aircraft that are known to be inaccurate if improperly placed or if the airflow over them is abnormally disturbed. They are for proving runs only.
This allows modern aircraft to be RVSM capable out of the box. I remember having to help set this rig up on DC-9's within the last decade when US airspace was to be mandated RVSM as part of a measure to qualify them.
Currently a designated diamiter around the static port needs to be inspected for dents or abnormality's to maintain RVSM status.