swallow with collar

swallow with collar

Observation - swallow with collar - UK and Ireland. Description: one swallow from a group greatly enlarged to show what seems to be a 'collar'. Is it something the swallow has picked up or is it some sort of tracking device ?

one swallow from a group greatly enlarged to show what seems to be a 'collar'. Is it something the swallow has picked up or is it some sort of tracking device ?