Last recorded
In a location that you monitor regularly (at least once a week), record the date on which you last see a swallow before it migrates in September.
As the departure time gets closer keep a note of the dates when you do see the birds. Do this until you no longer see them and then you have your ‘last recorded’ date.
Available historic data: we have very good records from 1891-1999 (7384 records)