Nice observation. I see that BTO Nest record scheme says for First Clutches Laid average date is 21 Apr and range is 21 Mar – 21 Jun. Incubation and fledging are each 14 or 15 days. So juveniles on April 20th is certainly at the early end of the range, but we are in the south, the site is lowlands, and it’s getting warmer.
Glamorgan Bird Club 
Nice observation. I see that BTO Nest record scheme says for First Clutches Laid average date is 21 Apr and range is 21 Mar – 21 Jun. Incubation and fledging are each 14 or 15 days. So juveniles on April 20th is certainly at the early end of the range, but we are in the south, the site is lowlands, and it’s getting warmer.