i was undertaking a common birds census last week in Somerset and came across a Willow Warbler which was singing unusually. The bird would sing a series of songs, some of the series would be pure Willow Warbler, others would be Willow Warbler to start and finish but in the centre there would be an exact imitation of a Chiffchaff. The amount of Chiffchaff notes varied within any given series of songs. The bird looked and acted, to my eyes, like a Willow Warbler.
I have attached a photo of the bird singing and i have also uploaded two videos of the bird singing onto youtube, link below:
During a search of the internet i have found very similar songs on the Dutch birding website, where a song starts and finishes Willow Warbler with a Chiffchaff middle section. On the website these have been labelled as hybrids.
Has anyone else had experience of similar birds?. Also does anyone know whether hybrids between the two species have ever been proved for definite.