This morning I spent some time at Stanford Reservoir with the Stanford Ringing Group. With summer all but over, the number of birds trapped was relatively low with 48 new birds of 13 species ringed (25% of which were Swallows) and 18 retraps of 11 species.
When not singing or calling, two of these species, Chiffchaff and Willow Warbler, present perennial ID difficulties for many birders in the field but not, of course, in the hand where biometrics and key features not obviously visible to field observers make separation easy. However, one of these features – namely wing structure – can be used in the field, if the bird stays still long enough for it to be assessed! Willow Warbler has longer wings than Chiffchaff, which is illustrated well in the below set of photos of two of the individuals trapped at Stanford this morning.
These two photos show the diagnostic emarginated 6th primary of Chiffchaff and its absence in Willow Warbler – not visible in the field, of course – but in the image of Chiffchaff the short first primary is tucked away and not visible and the second primary is barely visible behind P3.
The above two show the difference in length of primary projection: in the Chiffchaff it is little more than half the length of the visible tertials while Willow Warbler has a much longer primary projection – often the same length as the tertials and at least three quarters the length in the shortest instance (click on image to enlarge).
Now have a go and apply this to the bird below, recently photographed by Doug McFarlane in Moulton. See here for the correct answer!
The final image is a bright juvenile Willow Warbler which, with its vivid yellow upper breast and whitish belly, is a pitfall for the unwary, sometimes accounting for erroneous reports of Wood Warblers in autumn …
Many thanks to John, Mick, Adam and Dawn for putting up with me and allowing me to photograph ‘their’ birds at Stanford this morning!