Sunday, 11 April 2010

A fairly quiet sunny day was nevertheless a pleasant one with reasonable numbers of migrant birds to be seen. Ringing was again carried out for most of the day and a fair total of birds were caught – the highlight of which was the unexpected appearance of two Grasshopper Warblers in a mist net at Cristin.

Grasshopper Warblers (c) Richard Brown
Grasshopper Warblers are very variable in their colouration. The undersides of the birds can be almost creamy-white through to very rich and deep yellow. Some birds can have no streaks on the breast while others can be very boldly marked. Neither of these two birds were extremes of the variation that we have seen with the birds we have trapped over the years, although the differences can be seen between these individuals.

In total18 Blackcaps were also ringed, illustrating the number of these birds that must go unnoticed when the mist nets are not open: only a couple were seen in the field all day. 60 Willow Warblers and 39 Chiffchaffs were logged, along with 11 Goldcrests, 123 Meadow Pipits, a Tree Pipit, seven Wheatears and five White Wagtails. The first Sedge Warbler of the year was in song at Nant, and the Tree Sparrow was seen briefly at the Observatory. Two fairly late Fieldfares and a Redwing were present in the lowlands, while a trickle of hirundines passing through included a couple of House Martins among 29 Sand Martins and 15 Swallows. Four Ringed Plovers were on the coast and the Jacksnipe and a single Common Snipe were again flushed from the ‘Badlands’. Finch passage continued with 26 Goldfinches, 14 Lesser Redpolls, seven Siskins and two Greenfinches.

Identification of Willow Warblers and Chiffchaffs
We are often asked the differences between Willow Warbler and Chiffchaff when we have birds in the ringing lab. Here is a quick summary of the main differences that are visible for identification in the field (in-hand features such as wing formulae will be covered at some other time).

Willow Warblers are generally larger birds and are normally paler in their overall appearance. The Willow Warbler on the left looks cleaner and less  'dingy'. Its face is more clearly marked with a stronger, paler supercilium and it has a slightly stronger bill.

As well as the slight plumage differences there are structural differences that can be seen in the field. The length of the exposed primaries (long flight feathers) beyond the tertials is one of the better structural features to use. On Willow Warblers (left) the  primaries extend almost 2/3 the length of the exposed tertials, whereas Chiffchaffs have a much shorter primary projection (about 1/3 the length of the exposed tertials). The primary tips on the Chiffchaff are also worn, these having been used by the bird to migrate to and from its winter quarters. The Willow Warbler, however, having undergone a complete moult in Africa, has much fresher primary tips.

Another useful feature (though not completely 'fool-proof') is the leg colour. The Willow Warbler on the left has much paler, fleshy brown legs, whereas the Chiffchaff has blackish legs. Some Willow Warblers can, however, show rather dark legs, though Chiffchaffs never have pale legs.

Willow Warbler (left bird in all images) and Chiffchaff (c) Steven Stansfield

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