Friday, 15 July 2011

Grasshopper Warblers are on the Move

My first ringing session of the autumn at Titchfield Haven this week, and was somewhat surprised by the numbers of some of the birds we were catching. Our ringing site is set within an area of damp scrub, and adjacent to an area of grazed pasture, reedbed and the lower reaches of the River Meon. Typically, as you can probably image, it is a great site for warblers, in particular Reed and Sedge, but over the last few years it has also been an excellent site for catching large numbers of migrating Grasshopper Warblers. Our highest total for this species is over 550 birds in one autumn, with the earliest capture date being the 15th July. Well on Thursday 14th July, we caught just over 70 birds, of which 36 of them were Grasshopper Warblers, but even more surprising was that prior to this session 35 had already been captured that week. So in our first weeks trapping of the season we have captured over 70 Grasshopper Warblers already.

Juvenile Grasshopper Warbler

Grasshopper Warblers are a relatively easy species to identify having generally brownish upperparts, and variably coloured underparts which range from white to bright yellow. The crown, back and rump are faintly tipped white, and the under-tail coverts are diffusely streaked down the centre.

Grasshopper Warbler Under-tail Coverts 

Ageing Grasshopper Warblers in the autumn is very straight forward; juvenile birds have very fresh plumage, and the primaries are not worn or bleached............. 

Fresh Primaries of Juvenile Wing 

                               ...................whereas adult birds show varying amounts of old and new feathers, with the old features being very worn and bleached. 

Abraded Wing of Adult Bird

Adult Grasshopper Warblers undergo a partial summer moult, which is why they show both old and new feathers in the autumn. There does not seem to be a pattern to this moult strategy with feathers being replaced anywhere on the body and within the wings and tail.

Worn and Fresh Feathers of Adult Bird

Both adult and Juvenile birds undergo a complete moult on there wintering grounds, and therefore cannot be aged when they return in the spring.

Partial Albino Wing of Juvenile Grasshopper Warbler

The most interesting bird captured on the 14th July was a Grasshopper Warbler with the outer five primaries on each wing pure white. I have seen many partially albino birds over the years, but usually there are just a few random body feathers or a white head, and the most common species has been the Blackbird. Last year I captured a Song Thrush and a Blackcap that both had varying degrees of albino in their primary feathers, but I have never seen a Grasshopper Warbler with all white primaries like this.

Partial Albino Wing of Grasshopper Warbler

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