Thursday, 8 August 2013

At last a juvenile Cetti's warbler .............

In order to try and maximise our capture of reedbed warblers at Oxwich, Barry Stewart and I visited Oxwich this morning.  Having arrived at around 05:30, we had five sixty-foot nets up by 06:15, allowing us an hour and a half of catching time before work.  The weather was perfect, with very light winds, although mist lingered for a while over the reeds and seemed to supress early bird activity.

Despite this, the overall catch numbered 28, of which 10 birds were reed warblers (two were retraps from previous sessions) and 7 were sedge warblers (all new birds).  Other species included two bullfinches (both juveniles), a song thrush and an adult female reed bunting, the brood patch of which was just beginning to feather over.  However, the star bird of the morning was a Cetti's warbler.  This is not an unusual species at Oxwich: there are several territorial males around the marsh, and birds were trapped in the spring.  However, the bird trapped was a juvenile, the only one of the year to date.  This provides strong evidence of breeding on the marsh, as the post juvenile moult of the bird was not well advanced, and there is no other suitable breeding habitat close by.

The Cetti's warbler is shown below.  The ten (as opposed to twelve in all other UK passerines) tail feathers characteristic of this species can be clearly seen.


Another interesting observation was the tail of this reed warbler (photo below).  While reed warblers are not a difficult species to age, the tail bar on this bird was incredible.  As young birds grow their tail feathers simultaneously, a strong fault bar (indicating a period of environmental stress) that goes across the tail indicates a juvenile.  Adults replace feathers sequentially.  Therefore, while individual feather might show fault bars, in adults these bars will not tend to line up.
 
The final photograph is of a song thrush.  The first for the site in 2013, and still in full juvenile plumage (the belly had not begun to feather up).


Owain Gabb
08/08/2013

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