Oxwich Marsh 27 July

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A steady session this morning.  Less than thirty birds were caught, but fifteen of these were reed warblers, and there were also three sedge warblers, a whitethroat and two blackcaps.  The remainder of the catch comprised wrens, blue tits and a robin.

Following a steady run of Cetti’s warblers earlier in the season, none have now been trapped in the last three sessions (although males were heard singing at two locations within the reedbed).  Also notable by their absence in the nets were Phylloscopus warblers: a willow warbler was calling near the ringing station for much of the morning, and both willow warbler and chiffchaff have featured heavily in the overall totals in the past few weeks.

One of the more interesting captures this morning was a male blackcap (pictured below).  Juveniles undergo a partial moult which takes in the body feathers and some of the wing feathers (but not the primaries, secondaries or tail).  This bird had fresh primaries and secondaries, but was in the process of moulting its greater coverts and body, and had a unworn tail with narrow feathers, hence the conclusion that it was a juvenile.  However, most juveniles cannot be sexed until later in the autumn, as the black head of a male typically only shows through after the head feathers have started to wear.

The number of reed warblers in the catch allowed us to compare adult and juvenile plumage.  The juveniles were warm brown in colour and showed minimal wear to their wing and tail feathers.  The adults, by contrast, were a dull brown, their wing feathers were heavily worn, and some of their central tail feathers were very tatty.  This general scruffiness was accentuated by moult of body feathers, particularly those around the head and neck.

Interestingly (for us at least!) a number of the adults showed reasonable fat deposits in their tracheal pits, suggesting they were feeding up in preparation for migration.  The juveniles had less obvious deposits (and would be expected to migrate later).  In the two photographs below, the adult is on the left and the juvenile the right.

Thanks to Keith Vaughton and Cerian Thomas for the assistance this morning and for an enjoyable session.

Owain Gabb


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