Oxwich Marsh: early August warbler movements …

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Despite some showers around dawn, the weather was good at Oxwich, only becoming breezy at around 09:30.  A total of five sixty-foot nets resulted in an overall catch of 40 birds, most of which were trapped in the first couple of hours.  The catch included 20 sedge warblers, 11 reed warblers, 2 blackcaps and a range of other species including reed bunting, robin, wren and blue tit. 

The highlight of the session was the capture of a juvenile grasshopper warbler, the first caught at the site in 2013.  This bird was carrying significant fat (fat score 5), indicating it was feeding up prior to migration.  Fat is assessed using a scale ranging between 0 (no visible fat) and 8 (a layer of fat overlying all of the breast muscles), and can be easily assessed by blowing the breast feathers to expose the underlying skin.  The fat score for this bird (5) indicates that the tracheal pit was full (convex), and fat was slightly overlapping the breast muscles.  A picture of the bird is below.

Sedge warbler has not been commonly captured at Oxwich this year: the number of territories around the reedbed earlier in the season appeared to be low.  This, and the extensive fat deposits visible on many of the sedge and reed warblers trapped, indicated that these birds had predominantly moved into the reedbed to feed up in preparation for migration (and were unlikely to have bred close by). 

Fourteen of the sedge warblers were juveniles.  These typically showed very little wear in the wing feathers and tails (some of which also showed fault bars and castellations).  Fault bars that run across all of the tail are indicative of juvenile birds, which grow their tail feathers synchronously.  Adult birds replace tail feathers one-by-one, so while individual feathers may have fault bars, these are very unlikely to form a continuous line.  Castellations are ‘nicks’ in tail feathers thought to result from damage in the nest. 

Oxwich is an excellent site for reptiles and invertebrates.  Today, a hummingbird hawkmoth was seen nectaring on bramble close to the ringing site, a buff tip (moth) caterpillar was moved from the adjacent road into nearby shrubs, and the hoverfly Sericomyia silentis (a substantial insect) was noted on ivy, where it posed for this photograph.  A brief search on Oxwich Marsh after the session resulted in three adders being seen.

Thanks to Keith Vaughton and Barry Stewart for making it another enjoyable session this morning.

Owain Gabb

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