Tuesday, 9 August 2016

Oxwich Marsh: early August warblers and hirundines

A couple of sessions in short periods of settled weather at the start of August have proved reasonably productive. The first, on Saturday 6 August saw us deploy a total of 840 feet of net, predominantly in the reed bed and nearby rushy ground. The second was a roost visit for swallows, and we limited ourselves to a line of nets along a bund through the marsh (total length 280 feet).

The total catch over the two visits was as follows:

Species
Ringed
Recaptured
Total
Sand Martin
1
0
1
Swallow
107
1
108
House Martin
1
0
1
Wren
3
0
3
Robin
1
2
3
Song Thrush
1
1
2
Grasshopper Warbler
1
0
1
Sedge Warbler
13
4
17
Reed Warbler
18
8
26
Whitethroat
4
0
4
Blackcap
4
1
5
Chiffchaff
6
0
6
Willow Warbler
7
3
10
Blue Tit
0
1
1
Chaffinch
1
0
1
Greenfinch
1
0
1
Siskin
1
0
1
Reed Bunting
5
4
9
Total:
175
25
200

Whitethroat
Swallows have been roosting in the marsh for some time (a roost began to build in mid-July), but this was the first time we have managed to get an evening session in. The catch was a slightly under par 105 birds, with singles of both sand martin (after dusk) and house martin (before dusk), and a few other odds and sods, including two willow warblers and single sedge and reed warblers.

The swallows were mainly juveniles. Only four adults were noted among them. We caught a few birds prior to dusk, which were released following processing, and over-nighted the remainder in purpose made boxes. The house martin was a bonus. It is only the third we have caught since February 2013.

The highlights of Saturday's catch were the eleventh grasshopper warbler of the year, steady numbers of reed and sedge warbler, a few whitethroat and siskin number 150 of the year (a record for the site by some distance). Some overhead movement of siskin was noted, which may indicate dispersal following breeding.

We also noted a reed bunting that resulted in some discussion with regard to sexing.
Reed bunting (in main moult)
Back of reed bunting showing white neck collar
Brood patch of reed bunting (the feathers have been gently blown to reveal the brood patch in this photo)
The reasons for this may be clear from the photos above. Photos 1 and 2 might suggest a male: the bird is scruffy as in main moult, but has a predominantly black head (albeit with some brown flecking) and a marked white collar. The third photo is of its brood patch. This was very well defined and wrinkled, suggesting recent engorgement (a female characteristic). The bird also had a short wing (74 mm), which also suggests a female. We tend to handle over 150 reed buntings a year at Oxwich, but don't often see a bird showing such a confusing set of features. However on the basis of the brood patch we concluded it could only be a female. Any comments with regard to experience of similar birds would be useful (thanks to Jerry Lewis for some very useful notes via email).

Thanks to all who attended one or more of the two sessions: Heather Coats, Keith Vaughton, Cedwyn Davies, Emma Cole, Val Wilson and Lynn Watts.

Owain Gabb
09/08/2016

Swallow on overhead line (Keith Vaughton)
[we didn't manage a decent shot of a bird during the session]

View towards the marsh at dusk (Emma Cole)


2 comments:

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  2. David Norman has commented usefully via email and has directed anyone interested in further discussion about ageing and sexing difficult reed buntings to a couple of articles. The links are:

    http://www.merseysiderg.org.uk/mid%20August%20mixture.htm and: http://www.merseysiderg.org.uk/Another%20autumn%20season.htm

    David has also noted that Adrian Blackburn wrote a piece for the Ringer's Bulletin in 2006 on the same topic. See: http://blx1.bto.org/ringers/ringers/r_bulletin/rb_11_10.pdf

    David has indicated that old-age and increased testosterone in females is not, in his view, (necessarily / always) the explanation for 'apparently male' (my wording) plumage traits, as some birds of known age appear to develop them within a few years. He has also noted that he captured a similar bird on 30 July this year, also an adult female with an active brood patch, but not yet having started post breeding moult. The bird had a typically short (74.5mm) wing length, and a greyish, off-white collar that did not extend right round the nape.

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