Tuesday, 5 June 2012

Birds and Bugs

A long weekend and the time to do some quality ringing was ruined by heavy rain, strong winds and Jubilee BBQ hangover. Nonetheless I did manage to get to Botley Wood on one occasion and was rewarded with a small but varied catch. My main target was once again Common Nightingale, but this species eluded me this time, although I did see three, and heard another three singing. The session was worth the effort and included several migrant species, one of which being my first Garden Warbler of the year. There were several birds singing, but the individual I caught was presumably a female since it had a well developed brood patch.

Garden Warbler

Both adult and juvenile Garden Warblers undergo a complete moult on their wintering grounds and therefore in the spring juvenile birds appear as fresh as adults.

Garden Warbler
This bird was extremely fresh, although there was some slight bleaching to the tips of the primaries, but the tips were not worn at all.

Wing of Garden Warbler

Similarly the tail feathers were very fresh, and surprisingly pointed, given that adult feathers are usually more rounded.

Tail of Garden Warbler

I also caught a couple of Common Whitethoats, one was a female, again with a very well developed brood patch, the other was a male. Common Whitethroat is a species where adults and juveniles undergo a partial moult on their wintering grounds, and therefore it is only possible to age birds by the extent of abrasion on the wing feathers.

Male Common Whitethroat

The primaries of the male bird (below) were very worn and faded at the tips, and the primary coverts were pointed. In addition the third, and largest, alula feather appeared to be of a different generation than the second, thereby indicating that this bird was a second year.

Wing of Male Common Whitethroat

The tail feathers were also worn and abraded, but they did show extensive white on the outer tail feather and on the tip of the second, a feature typical of an adult bird, in the autumn anyway.

Tail of Male Whitethroat

A Willow Warbler was the next interesting species to catch. Both adult and first year birds undergo a complete moult on their wintering grounds, and therefore there should be no discernible differences in their plumages in spring. However, Svensson states that some birds return in the spring with quite worn wing and tail feathers. This is attributed to one of three things;

1). a second year bird retaining juvenile flight feathers;

2). an adult which has inhibited its winter moult; or

3). a bird which has moulted very early in the winter. This is considered by Svensson to be the most likely. 

Worn Willow Warbler

As can be seen in the image below the tips of the primaries are extremely worn, not what would be expected from a bird that had moulted on its wintering grounds.

Abraded Wing of Willow Warbler

In addition the tail was extremely abraded, and interestingly there appeared to be two generations of feathers, with the central feathers showing what looked to be evidence of a fault bar, and therefore possibly retained juvenile feathers. 

Abraded Tail of Willow Warbler

Surprisingly this is now the third time I have captured a Willow Warbler in the spring at Botley Wood with extremely worm wing feathers. The first bird (below) had replaced a few secondaries, but all of the primaries and tertials, and the majority of the secondaries were old.

Another Spring Willow Warbler with abraded Wing

The wing of the other bird was so worn it was surprising this bird had made it back at all! In Svensson this is described as a rare occurance but given that I have encountered it three times since 1995, it presumably not that rare. Could it be evidence of an external factor on their wintering grounds that is preventing birds from moulting?

Another Spring Willow Warbler with abraded Wing

Other interesting species encountered during the session were a glow-worm Lampyris noctiluca larvae. Only three species of Glow-worm have been recorded in the British Isles, and this species is the commonest, but in decline itself.

Glow-worm Lampyris noctiluca larvae 

On returning home I found this male Oedemera nobilis beetle in the garden. This species is most commonly found in grassland habitats, the male have greatly enlarged hind femora, as can be seen in the image below.

Oedemera nobilis male

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