Monday, 28 January 2019

Clues to the survival of a Blyth's Reed Warbler in winter

Early in its stay, the Blyth's reed warbler at Hope Carr disappeared for three and a half days during a period of harsh, freezing conditions, the worst of the winter so far, and led to speculation that it had either moved on or more likely died. After all, what could an insectivorous species find to feed on in such harsh conditions? I think that this photo helps answer the question.

The prey item is I think a spider egg sac, or possibly a moth pupa, both of which I guess form a staple part of its diet especially when temperatures are sub-zero and adult invertebrates are inactive. The brambles are probably full of egg sacs or pupa such as this, attached to the bottom of leaves or bramble stems or other vegetation and they don't disappear or die just because of a few freezing nights or heavy snow. For the warbler it's like visiting the frozen food section at the supermarket! I don't know enough about the ecology of reed warblers to know if feeding on egg sacs and/or pupa is just a winter thing or if it happens all year. It would be interesting to know, but I guess that it's not that common or surely more insectivorous birds would over winter? Just as blackcaps change their diet from insects to berries in the winter, perhaps Blyth's reed warblers change from adult prey to pupa / eggs in winter? Actually the thought occurs to me that since this is probably the first ever overwintering Blyth's reed warbler in the UK and possibly even Europe, that this is probably also the first occasion that a winter food item of the species has been recorded in this country / continent. In the afternoon when temperatures rose slightly the bird was also seen briefly flycatching.

The Blyth's reed warbler showed better than ever today in glorious sunshine at Hope Carr, but it's still a difficult bird. Plenty left saying that they only had fleeting glimpses, and even more left without a photo, so I'm happy with these photos from my little bridge camera! The weather forecast for the next two days is for heavy rain / sleet / snow / sub-zero temperatures so it was good to see it feeding up so well today.

At about 10am I was viewing the filterbeds on the sewage works about 50m down the track along the perimeter fence when I heard the warbler calling, "tak, tak" and then I saw it at close range. It flew almost immediately and I lost it. About 5 minutes later it was picked up again by the assembled masses in the usual spot by the gate and it showed very well for a few minutes and then on and off all day, though it seemed more elusive in the afternoon. In some ways I'd prefer it to leave now, because whilst it remains I'm constantly tempted back for another look and probably neglect the rest of the site. Still, I recorded 47 species today including green sandpiper, tree sparrow and willow tit, so not a bad day.

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