Monday, 1 September 2014

Redstart Ageing

Yesterday sounded like a great day at Beer Head, with Phil noting 40+ Willow Warblers, four Whitethroats, three Spot Flies, three Tree Pipits, two Whinchats and a Reed Warbler. As a result I was eager to get up there this morning, especially as a veil of cloud had rolled over and even produced some pre-dawn drizzle. Perfect.

Sadly though in reality it was soon pretty obvious that most/all of  yesterdays birds had moved off - probably during the clear weather ahead of the front - and very little had dropped in to replace them. Saying that though we still ringed 23 birds from two mist nets, with two Dunnocks and a Wren being the only (presumed) resident birds.

The best of the captures was this lovely Redstart that appeared in front of us several hours after dawn, and flew straight into my 60 foot net...



There's more than enough visible in the above photo to age and sex this bird. What do you think it is? 



This photo should do it if the above didn't.  A bird with this much orange on its breast has to be a male, along with those black centred throat feathers.  You may notice though it's lacking a white forehead stripe, has a dull greyish brown crown, and the black throat feathers have broad white/off-white tips - well this makes it a first year bird.

And now I'm going to break with tradition for this blog and get a bit technical...

All juvenile passerines under go a post juvenile moult which starts not that long after fledging. Unlike adult complete moults, post juvenile moults are partial moults and include only a small numbers of wing feathers and only very rarely the main flight feathers (primaries and secondaries)*.  The wing feathers most often replaced during post juvenile moults are the greater coverts (GC's), with some birds replacing all of these, some replacing none, but most replacing a few. This gives ringers a very helpful ageing tool as in theory any passerine showing two different age classes of GC's is a year old or less. This moult limit will show in a bird until it undergo its first complete adult moult when ALL the GC's are replaced.  *Some species do a complete post juvenile moult (House Sparrow, Long-tailed Tit to name two), and some individuals of other species by some freak have been known to have complete post juvenile moults when they really shouldn't.

Anyway, here's the greater coverts of this mornings Redstart...



And here they are again with some red lines and words...



Notice how different they are. The new inners ones (the GC's are always moulted from the inside out) are rounder, almost blue in colour, grey edged with narrow brown tips, with the seven remaining juvenile GC's being dull with pale brown edges and tips.  They will stay this way for a little under a year, although as the feathers wear and fade it will become less obvious.  Now look at the first photo again and see if you can spot it on the closed wing.

Tails are also often big helpers in ageing passerines, with young birds tails often being pointed and more worn compared with rounded and newly grown adult ones.  This tail shape difference isn't so obvious in Redstarts, but look how abraded the tips are, plus I think the dull/dark tips and marks on some of the feathers is an age thing too and wouldn't be seen on an adult Redstart....



I think that's enough for today's lesson!

We also caught six Blackcap, six Robin, four Chiffchaff, two Whitethroat, one Willow Warbler and a Goldcrest.  Apart from a few large flocks of hirundines and the odd Yellow Wag, very little was flying over. Two Whinchats and a Wheatear were the only other grounded migrants I saw.

The weather looks good for the rest of the week, so I hope to fit in another ringing session at some point.

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