20 July 2014

Fault bars and a mystery feather

A good-sized team congregated this morning to run Visit 8 of the CES at Gunwalloe, in perfect conditions. A few hundred Swallows coming out of roost near the nets gave us a few stragglers (including an adult we'd ringed at the roost last week), but the majority of the catch was, predictably, Reed Warblers. The final total of 55 birds caught was close to the visit average of 60 birds, including the welcome return of a few Sedge Warblers, the less welcome return of reedbed Blue Tits and a lone juvenile Cetti's Warbler.

Symptomatic of a spell of poor weather a few weeks ago, we caught two birds both showing really obvious 'fault bars' in the tail. These occur when a period of reduced food supply in the nest means that growing feathers are lacking substance, so show up as weak bars along the tail: akin to tree rings in fact. The fact these bars are all the same distance from the tail tip indicates that these tail feathers were all grown all at the same time (ie in the nest), so is a very good indication these birds are juveniles.

Sedge Warbler fault bar
These fault bars are obviously also weak points in the feather and in this second example you can guess that the bird will be left with a rather stubby tail in the not-too-distant future... Not a problem if you're a local resident, but rather more of a problem for a long-distance migrant.

A rather more extreme fault bar on a Blue Ti
Heading back to the cars we also spotted this rather unusual feather, so any guesses as to it's identity please post a comment...

1 comment:

  1. Looks like the mystery feather was a primary covert from a young Great Black-backed Gull