29 November 2013

Crossbill, Avocet or Wrybill?

Beak deformities in wild birds are unusual, with fewer than one in 200 adult birds thought to be affected. As long as the bird can feed and preen once a bill is deformed enough that the tip isn't constantly worn away, it can grow to amazing lengths.

Juvenile Blue Tit with the a bill deformity. Ipsley Alders Nature Reserve, Mike Barstow

Whilst ringing at Ipsley Alders Nature Reserve, Redditch, Mike Barstow retrapped a Blue Tit on 25/11/2013 that he previously ringed on 19/01/2012 with no deformity noted. This bird now had a severely deformed bill (below). The bird appeared healthy with good plumage, muscle and even had a trace of fat. There are therefore two birds (above and below) that have been recorded at this site which have developed a beak deformity within a few years.The research continues.

Adult Blue Tit. Ipsley Alders Nature Reserve, Mike Barstow

Measuring bill length to investigate growth rate for future captures- Mike Barstow

 Types of beak deformities in the wild:
  • Crossed mandibles: similar to that of a Common Crossbill perhaps caused by slight asymmetry of the jaw. Straight beaks have been recorded as a deformity in Crossbills.
  • Decurved upper mandible: can occur when the tip of the lower mandible is damaged so that the beak does not close correctly.
  • Upcurved lower mandible: may occur when the tip of the upper mandible is missing (i.e. broken off).
  • Elongation of both mandibles: produces ‘Curlew-type’ beaks, although the degree of curvature varies considerably between individuals.
  • Bent to the side: seemingly uncommon, with one or other mandible warping sideways.
  • Gapped: where the upper and lower mandibles do not close fully leaving a visible gap.
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