I came across this sorry-looking first-calendar year Lesser Black-backed Gull today, with a heavily deformed upper bill:
First-calendar year Lesser Black-backed Gull (Larus fuscus graellsii) with deformed upper mandible. Leiden, The Netherlands, 28 August 2012.
It looked to be in good shape though and was able to eat well from the bread that we fed.
Such sights are not very common but not rare either. See also these examples here, here, and here.
It is the first time that I have seen this type of deformity in a gull; it’s usually very long-grown bills that I come across.
Second-calendar year European Herring Gull (Larus argentatus) with a long-grown bill. IJmuiden, The Netherlands, 12 July 2010.
See another example of a long-grown bill here.
What is causing it?
This seems to be a mystery still. Reports seem to point from anything to a vitamin deficit to an influence of man-made chemicals.
More often than not, affected gulls are no older than a few months old. This immediately indicates that such individuals don’t survive for very long. Not only because food intake is hampered, but taking care of their plumage is also problematic (with the feathers often infested with lice).
Having said that, exceptions always occur. Here is an example of an a 3rd-calendar type Herring Gull (Larus argentatus) and here an example of an adult Kelp Gull (Larus domanicus).
American Birding Association – The Mystery of Deformed Beaks
PBS Frontline – Fooling with nature
Seattle pi – Deformed beaks mean slow starvation for region’s birds; cause a mystery
Science alert – Developmental Malformations in Avian Species. Manifestations of Unknown or Genetic Etiology-A Review
A Herring Gull Chick (Larus Argentatus) with an Abnormal Bill (PDF)
avibirds.com – Skull deformity in a Herring Gull chick (PDF)