The keratin layer of the bill can be overgrown. The deformity shows mostly up in adult birds, and most often occurs in the upper beak but sometimes in the lower beak or both.
The cause of the deformity – called “avian keratin disorder” – hasn’t been determined, but is thought to be the result of environmental pollutants in the birds’ environment. The abnormality – sometimes accompanied by elongated claws, abnormal skin or variations in feather color – often impacting a bird’s ability to feed and clean itself.
In the past, large clusters of beak deformities have been associated with environmental pollutants such as organochlorines in the Great Lakes region and selenium from agricultural runoff in California. More recently, crows, chickadees, nuthatches and magpies in Alaska have been afflicted along with numerous other species to a lesser degree.
Uneven beak wear is often noticeable, especially along the occlusal or biting surfaces of the upper and lower beaks, usually occurring on both surfaces.
An overgrown beak can also be the result of health problems: For instance if an infestation of Knemidokoptes pilae has occurred. In the initial stage, an infestation with burrowing mites is hardly visible and hardly affects the general condition of the diseased bird. There are initially crusty plaques in the corners of the beak and on the beak itself.
In the early stages, they have the appearance of a bright white deposit that becomes thicker and crustier over time. The upper mandible becomes increasingly cavernous since the burrowing mites dig subtle tunnels (paths). If the skin around the eyes, on the legs or around the vent is also affected, this is accompanied by strong itching.
Two examples of shrikes with deformed bills
A Loggerhead Shrike (Lanius ludovicianus) at February 5th 2001 n Cochise County, Arizona
A Great Grey Shrike (Lanius excubitor) at January 8th 2014 in Spanga, Weststellingwerf, Friesland. This bird has an infestation of Knemidokoptes ssp.- see the bright light crusty plaques on the upper mandible.