Alaska Science Center
Physical Description of Deformities
In a normal Black-capped Chickadee, the upper (maxilla) and lower (mandible) parts of the beak are straight and meet at the tips (Figure 4). In most affected chickadees, the maxilla is overgrown and often curves downward (Figures 5 and 6). In some cases, the beak is crossed (Figures 7 and 8) or shows a gap between the maxilla and mandible (Figure 9). The overgrown part of the beak may be thin and brittle and sometimes breaks off, leaving a blunt tip (Figure 10). Other birds have extremely thickened (Figure 11) or laterally curved (Figure 12) beaks. The deformities appear to result from overgrowth of the rhamphotheca, which is made of keratin. Like human fingernails, the rhamphotheca grows throughout a bird’s life and is constantly worn down through pecking and feeding. X-rays indicated that neither the cranium nor the underlying bones of the beak itself were malformed.
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Beak deformities in other species often have a similar appearance, but differ slightly among groups. Deformities in woodpeckers (Figure 13) and nuthatches (Figure 14) generally result from extreme overgrowth, without significant crossing or curvature.
Most affected crows (Figures 15, 16, and 17), magpies (Figure 18), jays (Figure 19), and ravens (Figure 20), have similar growth patterns as deformed chickadees. Overgrown and downward-curving maxillas, severely elongated maxillas and mandibles, and crossed beaks are the most common corvid deformities.
Other affected passerine species, including Ruby-crowned Kinglet, American Robin (Figure 21), Varied Thrush (Figure 22), Orange-crowned (Figure 23) and Yellow-rumped warblers, Savannah and Lincoln’s sparrows, Dark-eyed Junco, Pine Grosbeak, Common Redpoll, and Pine Siskin, typically exhibit beaks with crossed tips and/or varying amounts of overgrowth.
Some deformed birds also have feather or skin abnormalities (Figure 24).