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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.

Click on photos for larger views

Normal Black-capped Chickadee - photo by Sandy Talbot - USGS Deformed maxilla in a Black-capped Chickadee - photo by Sandy Talbot - USGS Elongated and decurved maxilla, Black-capped Chickadee, photo by Susan Fenn
Figure 4 - Normal Black-capped Chickadee, USGS photo Figure 5 - Elongated and decurved maxilla, Black-capped Chickadee, USGS photo Figure 6 - Elongated and decurved maxilla, Black-capped Chickadee, photo by Susan Fenn
Crossed beak in a Black-capped Chickadee - photo by USGS Elongated and crossed beak, Black-capped Chickadee, photo by John DeLapp Gap between maxilla and mandible in Black-capped Chickadee - photo by USGS
Figure 7 - Elongated and crossed beak, Black-capped Chickadee, USGS photo Figure 8 - Elongated and crossed beak, Black-capped Chickadee, photo by John DeLapp Figure 9 - Gap between maxilla and mandible, Black-capped Chickadee, USGS photo
Overgrown part of beak broke off leaving a blunt tip on a Black-capped Chickadee - photo by USGS Thickened beak on a Black-capped Chickadee - photo by USGS Laterally curved beak on a Black-capped Chickadee - photo by USGS
Figure 10 - Broken tip of elongated maxilla, Black-capped Chickadee, USGS photo Figure 11 - Thickened and elongated beak, Black-capped Chickadee, USGS photo Figure 12 - Laterally curved maxilla, Black-capped Chickadee, USGS photo

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. 

Elongated beak, Downy Woodpecker, photo by Jim Tinius Elongated beak, Red-breasted Nuthatch, photo by Diane Hendersen
Figure 13 - Elongated beak, Downy Woodpecker, photo by Jim Tinius Figure 14 - Elongated beak, Red-breasted Nuthatch, photo by Diane Hendersen

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.

Deformed beak on a Northwestern Crow - photo by Kevin Mack Deformed beak on a Northwesten Crow - photo by Judy Rowe Taylor Elongated and crossed beak, Northwestern Crow, photo by Heidi Cline
Figure 15 - Elongated maxilla, Northwestern Crow, photo by Kevin Mack leaningcedarstudio.com Figure 16 - Elongated and decurved maxilla, Northwestern Crow, photo by Judy Rowe Taylor Figure 17 - Elongated and crossed beak, Northwestern Crow, photo by Heidi Cline
Black-billed Magpie with curved beak - photo by Jules Tileston Steller's Jay with curved beak - photo by Terry Coddington Common Raven with a crossed beak - photo by Mark Prins
Figure 18 - Black-billed Magpie with curved beak, photo by Jules Tileston Figure 19 - Elongated and laterally curved maxilla, Steller's Jay, photo by Terry Coddington Figure 20 - Crossed and twised beak, Common Raven, photo by Mark Prins - Inada Images

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.

Elongated beak, American Robin, photo by Alan Estep Elongated beak, Varied Thrush, photo by Carolyn Heitman Orange-crowned Warbler with a crossed beak - photo by Dick Cannings
Figure 21 - Elongated beak, American Robin, photo by Alan Estep Figure 22 - Elongated beak, Varied Thrush, photo by Carolyn Heitman Figure 23 - Crossed beak, Orange-crowned Warbler, photo by Dick Cannings

Some deformed birds also have feather or skin abnormalities (Figure 24).

Scaly legs on a Black-capped Chickadee - Photo by USGS
Figure 24 - Scaly legs, Black-capped Chickadee, USGS photo

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