Physical Description of Beak Deformities

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Figure 3. Normal Black-capped Chickadee close up.
(Public domain.)

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In a normal Black-capped Chickadee, the upper (maxilla) and lower (mandible) parts of the beak are straight and meet at the tips (Figure 3). In most affected chickadees, the maxilla is overgrown and often curves downward (Figure 4). In some cases, the beak is crossed (Figure 5) or shows a gap between the maxilla and mandible (Figure 6a). The overgrown part of the beak may be thin and brittle and sometimes breaks off, leaving a blunt tip (Figure 6b). Other birds have extremely thickened (Figure 6c) or laterally curved (Figure 6d) beaks. The deformities appear to result from overgrowth of the rhamphotheca, the outer surface of the beak 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 of affected birds indicated that neither the cranium nor the underlying bones of the beak itself were malformed.

Figure 4. Elongated and decurved maxilla on Black-capped Chickadees.
(Public domain.)

Figure 5. A collage of 6 Black-capped Chickadees with crossed beaks. Taken at various times. Upper right taken by Joy Geiselman, lower middle by Rachel Richardson, all by USGS, Alaska Science Center.
(Public domain.)

Figure 6. Black-capped Chickadees with various forms of beak deformities: a) gap between maxilla and mandible; b) overgrown part of beak becomes brittle breaks off and leaves a blunt tip; c) extremely thickened beaks; and d) laterally curved beaks.
(Public domain.)

We conducted a captive study of Black-capped Chickadee to measure rates of beak growth in birds with AKD. We found that the beaks of affected birds grew, on average, twice as fast as those of birds with apparently normal beaks. Therefore, accelerated keratin growth appears to be the primary mechanism by which deformities develop.

Beak deformities in other species often have a similar appearance, but differ slightly among groups.  Deformities in woodpeckers, flickers, and nuthatches (Figure 7a-c) generally result from extreme overgrowth, without significant crossing or curvature.

Figure 7. Downy Woodpecker, Northern Flicker and Red-breasted Nuthatch with beak deformities.
(Public domain.)

Most affected crows (Figure 8), magpies, jays (also Figure 8), and ravens, 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.

Figure 8. Northwestern Crows and Steller's Jay with beak deformities: a) crow with elongated lower beak; b) crow with elongated and curved upper beak; c) crow with longer upper beak in Seward, Alaska, photo courtesy of Charlie Finn; d) Steller's Jay with longer and curved upward lower beak in La Grande, Oregon, photo courtesy of Susan Daugherty.
(Public domain.)

Other affected passerine species, including Ruby-crowned Kinglet, American Robin, Varied Thrush, Orange-crowned, Chestnut-sided (Figure 9a) and Yellow-rumped (Myrtle) warblers (Figure 9b), Savannah and Lincoln’s sparrows, Dark-eyed and Slate-colored Junco (Figure 9c), Pine Grosbeak, Common Redpoll, Boreal Chickadee (Figure 9d), and Pine Siskin, typically exhibit beaks with crossed tips and/or varying amounts of overgrowth.

Figure 9. Passerine species with beak deformities: a) Chestnut-sided Warbler in Newton Centre, Massachusetts, photo courtesy of John Neufeld; b) Yellow-rumped or Myrtle Warbler in Cape May, New Jersey, photo courtesy of John McNamara; c) Slate-colored Junco on Middleton Island, Alaska, photo courtesy of Rachel Richardson; and d) Boreal Chickadee, photo courtesy of Rachel Richardson.
​​​​​​​(Public domain.)

Some deformed birds also have feather or skin abnormalities (Figure 10).

Figure 10. Black-capped Chickadee with scaly legs.
(Public domain.)

See the Multimedia tab for more examples and full size versions of all of the images in the collages above.