Beak Deformity's Effects on Birds

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Birds with beak deformities often have difficulty foraging and preening, and may not be able to keep themselves warm and well-fed during cold winter months.  Although some birds with beak deformities breed successfully, they typically encounter more challenges than normal birds.

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Small bird with crossed long beak and ruffled feathers

Black-capped Chickadee with crossed beak and ruffled feathers. (Public domain.)


Physical limitations associated with beak deformities can change normal behavior patterns.  For example, affected birds are often seen picking up food with their heads turned sideways because an overgrown or crossed beak prevents them from eating normally.  Birds with deformities may also change their foraging habits to include more easily acquired foods. Severe deformities make it difficult for birds to forage and they may instead rely heavily on feeders and other sources of supplemental food.  Affected chickadees, nuthatches, and woodpeckers with beak deformities often feed on the snow beneath feeders, picking up scraps dropped by other birds.  Affected crows, ravens, magpies, and jays are frequently seen outside of grocery stores or fast food restaurants or near city dumps.

In addition to changes in foraging behavior, some birds with beak deformities also exhibit abnormal behavior during breeding.  We documented several cases in which a female parent with a beak deformity abandoned the nest after she was banded; most unaffected females tolerated such disturbance quite well and returned to incubate eggs immediately after they were released.  In other cases, females with beak deformities behaved erratically, and eggs were scattered about the nest box haphazardly rather than arranged neatly in a nest cup.  It is still unclear whether this behavior results from a physical limitation imposed by the bill deformity or from hormonal or physiological disruption of incubation behavior.  Analysis of nest box video data will help us learn more about the causes of abnormal behavior among chickadees with avian keratin disorder (AKD).


Mortality rates of birds with beak deformities are apparently higher than those of normal birds, especially during the shortest, coldest days of winter. Chickadees need to consume 10% of their body weight in food every day just to survive the night during the Alaskan winter. Beak deformities may inhibit foraging and feeding, making it more difficult for these birds to consume sufficient food.

Affected birds spend more time at feeders and near human sources of food, which are typically in open areas without trees or dense vegetation. Because of this increased time away from cover, they are more easily seen and therefore more susceptible to predators.

Some affected birds have difficulty preening and many have dirty, matted plumage by late winter.  Preening is important for maintaining the insulating ability of feathers.  If birds cannot do this, they have trouble staying warm during cold winter months.  Chickadees with beak deformities and dirty, almost jet black breast feathers were found dead at residences in winter, most likely due to starvation or hypothermia.

Additionally, birds with AKD may be more susceptible to other diseases and opportunistic infections. For example, we detected Plasmodium, a blood parasite infection responsible for causing avian malaria, in affected Black-capped Chickadees at nearly three times the rate of birds with normal beaks (Wilkinson et al. 2016). Similarly, bacterial and fungal infections occur more commonly in birds with beak deformities (Van Hemert et al. 2013).


Chickadee with a deformed beak in a bush

Black-capped Chickadee with elongated upper beak.

(Credit: Andrew Reeves, USGS, Alaska Science Center. Public domain.)

Although many Black-capped Chickadees in the south-central Alaskan population successfully raise 6-8 young each year, parents with beak deformities face greater challenges.  In our 2000-2004 breeding study, 305 nest boxes were used by Black-capped Chickadees.  Of these, 33 were occupied by pairs in which either the male or the female had a beak deformities.  In one unusual case, both the male and the female from a nest were affected.

For nests in which the female had a beak deformity, fewer eggs hatched on average. This may have been due to a physical problem with the eggs, such as thinner eggshells, or a behavioral change, such as reduced incubation by the female.

For nests in which the male had a beak deformity, a smaller proportion of the young survived to leave the nest.  It is likely that the physical deformity hindered the male’s ability to gather enough food for the nestlings.  In addition, we were surprised to find that affected males had to cope with a different problem—being tricked into raising someone else’s young!  Genetic studies showed that nests used by an affected male contained a higher proportion of eggs from a different male or from different parents entirely.  Although the reason for this pattern is not known, we suspect that affected males were less able to defend their territories due to difficulty foraging, which resulted in long periods away from the nest.

Despite these problems, a surprising number of chickadees with beak deformities do appear to find mates and breed.