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Effects on Birds

| Behavior | Survival | Reproduction |


Behavior

Physical limitations associated with beak deformities can change normal behavior patterns.  For example, deformed birds are often seen picking up food with their heads turned sideways because an overgrown or crossed beak prevents them from eating normally (Figures 25 and 26).  To see a video of a Northwestern Crow feeding with a deformed beak, click here (video by Aquetec Water Taxi).  To see a video of a Black-capped Chickadee with a crossed beak, click here (video by Lee Tibbitts), a Black-capped Chickadee with an elogated beak, click here (video by Wayne Hall). Deformed birds may also change their foraging habits to include more easily acquired foods.  Severely deformed birds have difficulty foraging and spend large amounts of time at feeders.  Deformed chickadees, nuthatches, and woodpeckers often feed on the snow beneath feeders, picking up scraps dropped by other birds.  Deformed crows, ravens, magpies, and jays may rely heavily on human sources of food, including dumpsters and feeders.  These birds are frequently seen outside of grocery stores or fast food restaurants or near city dumps.

Feeding deformed Northwestern Crow - photo by Judy Rowe Taylor Feeding deformed Black-capped Chickadee
Figure 25 - Northwestern Crow, photo by Judy Rowe Taylor Figure 26 - Black-capped Chickadee, photo by Don Kuhle

In addition to changes in foraging behavior, some deformed birds have also exhibited abnormal behavior during breeding.  We documented several cases in which the deformed female parent abandoned the nest after she was banded; most normal females tolerated such disturbance quite well and returned to incubate eggs immediately after they were released.  In other cases, the deformed females 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 a hormonal disruption of incubation behavior.  Other studies, however, have documented abnormal breeding activity associated with contamination by DDEs and PCBs (Haegele and Hudson 1977, Fox et al. 1978, McCarty and Secord 1999).  Analysis of nest box video data will help us learn more about the causes of abnormal behavior among deformed chickadees.


Survival

Mortality rates of deformed birds are probably proportionately higher than those of normal birds, especially during the shortest, coldest days of winter. 

Deformed 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 deformed 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.  A few deformed chickadees with dirty, almost jet black breast feathers were found dead at residences in winter, most likely due to starvation or hypothermia.


Reproduction

Although many Black-capped Chickadees in the south-central Alaskan population successfully raise 6-8 young each year, nests with a deformed parent 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 was deformed.  In one unusual case, both the male and the female from a nest were deformed. 

For nests in which the female was deformed, fewer eggs hatched on average. The fact that fewer eggs hatched may have been due to a physical problem with the eggs, such as thinner eggshells, or reduced incubation by the female. 

For nests with a deformed male, 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 deformed males had to cope with a different problem—being tricked into raising someone else’s young!  Genetic studies showed that nests used by a deformed 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 deformed 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 deformed birds do appear to find mates and breed.

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