Wild Neighbors: The Strange Case of the Sickle-billed Chickadees

By Joe Eaton
Monday March 14, 2011 - 04:11:00 PM
Alaskan black-capped chickadee with avian keratin disorder.
US Geological Survey
Alaskan black-capped chickadee with avian keratin disorder.

There’s a web site that compiles observations posted to birding newsgroups all over California; I try to check it out every couple of days. That’s how I heard about the deformed raven in Redwood City. On March 7, Ken Schneider noticed a raven with “a pronounced bill deformity,” “greatly elongated and decurved,” eating cat food on the sidewalk near his garage. Schneider thought this was noteworthy because he was aware of a recent phenomenon first described in Alaska but now documented in British Columbia and Washington State as well: a growing incidence of birds with misshapen beaks. 

The US Geological Survey, whose scientists have been investigating whatever is going on, has representative images on its web site . The most common variation seems to be an extended and down-curved upper mandible, comparable to that of a curve-billed thrasher or a curlew. Sometimes the mandible tips cross. Some birds have a gap between the cutting edges of the upper and lower mandibles. In at least one specimen, the tip of the bill curves to one side. Needless to say, none of these anomalies are very functional. Affected birds have trouble feeding and maintaining their plumage. The condition involves only the rhampotheca, the keratinous layer that overlies the bones of the beak; the bone itself is unaffected. 

The USGS team (yes, the agency’s purview goes far beyond geology; other USGS biologists study ducks and rails in San Francisco Bay) has a pair of articles about the rash of deformities in a recent issue of The Auk, the journal of the American Ornithologists’ Union. The authors report that beak deformities have been documented in 30 species of Alaskan birds—waterbirds, raptors, woodpeckers, and songbirds--since 1979. 

The most severely affected species are the black-capped chickadee (2160 cases from 1991 to 2008) and the northwestern crow (148 cases from 1979 to 2009.) The chickadee is a common northern-forest bird whose range barely extends into coastal northern California. The crow, a smaller edition of the ubiquitous American crow, occurs from Puget Sound northward. The seed- and insect-eating chickadee and the omnivorous and opportunistic crow would seem to have little in common ecologically. (Only a handful of deformities have been observed in boreal and chestnut-backed chickadees, close relatives and sometimes neighbors of the black-capped.) 

Lead author Colleen Van Handel and colleagues have been trying to narrow down the list of suspected causes. So far, Sarah Palin has not been directly implicated. Although affected birds show significant DNA damage, the scientists don’t think the condition, which they’ve named avian keratin disorder, is congenital. It’s seen only in adult birds, not in embryos or hatchlings. Some chickadees that had normal beaks when first trapped and banded turned up with malformations the following year. A few subjects had a fungal infection; most did not. There was no evidence of infection by bacteria, viruses, or bites, all of which have been known to cause beak deformities. 

What about contaminants? The USGS web site refers to a number of possible sources: pesticides used against bark beetles, fire retardants deployed in several large forest fires, agricultural pesticides and herbicides, chemicals from military installations in Anchorage, and multiple superfund sites. Superfund sites in Alaska?! In terms of known problematic chemicals, the birds don’t show the symptoms associated with selenium poisoning (remember the Kesterson National Wildlife Refuge scandal?) or with exposure to PCBs. However, PCB levels were high in chickadees at all life stages, and were correlated with deformities in adults. But correlation doesn’t establish causation. 

Could a dietary deficiency be responsible? Black-caps, like other chickadees, appreciate bird feeders. Apparently an all-seed diet can be low in vitamins A and D and folic and pantothenic acids, and its high fat content can interfere with calcium uptake. Again, though, the symptoms are different from those indicative of low calcium. which would also have affected the bone. or vitamin deficiency. Chickadees are not long-distance travelers, and deformed individuals have been found in remote areas of Alaska where bird feeders are few and far between. And an excess of seeds wouldn’t account for the northwestern crows, which, like most crows, will eat just about anything. 

The fact that some chickadees also had overgrown claws (which, like our nails, are made of keratin) suggests some kind of systemic keratin problem. But the incidence of deformed claws, less than one percent of the sample, was much lower than that of deformed beaks. Research continues, both on how the condition develops and how well its victims are able to survive and reproduce. One intriguing line of work involves stable isotope analysis of the blood and feathers of crows and chickadees to see if the diets of normal and deformed birds differ. 

Since media coverage of the phenomenon has increased, USGS has received a handful of deformed chickadee reports from Washington State to Maine and Quebec. Most sightings of deformed northwestern crows are Alaskan, but some have been seen in British Columbia and Washington State. We can only conjecture where the Redwood City raven came from, or what was wrong with it. Any observations of beak anomalies, in any species of bird, should be reported to the USGS website.