Passerines with Deformed Bills

Julie A. Craves, Rouge River Bird Observatory, Natural Areas Dept., University of Michigan-Dearborn,
Dearborn, MI 48128

Pomeroy (1962) stated that abnormal bills—subjectively defined as being noticeably "different from the normal"—in wild birds are rare, with a frequency estimated at less than 0.5%. No doubt the degree of deformity affects individual survival and the resulting low frequency. For example, while some birds can adjust to their handicap and may feed by turning their heads to one side (Brown 1976; Sharp and Neill 1979; Steffee 1968), others apparently starve or are plagued by numerous parasites, such as body lice (Thompson and Terkanian 1991; Arendt and Arendt 1986). Deformed bills can take many forms. Mandibles may be crossed, upper and/or lower mandibles curved downwards, upper and/or lower mandibles curved upwards, or mandibles elongated (nearly always also curved downward as well).

I searched the literature for reports of bill abnormalities in passerines. Table 1 summarizes a number of these reports. I noticed that bill deformities seemed more prevalent in certain groups. The families Icteridae and Mimidae as well as abundant species (such as European Starlings [Sturnus vulgaris] and House Sparrows [Passer domesticus]) seem over represented with a high incidence of reported deformities. This paper offers a review of my research.

As an example of multiple reports for common species, Pomeroy (1962) cited more than 60 cases of European Starlings with bill deformities, but concluded that deformities are probably no more prevalent in starlings than among other species. Corroborating this conclusion is the 0.038% frequency of abnormal bills in starlings found by Hicks (1934) in a large (about 10,000), U.S. sample. The high number of reports for these species is very likely due, simply, to their abundant numbers.

Multiple reports of bill deformities in the family Icteridae were also found. Sharp and Neill (1979) studied populations of wintering blackbirds, most of which were Red-winged Blackbirds (Agelaius phoeniceus). They found a frequency of bill deformity of 0.6% (18 cases). A search of the literature and the collection of study skins at the Museum of Zoology, University of Michigan (UMMZ) revealed reports of bill deformities in 9 species of icterids (see Table 2).

I also found multiple reports of bill abnormalities among the Mimidae.  My encounter with a deformed Gray Catbird (Dumetella carolinensis) prompted my interest to review bill deformities and I found 25 references in the literature of bill deformities in this family, and one other instance at UMMZ (see Table 3).  Unfortunately, authors did not give sample sizes and a frequency rate cannot be determined.  Mimids do not gather in large flocks, nor are they considered abundant if compared to starlings, House Sparrows, or blackbirds.  The high number of reports suggest a higher than "normal" frequency of deformities for the Mimidae.

To understand bill abnormalities, it is useful to review the anatomy of a bird's bill. The structure of a bird's bill, from the inside out, is as follows: 1 ) an underlying bony structure, 2) the dermotheca, a thin, cutaneous layer containing blood vessels and nerves, and 3) the rhamphotheca, the visible horny sheath. The rhamphotheca is formed when cells from the dermotheca are produced, move outward, and become keratinized and hard.

Possible reasons for bill deformities include genetic or developmental causes, injury, or disease. Several researchers (Fox 1952, Pomeroy 1962, Sharp and Neill 1979) could not produce conclusive evidence to support any of these causes. In fact, some evidence seems contradictory. For example, if bill deformity were genetic in nature, some abnormality in bony structure might be expected. Yet typically (Thompson and Terkanian 1991; Fox 1952; Taylor and Anderson 1972; Easterla and Todd 1971), the abnormality was due to the excessive growth of the dermotheca and rhamphotheca; the underlying bony structure was normal. Bill deformities due to injuries might be expected to produce scars on the underlying bony structure, but again, bone has appeared normal. Additionally, if injury was the cause, then uniform, correlated growth to each mandible would not be expected (Fox 1952).

The idea that bill deformities may be due to chemical pollutants such as pesticides is worth exploring. Birds such as gulls, cormorants, and some raptors with malformities stemming from chemicals are well-documented. The dietary habits of these birds, high on food chains, make them recipients of concentrating environmental mutagens, a possible cause for deformities. Since passerines tend to be lower in the food chain, the possible connection between environmental causes and bill deformities is not as strong.

Still, some researchers have offered chemical pollutants as a potential explanation for bill deformities in the birds they studied. Sharp and Neill (1979) felt that the deformities they found among blackbirds may have resulted from avicidal chemicals. Brown (1976) also mentioned this possibility as the cause of the numerous deformed Brown Thrashers (Toxostoma rufum) in central Florida, a prime agricultural area. The high incidence of specimens of Brown Thrashers from the late 1960s to the early 1970s — the era of widespread DDT use — also lends credence to this theory.

Gochfeld (1972) urged monitoring of bill deformities to detect if the incidence of and distribution of defects was changed, especially considering the potential mutagenic, teratogenic, and carcinogenic effects of chemical pollutants. Yet fewer reports of birds with deformed bills are appearing in the literature; whether this is due to a real decline in occurrence or a lack of interest in reporting is not known (see Parkes 1969).

It appears that bill deformity rates may vary. Banders keeping appropriate records can provide basic data on frequency of deformities. Changes in the frequency or degree of morphological deformities suggest concerns for closer looks for possible causes. It may be worthwhile to secure blood and tissue samples from badly deformed birds for analysis, since it is likely these birds would not survive in the wild. Banders examining birds will likely be the first to be aware of potential environmental concerns.

LITERATURE CITED

Allard, H.A. 1930. Bill deformity in a catbird. Auk 47:93.

Arendt, W.J., and A.I. Arendt. 1986. Bill deformity in a Pearly-eyed Thrasher from Montserrat, West Indies. N. Am. Bird Bander 11:51-52.

Barlow, J.C. 1967. A bill deformity in a European Tree Sparrow. Can. J. Zool. 45:889-891.

Brown, L. N. 1976. Prevalence of bill abnormalities in Florida Brown Thrashers. Florida Field Natur. 4:11-13.

Carothers, S.W. and R.P. Balda. 1970. Abnormal bill of awestern Meadowlark, Sturnella n. neglecta.  Auk 87:173-174.

Easterla, D.A. and J.M. Todd. 1971. Bill deformity in a Yellow-headed Blackbird. Auk 88:677-678.

Easterla, D.A. and R.H. Wauer. 1972. Bronzed Cowbird in west Texas and two bill abnormalities. Southwestern Natur. 17:293-295.

Flanigan, A. B. 1976. A mutant Common Yellowthroat and other species with abnormalities banded at an Ohio banding station in 1975. inland Bird Banding News 48:45-48.

Fox, W. 1952. Behavioral and evolutionary significance of the abnormal growth of beaks of birds. Condor 54:160-162.

Gochfeld, M. 1972. Avian abnormalities and the scientific literature. Amer. Birds 26:705.

Goertz, J. W., and E. E. Mowbray. 1969. Brown Thrasher with a bill deformity. Southwestern Natur. 14:254-255.

Hicks, LE. 1934. Individual and sexual variations in the European Starling. Bird-Banding 5:103-118.

Hodges,  J. 1952. Two nestling robins with abnormal beaks. Condor 54:359.

Johnson,C.E. 1929. Bill deformity in a Blue Jay. Auk 46:241-242.

Long Point Bird Observatory. 1992. Annual Report.

Morton, E. S. 1963. A partial albino Red-winged Blackbird with a deformed bill. Wilson Bull. 75:281.

Mutchler, T.M. 1976. Common Grackle with malformed bill. N. Am. Bird Bander 1:145.

Parkes.K.C. 1969. On abnormally crossed mandibles in birds. Wilson Bull. 81:342.

Pomeroy,D.E. 1962. Birds with abnormal bills. Brit. Birds 55:49-72.

Prescott, K.W. 1968. Bill deformity in a Brown Thrasher. Bird-Banding 39:134.

Riggins, J.N.,and H. Riggins. 1976. Common Yellowthroat with bill deformity. Inland Bird-Banding News 48:22.

Sharp, M.S., and R.L. Neill. 1979. Physical deformities in a population of wintering black birds. Condor 81:427-430.

Steffee, N.D. 1968. Numbers of deformed Brown Thrashers (Toxostoma rufum) reported from scattered locations. Florida Natur. 41:126-127.

Taylor, W.K., and B.H.Anderson. 1972. Brown Thrasher (Toxostoma rufum) with a bill abnormality in Seminole County. Florida Natur. 45:129.

Thompson, C.W.,and B.A.Terkanian. 1991. Abnormally long bill in a young Curve-billed Thrasher. J. Field Omithol. 62:157-161.