Deformed bills in songbirds (passerines) are uncommon, perhaps because they are further down the food chain and have less opportunity to accumulate toxins, perhaps because bill deformities pose a more serious threat to survival for small birds, or for a host of other reasons. There are many factors that may contribute to deformed bills in birds.

After banding a Gray Catbird with a grossly crossed bill in 1993, the Rouge River Bird Observatory’s Julie Craves looked into the incidences of passerines with deformed bills. Her article, Passerines with deformed bills (pdf), was published in the journal North American Bird Bander. It provides a primer on the anatomy of bird bills, discusses possible causes for bill deformities, and several tables summarize reports of passerines with deformed bills from the literature. As you’ll see in her paper, Icterids (the blackbird family) and Mimids (catbirds, mockingbirds, and thrashers) appear to have a higher rate of bill deformities than other families.

Since the publication of the article, other reports of birds with deformed bills have surfaced. The Tables below list reports that the author has become aware of.  You may report bill deformities (primarily crossed or overgrown bills) in passerines (not woodpeckers, waterfowl, hawks, or other non-songbirds) to Julie Craves (jac AT rrbo.org); this page will be periodically updated. Please note that due to the high rate of bill deformities on eastern House Finches (which may be connected with the prevalent conjunctivitus disease or genetic bottlenecking) that House Finch reports will not be included.

If you post your photos on the online photo sharing site Flickr, there is a photo pool called Birds with Deformed Bills. You can add your photos of any species of bird with a deformed bill by joining the pool.

2011 update for the UK: The British Trust for Ornithology sponsors a project called Garden BirdWatch. After an increase in sightings of birds with deformed bills, they started the Big Garden Beak Watch project to compile information. British birders are encouraged to participate; more here.

This compilation of bill deformities is property of the Rouge River Bird Observatory. Do not use or cite without written permission. Copyright for photos is held by photographers. Please do not use without permission from individual listed.

Flycatchers to European Starling (below; updated 12 Feb 2013)
Warblers through House Sparrow + Literature Cited (pg 2; updated 14 Mar 2013)
Bill Tumors, Most Due to Avian Pox (pg 3; not being updated)

Flycatchers to European Starling