Sketches by David Sibley reveal similarity and variation in bill shape

Northern Cardinal by David Allen Sibley.
Northern Cardinal by David Allen Sibley.

When we talk about identification, we tend to focus on differences, but it can be instructive (and reassuring) to think about similarities. In fact, understanding differences often requires us to appreciate the similarities between species.

Ruby-throated Hummingbird by David Allen Sibley.
Ruby-throated Hummingbird by David Allen Sibley.

Bill shapes vary widely, and nearly every species can be identified by its bill alone. The shapes are closely related to a bird’s foraging habits, and each species’ niche and preferred prey influence subtle variations in bill shape and size.

Bald Eagle by David Allen Sibley.
Bald Eagle by David Allen Sibley.

The adaptability of shape is demonstrated by such rapidly evolving populations as the Hawaiian honeycreepers or Galapagos finches, as well as by studies showing that, on average, Snow Geese wintering in corn fields have larger bills than those wintering in salt marshes. That the small differences are consistent enough to identify populations demonstrates the value of shape for identification.

American Robin by David Allen Sibley.
American Robin by David Allen Sibley.

All of this leads to a strong emphasis on bill shape as a field mark. We are remarkably good at discerning small differences in shape and proportions. Anyone can see the difference in shape between a hummingbird, warbler, and cardinal; an experienced birder can discern differences between species of hummingbird or warbler.

The sketches here illustrate the incredibly varied bill shapes of five unrelated species. In spite of their differences, all the bills share the same basic structure. Looking for the similarities can be a big help in understanding and interpreting other aspects of variation.

Mallard by David Allen Sibley.
Mallard by David Allen Sibley.

For example, while the Bald Eagle’s bill is prominently hooked, a slight hook is a common feature of every bill. Notice that the feathering on the chin always extends forward on the underside of the bill, and that the feathering on the base of the upper mandible (near the nostril) always forms a more or less convex edge, “bulging” onto the bill.

If you have an opportunity to see birds at close range this winter, take a few minutes to study bills. Appreciating broad similarities between species will help you discern subtle differences.

 

This article from David Sibley’s “ID Toolkit” column appeared in the January/February 2016 issue of BirdWatching.

 

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David Sibley

David Sibley

David Sibley writes the column “ID Toolkit” in every issue of BirdWatching. He published the Sibley Guide to Birds in 2000, the Sibley Guide to Bird Life and Behavior in 2001, and Sibley’s Birding Basics in 2002. He is also the author of the Sibley Guide to Trees (2009), the Sibley Guide to Birds-Second Edition (2014), and guides to birds of eastern and western North America (2016). He is the recipient of the American Birding Association’s Roger Tory Peterson Award for lifetime achievement in promoting the cause of birding and a recognition award from the National Wildlife Refuge System for his support of bird conservation.

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