Why do some birds hop while others walk or run?

Ever notice that? Sparrows hop. Crows walk.

Sparrows are small and crows are big. Small birds, particularly songbirds, have short legs. Hopping is the most efficient way for them to move (if not flying). Larger birds, with longer legs, find hopping to be energy-intensive, so they walk.

This, of course, is a generalization. I've watched robins run as well as hop, and crows hop as well as walk. All animals are practical — whatever works.

That expediency comes to the fore in foot design. One size does not fit all birds. Feet and legs are finely tuned tools that vary from species to species to serve their needs.

When birds walk, they do so on their toes. What you see when you watch a crow walk are its toes and a fused foot-ankle that disappears up into body feathers. Modestly hidden there are knees and thighs. The joint you see behind the toes is not a knee.

The birds we commonly see here have one of nine different foot configurations, the differences basically being the arrangement of toes.

As tools, toes are excellent statements on bird lifestyles. They facilitate movement on the ground, plus perching, scratching (in sand or leaves, for example), swimming, gripping and killing. They are shock absorbers when the bird lands.

When gripping, say a branch, perching birds have toes that work in reverse of ours. Bird toes automatically grip tightly until the bird leaves.

Most perching birds share toe arrangement — three toes to the front, one pointing back.

Owls, woodpeckers, osprey and cuckoos have two toes forward, two backward.

Kingfishers have two toes fused by webbing. This helps them dig the nesting burrow.

Swifts have all four toes pointing forward. Swifts don't perch. They hang, inside a chimney, for instance, toes hooked to rough surfaces.

Ducks, geese, swans, gulls and terns have webbing between their three front-facing toes. Webbing equates to paddles.

Some birds have narrow strips of webbing on toe edges — palmating. Sandpipers and grouse are semipalmated, with partial webbing. Grebe toe edges are webbed all around. When the grebe swims, that webbing flares on the back stroke, to propel, then collapses on return to eliminate drag.

Hawks and owls and eagles have strong toes with long, sharp talons that catch, hold and kill.

Bird legs and feet are covered by a tough layer of plates or scales made of keratin, like your fingernails. The bottom of a bird foot is covered with small fleshy pads. Pad arrangement depends on species. In some cases, pad patterns are unique to the individual. They can be used for identification, like fingerprints.

Why don't bird feet and legs freeze in cold weather? There's not much flesh on a bird foot, so there's little moisture to freeze. Plus, circulation moves through the foot so fast that the blood does not cool to freezing.

Why do some sandpiper species have short legs and others have long legs? That helps determine where species feed — on a muddy shore or in water, shallow or deep.

Birds are descendants of dinosaurs, but not all from the same dinosaur lineage. Bird families evolved similar traits more than once to meet similar needs.

Each bird species has hopped or walked into its own niche. Design has been driven by need.

Read Jim Williams' birding blog at startribune.com/wingnut.