More than half of all bird species (5000+) are classified in order Passeriformes and referred to as passerines. Passerines, sometimes known as perching birds or less accurately as songbirds, have four toes – three of them directed forward and one toe directed back.
This arrangement allows for stable perching on structures such as small branches and herbaceous stems because the hind toe is opposable to the others (much like our thumbs are opposable to our fingers) which allows “grasping”.
Interestingly, the tendon/bone anatomy of a passerine causes automatic closing of the foot (grasping) when the leg bends – such as when it lands on a typical perch. This arrangement also allows passerines to sleep on a perch without falling off.
Here you can see the typical “three toe forward, one toe back” arrangement on this Savannah Sparrow that allows passerines to grasp narrow perches like branches or wires and remain stable on them. Animals without something like this toe arrangement would be “walking a tightrope” but these birds have no problem with it.
Maintaining stability on a perch such as this would be virtually impossible without the ability to grasp, particularly in a breeze (this is a Loggerhead Shrike).
This shot of a juvenile White-crowned Sparrow gives us a better look at all four toes.
One more look at the opposable rear toe, this time on a Western Meadowlark.
Ok, now to my point. Many birds that do not typically ”perch” (with exceptions) are waterfowl and shorebirds. The Killdeer is a plover but because of the habitats it prefers it’s often found with passerines and many of us associate them with the perching birds. But think about it, have you ever seen a Killdeer perched in a tree, on a branch, narrow stem or wire? I don’t think I have and I’ve spent a lot of time over the years with them.
And here’s why. Like most plovers, Killdeer lack the hind toe so they rarely perch (except on something like a rock). They do not have the ability to grasp a narrow perch for stability.
Another view of the “toe that isn’t there”.
I realize that this is probably elementary information for most folks who are well-schooled in birds but I remember when I finally realized that I’d never seen a Killdeer perched and eventually figured out why.
Perhaps there are others in the same boat as I was…