Obviously the foot to the left of the White-throated Sparrow's is adapted to do something beyond just grasp a twig, and it's not the foot of a songbird. No perching songbird would possess such powerful-looking feet with such dangerous-looking talons. This foot belongs to a Barred Owl nestling and it is beautifully adapted for grabbing victims such as rodents and small squirrels, wounding them or even killing them in the process, and later tearing the prey to shreds.
Therefore, each time you see a bird species that is new for you, it's a good idea to notice its feet, for their adaptations may cue you to something special the bird does. Maybe the most famous non-perching feet are those of ducks, who possess webs between the front three toes, for paddling water. Woodpeckers cling to the sides of trees and thus need very powerful grips, and have their third toes swung around so that their feet are equipped with two toes up front and two in the back. Ground-living birds like pheasants and chickens possess very thick, powerful toes with well developed nails, perfect for scratching the ground.
The average bird foot has four toes (the technical name for that condition is anisodactyly), and typically the first big toe (the hallux) is turned backward, while the other three toes face forward. You can see this clearly in the above pictures. Usually the hallux grows at the same level as the other toes so that it can grasp an object from the opposite direction of the other toes. Some birds -- cranes, many rails, and members of the Pheasant Family, for example -- have their halluces growing higher up their legs so that they never touch the ground. These birds usually walk or run on the ground, so a hallux dragging along behind the main part of their foot would only get in the way.
Swifts, which may rest hanging on vertical walls, have all their toes turned forward, or can turn them forward when they want. Among kingfishers, two toes are fused for part of their length. In ducks, the three front toes are connected with a web of skin. Among cormorants, gannets and pelicans, all four toes are connected by webbing.
Claws on a bird's foot are actually specialized scales formed into horny sheaths. Claws grow continuously and are worn away through daily activity. Claws are usually called talons when we are referring to birds of prey such as owls, eagles and hawks.