Woodpeckers are not mammals. They are aves that belong to the Kingdom Animalia Phylum Chordate Subphylum Vertebra Class Aves. Birds such as the woodpecker are bi-pedal, warm blooded, oviparous vertebrates that can be characterized primarily by their feathers, modified wings, and hollow bones. Some common characteristics of birds include a bony beak with no teeth, the laying of hard-shelled eggs, high metabolic rate, and a light but strong skeleton. These creatures can also be characterized by their flight, though there are a few species of birds that have lost this ability. Birds feed their young by regurgitating partially digested material.
On the other hand, mammals can be characterized by having hair or fur, giving birth to live young and nursing them with their mammary glands, and most mammals also usually have sweat glands. Mammals possess stronger and denser bones than birds, have specialized teeth, and have been known to carry their young in their womb or pouch. Mammal young are usually born one at a time whereas birds such as the woodpecker are often born in pairs, with the stronger either killing or pushing the other out of the nest.
Interestingly enough birds and mammals both evolved from reptiles, however each from a different type of reptile. Mammals first appeared about two hundred and twenty five million years ago and dramatically increased in number, diversity and size after the distinction of dinosaurs and larger reptiles about 65 million years ago.
The oldest bird fossil that we know of today is from a bird called Archaeopteryx. This bird lived about one hundred and fifty million years ago and also underwent many changes such as mammals, however they underwent these changes much earlier than mammals did.
One of the biggest differences between birds and mammals is the fact that birds have wings in place of the arms or forelegs that mammals have. Mammals usually walk upon all four limbs or have adapted to walk upright such as humans do. (take for instance, apes). The Woodpecker also possesses a large vertical keel on their sternum that mammals do not have. Attached to this sternum are the strong, robust breast muscles that the bird uses to pull its wings during flight.