The Stirge is a monster from AD&D. My information comes from the 1996 printing of the Monstrous Manual, as well as from my own experience DMing the creatures.
Stirges are small bird-like creatures that drink blood to sustain themselves. They have bat-like wings and four small, pincer-like legs, a well as a long, dangling proboscis. Stirges have a 2 foot wingspan. The body of the Stirge is rusty red to reddish-brown in color, the legs yellow, and the proboscis, pink at the tip, fading to grey at the base (near their heads).
Stirges form nest-like colonies in attics, dungeons, and groves of trees. Stirges sleep upside down, which, in additon to their bat-like wings, leads many to surmise that they may be somehow related to vampire bats. Stirges may be bred in captivity, but this requires a constant supply of blood.
Stirges have an acute sense of smell, can see in the dark, and can sense heat sources within 200 feet. With these senses, a stirge may find even the smallest gap in armor, hide, or other protection, and exploit it to gain a meal. After a stirge has gorged itself on blood, it flies away to sleep for a period of time lasting anywhere from a day to a week. It is during this rest period that the beasts are most vulnerable, and may be dispatched easily.
The territory of a colony of stirges is generally only a mile in diameter. Due to this, the stirges, having drained an area of available blood supplies, move quite frequently. Often, it is only discovered that stirges inhabit an area long after they have disappeared, making them difficult to track and destroy. Stirges have a difficulty piercing thick animal hides, and because of this, some large colonies of stirges have developed a symbiotic relationship with evil dragons.
Large groups of stirges may prove a danger to civilized areas, as well as low level adventurers. However, any relatively well equipped group of adventurers or town militia should be able to deal with all but the largest colonies of stirges.
Rumors persist of exceptionally large varieties of stirges deep in the tropical jungles. Reports tell of stirges with wingspans of as much as 8 feet. They are reported to have a paralyzing poison on the tip of their sharp snouts, which are highly prized by tribesmen. None of these large versions have ever been captured for study, so all information known about them comes from tribesmen, explorers, and missionaries in the jungle regions.