The Common Teal is the smallest dabbling duck alive, at 34–43 cm (13-17 in) length and with an average weight of 360 g (13 oz) in drake (males) and 340 g (12 oz) in hens (females). The wings are 17.5-20.4 cm (6.9-8.0 in) long, yielding a wingspan of 53–59 cm (21-23 in). The bill measures 3.2–4 cm (1.3-1.6 in) in length.
From a distance, the drakes in nuptial plumage appear grey, with a dark head, a yellowish behind, and a white stripe running along the flanks. Their head and upper neck is chestnut, with a wide and iridescent dark green patch of half-moon- or teardrop-shape that starts immediately before the eye and arcs to the upper hindneck. The patch is bordered with thin yellowsh-white lines, and a single line of that colour extends from the patch's forward end, curving along the base of the bill. The breast is buff with small round brown spots. The center of the belly is white, and the rest of the body plumage is mostly white with thin and dense blackish vermiculations, appearing medium grey even at a short distance. The outer scapular feathers are white, with a black border to the outer vanes, and form the white side-stripe when the bird is in resting position. The primary remiges are dark greyish brown; the speculum feathers are iridescent blackish-green with white tips, and form the speculum together with the yellowish-white tips of the larger upperwing coverts (which are otherwise grey). The underwing is whitish, with grey remiges, dense dark spotting on the inner coverts and a dark leading edge. The tail and tail coverts are black, with a bright yellowish-buff triangular patch in the center of the coverts at each side.
In non-breeding (eclipse) plumage, the drake looks more like the hen; it is more uniform in colour, with a dark head and vestigial facial markings. The hen itself is yellowish-brown, somewhat darker on wings and back. It has a dark greyish-brown upper head, hindneck, eyestripe and feather pattern. The pattern is dense short streaks on the head and neck, and scaly spots on the rest of the body; overall they look much like a tiny Mallard (A. platyrhynchos) hen when at rest. The wings are coloured similar to the drake's, but with brown instead of grey upperwing coverts that have less wide tips, and wider tips of the speculum feathers. The hen's rectrices have yellowish-white tips; the midbelly is whitish with some dark streaking.Immatures are coloured much like hens, but have a stronger pattern. The downy young are coloured like in other dabbling ducks: brown above and yellow below, with a yellow supercilium. They are recognizable by their tiny size however, weighing just 15 grams (about half an ounce) at hatching.
The drake's bill is dark grey, in eclipse plumage often with some light greenish or brownish hue at the base. The bill of hens and immatures is pinkish or yellowish at the base, becoming dark grey towards the tip; the grey expands basewards as the birds age. The feet are dark grey in males and greyish olive or greyish-brown in females and immatures. The iris is always brown.
This is a noisy species. The male whistles cryc or creelycc, not loud but very clear and far-carrying. The female has a feeble keh or neeh quack.
Males in nuptial plumage are distinguished from Green-winged Teals by the horizontal white scapular stripe, the lack of a vertical white bar at the breast sides, and the quite conspicuous light outlines of the face patch, which are indistinct in the Green-winged Teal drake. Males in eclipse plumage, females and immatures are best recognized by their small size, calls, and the speculum; they are hard to tell apart from the Green-winged Teal however.
The Common Teal usually feeds by dabbling, upending or grazing; it may submerge its head and on occasion even dive to reach food. In the breeding season it eats mainly aquatic invertebrates, such as crustaceans, insects and their larvae, molluscs and worms. In winter, it shifts to a largely granivorous diet, feeding on seeds of aquatic plants and grasses, including sedges and grains. Diurnal throughout the breeding season, in winter they are often crepuscular or eaven nocturnal feeders.
Males call to Females to mate. Shortly after mating, females lay egges in ground nests.It nests on the ground, near water and under cover. The pairs form in the winter quarters and arrive on the breeding grounds together, starting about March. The breeding starts some weeks thereafter, not until May in the most northernly locations. The nest is a deep hollow lined with dry leaves and down feathers, built in dense vegetation near water. After the females have started laying, the males leave them and move away for shorter or longer distances, assembling in flocks on particular lakes where they moult into eclipse plumage; they will usually encounter their offspring only in winter quarters. The clutch may consist of 5-16 eggs, but usually numbers 8-11; they are incubated for 21–23 days. The young leave the nest soon after hatching and are attended by the mother for about 25–30 days, after which they fledge. The drakes and the hens with young generally move to the winter quarters separately. After the first winter, the young moult into adult plumage. The maximum recorded lifespan – though it is not clear whether this refers to the Common or the Green-winged Teal – was over 27 years, which is rather high for such a small bird.