In the context of birds, usually expressed in terms of total length, from tip of bill to end of tail with the neck extended and the bill in line, as in a museum specimen. This gives an indication of variations in size within or between species in a group, but has the disadvantage of being influenced by differences in shape or posture and the length of bill, neck and tail, giving an exaggerated impression of long-billed, -necked and/or -tailed birds. The size can also be expressed by the weight, which gives a better indication of the general size independent of proportions, but has the disadvantage of being very variable, even in the individual at different times. Another size indication is the 'span' (or 'spread' or 'expanse'), being the distance between the wing-tips when they are fully extended. The size range of birds is substantial, ranging from 6,3 cm long and less than 3 gram in small hummingbirds, to about 2,3 meter high and 130 kg in the Ostrich. The size of birds is thought to be related to such items as energy conservation (the bigger an animal, the less energy loss) and predator-prey relationships (e.g., a large size confers some immunity from enemies).

Alternative forms for size : sizes, span, spread, wingspan.