The largest living bird, as well as the largest member of the Struthioniformes, is the ostrich (Struthio camelus) reaching a height of over 2.7 m (9 ft) and weighing over 156 kg (345 lb). Eggs laid by the Ostrich can weigh 1.4 kg (3 lb) and are the largest eggs in the world today (and are also the largest single cell of any organism, although Caulerpa (a sea plant) could be considered the largest active cell).
The largest bird in the fossil record may be the extinct Elephant Birds of Madagascar, which were related to the Ostrich. They exceeded 3 m (10 ft) and 500 kg (1,120 lb). The last of the Elephant Birds became extinct about 300 years ago. Of almost the exact same proportions as the largest Elephant Birds was Dromornis stirtoni of Australia, part of a 26,000-year old group called mihirungs of the family Dromornithidae.
The tallest bird ever however was the Giant Moa (Dinornis maximus), part of the moa family of New Zealand that went extinct about 200 years ago. This moa stood up to 3.7 m (12 ft) tall when it was erected in an unnatural position, but weighed about half as much as a large Elephant Bird or mihirung due to its comparatively slender frame.
The largest carnivorous bird was the phorusrhacid Brontornis, an extinct flightless bird from South America which reached a weight of 350 to 400 kg (770 to 880 lb) and a height of about 2.8 m (9.2 ft).
The largest birds ever capable of flight was Argentavis magnificens, a now extinct member of the Teratornithidae group, with a wingspan of up to 8.3 m (28 ft), a length of over 3 m (10 ft) and a body weight of 80 kg (176 lbs.).
- Waterfowl (Anseriformes). The largest species in general is the Trumpeter Swan (Cygnus buccinator), which can reach an overall length of 1.82 m (6 ft), a wingspan of 3 m (10 ft) and a weight of 17.3 kg (38 lb). However, the heaviest waterfowl ever recorded was an overweight Mute Swan (Cygnus olor) from Poland, who weighed nearly 23 kg (50 lb). The members of the previously mentioned Dromornithidae are now classified as members of this order, making them the largest "waterfowl" that ever lived.
- Swifts & allies (Apodiformes). The White-naped Swift (Streptoprocne semicollaris) and the Purple Needletail (Hirundapus celebensis) reach similar large sizes, at up to 225 g (8 oz) and 25 cm (10 in). The hummingbirds are also traditionally included in this order, the largest species of which is easily the Giant Hummingbird (Patagona gigas).
- Nightjars & allies (Caprimulgiformes). The largest species of this order is the Great Potoo (Nycitbius grandis), the maximum size of which is about 680 g (1.5 lb) and 60 cm (2 ft).
- Shorebirds (Charadriiformes). The largest species in this diverse order is the Great Black-backed Gull (Larus marinus), attaining a size of as much as 85 cm (34 in) and 2.5 kg (5.5 lb).
- Herons & allies (Ciconiiformes). The largest species, if measured in regard to body weight and wingspan, is the Andean Condor (Vultur gryphus), reaching a wingspan of 3.2 m (10.7 ft) and a weight of 15 kg (33 lb). The longest-bodied species is probably the Saddle-billed Stork (Ephippiorhynchus senegalensis), which often exceeds 1.5 m (5 ft) tall. Most of the largest flying birds in the fossil record, including the largest, Argentavis magnificens, were members of the Ciconiiformes.
- Mousebirds (Coliiformes). The mousebirds are remarkably uniform, but the largest species is seemingly the Speckled Mousebird (Colius striatus), at 57 g (2 oz) and over 35 cm (14 in).
- Pigeons (Columbiformes). The largest species of the pigeon/dove complex is the Victoria Crowned Pigeon (Goura victoria). Some exceptionally large ones have reached 3.7 kg (8.2 lb) and 85 cm (34 in). The extinct dodo (Raphus cucullatus), however, was the largest bird of this order of all time, weighing about 23 kg (50 pounds).
- Kingfishers & allies (Coraciiformes). The largest species is the Southern Ground Hornbill, reaching sizes of as much as 6 kg (13.5 lb) and nearly 1.2 m (4 ft) in length.
- Cuckoos & allies (Cuculiformes). The largest species of this order is the Great Blue Turaco (Corythaeola cristata), a cousin of the cuckoos. This species, which can weigh over 1.2 kg and measure over 74 cm in length, is much larger than other turacos.
- Birds of prey (Falconiformes). The largest species is the Eurasian Black Vulture (Aegypius monachus), attaining a maximum size of 14 kg (31 lb), 119 cm (47 in) long and 3 m (10 ft) across the wings. Slightly larger, at up to 1.4 m (4.7 ft) long, 3 m (10 ft) across the wings and weighing at least 15 kg (33 lb), was the now extinct Haast's Eagle (Harpagornis moorei), the largest eagle ever.
- Gamebirds (Galliformes). The largest member of this diverse order is the Wild Turkey (Meleagris gallopavo). The largest specimen ever recorded was just short of 14 kg (31 lb) and 130 cm (52 in). The heaviest domesticated turkey on record weighed in at 37 kg (81 lb). The longest species, if measured from the tip of the bill to the end of the long tail coverts, is the Green Peafowl (Pavo muticus), at up to 3 m (10 ft) long. A prehistoric, flightless family, sometimes called (incorrectly) "giant megapodes" (Sylviornis) were the biggest galliformes ever, having reached 1.70 m (5.6 ft) long and weighed up to about 30 kg (66 lb).
- Loons (Gaviiformes). The largest species on average is the Yellow-billed Loon (Gavia adamsii), at up to 1 m (3.3 ft) and 7 kg (15.4 lb). However, one exceptional Common Loon weighed 8 kg (17.6 lb), heavier than any recorded Yellow-billed Loon.
- Cranes & allies (Gruiformes). The Great Bustard (Otis tarda) and Kori Bustard (Ardeotis kori) are the heaviest birds capable of flight, both occasionally reaching 21 kg (46 lb) and 1.5 m (5 ft) long. The tallest and longest bird flying bird on earth, also represented in the Gruiformes, is the Sarus Crane (Grus antigone), sometimes standing almost 2 m (6.6 ft) tall. The largest ever gruiform was a species of "terror bird", highly predatory, flightless birds of South America: Brontornis burmeisteri. This species stood about 2.8 m (9.2 ft) and weigh up to about 400 kg (880 lb).
- Songbirds (Passeriformes). Both the Common Raven (Corvus corax) and the Thick-billed Raven (Corvus crassirostris) are enormous by songbird standards. Both of these birds can exceed 1.5 kg (3.3 lb) and 70 cm (28 in).
- Cormorants & allies (Pelecaniformes). The largest species is the Dalmatian Pelican (Pelecanus crispus), which attains a length of 1.82 m (6 ft), a wingspan of 3.5 m (11.5 ft) and a body weight of 15 kg (33 lb). A pelecaniform of the last Miocene, Osteodontornis, was the largest-winged bird ever, after Argentavis, at up to 6 m (20 ft) across and 2.1 m (7 ft) long.
- Flamingos (Phoenicopteriformes). The largest flamingo is the Greater Flamingo (Phoenicopterus ruber), at up to 1.5 m (5 ft) tall and 4 kg (8.8 lb).
- Woodpeckers & allies (Piciformes). The largest species of this diverse order is the Toco Toucan (Ramphastos toco). The maximum size of this tropical forest bird is possibly as much as 1 kg (2.2 lb) and 75 cm (30 in).
- Grebes (Podicepiformes). The largest species of grebe is the Great Grebe (Podiceps major). It can reach a length of 80 cm (32 in) and a weigh of over 2 kg (4.4 lb).
- Tubenoses (Procellariiformes). The largest species is Wandering Albatross (Diomedea exulans), also the bird with the longest wingspan of any living bird. The maximum dimensions are a length of 1.44 m (4.6 ft) and a wingspan of 3.63 m (nearly 12 ft). Immatures have weighed as much as 13.8 kg (35 lb) at the time of their first flights. The largest-ever tubenose was the albatross-like Gigantornis eaglesomei, with a wingspan of about 6 m (20 ft).
- Parrots (Psittaciformes). The longest and largest overall parrot is the endangered Hyacinth Macaw (Anodorhynchus hyacinthinus), reaching nearly 1.2 m (4 ft) long and 2 kg (4.4 lb). However, the heaviest parrot is the nearly-extinct Kakapo (Strigops habroptilus), which can weigh over 4 kg (8.8 lb), but doesn't much exceed 60 cm (2 ft).
- Sandgrouse (Pterocliformes). The Black-bellied Sandgrouse (Pterocles orientalis) is the largest sandgrouse, at a maximum size of 634 g (1.4 lb) and 45 cm (18 in).
- Penguins (Sphenisciformes). The largest species is easily the Emperor Penguin (Aptenodytes forsteri), with a maximum size of 1.35 m (4.3 ft) and 46 kg (102 lb). At one time, possibly to compete with the mammalian pinnipeds, a number of giant penguins existed. The largest is believed to be Anthropornis nordenskjoeldi, having reached a height of nearly 2 m (6.7 ft) and a weigh of up to 135 kg (300 lb).
- Owls (Strigiformes). Both the Eurasian Eagle Owl (Bubo bubo) and Blakiston's Fish Owl (Bubo blakistoni) reach very large sizes. Record-sized specimens of both species have weighed in the ballpark of 4.5 kg (10 lb) and measured over 80 cm (32 in) long. The largest owl known to have existed was Ornimegalonyx oteroi of Cuba, having measured over 1 m (3.3 ft) tall.
- Tinamous (Tinamiformes). The largest species of tinamou, a group of chunky, elusive ground-birds from tropical America, is the Grey Tinamou (Tinamus tao). It can reach a weight over 2 kg (4.4 lb) and length of over 55 cm (22 in).
- Trogons (Trogoniformes). The Resplendent Quetzal (Pharomachrus mocinno) is much larger than most trogons. It often exceeds 225 g (8 oz) and is at least 35 cm (14 in) along the head-and-body, with the remarkable tail of the male adding 60 cm (2 ft) to the length.