INTRODUCTION
The ability to fly has enabled birds to occupy a wide diversity of habitats and develop many adaptations for feeding. This has led to a large number of about 9700 extant species belonging to the class Aves, divided into about 27 avian orders.
The largest order of all is the Passeriformes with over 5712 species and the smallest is the Struthioniformes with one species, the ostrich (King & McLelland 1984). Table 6.1 shows the types of bird most commonly seen in a veterinary clinic.Birds evolved from reptiles and many similarities still remain. Like reptiles, birds have scales on their beak, legs and feet, a single occipital condyle, a single middle ear bone, the columella and a jawbone made up of five bones fused together (Quesenberry et al. 1997). They also have nucleated erythrocytes, a renal portal system and excrete uric acid (Maina 1996).
While reptiles and mammals show incredible diversity, the constraints of flight means the basic bird design varies very little from species to species. In fact there are fewer morphological variations among all bird species than among, for example, the mammalian order of Carnivora (with nearly 300 species) (Maina 1996). For this reason, this section will cover the anatomy and physiology of birds in general. The
Table 6.1 Common avian orders seen in veterinary practice
| Order approx no | Species | Examples of species |
| Galliformes | 214 | Pheasants, domestic fowl, guinea fowl, quail |
| Anseriformes | 161 | Swans, geese, ducks |
| Psittaciformes | 358 | Cockatoos, budgies, cockatiels |
| Columbiformes | 310 | Pigeons, doves |
| Passeriformes | 5712 | Songbirds, canaries, zebra finches |
| Falconiformes | 285 | Eagles, hawks, falcons |
| Strigiformes | 178 | Owls, nightjars |
differences between the common orders can be seen in Table 6.6 at the end of the chapter.
Size range
In size, birds range from the hummingbird (Trochilidae spp.), which weighs 2 g, to the flightless ostrich (Struthio camelus) which can weigh up 120 kg.
The largest flying birds weigh up to a maximum of 15 kg and range from the Mute swan (Cygnus olor) to the Andean condor (Vultur gryphus) (Kirkwood 1999).Longevity
Birds tend to have longer life spans than mammals of similar size (Kirkwood 1999). Pigeons and swans can live for up to 30 years, while psittacines like African grays (Psittacus eritha- cus) and cockatoos (Cacatua spp.) commonly live for over 40 years. In the passerines the larger birds live longer than smaller ones: the raven can live for well over 40 years whereas canaries live from 8 to 16 years (Dorrestein 1997b).