These centres become more clearly defined and increase in the number of distinct nuclei as amphibians and reptiles are examined, and they are as extensive in birds as they are in mammals.
Although there is an extensive heritage of bird foraging theory in ecology, most of it addresses only the individual level and is too simple for our context.
Whether different mechanisms of colour production have different intrinsic levels of variability, is an intriguing issue that could be pursued further based on a more extensive sampling across bird species and coloured patches.
Although some birds are still kept in small flocks in extensive systems, most birds available in the market today are reared in intensive commercial enterprises.
In extensive systems, the birds are exposed to adverse weather conditions and are vulnerable to predators and disease-carrying wild birds.
Some advanced beaked forms were toothless, such as the pteranodonts and azhdarchids, and had larger, more extensive, and more bird-like beaks.
The grasslands, heathland, meadows and mire support extensive populations of birds such as Barn owls (Tyto alba) and nightjar, with butterflies including Marbled White (Melanargia galathea), Green Hairstreak (Callophrys rubi) and the Gatekeeper Butterfly (Pyronia tithonus).
Letter X: "On snakes and on the humming-bird" — Extensive detailing of a wide variety of snakes, including the cultural practices surrounding them; it also mentions their habits and stories that have been told in America, warning people about certain ones.
