What are birds?
The class Aves places second in the number of species among vertebrates. There are approximately 8,600 species of birds in the world. It is estimated that the there are 100 billion birds in all. There are 300 species in Estonia and they belong to 20 orders. Read about them in Estonian Birds.
What do birds look like?
Birds have an exceptional streamlined body that is covered by feathers. A lot of birds have very colourful feathers, the others are hardly noticeable in the nature due to their modest appearance. Front limbs are modified into wings. Apart from a few exceptions, all birds can fly. Hind limbs have four toes in most cases. Jaws are covered by a horny beak. Weight varies greatly. The ostrich is the largest among birds (weighs up to 150 kg), the hummingbird is the tiniest bird (weighs from 1.6 g). The mute swan is the largest Estonian bird (weighs up to 13 kg) and the goldcrest is the smallest bird in Estonia (weighs from 3.5 g).
Where do birds live?
Birds live in diverse habitats: deserts, mountains, forests, tundras, near the bodies of water, etc. A lot of birds never leave their living places, the others migrate to warmer countries when the weather changes. Migrations may be tens of thousands of kilometers long.
How do birds move?
Most birds can fly by means of feathered wings. Powerful breast muscles move wings. Tail helps birds direct the flight. Birds' light rigid skeleton and fast digestion favours flying. Hind limbs - legs - are for walking on land. Water birds have webbed-swimming feet. Several birds can swim, walk and fly but most of them are still best adapted for one or two of these.
How do birds breathe?
Birds' loss of energy is enormous in flying and it requires a great amount of oxygen for the organism of the bird. Therefore birds have special organs - air sacs - in addition to lungs. Air always passes through lungs in one direction, it enables maximum oxygenation of the blood.
What is the circulatory system of birds like?
Birds have four-chambered heart. Thus the systemic circulation and pulmonary circulation are completely separate, consequently arterial and venous blood cannot get mixed. This is one of the reasons why birds are warm-blooded, i.e. their body temperature does not depend on the temperature of the environment. Birds' average body temperature is 41 - 42 °C .
What do birds eat?
Birds have diverse diets: they eat both plants and animals from air, water, or land. Birds have no teeth - food moves into two-chambered stomach. In the first chamber food is mixed with digestive enzymes, the second chamber is the gizzard functioning as a substitute for teeth. Birds must eat large quantities of food to generate the energy needed for both flight and temperature regulation especially in winter and during migration. Some small birds have to eat larger quantities of food daily than their own weight.
What are the main organs of senses of birds?
The organs of senses are unevenly developed. They depend more on their vision than any other sense. Birds have keen senses of vision and hearing but poor senses of smell and taste. The majority of birds does not distinguish smells.
How do birds reproduce and develop?
All birds are bisexual, fertilization is internal. Most birds build nests. These are diverse and are situated either on land, in trees, in hollows of trees, in burrows, or on the sides of cliffs, etc. A lot of birds begin nesting with complicated rituals - courtship. Males and females of most bird species form pair bonds - i.e., they stay with one another throughout the reproductive season. Laying eggs is followed by incubating and hatching. Young hatchlings are capable to follow their mother almost immediately after having been hatched in case of several bird species. Songbirds and most sea birds have altricial young - i.e., the newly hatched birds are blind, naked, and helpless and must remain in the nest for a time because they require constant feeding and care. Most our birds nest only once a year but there are species that can nest up to three times a summer in case of favourable weather conditions.