Question

Which is the largest living bird?
A. Emperor Penguin
B. Ostrich
C. Brown Kiwi
D. Greater Rhea

Answer 1

Hint: Birds come under class aves. Struthio is the bird which is the biggest in size. They weigh the most among all the birds. It is their long neck and legs which make them the largest birds in their class.

Complete answer: The five kingdom classification was given in 1969 by R.H. Whittaker. He classified organisms on the basis of mode of nutrition, phylogenetic relationship, cell structure, reproduction, and body organization.
The five kingdoms proposed by him were Kingdom Monera, Kingdom Protista, Kingdom Fungi, Kingdom Plantae, and Kingdom Animalia. His classification did not include viruses and lichens. Birds were included in kingdom Animalia. Birds belong to class aves under phylum Chordata. The word aves is derived from the word ‘Aves’ which means bird. Phylum Chordata includes all those animals which have notochord including birds. Birds come under subphylum Vertebrata under phylum Chordata which means all those organisms whose notochord is converted to the vertebral column. Under Vertebrata, they belong to superclass Tetrapoda, which are organisms with four legs.
Birds are warm-blooded animals with four-chambered hearts and a single monocondylic skull. The hepatic portal system in birds is functional whereas their renal portal system is vestigial.
The scientific name of an ostrich is ‘Struthio’. It is the largest living bird. The long neck and the legs give it its large appearance. These are flightless birds. The egg of an ostrich is the largest single cell ever existing.
The forelimbs of the ostrich are modified into wings. Hind limbs are used for walking and running. Their skin is thin, dry, and non-glandular. Their bones are hollow that is air-filled (pneumatic). It makes their body lightweight. Their jaws are modified into beaks and their teeth are absent. They also possess special structures like crops and gizzards which are absent in other animals except for cockroaches. The heart of an ostrich is completely four-chambered, that is with two auricles and two ventricles. The presence of four-chambered hearts makes their circulation a complete double type of circulation. The respiration in ostrich is by lungs, that is they have pulmonary respiration. Their lungs have air sacs which help to increase their buoyancy. This feature is found in all birds and only in birds.
The brain of an ostrich is enlarged. Their brain has a well-developed cerebellum which helps them to keep their equilibrium. Their optic lobes are well developed while olfactory lobes are poorly developed which makes their sense of smell poor.
All birds are unisexual including ostrich. They have a distinct sexual dimorphism. Ostrich is oviparous, that is they lay eggs that are shelled containing yolk and albumin. They have internal fertilization.
Hence, the correct answer is option B.

Note: The urinary bladder is absent in all birds, unlike other animals. That means they do not store their urine, it is excreted out as soon as it is formed. The female bird has the presence of only the left ovary and left oviduct. Also, their retina has only cone cells except for owls, kiwi, penguin, and ostrich, all are flightless birds.