What are the modifications that are observed in birds that help them fly?

Birds have undergone many structural adaptations to suit their aerial life. Some of these adaptations are as follows:
(i) Streamlined body for rapid and smooth movement and to counter the resistance offered by the air current.
(ii) Covering of feathers for insulation.
(iii) Forelimbs modified into wings and hind limbs used for walking, perching, and swimming.
(iv) Presence of pneumatic bones to reduce weight.
(v) Presence of additional air sacs to supplement respiration.
(vi) Endoskeleton has hollow long bones to reduce weight that favors flying.
(vii) Uricotelic excretion that helps to reduce storage of water to reduce weight.

C.

Annelida

The numbers of eggs produced by an oviparous mother will always be more than the young ones produced by a viviparous mother. This is because in oviparous animals, the development of young ones takes place outside the mother's body. Thus, their eggs are vulnerable because of the harsh environmental conditions and predators. Therefore, to overcome the loss, more eggs are produced by mothers so that even under harsh environmental conditions, some eggs are able to survive and produce young ones.
On the other hand, in viviparous organisms, the development of young ones takes place inside the body of the mother. Since the development takes place inside the body the young one is less exposed to environmental conditions and predators. Therefore, there are more chances of their survival and hence, less number of young ones is produced compared to the number of eggs.

Liver fluke
Fasciola hepatica
Taenia solium
Ascaris lumbricoides
Wuchereria bancrofti
Ancyclostoma
Anopheles
Bedhug
Lice