Polygyny is the form of polygamy in which the male has more than one female mate.
For example- Certain species of grouse show such behavior. In this case, male gather in a collective display ground, the lek, which is divided in to individual territories. Each territory is defended by male displaying behavior. There is competition among the males for mates and females tend to choose the dominant male.
Polyandry is the form of polygamy in which a female mate with several males and males incubate the eggs. Polyandry is seen in species of spotted sandpipers. In this case male incubates the eggs with in the territory of female and provides most parental care.
Monogamy is much more common among birds than among mammals. This is because male and female birds are equally adept at most aspects of parental care.
In mammals, males cannot gestate and lactate. Hence they can provide little help in caring for the young.
In birds, both the female and male birds can alternate care of the nest and young, which permits one parent to be at nest at all times. The constant attendance of either male or female birds in a common nest gives protection to the eggs or to the young.
The demand on the male to care for the young and his mate gives him no opportunity to mate with other females.
The Archaeopteryx fossils helped in understanding the ancestry of birds. As the fossil record of birds was very obscure, the discovery of Archaeopteryx was quite significant.
Archaeopteryx possesses morphological characters that look like a combination of reptilian and avian characters. It has teeth like reptiles and a beak like birds. The body is covered with feathers as in birds. Its skeleton is similar to reptiles with the presence of a bony tail.
So Archaeopteryx is a connecting link between birds and reptiles and demonstrates that birds are grouped phylogenetically with dinosaurs.