Though I was/am not a great nature lover, this series succeeded in keeping me interested right throughout with its excellent commentary (credit to Attenborough). The documentary covers the origins, types( sea/land, veg/meat-eating), acquisition of food, communication methods, mating rituals, laying of eggs, raising family, and finally dangers to its existence in modern world.
Except the goose and swans, most of them seem are prone to either infidelity/polygamous relationships or seasonal relationships, treacherous behaviour to obtain food etc. And all of them are territorial.
Some of the stuff which was fascinating:
-how the bower birds decorate their nests to attract mates
-polygamous birds especially the sparrow .
-how one mother bird (cant remember which one) kills one of its weakest child for survival
-how a crow breaks a nut in middle of a busy Tokyo city street
Not much is covered on migrant birds behaviors. For that I would recommend the movie 'The winged migration'.