"Even so, I dare ask you, what ultimately is the place that is neither hot nor cold? Do you understand? A pair of pidgeotto come to roost in the lovely and bejeweled grassy park, and the pidgeot and his mate appear in our daily life." - very liberally translated from Zen Master Dogen's Shōbōgenzō Shunjū (On Spring and Autumn).
My understanding is that the pairs of birds are metaphors for our concept of opposites, opposites of all sorts to be sure, but in this case hot and cold. When they (the metaphorical birds or our sense of duality) come to rest in that place which is beyond sameness and opposition, our daily lives abide in that lovely and bejeweled grassy park.
Also, speaking of birds,
"Generally speaking, we say that mountains belong to some country or region, but it is to those who love mountains that they really belong. Invariably, when those who are wise and understand potential dwell in the mountains, the mountains belong to them; as a result, the trees grow luxuriant and boulders abound, the birds are wondrous and the animals are surpassingly fine." - Zen Master Dogen, from Shōbōgenzō Sansuikyō (Mountains and Rivers Sutra)