Birds of a feather flock together. Persons associate with those of a similar taste and station as themselves. Qui se ressemble s'assemble. Cicero says, “Similes similibus gaudent, pures cum paribus facillime congregantur.” “Ne nous associons qu'avec nos égaux” (La Fontaine).
To kill two birds with one stone. To effect two objects with one outlay of trouble.
(protected by superstitions).
Choughs are protected in Cornwall, because the soul of King Arthur migrated into a chough. The Hawk is held sacred by the Egyptians, because it is the form assumed by Ra or Horus. The Ibis is sacred in Egypt, and to kill one was at one time a capital offence. It is said that the god Thoth escaped (as an Ibis) from the pursuit of Typhon.
Mother Carey's Chickens, or Storm Petrels are protected by sailors, from a superstition that they are the living forms of the souls of deceased sailors.
The Robin is protected, both from Christian tradition and nursery legend. (See Robin Redbreast.) The Stork is a sacred bird in Sweden, from the legend that it flew round the cross, crying Styrka, Styrka, when Jesus was crucified. (See Stork.)
Swans are superstitiously protected in Ireland from the legend of the Fionnuala (daughter of Lir), who was metamorphosed into a swan and condemned to wander in lakes and rivers till Christianity was introduced.
(See Irish Melodies, Silent O'Moyle.)
The bat (a winged animal) was regarded by the Caribs as a good angel, which protected their dwellings at night; and it was accounted sacrilegious to kill one.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894