The great crested grebe has been found by Julian Huxley to have a complicated courtship-display in which both sexes partic ipate. A pair approach, facing one another with head wagging from side to side, then raise one wing and preen the feathers beneath, an action varied in many ways to culminate finally in a weird "dance" in which the pair rise erect, treading water, and remain bolt upright with breasts nearly touching for a brief space. In some species of albatross mutual courtship-displays have pro gressed to a point where they have far exceeded mere mating antics, and have become social customs that continue through a period of months, beginning with the arrival of the birds on the breeding grounds, and continuing until the young are grown. In the Laysan albatross (Diomedea immutabilis) of the Leeward islands of Hawaii, the display begins with two birds approach ing with quick bows, shaking their heads rapidly from side to side, raising the wing to preen the feathers beneath, and finally throw ing the head and neck fully erect, with the bill pointing perpen dicularly to the sky, while both birds emit a hollow groan. This action is of daily occurrence when the birds are on land, from October to May, and is participated in by mates or neighbours. It has thus developed into an interesting racial custom.
As an example of the third type of display there may be men tioned the courtship of the phalaropes, in which the female has brighter plumage than the male, and takes much of the initiative. In Wilson's phalarope (Plialaropus tricolor) both birds bow and nod, but in the culmination of this action the male flees with one, or sometimes two, females in close pursuit.
The site for the nest that is to contain the eggs is sometimes chosen by the female, sometimes indicated by the male, though the female may exhibit certain supposed prerogatives of her sex in such matters by modifying considerably the original plans pro posed by her mate. The male house sparrow (Passer domesticus) selects some hollow suitable for a nest, and rests beside the en trance, calling and chattering until a female comes to inspect the premises. If these appear suitable, nest-construction may proceed. With birds that nest in trees or bushes, or on the ground, where there is greater latitude of site, various locations may be examined before definite selection is made, both male and female apparently exhibiting directive impulse in choice. When suitable sites are numerous it is probable that final selection comes frequently through the chance that directs the placing of the first nesting material when the courtship shall have proceeded to the point of actual nest ' construction. In some instances, where numerous
sites all exactly alike are available, as where barn swallows (Hirundo) make their nests of mud on the rafters of a building, there is often confusion, so that birds may carry pellets of mud to a dozen rafters until, finally, one of the several foundations assumes more importance in their eyes than the others and a nest is brought to completion. In such species as the ruby throated hummingbird (Archilochus colubris) and prairie-hen (Tympanuchus americanus), where mating takes place at a point away from the nest and the male does not visit the nest-site, selection of a suitable spot rests entirely with the female.
With most birds mating is temporary, and may be for the period of a single brood, or, where two or more families are reared, for one nesting season. Some interesting information has been secured recently on this subject through marking birds with numbered bands which serve to distinguish such individuals from others of their kind. Experiments made by S. P. Baldwin on the house wren (Troglodytes aedon) in Ohio have been particularly instructive. In these studies one male house wren was found mated with three different birds in three successive years, and this same individual mated with two different mates for first and second broods during one season. Another wren mated with one corn panion for one brood, and in the following year took a new mate, with which it was found in the year succeeding. This shifting in the mating relation is not due to the death of one of the birds.
Though the majority of birds associate in pairs only during the nesting season there are some exceptions to this. The white breasted nuthatch (Sitta carolinensis) of North America is found so invariably in pairs that when one appears alone the assump tion is that something has killed its mate. Cardinal grosbeaks (Cardinalis cardinalis) also are usually found in pairs throughout the year. It is commonly stated that some birds, as eagles, mate for life, but this remains to be definitely proven.
Polygamy is practised by numbers of birds, particularly among the pheasant-like species. The males of the capercaillie (Tetrao urogallus), black grouse (Lyrurus tetrix) and wild turkey (Melea gris gallopavo) regularly take several mates, though the bobwhite (Colinas virginianus), a related species, is monogamous. Poly andry has been alleged for a number of species but needs veri fication as to regular occurrence since one of the forms concerned, the cowbird (Molotlirus ater), where the female has been said to mate with several males, has recently been found to pair as regularly as is normal in small birds.