BIRD NUPTIALS AND NESTINGHaving secured for himself a territory in which to rear a family, our cock bird, defending it vigorously, awaits the arrival of the hens. These normally come in after the cocks by a period which varies with different species, and in different years, but is usually between 9 and 15 days later. When the hen birds arrive, how precisely does this pair formation take place? We know that the hen birds will be making for the spot near to where they nested last year, so that there is a possibility, though no certainty, that the same birds may pair together, in the same spot, as did so last year, even though they have migrated in the meantime, over 3000 miles or more. But, through deaths and other causes, a cock bird is equally liable to get a new mate in his territory, though she may be one of his brood from the previous year or even a sister. Birds of one species inhabiting a given area are almost certain to be closely inter-related, and to be derived from intensive in-breeding. But contrary to popular opinion, in-breeding as such is not a bad thing. On the contrary, provided the strain is good and no hereditary defects are present, in-breeding coupled with rigid selection tends to improve a stock rather than otherwise, and is the method used by live-stock breeders to build up pure pedigree strains. Since, under natural wild conditions, defects are automatically eradicated by stringent natural selection, brother and sister crosses and the like would tend to strengthen rather than weaken, a group of birds of a given species. The period of sexual stimulation through visual patterns has commenced, and from now on until final mating both birds will keep close together, and through the gradual effect of visual stimuli of ever-increasing complexity and emotional significance, will approach the climax of this first phase of the breeding cycle. Mutual stimulation towards a sexual goal has been well demonstrated from studies of the behavior of colony-nesting birds in the breeding season. With birds like the gulls, the mutual stimulation towards sexual maturity is one, not only of the individuals of a pair for one another, but of the whole mass of birds for each individual within the mass. This has led one ornithologist to put forward a valuable concept which we can call the " mass stimulation " idea. By this, it is suggested that, for birds which nest together in large groups, a very important factor in the safety of the colony is the need for most of the eggs to be laid at approximately the same time, so that a large proportion of the adults may be incubating eggs or feeding young together, and in this way, the time during which downy chicks are exposed to predatory birds is cut to a minimum. In order that this may occur, the majority of the pairs must come to full breeding condition at the same time, and this is accomplished by the sexual excitation produced by the mass of birds in the colonial unit. Contained in this theory is also the idea that for colony-nesting birds there will be a minimum number below which the effect of mass stimulation will be so slight that the birds find it difficult to reach the sexual maturity necessary for successful breeding. As an example of the latter, the case of the fulmar petrel is often quoted. This bird which formerly nested mainly on St. Kilda, has spread during the present century in a remarkable manner, and now bids fair to reach the south coast of England. Its methods of colonizing the cliffs on which it breeds have been noted, and it is usual for a preliminary inspection by one or two birds to take place in one year, to be followed by the regular presence of several non-breeding birds in the next, until finally, when a little group has collected, regular breeding commences. During the first year or so, the birds may behave as though breeding, and
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