But here is a serious difficulty urged by some writers as a powerful argument against the sexual impulse as the great factor in the return journey. Many of the birds which migrate northwards or homewards are sexually immature, and others of them are undoubtedly to be classed as "non-breeders," which means that during that particular summer they will not be engaged in the work of reproduction; why, then, should young birds or non-breeders migrate from the winter base. Possibly in the early days of migration only the mature birds did return; that we cannot state one way or the other. But it is reasonable to argue that once a regular migration habit has become not only confirmed by heredity but a very true advantage to the species, its influence will be felt by each and every individual. Again it is clear that the sexual impulses, in an undeveloped form, are appreciated by the adolescent, and in many animals by even the most juvenile. The play of all young animals is either an imitation or reflection of the search for food--the hunting instinct--or the love-making and sexual quarrels pertaining to reproduction, the pretended competition by the young for the favours of the opposite sex. They may play at and actually perform a migration which is so closely bound up with the life of the species. That this impulse has not always sufficient strength to force them to perform the whole journey is apparent from the fact that many non-breeders, young or sexually mature, on their northward journey through our islands or along our coasts, never reach the breeding area; the food supply on the way attracts them more than the memory of home; they linger with us until the breeding season is over and the return journey has begun. Knots, sanderlings, turnstones and many other waders may be seen on passage late in June, and some remain on our mud-flats throughout the summer; in July the tide of migration has turned.