For an animal to return to a specific spot after a lengthy migration, it must use true navigation to get there. That is, it needs to not only travel in a given compass heading and know where it is at any given time so the course may be altered when necessary but also be able to recognize its goal when it has arrived. It is dangerous to generalize on the means of orientation and navigation in migration; different groups of birds with different modes of existence have evolved different means of finding their way from one place to another (Pettingill 1970). We are only beginning to realize the complexities involved in the many modes of bird orientation and navigation. All we can do in this section is present a brief summary of some of the more important principles involved and the studies that have enhanced our knowledge in the area. Ability to follow a more or less definite course to a definite goal is evidently part of an inherited faculty. Both the direction and the goal must have been implanted in the bird's genetic code when the particular population became established at its present location. The theory is sometimes advanced that older and more experienced birds lead the way and thereby show the route to their younger companions. This explanation may be acceptable for some species such as geese, swans, and cranes because they stay in family groups, but not for species in which adults and young are known to migrate at different times, especially when young migrate ahead of the adults. As indicated in a previous section on segregation, many North American shorebirds as well as the cuckoos of New Zealand do this. An inherited response to its surroundings, with a definite sense of the goal to be reached and the direction to be followed, must be attributed to these latter birds. It is well known that birds possess wonderful vision. If they also have retentive memories subsequent trips over the route may well be steered in part by recognizable landmarks. Arguments against the theory of landmark memory are chiefly that unescorted young birds, without previous experience, can find their way to the winter quarters of their species, even if the wintering area has a radically different landscape and vegetation than the breeding grounds. Experimental findings and field observations indicate landmarks are used in navigation by certain birds, but the degree of use varies considerably among the species (Bellrose 1972a).