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migration was over, and the birds would normally have been nesting in their northern haunts had they been allowed to proceed. The gonads were developed by this time to a maximum. On being released, however, these birds did not remain in the district, but immediately resumed their migratory journey northwards. Therefore the idea that the migratory urge is associated merely with a state of change in the sex organs is not entirely satisfactory. Psychological, as well as physiological factors enter into the problem. Thus in the group of birds known as the waders (knot, turnstone, sanderling and the like), a large number of immature birds which have wintered in the tropics or even in South Africa, come north with the main tide of spring migrants. Some may be seen about our coasts all summer, yet they do not go on to breed in the Arctic tundra. Such birds, although examination of their sex-organs shows them to be in an immature state, have been induced by some factor, as yet unappreciated, to embark on a long migration. In addition it has been shown that castrated birds will migrate ! A great deal of accurate experimental work will, no doubt, be done in years to come on this problem and we may one day come to the full answer. At the moment, the cause or causes which induce the migratory urge are still undiscovered, and we can at best postulate some inherent rhythm, which induces the restlessness so apparent in birds at the migratory season. This restlessness gradually increases, until a combination of internal and external factors suddenly reaches a critical pitch, the " spring " is released, and the birds are off. |
