Owen D.F. (1969) The migration of the Yellow Wagtail from the equator. ARDEA 57 (1-2): 77-85

1. Curry-Lindahl and Marshall & Williams have postulated that the departure of the Yellow Wagtail from its equatorial wintering grounds in Africa is determined by either an innate internal timer or by conditions on the breeding grounds during the previous year, or by both. They argue that there are no environmental factors in Africa that could stimulate departure. Marshall & Williams specifically rule out fluctuations in insect numbers as a possible stimulus. 2. But insect numbers do fluctuate near the equator. At Kampala, Uganda, the major fluctuations occur in February-April, the period when Yellow Wagtails are preparing to depart from the wintering grounds. 3. It is suggested that fluctuations in insect numbers acting as an environmental trigger provide a better hypothesis than the existence of an internal timer to explain the departure of the Yellow Wagtail from the equator.