Leap-frog patterns have been demonstrated by a variety of methods. The pattern in Fox Sparrow populations on the Pacific coast of North America has been known about for almost 100 years, and was worked out using museum specimens collected (i.e. shot) in their wintering range on the US west coast, whose origins could then be traced by plumage characteristics. The Canada Goose varies in plumage colouration across its breeding range, and also varies greatly in size, so that different breeding populations can be identified in the field. Bar-tailed Godwit, like many wader species, has a migration schedule that is very synchronised, with well-defined stopover sites, so that different populations can be observed departing and arriving at different times along the migration route.

 

To understand why leap-frog migration happens, it's helpful to consider the pattern in trans-Saharan migration of the Yellow Wagtail:

 

 

The pattern and colour of the crown in male Yellow Wagtails varies geographically throughout the breeding range, so the origin of birds seen in Africa in winter is easy to work out. Populations breeding in the Mediterranean such as Black-headed (Balkans, Anatolia), Ashy-headed (Italy) and Spanish Wagtails winter in a narrow band of territory just south of the Sahara (yellow on the map). The Blue-headed Wagtail (purple) occupies the whole of the winter range, outlined in red, while the Grey-headed Wagtail (orange) winters south of about 10^o north. The position of this northern boundary to the Grey-headed Wagtail's winter range is the key to understanding leap-frog migration. There is no natural barrier or discontinuity at 10^o north, and the behaviour, physiology and habitat requirements of Grey-headed Wagtails is no different to Blue-headed or any of the other subspecies. What could possibly prevent Grey-headed Wagtails from wintering further north?

 

To answer this question, we need first to look at the timing of spring migration back to the breeding area:

 

 

The timing of breeding among temperate bird populations is later at higher latitudes. Songbirds typically breed in April around the Mediterranean, but not until June in the arctic region of northern Europe. Because of this, spring migration is later among more northerly breeding populations.

 

Next, consider the climatic regime of the west African wintering area. As in the rest of the tropics, the weather is governed by movements of the 'Inter-tropical Convergence Zone', which is a stable line of rain-bearing fronts that oscillates alternately north and south of the equator. In the northern spring, the 'ITCZ' moves north until it reaches about 15^o north at mid-summer, before retreating back towards the equator. The environment therefore changes very profoundly during the period of spring migration.

 

 

 

While southern breeders are migrating north in March, the west African savannas are parched and dry, with rainfall occurring only in the rain forest region of the extreme south. Fattening for migration is difficult everywhere at this time of year, so southern breeders minimize their costs by wintering just south of the desert.

 

By early April, when populations breeding across central Europe are beginning to migrate north, the ITCZ is over the southern savannas, and the accompanying rainfall stimulates insects to emerge, particularly termites, which swarm in huge numbers. Since food is plentiful in the south, it is touch and go whether costs are minimized by fattening in the north where food is scarce, or by extending migration to the region where food becomes plentiful at just the right time before migration.

 

By mid-April, when the northernmost breeding populations are beginning to head north, the ITCZ is further north still, so migrants do not have to travel so far to reach it, tipping the balance in favour of all northern breeders extending migration. The latitude of the ITCZ in mid-April is 10^o north, and this is why northern breeding Grey-headed Wagtails winter no further north than this latitude. In effect, they are 'waiting' for the abundant food that accompanies the arrival of the ITCZ just before they have to embark on migration to ensure timely arrival on their breeding grounds.