Timing of pre-nuptial migration and leap-frog patterns in Yellow Wagtails (Motacilla flava)

Leap-frog migration may be caused by a surge in food availability late in the pre-nuptial migration period, in parts of the wintering grounds most distant from the breeding area. Birds breeding at the highest latitudes are the latest to depart on pre-nuptial migration, and so can take advantage of this surge by extending their post-nuptial migration to pass the winter in the most distant wintering areas. Such populations should fatten more quickly prior to migration than low-latitude breeders, which migrate earlier in the year and winter closer to the breeding area.

In April 2004, I conducted a pilot study into the feasibility of comparing fattening rates ahead of pre-nuptial migration among populations of a leap-frog migrant, the Yellow Wagtail, at different latitudes in Nigeria. A population in central Nigeria was estimated to be fattening at up to 0.47g d⁻¹, which is greater than estimates obtained from data collected at a site in northern Nigeria. More northerly breeding populations wintering in central Nigeria may therefore be able to add sufficient fat to cross the Sahara up to 7.5d more quickly than would be possible if they remained in northern Nigeria.