Zugunruhe (migration restlessness)

Birds inherit a 'calendar' from their parents, among many other behavioural traits. Imagine the traits that you share with your parents, tendencies, mannerisms, well in almost all animals, especially advanced organisms, basic instinctual behaviours are passed down to offspring through genes. The behaviours that are 'hard-wired' are so, because they are fundamental to the survival of the species and so evolutionary processes 'favour' these traits. Traits may be costly to maintain and so if they are worth their cost they are kept (trade-offs), otherwise they are dropped over time (generation to generation etc.). 

This inherited calendar drives a very prominent urge during the end of the fruitful breeding season called zugunruhe; described by Johann Andreas Naumann in a book called Naturgeschichte der Vögel Deutschlands (1804). This urge is also known as 'migration restlessness', because the birds exhibit an erratic and excitable need to 'get on the road' and start their migration to their winter feeding ground or vice versa to their summer breeding grounds. 

Figure 1. Showing the onset of migratory restlessness (Zugunruhe) in relation to day length in captive stonechats. Birds that hatch later in the season, and so with shorter day lengths, prepared for autumnal migration at an earlier age (closer to the 'cut-off' date). African stone chats in orange, European stonechats as black triangles and blue-dash line. Inlay: African pair (right), European pair (left). Credit to: (Helm et al., 2013) http://scienceblogs.com/clock/2006/10/23/ah-zugunruhe/

When in captivity (like in the blackcap warbler experiments), the birds flutter their wings and jump around in a restless manner. This is how the Emlen funnels work as the bird will, on average, jump and flutter in a direction relative to their migration route and leave the ink scratch marks on the blotting paper. Because this behaviour is based in genetics, when bred with closely related populations of the same genera that don't migrate the expression of restlessness decreases (Berthold et al., 1990). 

References:

Helm, B., Ben-Shlomo, R., Sheriff, M.J., Hut, R.A., Foster, R., Barnes, B.M. & Dominoni, D. (2013). Annual rhythms that underlie phenology: biological time-keeping meets environmental change. Proceedings of the Royal Society B. 280: 20130016.

Berthold, P., Wiltschko, W., Miltenberger, H. & Querner, U. (1990). Genetic transmission of migratory behaviour into a nonmigratory bird population. Experientia. 46(1): 107-108.