Figure 1. First autumn orientation data (seasons 2010, 2011 and 2013) of all birds when they were tested in the normal magnetic field without access to visual cues.

The birds had not developed a functional magnetic compass during their first autumn (mean = 23°, r = 0.17, n = 41, P = 0.295). Black dots denote individual mean headings of the birds, which later became the experimental group; open circles denote individual mean headings of the birds, which later were part of the control group. The arrow indicates the overall group mean orientation direction relative to magnetic North (mN). The dotted circle indicates the mean vector length required for 5% significance according to the Rayleigh test.