The revealed correlation between song level and background
noise does not necessarily imply causation.
However, birds recorded in noisier territories were
neither bigger (
β
=
−
0·232,
t
=
−
0·563,
P
= 0·597; Fig. 3)
nor heavier (
β
= 0·329,
t
= 0·859,
P
= 0·430; Fig. 4).
Furthermore, the additional measurements of song
levels on weekend days (when there was less traffic
in the morning hours and environmental noise levels
were lower) yielded intra-individual changes in vocal
intensities dependent on natural variation of noise
levels, indicating that the birds indeed adjusted the
sound pressure levels of their songs depending on the
background noise (Fig. 5).