Multiple studies in sticklebacks (Gasterosteus sp.)
have found that aggression and boldness were correlated
in some populations, but uncorrelated in populations
that appeared to have lower predation risk
(Bell 2005; Dingemanse et al. 2007), and exposure
to predation caused the correlation between boldness
and aggression to arise (Bell & Sih 2007). These
studies on sticklebacks demonstrate that not all correlations
between aggression and boldness reflect
fixed behavioural syndromes, but that behavioural
plasticity can play an important role in generating
correlations between behaviours. If predation pressure
is lower on the urban song sparrows, we may
be observing a similar phenomenon. At present,
however, it is unknown if predation pressure differs
between our urban and rural populations.