The challenge hypothesis (Wingfield et al. in
Am Nat 136:829-846, 1990) predicts that circulating
testosterone increases when socially monogamous male
birds are challenged during breeding. Although the chal-
lenge hypothesis has been confirmed in large-scale
interspecific comparisons of seasonal hormone profiles,
experimental tests of the challenge hypothesis are still
uncommon and the results equivocal. We tested one of
the predictions of the challenge hypothesis by investi-
gating the behavioural and hormonal response of free-
living European robins during simulated territorial
intrusions (STIs) in the breeding season. We conducted
STIs by placing a stuffed decoy in a territory and playing
robin song. After the behaviour of the focal male had
been recorded for at least 10 min, it was captured and a
blood sample was taken immediately. Controls were
caught within 10 min of the first response of the territory
owner. Hormone concentrations were measured by radio-
immunoassay. Although previous studies have shown that
testosterone has an impact on aggression, European robins
do not respond to STIs by increasing circulating levels of
testosterone.