Function of juvenile plumage in the northern goshawk (Accipiter gentilis): Aggressive mimicry hypothesis

Multiple raptors show juvenile plumage that is substantially different
from that of their parents. Here, we test the prediction that the
colouration of the juvenile northern goshawk (Accipiter gentilis)
resembling the colouration of the common buzzard (Buteo buteo) acts as a
form of aggressive mimicry. The goshawk specialises in hunting larger
birds and mammals up to the size of geese or hares, while the buzzard
preys mostly on small rodents. Potential prey may thus consider juvenile
goshawks as less dangerous raptors, and the juvenile goshawk may thus gain
an advantage when hunting. We used the Eurasian magpie (Pica pica), a
common prey of the goshawk, to test this prediction. We compared the
behavioural responses of magpie parents defending their freshly fledged
young towards stuffed dummies of an adult goshawk, juvenile goshawk, and
buzzard. To be able to assess whether this behaviour differs from
responses to a nest predator and a harmless bird we also presented a
common raven (Corvus corax) and common pheasant (Phasianus colchicus) as
baseline stimuli. The overall intensity of antipredatory behaviour towards
the juvenile and adult goshawks did not differ, but magpies took more
risks facing juvenile goshawks. Additionally, the intensity of
antipredatory behaviour towards the juvenile goshawk was higher than
towards the buzzard, but the willingness to take risks did not differ in
relation to the two predators. The overall intensity of antipredator
behaviour thus supports the conclusion that magpies do not distinguish
between juvenile and adult goshawks, while they do distinguish juvenile
goshawks from buzzards. Conversely, the willingness to take risks supports
the conclusion that magpies do not distinguish juvenile goshawks from
buzzards.