sub setFlyBehaviour {
    caller(0)->isa(__PACKAGE__) || confess "setFlyBehaviour is protected";
    my $closure      = shift;
    my $flyBehaviour = shift;
    &{ $closure }("FLYBEHAVIOUR", $flyBehaviour);
}
    
sub setQuackBehaviour { 
    caller(0)->isa(__PACKAGE__) || confess "setQuackBehaviour is protected";
    my $closure      = shift;
    my $quackBehaviour = shift;
    &{ $closure }("QUACKBEHAVIOUR", $quackBehaviour);
}