I photographed this Blackbird in a hedge at the Hermitage a couple of days ago. A smart male, like many of the birds along the burn, it seemed unconcerned by my presence, but as I fired off a couple of shots it wing-flicked.
The wing-flick, which is caught in the shot to the right, is a rapid extension of the primaries, of one wing in this case. The bird probably uttered a soft alarm type call simultaneously, but I can't remember for sure.
Now, this isn't an uncommon behaviour, but it is one that in the past I had dismissed as an indication of the bird being 'caught in two minds' - like it can't quite decide whether to flee or not. This time, though, I had a couple of images to study on the camera LCD, and the images reveal what appears to be a very deliberate and controlled behaviour - the bird is clearly moving one wing only, and does not appear to be preparing to flee. Also, the wing that is being flicked is the one that very visible from my viewpoint. The penny dropped - the wing-flick is a pursuit-deterrent signal...
A pursuit-deterrent signal is a communication between a potential prey animal and its potential predator. In doing so, the prey individual makes itself more obvious to the predator, but in doing so it communicates that the predator has been spotted, has lost the element of surprise and if it continues its pursuit is unlikely to be successful.
Notice that this pursuit-deterrent signal also appears to be an honest signal - that is, the bird is showing off the quality of its wing feathers as well as its speed of wing movement - the very features that are going to be utilised in any escape attempt. A sharp-eyed predator would end up knowing it has been spotted and also have it rammed home that this prey is not likely to be caught.
I am not aware of any studies of this behaviour in thrushes, but it seems that the behaviour has been studied in the Stonechat. This study found that although wing-flicking had been assumed to be a flight-intention signal, it generally did not precede flight. Instead, it was a signalling behaviour used during the establishment of territory and in the distraction of nest predators.
No doubt there are many examples of pursuit-deterrent signals out there. Here is one more, a Great Basin Fence Lizard, photographed in the Joshua Tree National Park, California, honest signalling using its push-up display. This was repeated rapidly several times to make sure that I got the message that this lizard had definitely been working out and was no push over! Notice that it emphasised the message by flashing a neon blue throat at the same time. For more honest signalling push up displays in lizards, see here.