THE USE OF PLAYBACK
You really need to judge each case separately, but should always follow local restrictions (some areas/reserves don’t allow playback). Recently, there was a lengthy discussion on a scientific Neotropical board and the conclusion reached was comparable to the following:
First, there apparently have been NO THOROUGH SCIENTIFIC SURVEYS actually checking the possible effect playback has on birds. Overall, most scientists agree that playback if done using common sense, pose few problems. The big problem often arise when people keep on playing the tape endlessly - if the bird doesn't react after 15 seconds or so, forget it and stop. And if a bird reacts let it be, no continuous driving it back and forth causing far more stress than needed. If done in a sensible manner, the bird will think another individual entered its territory, but it rapidly got chased out, and it will continue with its usual life almost immediately afterwards. Note that this happens naturally on a daily basis in most bird species, and really doesn't pose a big problem in that case (playback is basically imitating nature). Special caution should be taken when dealing with rare birds or birds during the breeding season, and regularly a complete "no" to playback may be the best approach in those situations. Furthermore, areas visited continuously by birders pose a particular problem. In areas (both in the Neotropics and in Asia; I'm sure elsewhere too) where playback is used more or less continuously, it has been observed that birds get "played out". They quite simply don't respond anymore. On the other hand there is no proof that the birds actually disappear (but again note that no thorough scientific surveys have been done, meaning that we don't know for sure). However, in my opinion the question isn't if the birds actually disappear. The fact that their behaviour is significantly altered should be enough to ban it completely in those cases. So, in the end I have no prob's using playback - and yes - have used it in various parts of the World. If you ever are to use it, check above and follow the basic rules – common sense is the clue…