Which small bird has an alarm call which is a sharp "tchk" repeated frequently at short intervals ? There's one in the garden that does this as soon as I and the dog come near the trees and shrubs. I feel sure it's a small bird because one flew out and the call stopped.
If it's like a teeny tiny nachine gun it's a wren. If it's like an annoyed tut repeated at 1-2 second intervals it's a blackburd. The blackbird is also louder than the wren (bigger bird).