It is widely recognized that the accuracy of bird counts are subject to significant biases when conducted by different observers, unless a correction procedure is used. How best we can correct the bias?
There si no biais if obsevers share their respective observations at the moment they observe the birds. Usually observers are looking in oposit directions following a circular screening of the counted location. Each must survey during a defined time. When a bird / animal is sighted by one observer, he immediatly informs the second observer. They both stop their time of observation to share this sighting. Then they start again where they stopped. You need for that to have chronometers. DOing so you avoid double counting an the time of obseravation / observer is respected.