Thanks Dibakar Pal, my question was how to reduce bias when point counts of bird are done by 2 or more different persons?

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.

cheers

Yves

You can train the observers prior to data collection. Use objects at known distances in an environment as close as possible to the study sites. Once they all are measuring distances similarly, the team is ready to go to the field. In our experience, that is very effective.

In my experience, especially without training, a single observer has plenty of variability in distance estimations (across habitats and even across sampling sessions). I've tested for the effect of observers in animal abundances in forests, and they were negligible when sampling size was large enough.

Cheers,

Gustavo