This paper analyzes the variations in the detectability of eight bird species censused by car in open areas of Central Spain during winter. Species-specific detectability was highly and positively correlated with bird size, bird-group size and percentage of time spent flying. These three variables explained 98 % of the variance in species detectability. We conclude that interspecific comparisons of the abundance of similar sized large bird species censused visually by car may not be valid due to their different behaviour.
