Many analyses do not consider the problems associated with the effects of population size on encounter recording. Population size could impact on the detection of bird arrival time as there is a higher probability of observing earlier arrival when the population size is greater and the song activity of birds is increased, as occurs with a larger population. As a case study, we have analysed data on the red-backed shrike Lanius collurio collected in Western Poland during 1983-2000. In this period the red-backed shrike's return to its breeding sites became significantly earlier whilst the contemporary population size increased significantly. To eliminate linear trends through time we have worked on the standardised residuals from regression of both arrival time and population size on year. The correlation between arrival time and population size residuals was significantly negative, further supporting the link between detection and population size. This finding suggests that, in studies of avian migration and its changes over time, the relationship between arrival date and population size needs to be considered.