Effect of a covariate for detection and subsampling on bias of population trends.

<p>Bias (%year^-1) in estimated trend in migration counts (estimated—simulated trend), when trend was estimated with and without a covariate for detection, and with and without subsampling to every third or fifth observation day. All datasets were simulated to have a declining population trend of 1.2%year^-1 and a systematic increase in stopover duration (indexed by a linear increase in daily survival probability between 0.40–0.50, 0.35–0.55, 0.30–0.60, 0.25–0.65 or 0.20–0.70) over a 20-year time series. Lines of the boxplots represent the 25^th percentile, median and 75^th percentile of bias estimates across 100 simulated datasets. The horizontal dashed line depicts no bias in estimated trend.