| Summary: | The rock ptarmigan (Lagopus muta) is the most important game bird in Iceland and a keystone species within the terrestrial food web, making monitoring of its population essential to assure a sustainable harvest. One of the population factors monitored is spring density of territorial cocks, surveyed yearly in all parts of Iceland. Since 1999, ptarmigan counts have been conducted also on road transects using the distance sampling technique. The goal of this study is to validate the road transect approach and use the obtained population indices to study rock ptarmigan population dynamics, trends, and synchrony in numbers among populations. The data analysed in this study was collected on 18 road transects for 1999–2020. Comparison of time series derived using road transects and time series from same areas but surveyed with different techniques, confirms the reliability of the road transects approach. The population dynamics were studied using nonlinear time series modelling. Only four of the time series show significant signs of cyclicality (significant autocorrelation at half the dominant period), with a period of 5 (in two cases), 10 and 14 years. Fitted ARX models show no trend in density for nine time series, while three series show a positive trend and two a negative trend. Synchrony among populations is common between adjacent areas, while one-year lag is more common with increasing distance between areas. The derived time series are still relatively short and this should be kept in mind when interpreting the results. Rjúpan (Lagopus muta) er mikilvægasta bráð skotveiðimanna á Íslandi og hún er að auki lykiltegund í fæðuvef á þurrlendi. Vöktun tegundarinnar er stór þáttur í því að tryggja sjálfbæra nýtingu stofnsins. Þéttleiki óðalskarra er einn af þeim stofnþáttum sem vaktaður er árvisst. Frá 1999 hafa rjúpur verið taldar á vegsniðum samkvæmt aðferðafræði sem kölluð er distance sampling. Markmið þessa verkefnis er að sannreyna gildi vegsniða og nota stofnvísitölur byggðar á slíkum talningum til að ...
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