ALTENBURG JF, HUIZENGA H & LOONSTRA J (2017) Frequent counts and resightings of colourringed Blacktailed Godwits. LIMOSA 90 (1): 3-12.

Black-tailed Godwits have been reported to stage in freshwater-wetlands during spring and autumn migration. The Everdingerwaard is a well-known staging area for Black-tailed Godwits located near Utrecht, but due to a high turnover rate of birds it remains unclear how many individuals actually use this site. In 2015 and 2016, turnover rates between subsequent roost counts were estimated on the basis of observations on colour-ringed individuals, during pre-breeding (from arrival until 1 May) and postbreeding (from 1 May until 10 June; this period mainly includes failed breeders). This revealed that 1013 (95% CI: 813- 1.365) and 746 (95% CI: 523-1071) Black-tailed Godwits used the Everdingerwaard during pre-breeding, in 2015 and 2016, respectively. These numbers were 190% and 174% higher than the maximum roost counts during the pre-breeding period in the two years. During post-breeding, 580 (95% CI: 425-995) and 336 (95% CI: 221-501) Black-tailed Godwits were estimated to use the area, in 2015 and 2016, respectively, which was 140% and 165% higher than the maximum roost count. In addition, minimum staging duration was estimated. Despite a limited sample size, we were able to show that Black-tailed Godwits breeding within a range of 50 km of the Everdingerwaard stayed longer in the area during the pre-breeding period of 2016 and the post-breeding period of 2015 compared to birds breeding elsewhere. Our results illustrate the importance of combining frequent counts with observations on colour-ringed birds in order to accurately estimate the true number of birds using a staging area. Results on minimum staging duration raise new questions about the individual use of the Everdingerwaard during the different periods.

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