OOSTERHUIS R (2020) Dramatic decline of Eurasian Tree Sparrow Passer montanus in Groningen, the Netherlands. LIMOSA 93 (1): 28-30.

From 2010 onwards I counted the number of Tree Sparrows on two feeding spots in gardens in the villages of Leek and Lettelbert (Table 1), which were also visited by House Sparrows. I tallied bird numbers from pictures made by motion detection cameras.
By means of colour marked House Sparrows, I was able to estimate the yearly number of sparrows visiting the feeding spots. House Sparrow populations amounted to 179-207 individuals in Leek and 83-111 individuals in Lettelbert, without any apparent trend (Fig. 1). The proportion of Tree Sparrows in Leek decreased from 5.6% in the winter of 2010/11 to 0.03% in 2018/19. In Lettelbert the proportion declined from 9.2% in 2010/11 to 0.16% in 2018/19 (Fig. 1). The annual decrease for Leek was 44%, for Lettelbert 47%. As during the study period House Sparrow numbers remained stable, I assume the decline in proportion of Tree Sparrows reflects a population decline. Despite major differences in land use between Leek and Lettelbert (Tab. 1), the decline progressed at the same dramatic rate. Feeding ad libitum obviously could not prevent these Tree Sparrow populations from collapse.

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