White-throated Dipper and Grey Wagtail are typical inhabitants of rivers and streams. Both feed to large extent on freshwater invertebrates. The Dipper is highly specialized, with larvae of caddisflies as the main prey, followed by stoneflies, mayflies, net winged midges and blackflies. Freshwater invertebrates are an important prey for the Grey Wagtail, too. However, the species is less specialized and terrestrial insects can make up a large part of its prey. In order to analyse the relationships between the occurrence of Dipper and Grey Wagtail and the quality and abundance of the invertebrate community at a country-wide level, we combined data from the Swiss Breeding Bird Atlas 2013-2016 with data issued from various national and cantonal monitoring projects focused on freshwater invertebrates. Both Dipper and Grey Wagtail had the highest abundance at rivers and streams with a good or very good biological condition according to the occurring freshwater invertebrates. Our results show that the standard index used in Switzerland for calculating the biological condition of watercourses (Indice biologique Suisse, IBCH) correlates with the occurrence of Dipper and Grey Wagtail at the landscape level. The Dipper appears to be much more demanding than the Grey Wagtail with respect to the biological condition of watercourses; it is completely lacking in rivers or streams of unsatisfactory condition. This can be well explained by the stronger specialization on freshwater invertebrates as prey. Our results confirm the indicator value of the Dipper for rivers and streams with good biological condition. Our results also show that prey abundance has a larger influence on the occurrence of Dipper and Grey Wagtail than the structural quality of running waters. A possible explanation could be the partly positive effect of artificial constructions. Bank extensions, bridges and tunnel entrances are often used as nesting sites by both species.

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