Importance of shallow river topography for inland breeding Common Terns

An increasing number of inland Common Tern Sterna hirundo populations nowadays breed in artificial habitats. Inland breeding Common Terns still intensively use rivers while foraging, but the characteristics of their preferred foraging habitats are not well known. In this study we combined telemetric field research and river hydrology modelling to investigate whether Common Terns breeding near Zagreb show a preference for shallow river topography. Our results show that Common Terns breeding close to a preferred foraging area visit shallower parts of the river more when foraging than a random distribution of points along the river would suggest. Furthermore, we show that terns breeding on an artificial island further away from a preferred foraging area exhibit higher distance and duration of foraging trips. They also have a larger foraging range and shorter nest presence. The observed changes in foraging and nesting efforts may affect breeding success as well as adult fitness and survival, influencing the health of the entire population. These findings facilitate the identification of preferred feeding areas as those shallower than 1 m, and indicate that positioning artificial breeding sites closer to these areas could increase population fitness. Furthermore, our results offer useful input for river restoration projects, as well as for analysing the environmental impact of interventions in the riverbed.

Automated summary

An increasing number of inland Common Tern populations breed in artificial habitats. This study investigated the preference of Common Terns for shallow river topography by combining telemetric field research and river hydrology modelling. The results showed that breeding Common Terns near a preferred foraging area visit shallower parts of the river more frequently, indicating a preference for shallow water.