Reduced breeding success of Pied Flycatchers Ficedula hypoleuca along regulated rivers

Abstract

Most large rivers in northern Sweden are regulated to produce hydropower, with subsequent effects on flow dynamics and aquatic insect communities. Several studies have shown that aquatic and terrestrial systems are intimately connected via the export of emergent aquatic insects, but few have assessed how human modifications of aquatic habitats may influence this connection. We compared breeding success of the insectivorous Pied Flycatcher Ficedula hypoleuca in near‐riparian upland forests along two regulated and two free‐flowing large rivers in northern Sweden over 3 years. The regulated rivers showed lower aquatic insect export to the surroundings, as a consequence of regulation‐induced loss of suitable aquatic insect habitats. Survival of Pied Flycatcher nestlings was 10–15% greater along the free‐flowing rivers. Females breeding near the free‐flowing rivers also started egg‐laying earlier and with greater synchrony than those at the regulated rivers, and showed a smaller decrease in weight during breeding than did females along the regulated rivers. However, there were no differences in occupation rate, clutch size or number of successfully hatched juveniles between regulated and free‐flowing rivers. As regulated rivers showed lower abundance of flying aquatic insects, which may also reduce the abundance of terrestrial invertebrate prey, regulation‐induced changes in the export of emergent aquatic insects may explain both directly and indirectly the observed reduction in Pied Flycatcher breeding success along regulated rivers. Large‐scale river regulation may therefore impair the breeding success of insectivorous birds through impacts on prey availability.