Exploitation of prey by a river bird, the dipper Cinclus cinclus (L.), along acidic and circumneutral streams in upland Wales

Abstract

SUMMARY. 1. The diet of the Eurasian dipper Cinclus cinclus. a riverine bird from a globally widespread genus, was assessed through all stages of its annual cycle using published data and field studies from streams of contrasting chemistry. Time‐activity budgets were also compiled throughout the year and used to estimate annual energy requirements from the stream ecosystem.2. The annual energy requirements for a territorial pair ranged from 148,000 to 158,000 kJ yr−1 depending on whether one or two broods were reared. After allowing for assimilation efficiency, these requirements were estimated to represent 10.5–11.0 kg dry mass of fish and invertebrates. Using representative values for territory size (4680–11,250 m2), annual exploitation of secondary production was estimated at 0.93–2.35 g dry mass m−2.3. Several features combined to focus the predatory load on certain organisms over different stages of the annual cycle. These included the availability and selection of alternative prey, the need to provision nestlings with large items such as trichopteran larvae, and the use by females of calcium‐rich prey such as fish prior to egg formation.4. Across their range of territory size, annual exploitation (dry mass) by dippers was estimated at 0.06–0.29 g m−2 for Plecoptera. 0.02–0.22 g m−2 for Ephemeroptera. 0.59–1.11 g m−2 for Trichoptera and 0–0.78 g m−2 for fish. Exploitation of Ephemeroptera. I richoptera and fish were all highest in circumneutral streams, but exploitation of Plecoptera was highest in acidic streams because other prey were scarce.5. Cottids dominated the fish component of the diet. Influences on their density could be substantial according to available data on production in Welsh streams. Hydropsychids and limnephilids dominated the trichopteran component, with exploitation again representing potentially substantial amounts of production. Contemporaneous data are required on benthic production and exploitation by the birds.6. The ecological role of birds in rivers particularly, and aquatic ecosystems generally, is currently neglected but worthy of considerable research effort.