The influence of a river bird, the dipper (Cinclus cinclus), on the behaviour and drift of its invertebrate prey

Abstract

1. We examined the behavioural response of stream macroinvertebrates to real and simulated predatory activity by a river bird, the Eurasian dipper, Cinclus cinclus L.2. In the field, we assessed whether invertebrate drift changed in response to live dippers in enclosures; we found no effects on the drift of any of the five families for which individual analyses were possible, either because it was infrequent, or involved distances too short to be detected (< 0.5 m).3. In a laboratory stream, we observed prey during encounters with a model dipper which simulated flight, swimming, bill contact with the prey, and stone turning. Invertebrate families varied in their response. Simuliids and hydropsychid caddis lacked effective escape behaviour, consistent with heavy losses through predation by dippers in the wild. Other families either drifted (Baetidae, Gammaridae) or moved away (Heptageniidae, Ephemerellidae, Leuctridae, Perlidae) from the model dipper, but responded only to bill contact or simulated stone turning. Such delayed responses would not protect individuals directly targeted by foraging dippers and partly explain the lack of detectable effects by dippers on drift in the field.4. We suggest why invertebrates do not show more marked escape responses to this important predator.