Lake characteristics and their selection by breeding Slavonian Grebes Podiceps auritus in Scotland

Abstract

Capsule Slavonian (Horned) Grebes Podiceps auritus bred on moderately sized lakes, preferring those with small fish, clear water, and over-hanging trees. Aim To describe breeding habitat and investigate factors likely to influence lake selection by breeding Slavonian Grebes in Scotland, and thereby help potential conservation measures. Methods We described and compared ecological parameters at lakes that were, and were not, used by grebes. We tested (by logistic regression) whether selection was influenced by: (1) the main food species (invertebrates and two small fish – Three-spined Sticklebacks and Minnows) and the ability to hunt fish; (2) nesting habitat (dense Bottle Sedge-beds and branches of over-hanging trees); (3) presence of introduced Pike (a predatory fish); and (4) human disturbance. Results Slavonian Grebes in Scotland bred in moderately sized lakes (median size = 5.5 ha), in catchments dominated by moorland, and at an average altitude of 220 m. Most nest-sites were in beds of dense Bottle Sedge, and sometimes in the branches of trees that trailed in the water. On average, the pH of the water was neutral, nutrient status low and water clarity high. Most of the grebe lakes had small fish, and the main large invertebrates were amphipods, Ephemeroptera and Trichoptera. Grebe lakes tend to be hundreds of metres from roads and houses. Statistical analysis indicated that grebes selected lakes that had small fish and, to a lesser extent, clear water, which would make pursuit hunting easier. Lakes with Pike tended to have no small fish, thereby reducing the suitability of lakes for grebes. There was some evidence that lakes selected by grebes also had more shoreline trees, which can be used as nesting habitat, sometimes in conjunction with sedges. Conclusion Conservation measures to improve lakes for Slavonian Grebes should include ensuring Sticklebacks occur in lakes with clear water. Nesting habitat might be improved by planting Eared Willows in sedge-beds. Translocation of Pike should be stopped.