Correlates of Breeding Success of Horned Grebes in Scotland

A study was carried out on breeding Horned (Slavonian) Grebes (Podiceps auritus) in Scotland to investigate causes of low breeding success. Clutch and brood survival were examined separately, as well as overall breeding success. For those clutches where the cause of loss was known, most were lost to avian predators. Clutch survival was negatively correlated with crow (Corvus spp.) frequency and variability in water levels. Brood survival was lower where the introduced fish species, Pike (Esox lucius) was present and higher where the water was clearer. These factors may have affected predation rates and foraging efficiency, respectively. Overall productivity (young fledged per pair) was negatively related to the frequency of crows and number of bank anglers, who can disturb grebes. Remains of 19 fully-grown birds were found at five lakes. Some had been skinned and dismembered in a similar way, suggesting predation by a mustelid. Steps to improve productivity should include education of bank anglers, and perhaps selective removal of Pike and crows. Alternatively, breeding habitat could be improved at lakes where predators are less common.